Category Archive Anatomy A – Z

Cephalic Phase

The cephalic phase of gastric secretion occurs before food enters the stomach due to neurological signals.

The cephalic phase of gastric secretion occurs before food enters the stomach, especially while it is being eaten. It results from the sight, smell, thought, or taste of food; and the greater the appetite, the more intense is the stimulation.

Neurogenic signals that initiate the cephalic phase of gastric secretion originate from the cerebral cortex, and in the appetite centers of the amygdala and hypothalamus. They are transmitted through the dorsal motor nuclei of the vagi, and then through the vagus nerve to the stomach.

This phase of secretion normally accounts for about 20% of the gastric secretions that are associated with eating a meal. Since this enhanced secretory activity is brought on by the thought or sight of food it is a conditioned reflex—it only occurs when we like or want food. When one’s appetite is depressed this part of the cephalic reflex is inhibited.

The cephalic phase causes ECL cells to secrete histamine and increase HCl acid in the stomach. There will also be an influence on G cells to increase gastrin circulation.

Chain of Events for the Nervous System and Hormone System

• Thinking of food (i.e., smell, sight) stimulates the cerebral cortex.
• The cerebral cortex sends messages to the hypothalamus, the medulla, and the parasympathetic nervous system via the vagus nerve, and to the stomach via the gastric glands in the walls of the fundus and the body of the stomach.
• The gastric glands secrete gastric juice.
• When food enters the stomach, the stomach stretches and activates stretch receptors.
• The stretch receptors send a message to the medulla and then back to the stomach via the vagus nerve.
• The gastric glands secrete more gastric juice.
• Chemical stimuli (i.e., partially digested proteins, caffeine) directly activate G cells (enteroendocrine cells) that are located in the pyloric region of the stomach to secrete gastrin; this, in turn, stimulates the gastric glands to secrete gastric juice.

Gastric Phase

The gastric phase is a period in which swallowed food activates gastric activity in the stomach.

The gastric phase is a period in which swallowed food and semi-digested protein ( peptides and amino acids ) activate gastric activity. About two-thirds of gastric secretion occurs during this phase.

Ingested food stimulates gastric activity in two ways: by stretching the stomach and by raising the pH of its contents.

Stretching activates two reflexes: a short reflex is mediated through the myenteric nerve plexus; and a long reflex is mediated through the vagus nerves and brainstem.

Gastric Secretion

Gastric secretion is stimulated chiefly by three chemicals:

• Acetylcholine (ACh). This is secreted by the parasympathetic nerve fibers of both the short and long reflex pathways.
• Histamine. This is a paracrine secretion from the enteroendocrine cells in the gastric glands.
• Gastrin. This is a hormone produced by enteroendocrine G cells in the pyloric glands.

All three of these stimulate parietal cells to secrete hydrochloric acid and intrinsic factor. The chief cells secrete pepsinogen in response to gastrin and especially ACh, and ACh also stimulates mucus secretion.

As dietary protein is digested, it breaks down into smaller peptides and amino acids that directly stimulate the G cells to secrete even more gastrin: this is a positive feedback loop that accelerates protein digestion.

Small peptides also buffer the stomach acid so the pH does not fall excessively low. As digestion continues and these peptides empty from the stomach, the pH drops lower and lower. Below pH of 2, stomach acid inhibits the parietal cells and G cells: this is a negative feedback loop that winds down the gastric phase as the need for pepsin and HCl declines.

 

The gastric phase of digestion: During the gastric phase, gastrin is secreted. The stomach stretches and churns while enzymes break down proteins.

Intestinal Phase

The intestinal phase occurs in the duodenum as a response to the arriving chyme, and it moderates gastric activity via hormones and nervous reflexes.

The intestinal phase occurs in the duodenum as a response to the arriving chyme, and it moderates gastric activity via hormones and nervous reflexes. The duodenum initially enhances gastric secretion, but soon inhibits it. The stretching of the duodenum accentuates vagal reflexes that stimulate the stomach, and peptides and amino acids in the chyme stimulate the G cells of the duodenum to secrete more gastrin, which further stimulates the stomach.

Soon, however, the acid and semi-digested fats in the duodenum trigger the enterogastric reflex. That is, the duodenum sends inhibitory signals to the stomach by way of the enteric nervous system, while also sending signals to the medulla that inhibit the vagal nuclei. This reduces vagal stimulation of the stomach and stimulates sympathetic neurons that send inhibitory signals to the stomach.

 

Duodenum: The intestinal phase of digestion occurs in the duodenum, the first segment of the small intestine.

Chyme

Chyme also stimulates duodenal enteroendocrine cells to release secretin and cholecystokinin. These hormones primarily stimulate the pancreas and gallbladder, but they also suppress gastric secretion and motility. The effect of this is that gastrin secretion declines and the pyloric sphincter contracts tightly to limit the admission of more chyme into the duodenum. This gives the duodenum time to work on the chyme it has received before being loaded with more.

The enteroendocrine cells also secrete glucose-dependent insulinotropic peptides. Originally called a gastric-inhibitory peptide, it is no longer thought to have a significant effect on the stomach. Rather, it probably stimulates insulin secretion in preparation for processing the nutrients that are about to be absorbed by the small intestine.

Hormones of the Digestive System

There are five main hormones that aid and regulate the digestive system in mammals.

There are five main hormones that aid in the regulation of the digestive system in mammals. There are variations across the vertebrates, such as birds, so arrangements are complex and additional details are regularly discovered. For instance, more connections to metabolic control (largely the glucose-insulin system) have been uncovered in recent years.

• Gastrin is in the stomach and stimulates the gastric glands to secrete pepsinogen (an inactive form of the enzyme pepsin) and hydrochloric acid. The secretion of gastrin is stimulated by food arriving in the stomach. The secretion is inhibited by low pH.
• Secretin is in the duodenum and signals the secretion of sodium bicarbonate in the pancreas and it stimulates the secretion of bile in the liver. This hormone responds to the acidity of the chyme.
• Cholecystokinin (CCK) is in the duodenum and stimulates the release of digestive enzymes in the pancreas and stimulates the emptying of bile in the gallbladder. This hormone is secreted in response to the fat in chyme.
• Gastric inhibitory peptide (GIP) is in the duodenum and decreases stomach-churning in order to slow the emptying of the stomach. Another function is to induce insulin secretion.
• Motilin is in the duodenum and increases the migrating myoelectric complex component of gastrointestinal motility and stimulates the production of pepsin.

Appetite-Regulating Hormones

There are hormones secreted by tissues and organs in the body that are transported through the bloodstream to the satiety center, a region in the brain that triggers impulses that give us feelings of hunger or aid in suppressing our appetite. Ghrelin is a hormone that is released by the stomach and targets the pituitary gland, signaling to the body that it needs to eat.

PYY is a hormone that is released by the small intestine to counter ghrelin. It is released by the hypothalamus and signals that you have just eaten and helps to suppress our appetite.

The pancreas releases the hormone insulin that targets the hypothalamus and also aids in suppressing our appetite after we have just eaten and there is a rise in blood glucose levels.

The last hormone is leptin, which also helps to suppress appetite. Leptin is produced by adipose fat tissue and targets the hypothalamus.

 

Digestive hormones: The action of the major digestive hormones.

Function

Digestion is a process that converts nutrients in ingested food into forms that can be absorbed by the gastrointestinal tract. Proper digestion requires both mechanical and chemical digestion and occurs in the oral cavity, stomach, and small intestine. Additionally, digestion requires secretions from accessory digestive organs such as the pancreas, liver, and gallbladder. The oral cavity, stomach, and small intestine function as three separate digestive compartments with differing chemical environments. The oral cavity provides significant mechanical digestive functions and minor chemical digestion at a pH between 6.7 and 7.0. The oral cavity requires separation from the acidic environment of the stomach with a pH of 0.8 to 3.5. As such, enzymes such as alpha-amylase secreted by salivary glands in the oral cavity and also by the pancreas cannot function in the stomach, and thus digestion of carbohydrates does not occur in the stomach. However, in the stomach, significant digestion of proteins into polypeptides and oligopeptides occurs by the action of pepsin, which functions optimally at a pH of 2.0 to 3.0.

Minor digestion of lipids into fatty acids and monoacylglycerols also occurs by the action of gastric lipase secreted by chief cells in oxyntic glands of the body of the stomach. Importantly, this acidic environment of the stomach is also separated from the more basic environment of the small intestine by the tonically constricted pylorus. This functions to create an environment where the digestive enzymes produced by the pancreas and duodenum can function optimally at a pH of 6 to 7, a more basic environment than the stomach created by bicarbonate secreted by the pancreas. These separate yet coordinated digestive functions are essential to the body’s ability to absorb and utilize necessary nutrients. A defect in any aspect of this process can result in malabsorption and malnutrition amongst other gastrointestinal pathologies.

References

Show More

• https://www.ncbi.nlm.nih.gov/books/NBK544242/
• https://www.ncbi.nlm.nih.gov/books/NBK22172/
• https://www.ncbi.nlm.nih.gov/books/NBK537103/
• https://www.ncbi.nlm.nih.gov/books/NBK500174/
• https://www.ncbi.nlm.nih.gov/books/NBK482334/
• https://www.ncbi.nlm.nih.gov/books/NBK459366/

Anatomy of the Large Intestine

The large intestine absorbs water from the remaining indigestible food matter and compacts feces prior to defecation.

Function and Form of the Large Intestine

The function of the large intestine (or large bowel) is to absorb water from the remaining indigestible food matter, and then to pass the useless waste material from the body. The large intestine consists of the cecum and colon.

Large intestine: A schematic of the large intestine, with the colon marked as follows: cecum; 1) ascending colon; 2) transverse colon; 3) descending colon; 4) sigmoid colon, rectum, and anus.

Differences Between Large and Small Intestine

The large intestine differs in physical form from the small intestine in several ways. The large intestine is much wider, and the longitudinal layers of the muscular are reduced to three, strap-like structures known as the taeniae coli.

The wall of the large intestine is lined with simple columnar epithelium. Instead of having the evaginations of the small intestine (villi), the large intestine has invaginations (the intestinal glands).

While both the small intestine and the large intestine have goblet cells, they are more abundant in the large intestine.

Additional Structures

The appendix is attached to the inferior surface of the cecum. It contains the least lymphoid tissue, and it is a part of mucosa-associated lymphoid tissue, which gives it an important role in immunity.

Appendicitis is the result of a blockage that traps infectious material in the lumen. The appendix can be removed with no apparent damage or consequence to the patient.

On the surface of the large intestine, bands of longitudinal muscle fibers called taeniae coli, each about 0.2 inches wide, can be identified. There are three bands, starting at the base of the appendix and extending from the cecum to the rectum.

Along the sides of the taeniae, tags of peritoneum filled with fat, called epiploic appendages (or appendices epiploicae) are found. The sacculations, called haustra, are characteristic features of the large intestine and distinguish it from the small intestine.

Histology of the Large Intestine

The large intestine has taeniae coli and invaginations (the intestinal glands), unlike the small intestines.

Histology of the Large Intestine

Colon biopsy: Micrograph of a colon biopsy.

Sigmoid colon: A photograph of the large bowel (sigmoid colon) that shows multiple diverticula on either side of the longitudinal muscle bundle (Taenia coli).

In histology, an intestinal crypt—called the crypt of Lieberkühn—is a gland found in the epithelial lining of the small intestine and colon. The crypts and intestinal villi are covered by epithelium that contains two types of cells: goblet cells that secrete mucus and enterocytes that secrete water and electrolytes.

The enterocytes in the mucosa contain digestive enzymes that digest specific food while they are being absorbed through the epithelium. These enzymes include peptidases, sucrase, maltase, lactase, and intestinal lipase. This is in contrast to the stomach, where the chief cells secrete pepsinogen. In the intestine, the digestive enzymes are not secreted by the cells of the intestine.

Also, the new epithelium is formed here, which is important because the cells at this site are continuously worn away by the passing food. The basal portion of the crypt, further from the intestinal lumen, contains multipotent stem cells.

During each mitosis, one of the two daughter cells remains in the crypt as a stem cell, while the other differentiates and migrates up the side of the crypt and eventually into the villus. Goblet cells are among the cells produced in this fashion. Many genes have been shown to be important for the differentiation of intestinal stem cells.

The loss of proliferation control in the crypts is thought to lead to colorectal cancer.

Bacterial Flora

The largest bacteria ecosystem in the human body is in the large intestine, where it plays a variety of important roles.

Bacterial Flora

The large intestine houses over 700 species of bacteria that perform a wide variety of functions; it is the largest bacterial ecosystem in the human body. The large intestine absorbs some of the products formed by the bacteria that inhabit this region.

For example, undigested polysaccharides (fiber) are metabolized to short-chain fatty acids by the bacteria in the large intestine, and then are absorbed by passive diffusion. The bicarbonate that the large intestine secretes helps to neutralize the increased acidity that results from the formation of these fatty acids.

Bacteria and Vitamins

Bacterial flora: Escherichia coli is one of the many species of bacteria present in the human gut.

These bacteria also produce large amounts of vitamins, especially vitamin K and biotin (a B vitamin), for absorption into the blood. Although this source of vitamins, in general, provides only a small part of the daily requirement, it makes a significant contribution when dietary vitamin intake is low.

An individual who depends just on the absorption of vitamins formed by bacteria in the large intestine may become vitamin deficient if treated with antibiotics that inhibit other species of bacteria, as well as the disease-causing bacteria.

Other bacterial products include gas (flatus), which is a mixture of nitrogen and carbon dioxide, with small amounts of hydrogen, methane, and hydrogen sulfide. These are produced as a result of the bacterial fermentation of undigested polysaccharides. The normal flora is also essential for the development of certain tissues, including the cecum and lymphatics.

Bacteria and Antibodies

Bacterial flora is also involved in the production of cross-reactive antibodies. These are antibodies produced by the immune system against the normal flora, that is also effective against related pathogens, and prevent infection or invasion.

The most prevalent bacteria are the Bacteroides, which have been implicated in the initiation of colitis and colon cancer. Bifidobacteria are also abundant and are often described as friendly bacteria.

A mucus layer protects the large intestine from attacks from colonic commensal bacteria. Some factors that disrupt the microorganism population of the large intestine include antibiotics, stress, and parasites.

Digestive Processes of the Large Intestine

In the large intestine, a host of microorganisms known as gut flora help digest the remaining food matter and create vitamins.

Overview of the Large Intestine

Digestive processes in large intestine: This image shows the relationship of the colon to the other parts of the digestive system.

Gut Flora

Gut flora consists of microorganisms that live in the digestive tracts of animals—the gut is the largest reservoir of human flora. The human body, which consists of about 10 trillion cells, carries about ten times as many microorganisms in the intestines.

The metabolic activities performed by these bacteria resemble those of an organ, leading some to liken gut bacteria to a forgotten organ. It is estimated that these gut flora have around a hundred times as many genes in aggregate as there are in the human genome.

Bacteria make up most of the flora in the colon and up to 60 percent of the dry mass of feces. Somewhere between 300 and 1000 different species live in the gut, with most estimates at about 500. Ninety-nine percent of the bacteria probably come from about 30 or 40 species.

Research suggests that the relationship between gut flora and humans is not merely commensal (a non-harmful coexistence), but rather a mutualistic relationship. Though people can survive without gut flora, the microorganisms perform a host of useful functions, such as:

• Fermenting unused energy substrates.
• Training the immune system.
• Preventing growth of harmful, pathogenic bacteria.
• Regulating the development of the gut.
• Producing vitamins for the host (such as biotin and vitamin K).
• Producing hormones to direct the host to store fats.

Gut Flora and Specialized Digestion

Bacterial flora: Escherichia coli, one of the many species of bacteria present in the human gut.

Without gut flora, the human body would be unable to utilize some of the undigested carbohydrates it consumes. Some types of gut flora have enzymes that human cells lack for breaking down certain polysaccharides. Carbohydrates that need bacterial assistance for digestion include:

• Certain starches.
• Fiber.
• Oligosaccharides and sugars like lactose (in the case of lactose intolerance) and sugar alcohols.
• Mucus is produced by the gut.
• Various proteins.

Fermentation

Bacteria turn the carbohydrates they ferment into short-chain fatty acids (SCFAs) by a form of fermentation called saccharolytic fermentation. These SCFAs include acetic acid, propionic acid, and butyric acid.

SCFAs can be used by host cells as a major source of useful energy and nutrients for humans. They also help the body absorb essential dietary minerals such as calcium, magnesium, and iron. Evidence indicates that bacteria enhance the absorption and storage of lipids and produce and aid the absorption of needed vitamins, such as vitamin K.

Absorption and Feces Formation in the Large Intestine

The large intestine absorbs water from the chyme and stores feces until they can be defecated.

After the food has been passed through the small intestine, it enters the large intestine. Within the large intestine, digestion is retained long enough to allow fermentation via gut bacteria that break down some of the substances that remain after processing in the small intestine.

Some of the breakdown products are absorbed. In humans, these include most complex saccharides (at most, three disaccharides are digestible by humans).

Intestinal Bacteria

The large intestine houses over 700 species of bacteria that perform a variety of functions. The large intestine absorbs some of the products formed by the bacteria that inhabit this region.

Undigested polysaccharides (fiber) are metabolized into short-chain fatty acids by bacteria in the large intestine and get absorbed by passive diffusion. The bicarbonate that the large intestine secretes helps to neutralize the increased acidity from the formation of fatty acids.

Intestinal bacteria also produce large amounts of vitamins, especially vitamin K and biotin (a B vitamin), which are absorbed into the blood. Although this source of vitamins provides only a small part of the daily requirement, it makes a significant contribution when dietary vitamin intake is low. An individual that depends on the absorption of vitamins formed by bacteria in the large intestine may become vitamin-deficient if treated with antibiotics that inhibit other species of bacteria while targeting the disease-causing bacteria.

Other bacterial products include gas (flatus)—a mixture of nitrogen and carbon dioxide, with small amounts of the gases hydrogen, methane, and hydrogen sulfide. The bacterial fermentation of undigested polysaccharides produces these gases.

Intestinal flora is also essential for the development of certain tissues, including the cecum and lymphatics.

Water and Cellulose

The large intestine absorbs water from the chyme and stores feces until it can be defecated. Food products that cannot go through the villi, such as cellulose (dietary fiber), are mixed with other waste products from the body and become hard and concentrated feces.

The feces is stored in the rectum for a certain period and then the stored feces is eliminated from the body due to the contraction and relaxation of the anus. The exit of this waste material is regulated by the anal sphincter.

Defecation Reflex

Defecation is a combination of voluntary and involuntary processes that create enough force to remove waste material from the digestive system.

Defecation

For the adult human, the process of defecation is normally a combination of both voluntary and involuntary processes that create enough force to remove waste material from the digestive system.

The rectal ampulla acts as a temporary storage facility for the unneeded material. As additional fecal material enters the rectum, the rectal walls expand. A sufficient increase in fecal material in the rectum causes the stretch receptors from the nervous system, located in the rectal walls, to trigger the contraction of rectal muscles, the relaxation of the internal anal sphincter, and an initial contraction of the skeletal muscle of the external sphincter. The relaxation of the internal anal sphincter causes a signal to be sent to the brain indicating an urge to defecate.

Defecation reflex: The conscious and parasympathetic pathways of the defecation reflex.

If this urge is not acted upon, the material in the rectum is often returned to the colon by reverse peristalsis where more water is absorbed, thus temporarily reducing pressure and stretching within the rectum. The additional fecal material is stored in the colon until the next mass peristaltic movement of the transverse and descending colon. If defecation is delayed for a prolonged period, the fecal matter may harden and autolyze, resulting in constipation.

Once the voluntary signal to defecate is sent back from the brain, the final phase begins. The abdominal muscles contract (straining), causing the intra-abdominal pressure to increase. The perineal wall is lowered and causes the anorectal angle to decrease from 90 degrees to less than 15 degrees (almost straight), and the external anal sphincter relaxes.

The rectum now contracts and shortens in peristaltic waves, thus forcing fecal material out of the rectum and down through the anal canal. The internal and external anal sphincters, along with the puborectalis muscle, allow the feces to be passed by pulling the anus up and over the exiting feces in shortening and contracting actions.

The small intestine is the part of the gastrointestinal tract where much of the digestion and absorption of food takes place.

The Small Intestine

The small intestine is the part of the gastrointestinal tract that follows the stomach, which is in turn followed by the large intestine. The small intestine is the site where almost all of the digestion and absorption of nutrients and minerals from food takes place.

Small intestine: An illustration of the small intestine with the duodenum, jejunum, and ileum labeled.

The average length of the small intestine in an adult human male is 6.9 m (22 feet, 6 inches), and in the adult female 7.1 m (23 feet, 4 inches). It can vary greatly, from as short as 4.6 m (15 feet) to as long as 9.8 m (32 feet). The small intestine is approximately 2.5–3 cm in diameter, and is divided into three sections:

• The duodenum is the first section of the small intestine and is the shortest part of the small intestine. It is where most chemical digestion using enzymes takes place.
• The jejunum is the middle section of the small intestine. It has a lining that is designed to absorb carbohydrates and proteins. The inner surface of the jejunum, its mucous membrane, is covered in projections called villi, which increase the surface area of tissue available to absorb nutrients from the gut contents. The epithelial cells which line these villi possess even larger numbers of microvilli. The transport of nutrients across epithelial cells through the jejunum includes the passive transport of some carbohydrates and the active transport of amino acids, small peptides, vitamins, and most glucose. The villi in the jejunum are much longer than in the duodenum or ileum.
• The ileum is the final section of the small intestine. The function of the ileum is mainly to absorb vitamin B12, bile salts, and any products of digestion that were not absorbed by the jejunum. The wall itself is made up of folds, each of which has many tiny finger-like projections known as villi on its surface. The ileum has an extremely large surface area both for the adsorption of enzyme molecules and for the absorption of products of digestion.

The Villi

The villi contain large numbers of capillaries that take the amino acids and glucose produced by digestion to the hepatic portal vein and the liver. Lacteals are the small lymph vessels that are present in the villi. They absorb fatty acids and glycerol, the products of fat digestion, into direct circulation.

Layers of circular and longitudinal smooth muscle enable the digested food to be pushed along the ileum by waves of muscle contractions called peristalsis. The undigested food (waste and water) is sent to the colon.

Histology of the Small Intestine

The small intestine wall has four layers: the outermost serosa, muscularis, submucosa, and innermost mucosa.

The Small Intestine’s Layers

Section of duodenum: This image shows the layers of the duodenum: the serosa, muscularis, submucosa, and mucosa.

The small intestine has four tissue layers:

• The serosa is the outermost layer of the intestine. The serosa is a smooth membrane consisting of a thin layer of cells that secrete serous fluid and a thin layer of connective tissue. Serous fluid is a lubricating fluid that reduces friction from the movement of the muscular.
• The muscular is a region of muscle adjacent to the submucosa membrane. It is responsible for gut movement, or peristalsis. It usually has two distinct layers of smooth muscle: circular and longitudinal.
• The submucosa is the layer of dense, irregular connective tissue or loose connective tissue that supports the mucosa, as well as joins the mucosa to the bulk of underlying smooth muscle.
• The mucosa is the innermost tissue layer of the small intestines and is a mucous membrane that secretes digestive enzymes and hormones. The intestinal villi are part of the mucosa.

The three sections of the small intestine look similar to each other at a microscopic level, but there are some important differences. The jejunum and ileum do not have Brunner’s glands in the submucosa, while the ileum has Peyer’s patches in the mucosa, but the duodenum and jejunum do not.

Brunner’s Glands

Brunner’s glands (or duodenal glands) are compound tubular submucosal glands found in the duodenum. The main function of these glands is to produce a mucus-rich, alkaline secretion (containing bicarbonate) in order to neutralize the acidic content of chyme that is introduced into the duodenum from the stomach, and to provide an alkaline condition for optimal intestinal enzyme activity, thus enabling absorption to take place and lubricate the intestinal walls.

Peyer’s Patches

Peyer’s patches are organized lymph nodules. They are aggregations of lymphoid tissue that are found in the lowest portion of the small intestine, which differentiate the ileum from the duodenum and jejunum.

Because the lumen of the gastrointestinal tract is exposed to the external environment, much of it is populated with potentially pathogenic microorganisms. Peyer’s patches function as the immune surveillance system of the intestinal lumen and facilitate the generation of the immune response within the mucosa.

Intestinal Villi

Micrograph of the small intestine: A low-magnification micrograph of small intestinal mucosa that shows villi.

Intestinal villi (singular: villus) are tiny, finger-like projections that protrude from the epithelial lining of the mucosa. Each villus is approximately 0.5–1.6 mm in length and has many microvilli (singular: microvillus), each of which are much smaller than a single villus.

Villi increase the internal surface area of the intestinal walls. This increased surface area allows for more intestinal wall area to be available for absorption. An increased absorptive area is useful because digested nutrients (including sugars and amino acids) pass into the villi, which is semi-permeable, through diffusion, which is effective only at short distances.

In other words, the increased surface area (in contact with the fluid in the lumen) decreases the average distance traveled by the nutrient molecules, so the effectiveness of diffusion increases.

The villi are connected to blood vessels that carry the nutrients away in the circulating blood.

Digestive Processes of the Small Intestine

The small intestine uses different enzymes and processes to digest proteins, lipids, and carbohydrates.

Chemical Digestion in the Small Intestine

The small intestine is where most chemical digestion takes place. Most of the digestive enzymes in the small intestine are secreted by the pancreas and enter the small intestine via the pancreatic duct.

These enzymes enter the small intestine in response to the hormone cholecystokinin, which is produced in response to the presence of nutrients. The hormone secretin also causes bicarbonate to be released into the small intestine from the pancreas to neutralize the potentially harmful acid coming from the stomach.

The three major classes of nutrients that undergo digestion are proteins, lipids (fats), and carbohydrates.

Proteins

Proteins are degraded into small peptides and amino acids before absorption. Their chemical breakdown begins in the stomach and continues through the large intestine.

Proteolytic enzymes, including trypsin and chymotrypsin, are secreted by the pancreas and cleave proteins into smaller peptides. Carboxypeptidase, a pancreatic brush border enzyme, splits one amino acid at a time. Aminopeptidase and dipeptidase free the end amino acid products.

Lipids

Lipids (fats) are degraded into fatty acids and glycerol. Pancreatic lipase breaks down triglycerides into free fatty acids and monoglycerides. Pancreatic lipase works with the help of the salts from bile secreted by the liver and the gallbladder.

Bile salts attach to triglycerides and help to emulsify them; this aids access by pancreatic lipase because the lipase is water-soluble, but the fatty triglycerides are hydrophobic and tend to orient toward each other and away from the watery intestinal surroundings.

The bile salts act to hold the triglycerides in their watery surroundings until the lipase can break them into the smaller components that are able to enter the villi for absorption.

Carbohydrates

Some carbohydrates are degraded into simple sugars, or monosaccharides (e.g., glucose, galactose) and are absorbed by the small intestine. Pancreatic amylase breaks down some carbohydrates (notably starch) into oligosaccharides. Other carbohydrates pass undigested into the large intestine, where they are digested by intestinal bacteria.

Brush border enzymes take over from there. The most important brush border enzymes are dextrinase and glucoamylase, which further break down oligosaccharides. Other brush border enzymes are maltase, sucrase, and lactase.

Lactase is absent in most adult humans and for them, lactose, like most poly-saccharides, is not digested in the small intestine. Some carbohydrates, such as cellulose, are not digested at all, despite being made of multiple glucose units. This is because the cellulose is made out of beta-glucose that makes the inter-monosaccharide bindings different from the ones present in starch, which consists of alpha-glucose. Humans lack the enzyme for splitting the beta-glucose-bonds—that is reserved for herbivores and bacteria in the large intestine.

Pancreas

The pancreas is a gland organ in the digestive and endocrine systems.

The pancreas is a gland organ in the digestive and endocrine systems. As an endocrine gland, the pancreas produces several important hormones that include insulin, glucagon, somatostatin, and pancreatic polypeptide.

As a digestive organ, the pancreas secretes pancreatic juice that contains digestive enzymes that assist the absorption of nutrients and digestion in the small intestine. These enzymes help to further break down the carbohydrates, proteins, and lipids in the chyme.

Location

The pancreas is located posterior to the stomach and next to the duodenum. The pancreas functions as both an exocrine and endocrine gland. The exocrine function of the pancreas is essential for digestion as it produces many of the enzymes that break down the protein, carbohydrates, and fats indigestible foods.

Composition

The pancreas is composed of pancreatic exocrine cells, whose ducts are arranged in clusters called acini. The cells are filled with secretory granules containing the inactivated digestive enzymes, mainly trypsinogen, chymotrypsinogen, pancreatic lipase, and amylase, that are secreted into the lumen of the acini.

Structure and Function

Divisions

The pancreas is divided into 4 parts: head, neck, body, and tail.

The head of the pancreas is the enlarged part of the gland that is surrounded by the C-shaped curve of the duodenum. On its way to the descending part of the duodenum, the bile duct lies in a groove on the posterosuperior surface of the head or is embedded in its substance. The body of the pancreas continues from the neck passes over the aorta and L2 vertebra. The anterior surface of the body of the pancreas is covered with the peritoneum. The posterior surface of the body is devoid of peritoneum and is in contact with the aorta, the superior mesenteric artery (SMA), the left suprarenal gland, the left kidney, and renal vessels.

The neck of the pancreas is short. The tail of the pancreas lies anterior to the left kidney, where it is closely related to the splenic hilum and the left colic flexure. The main pancreatic duct carrying the pancreatic secretions joins with the bile duct to form the hepatopancreatic ampulla, which opens into the descending part of the duodenum. The hepatopancreatic sphincter of Oddi around the hepatopancreatic ampulla is a smooth muscle sphincter that controls the flow of bile and pancreatic juice into the ampulla and inhibits the reflux of duodenal substances into the ampulla.

Cell Types

The majority of the pancreas (approximately 80%) is made up of the exocrine pancreatic tissue. This is made of pancreatic acini (pyramidal acinar cells with the apex directed towards the lumen). These contain dense zymogen granules in the apical region, whereas the basal region contains the nucleus and endoplasmic reticulum (which aids in synthesizing the digestive enzymes). These enzymes are stored in secretory vesicles called the Golgi complex. The basolateral membrane of the acinar cells contains several receptors for neurotransmitters including secretin, cholecystokinin, acetylcholine, which regulate exocytosis of the digestive enzymes.

The pancreas also contains the islet of Langerhans, which contains the endocrine cells. Unlike the exocrine enzymes which are secreted by exocytosis, the endocrine enzymes enter the bloodstream via a complex capillary network within the pancreatic blood flow. There are 4 types of endocrine cells (A cells produce glucagon, B cells produce insulin, D cells produce somatostatin, and F cells produce pancreatic polypeptide).

Stellate cells are a direct formation of epithelial structures within the pancreas. In conditions like chronic pancreatitis, these cells promote inflammation and fibrosis.

Anatomy of the Pancreas

The pancreas lies in the epigastrium or upper central region of the abdomen and can vary in shape.

Variation

Pancreatic tissue is present in all vertebrate species, but its precise form and arrangement varies widely. There may be up to three separate pancreases, two of which arise from ventral buds, and the other dorsally. In most species (including humans), these fuse in the adult, but there are several exceptions.

Even when a single pancreas is present, two or three pancreatic ducts may persist, each draining separately into the duodenum (or an equivalent part of the foregut). Birds, for example, typically have three such ducts.

In teleosts, and a few other species (such as rabbits), there is no discrete pancreas at all, with pancreatic tissue being distributed diffusely across the mesentery and even within other nearby organs, such as the liver or spleen.

Anatomy of the Pancreas

The pancreas lies in the epigastrium or upper central region of the abdomen. It is composed of several parts.

• The head lies within the concavity of the duodenum.
• The uncinate process emerges from the lower part of the head and lies deep to superior mesenteric vessels.
• The neck is the constricted part between the head and the body.
• The body lies behind the stomach.
• The tail is the left end of the pancreas. It lies in contact with the spleen.

The superior pancreaticoduodenal artery from the gastroduodenal artery and the inferior pancreaticoduodenal artery from the superior mesenteric artery runs in the groove between the pancreas and the duodenum and supply the head of the pancreas.

The pancreatic branches of the splenic artery also supply the neck, body, and tail of the pancreas. The body and neck of the pancreas drain into the splenic vein; the head drains into the superior mesenteric and portal veins. Lymph is drained via the splenic, celiac, and superior mesenteric lymph nodes.

 

Parts of a pancreas: 1: Head of pancreas 2: Uncinate process of pancreas 3: Pancreatic notch 4: Body of the pancreas 5: Anterior surface of the pancreas 6: Inferior surface of the pancreas 7: Superior margin of the pancreas 8: Anterior margin of the pancreas 9: Inferior margin of the pancreas 10: Omental tuber 11: Tail of the pancreas 12: Duodenum.

Histology of the Pancreas

The pancreas serves digestive and endocrine functions, and it is composed of two types of tissue: islets of Langerhans and acini.

The pancreas is a glandular organ in the digestive system and endocrine systems of vertebrates. It is both an endocrine gland that produces several important hormones—including insulin, glucagon, somatostatin, and pancreatic polypeptide—as well as a digestive organ that secretes pancreatic juice that contain digestive enzymes to assist the absorption of nutrients and digestion in the small intestine. These enzymes also help to further break down the carbohydrates, proteins, and lipids in the chyme.

Under a microscope, stained sections of the pancreas reveal two different types of parenchymal tissue. Light-stained clusters of cells are called islets of Langerhans. These produce hormones that underlie the endocrine functions of the pancreas.

The dark-stained cells form acini that are connected to ducts. Acinar cells belong to the exocrine pancreas and secrete digestive enzymes into the gut via a system of ducts.

The pancreas is a dual-function gland that has the features of endocrine and exocrine glands.

The part of the pancreas with endocrine function is made up of approximately a million cell clusters called islets of Langerhans. Four main cell types exist in the islets. They are relatively difficult to distinguish using standard staining techniques, but they can be classified by their secretion

• α cells secrete glucagon (increase glucose in the blood ).
• β cells secrete insulin (decrease glucose in the blood).
• Delta cells secrete somatostatin (regulates/stops α and β cells).
• PP cells or gamma cells secrete pancreatic polypeptide.

The islets are a compact collection of endocrine cells arranged in clusters and cords and are crisscrossed by a dense network of capillaries. The capillaries of the islets are lined by layers of endocrine cells in direct contact with vessels, and most endocrine cells are in direct contact with blood vessels, either by cytoplasmic processes or by direct apposition.

Pancreatic Juice

The pancreatic fluid contains digestive enzymes that help to further break down the carbohydrates, proteins, and lipids in the chyme.

The pancreas is a glandular organ in the digestive system and endocrine systems of vertebrates. It is both an endocrine gland that produces several important hormones—including insulin, glucagon, somatostatin, and pancreatic polypeptide—and a digestive organ that secretes pancreatic juice that has digestive enzymes that assist the absorption of nutrients and digestion in the small intestine. These enzymes help to further break down the carbohydrates, proteins, and lipids in the chyme.

Pancreatic Juice

Pancreatic juice is a liquid secreted by the pancreas that contains a variety of enzymes, including trypsinogen, chymotrypsinogen, elastase, carboxypeptidase, pancreatic lipase, nucleases, and amylase.

 

Pancreatic fluid: A schematic diagram that shows pancreatic acini and the ducts where fluid is created and released.

The Liver

The liver makes bile, which is essential for the digestion of fats.

The Role of the Liver

The liver normally weighs between 1.3—3.0 kilograms and is a soft, pinkish-brown organ. It is the second-largest organ in the body, and is located on the right side of the abdomen.

The liver plays a major role in metabolism and has a number of functions in the body, including glycogen storage, plasma protein synthesis, and drug detoxification. It also produces bile, which is important for digestion.

The liver is supplied by two main blood vessels on its right lobe: the hepatic artery and the portal vein. The portal vein brings venous blood from the spleen, pancreas, and small intestine so that the liver can process the nutrients and byproducts of food digestion.

Bile

The bile produced in the liver is essential for the digestion of fats. Bile is formed in the liver, and it is stored in the gallbladder or released directly into the small intestine. After being stored in the gallbladder, the bile becomes more concentrated than when it left the liver; this increases its potency and intensifies its effect in digesting fats.

Anatomy of the Liver

The liver is located in the abdomen and has four lobes.

The Liver

The human liver is both the largest internal organ (the skin being the largest organ overall) and the largest gland in the human body. It is a soft, pinkish-brown, triangular organ normally weighing 1.44–1.66 kg (3.2–3.7 lb).

The liver has a wide range of functions including detoxification, protein synthesis, and the production of the biochemicals necessary for digestion. It is located in the right upper quadrant of the abdominal cavity, resting just below the diaphragm. The liver lies to the right of the stomach and overlies the gall bladder.

The liver is connected to two large blood vessels, the hepatic artery, and the portal vein. The hepatic artery carries blood from the aorta to the liver, whereas the portal vein carries blood containing the digested nutrients from the entire gastrointestinal tract, and also from the spleen and pancreas to the liver. These blood vessels subdivide into capillaries that then lead to a lobule.

Lobes of the Liver

Traditionally, the liver is divided into four lobes: left, right, caudate, and quadrate. The lobes are further divided into lobules, the functional units of the liver. Each lobule is made up of millions of hepatic cells that are the basic metabolic cells of the liver.

Histology of the Liver

Hepatocytes are the main tissue cells of the liver. The gallbladder contains the mucosa, muscularis, perimuscular, and serosa layers.

The Liver

A hepatocyte is the main tissue cell of the liver and makes up 70–80% of the liver’s cytoplasmic mass. Hepatocytes contain large amounts of rough endoplasmic reticulum and free ribosomes.

• Protein synthesis.
• Protein storage.
• The transformation of carbohydrates.
• The synthesis of cholesterol, bile salts, and phospholipids.
• The detoxification, modification, and excretion of exogenous and endogenous substances.

Hepatocytes also initiate the formation and secretion of bile. Hepatocytes are organized into plates separated by vascular channels (sinusoids) for blood vessels. The hepatocyte plates are one cell thick in mammals.

Hepatocytes are unique in that they are one of the few types of cell in the human body that are capable of regeneration. Hepatocytes are derived from hepatoblasts, the precursor stem cell of the liver that divides to produce new hepatocytes. The liver is capable of complete regeneration from as little as 25% of the original organ.

Blood Supply to the Liver

In the hepatic portal system, the liver receives a dual blood supply from the hepatic portal vein and the hepatic arteries.

In the hepatic portal system, the liver receives a dual blood supply from the hepatic portal vein and hepatic arteries. The hepatic portal vein carries venous blood drained from the spleen, gastrointestinal tract and its associated organs; it supplies approximately 75% of the liver’s blood. The hepatic arteries supply arterial blood to the liver and account for the remainder of its blood flow.

Oxygen is provided from both sources; approximately half of the liver’s oxygen demand is met by the hepatic portal vein, and half is met by the hepatic arteries. Blood flows through the liver tissue and empties into the central vein of each lobule. The central veins coalesce into hepatic veins that collect the blood leaving the liver and bring it to the heart.

A portal system is a venous structure that enables blood from one set of capillary beds to drain into another set of capillary beds, without first returning this blood to the heart. The majority of capillaries in the body drain directly into the heart, so portal systems are unusual.

The hepatic portal system connects the capillaries of the gastrointestinal tract with the capillaries in the liver. Nutrient-rich blood leaves the gastrointestinal tract and is first brought to the liver for processing before being sent to the heart. Here, carbohydrates and amino acids can be stored or used to make new proteins and carbohydrates.

The liver also removes vitamins and cofactors from the blood for storage, as well as filters any toxins that may have been absorbed along with the food. When any of these stored substances are needed, the liver releases them back into circulation through the hepatic veins.

Liver Function

The liver is thought to be responsible for up to 500 separate functions.

Functions of the Liver

The human liver is thought to be responsible for up to 500 separate functions, usually in combination with other systems and organs. The various functions of the liver are carried out by the liver cells or hepatocytes. Currently, there is no artificial organ or device capable of emulating all the functions of the liver.

The liver is the mainstay of protein metabolism— it synthesizes as well as degrades. It performs several roles in carbohydrate and lipid metabolism. The bulk of the lipoproteins are synthesized in the liver.

Mucosa

The mucosa, composed of simple epithelium cells, is the innermost layer of the gastrointestinal (GI) tract. It is the absorptive and secretory layer of the GI tract.

Layers of GI Tissue

The GI tract is composed of four layers. Each layer has different tissues and functions. From the inside out they are called: mucosa, submucosa, muscularis, and serosa.

The mucosa is the innermost layer, and functions in absorption and secretion. It is composed of epithelium cells and a thin connective tissue.

The mucosa contains specialized goblet cells that secrete sticky mucus throughout the GI tract. On the mucosa layer, small finger-like projections called villi and microvilli help to increase surface area for nutrient absorption.

 

Layers of GI tissue: Note the mucosa, located at the innermost layer.

Layers of Tissue Within the Mucosa

Since the mucosa is the innermost layer within the GI tract, it surrounds an open space known as the lumen. Food, mucus, and digestive juices pass through the lumen, and the mucosa comes in direct contact with digested food (chyme).

The general structure of the gut wall: This cross-section shows the mucosa in relation to the interior space, or lumen.

The mucosa is made up of three layers:

• The epithelium is the innermost layer and it is responsible for most digestive, absorptive, and secretory processes.
• The lamina propria is a layer of connective tissue that is Unusually cellular compared to most connective tissue.
• The muscular mucosa is a thin layer of smooth muscle and its function is still under debate.

The mucosae (singular: mucosa) are highly specialized in each organ of the gastrointestinal tract in order to deal with different digestive tract conditions. The most variation is seen in the epithelium tissue layer of the mucosa.

• In the esophagus, the epithelium is stratified, squamous, and non-keratinizing, for protective purposes.
• In the stomach. the epithelium is simple columnar and is organized into gastric pits and glands to deal with secretion.
• In the small intestine, the epithelium (particularly the ileum) is specialized for absorption, with villi and microvilli increasing surface area.

Submucosa

The submucosa is a dense, irregular layer of connective tissue with large blood vessels, lymphatics, and nerves that supports the mucosa.

The Layers of the GI tract

The GI tract is composed of four layers. Each layer has different tissues and functions. From the inside out they are called:

• Mucosa
• Submucosa
• Muscularis
• Serosa

The Submucosa

The submucosa is relatively thick, highly vascular, and serves the mucosa. The absorbed elements that pass through the mucosa are picked up from the blood vessels of the submucosa.

The submucosa also has glands and nerve plexuses. The submucosa lies under the mucosa and consists of fibrous connective tissue, separating the mucosa from the next layer, the muscular externa.

Layers of stomach lining: Stomach. The serosa is labeled at far right, and is colored yellow.

The general structure of the gut wall: The general structure of the gut wall is illustrated.

The submucosa consists of a dense irregular layer of connective tissue with large blood vessels, lymphatics, and nerves that branch into the mucosa and muscular externa. It contains Meissner’s plexus, an enteric nervous plexus, situated on the inner surface of the muscular externa.

In the gastrointestinal tract, the submucosa is the layer of dense irregular connective tissue or loose connective tissue that supports the mucosa. It also joins the mucosa to the bulk of underlying smooth muscle (fibers running circularly within the layer of longitudinal muscle).

Blood vessels, lymphatic vessels, and nerves (all supplying the mucosa) will run through here. Tiny parasympathetic ganglia are scattered around forming the submucosal plexus (or Meissner’s plexus) where preganglionic parasympathetic neurons create synapses with the postganglionic nerve fibers that supply the muscular mucosae.

Muscularis

The muscular is responsible for the segmental contractions and peristaltic movements in the gastrointestinal (GI) tract.

The gastrointestinal (GI) tract is composed of four layers of tissue, known as tunics. Each layer has different structures and functions. From the inside out, they are called the mucosa, submucosa, muscular externa, and serosa.

Structure of the Muscularis Externa

Muscularis mucosa of the submucosa: The muscular mucosa is adjacent to the submucosa, and should not be confused with the muscular externa.

General Structure of the gut wall: General structure of the gut wall—the muscular externa is labeled circular muscle and longitudinal muscle here.

Occasionally in the large intestine (two to three times a day), there will be a mass contraction of certain segments, moving a lot of feces along. This is generally when one gets the urge to defecate.

The pylorus of the stomach has a thickened portion of the inner circular layer: the pyloric sphincter. Alone among the GI tract, the stomach has a third layer of muscular externa. This is the inner oblique layer and helps churn the chyme in the stomach.

Serosa

Serosa consists of a secretory epithelial layer and a thin connective tissue layer that reduce the friction from muscle movements.

The Serous Membrane

In anatomy, the serous membrane (or serosa) is a smooth membrane that consists of a thin connective tissue layer and a thin layer of cells that secrete serous fluid. Serous membranes line and enclose several body cavities, known as serous cavities, where they secrete a lubricating fluid to reduce friction from muscle movements.

Serosa is not to be confused with adventitia, a connective tissue layer that binds together structures rather than reduces friction between them.

Layers of stomach lining: The serosa is labeled at far right, and is colored yellow.

Each serous membrane is composed of a secretory epithelial layer and a connective tissue layer underneath. The epithelial layer, known as mesothelium, consists of a single layer of avascular flat nucleated cells (simple squamous epithelium) that produce the lubricating serous fluid. This fluid has a consistency similar to thin mucus.

These cells are bound tightly to the underlying connective tissue. The connective tissue layer provides the blood vessels and nerves for the overlying secretory cells and also serves as the binding layer that allows the whole serous membrane to adhere to organs and other structures.

For the heart, the surrounding serous membranes include the outer, inner, parietal pericardium, and visceral pericardium (epicardium). Other parts of the body may also have specific names for these structures. For example, the serosa of the uterus is called the perimetrium.

The pericardial cavity (surrounding the heart), pleural cavity (surrounding the lungs), and peritoneal cavity (surrounding most organs of the abdomen) are the three serous cavities within the human body. While serous membranes have a lubricative role to play in all three cavities, the pleural cavity has a greater role to play in the function of breathing.

The serous cavities are formed from the intraembryonic coelom and are basically an empty space within the body surrounded by a serous membrane. Early in embryonic life, visceral organs develop adjacent to a cavity and invaginate into the bag-like coelom.

Therefore each organ becomes surrounded by a serous membrane—they do not lie within the serous cavity. The layer in contact with the organ is known as the visceral layer, while the parietal layer is in contact with the body wall.

Mouth

The mouth receives and mechanically breaks down food, produces saliva, and is the first portion of the alimentary canal.

The mouth is the first portion of the alimentary canal. It receives food and moistens the food with saliva, while the food is mechanically processed (mastication) by the teeth. The mouth is also known as the oral cavity, and within the oral cavity sits the tongue, the soft and hard palate, the uvula, and numerous salivary glands.

The oral mucosa is the mucous membrane epithelial tissue that lines the inside of the mouth. This membrane maintains a moist and lubricated environment within the mouth to prepare the digestive system for the entry of food.

The Mouth as a Communication and Breathing Tool

Inside of the mouth: An illustration of the inside of a human mouth. The cheeks have been omitted in the drawing and the lips pulled back for an unobstructed view of the teeth, tongue, jawbones, uvula, and alimentary canal.

Mechanical Food Breakdown by Teeth

In the digestive process, the mouth’s purpose is to prepare food for further digestion in the stomach and the small intestine. This process begins with the mechanical breakdown of food by the teeth, which fit into the alveolar arches. The front teeth (incisors and canines) are used to cut and tear food, while the teeth further back (bicuspids and molars) crush and grind.

Food Lubrication and Chemical Digestion By Saliva

Saliva is projected from three main pairs of salivary glands: the large parotid glands near the cheeks, the submandibular glands beneath the mandible, and the sublingual glands beneath the tongue.

Saliva keeps the mouth moist and lubricates the food, helping the tongue form the food into a soft wad, called a bolus. The fluid of saliva also contains several enzymes, notably lysozyme—an antibacterial agent—and amylase, which catalyzes large starch molecules into simpler sugars via hydrolysis.

Cross-section of the head and neck: A cross-section of the head and neck in mid-sagittal view, showing the structures of the mouth and throat.

Once properly chewed and lubricated, food and drink are swallowed into the esophagus, the tube that leads to the stomach.

The Structures of the Lips and External Mouth

Infant humans are born with an instinctual sucking reflex, by which they know how to gain nourishment using their lips and jaw. The philtrum, or bow of the lip, is the vertical groove or dip just below the nose.

The nasolabial folds are the deep creases of tissue that extend from the nose to the sides of the mouth. One of the first signs of age on the human face is the increase in prominence of the nasolabial folds.

Pharynx

The pharynx is part of the digestive and respiratory systems and consists of three main parts: the nasopharynx, oropharynx, and laryngopharynx.

The human pharynx (plural: pharynges) is the part of the throat situated immediately inferior to (below) the mouth and nasal cavity, and superior to the esophagus and larynx.

Head and neck overview: The human pharynx is situated immediately below the mouth and nasal cavity, and above the esophagus and larynx.

Nasopharynx

The nasopharynx is the most cephalad (toward the head) portion of the pharynx. It extends from the base of the skull to the upper surface of the soft palate. It includes the space between the internal nares and the soft palate and lies superior to the oral cavity.

The pharyngeal tonsils, more commonly referred to as the adenoids, are lymphoid tissue structures located in the posterior wall of the nasopharynx. Polyps or mucus can obstruct the nasopharynx, as can congestion due to an upper respiratory infection.

The eustachian tubes connect the middle ear to the pharynx and open into the nasopharynx. The opening and closing of the eustachian tubes serves to equalize the barometric pressure in the middle ear with that of the ambient atmosphere.

The anterior portion of the nasopharynx connects with the nasal cavities through openings known as choanae. The nasopharynx and its associated nasal tissues are lined with ciliated pseudostratified columnar epithelium, which is excellent for sweeping debris from the nasal passages.

The Pharyngeal Ostia

On the lateral walls of the nasopharynx are the pharyngeal Ostia of the auditory tube—triangle-shaped openings bound from behind by a firm prominence called the torus tuberous or cushion.

This binding is formed by a cartilaginous tube-like opening. Two folds arise from the cartilaginous opening:

• The salpingopharyngeal fold, a vertical fold of mucous membrane that extends from the inferior part of the torus.
• The salpingopalatine fold, a smaller fold that extends from the superior part of the torus to the palate.

Behind the Ostia of the auditory tube is a deep recess known as the pharyngeal recess (or fossa of Rosenmüller).

The posterior wall of the nasopharynx holds the pharyngeal tonsils, which can be especially marked in childhood. Superior to the pharyngeal tonsil, in the midline, an irregular flask-shaped depression of the mucous membrane sometimes extends upward; it is known as the pharyngeal bursa.

Oropharynx

The oropharynx or nasopharynx lies behind the oral cavity and extends from the uvula to the level of the hyoid bone. It opens anteriorly, through the isthmus faucium, into the mouth, and contains the palatine tonsil—another grouping of adenoid tissue.

The anterior wall consists of the base of the tongue and the epiglottis tissue. The lateral walls are made up of the tonsil and associated tonsilar tissues. The superior wall consists of the inferior surface of the soft palate and the uvula.

Because both food and air pass through the pharynx, a flap of connective tissue called the epiglottis closes over the glottis (tracheal opening) when food is swallowed to prevent accidental inhalation. The oropharynx is lined by non-keratinized stratified squamous epithelium.

Laryngopharynx

The hypopharynx or laryngopharynx is the caudal (most inferior) part of the pharynx; it is the part of the throat that connects to the esophagus. It lies inferior to the epiglottis and extends to the location where this common pathway diverges into the respiratory (larynx) and digestive (esophagus) pathways.

At that point, the laryngopharynx is continuous with the esophagus posteriorly. The esophagus conducts food and fluids to the stomach; air enters the larynx anteriorly. During swallowing, food has the right of way and air passage temporarily stops.

The laryngopharynx includes three major sites:

• The pyriform sinus.
• The postcricoid area.
• The posterior pharyngeal wall.

Like the oropharynx above it, the laryngopharynx serves as a passageway for food and air and is lined with stratified squamous epithelium.

The major transport passages of the alimentary canal include

• Pharynx: It is located at the back of the mouth and its function is to transfer the partially digested food bolus from the oral cavity to the upper part of the esophagus. This opening is known as Oropharynx. Pharynx also serves as a bridge that connects the air chambers of the nasal cavity and upper part of the trachea. This nasal opening of the pharynx is known as the nasopharynx. Nasopharynx also posses a collection of lymphatic tissue known as the tonsil. Epiglottis is the cartilage that prevents the entry of food particles into the air passage.

Esophagus

The esophagus is a muscular tube that moves food from the pharynx to the stomach via peristalsis.

The esophagus is an organ in vertebrates that consists of a muscular tube through which food passes from the pharynx to the stomach. It is a major component of the upper digestive system.

The word esophagus is derived from the Latin œsophagus, which derives from the Greek word esophagus, meaning entrance for eating. It is lined with mucus to aid in the passage of food.

Length and Location

In humans, the esophagus is continuous with the laryngeal part of the pharynx within the neck, and it passes through the thorax diaphragm and into the abdomen to reach the cardiac orifice of the stomach. It is usually about 10–50 cm long depending on an individual’s height. Due to the inferior pharyngeal constrictor muscle, the entry to the esophagus opens only when swallowing or vomiting.

Layers of Tissue

The esophageal tube in humans is comprised of two main layers of smooth muscle, though striated muscle comprises the tube near the pharynx. This combination of muscle tissue allows peristalsis to push food downward, and aids in regurgitation at the pharynx.

The innermost layer of smooth muscle is arranged in a series of concentric rings, while the outermost layer is arranged longitudinally.

In much of the gastrointestinal tract, smooth muscles contract in sequence to produce a peristaltic wave which forces a ball of food (called a bolus) from the pharynx to the stomach.

The Gastroesophageal Junction

The junction between the esophagus and the stomach (the gastroesophageal junction or GE junction) is not actually considered a valve in the anatomical sense, although it is sometimes called the cardiac sphincter.

Primary Function of the Alimentary Canal

The primary function of the alimentary tract or alimentary canal is to ingest food material and divide it into small fractions. What happens in the alimentary canal? A series of secretions, mainly enzymes act upon these smaller fractions and convert them into smaller molecules. These small molecules are thereby absorbed into the blood and lymph circulation. These small molecules are chiefly, amino acids, small peptides, sugars, and fatty acids, which are the building blocks in the synthesis of essential proteins, carbohydrates, and lipids.

Additional functions of the alimentary canal or gastrointestinal tract

The primary function of the alimentary canal is to carry out the process of digestion of food and absorption of nutrients from it. Apart from these primary functions, there are other supplemental yet essential roles of the alimentary canal. They are as follows:

• The alimentary canal acts as an immune barrier to various harmful microbes. This function is carried out by gut-associated lymphoid tissue (GALT) and variable pH conditions that are present throughout the alimentary canal.
• The colonic bacteria also help in maintaining immune homeostasis.
• The colonic bacterial colony also prevents the growth of harmful bacteria in our alimentary canal.
• Drug metabolism also occurs in the alimentary canal wherein the drug molecule is metabolized into smaller fractions and eventually eliminated from the body. Metabolism of antigens also occurs in the alimentary canal thereby detoxifying the body of the antigens.

The Main Lower Parts of the Human Alimentary Canal

Digestive tract

A. Stomach

The stomach forms the first part of the digestive tract. Food bolus from the esophagus enters the stomach. The stomach is the dilated portion of the digestive tract where fragmented food that has been received from the oral cavity via the esophagus, is retained, macerated, and partially digested. The pyloric sphincter present at the lower end of the stomach prevents the passage of food until it is converted into a thick semiliquid paste or pulp (better known as chyme). The pH of the stomach is generally acidic, although the presence or absence and nature of the food determines the pH of the stomach (Table 1).

Table 1: pH in different parts of the alimentary canal

The stomach is divided into five basic parts:

• Cardia: The foremost part of the stomach which is nearest to the esophagus
• Fundus: cardia is followed by the fundus part of the stomach and forms the upper part of the stomach
• Body (corpus): The body is the main part of the stomach, present between the upper and lower part of the stomach
• Antrum: The distal or the lower part of the stomach, which is near to the intestine. In this part of the stomach, mixing the food with gastric juices occurs.
• Pylorus: The last part of the stomach that acts as a regulator to control the emptying of the stomach contents into the small intestine

The stomach muscles are seldom inactive. The amount of food in the stomach determines the movement of the stomach muscles viz., expansion, or contraction. As soon as the food enters the stomach, the stomach muscle relaxes momentarily, thereafter the stomach muscle starts contracting.

Periodic stomach contractions result in churning and kneading of the food into a semisolid mixture called chyme. The rhythmic and periodic peristaltic movement of the stomach results in the movement of the food toward the pylorus and small intestine.

These cyclic and rhythmic movements of the stomach muscles are known as Gastric MMC(Migrating motor complex). Various secretions of the stomach are also released during this cycle and help in converting food bolus into chyme. These secretions include:

• Dilute acidic solution of hydrochloric acid (approximately 0.16 N)
• Proteolytic enzymes mainly, pepsin and minor quantities of other enzymes like rennin and gastric lipase
• Mucins

B. Small intestine

The small intestine is the longest part of the alimentary canal and the most important part of the alimentary canal wherein the majority of the digestive function is performed. The primary function of the small intestine is the digestion of the food followed by the absorption of the nutrients from the food.

The small intestine in adults measures approximately 6 m ~ 20-25 ft in length and have a surface area of approximately 250 m2. To increase the surface area, the small intestine exhibits significant architectural modifications in its mucosa and submucosa, i.e., folding in the mucosa and submucosa to form multiple finger-like projections or folds in the epithelium which are known as villi.

The large surface area ensures that the majority of the nutrients are absorbed during the passage of food through it. The small intestine is the main site for the absorption of amino acids, sugars, fats, and some larger molecules produced by digestion. The chyme from the stomach is transferred to the small intestine via the sphincter pylorus.

The small intestine is basically divided into three main parts. They are (1) duodenum, (2) jejunum, and (3) ileum. For the small intestine’s diagram.

The duodenum is the proximal 20–25 cm of the small intestine and is like a C-shape structure. The bile duct, liver, and pancreas open in the duodenum. Secretion from these glands neutralizes the acidic content of the chyme from the stomach and further promotes the digestion of proteins, fats, and carbohydrates.

The duodenum is characteristically retroperitoneal and the end which emerges from the peritoneum marks the beginning of the jejunum. The jejunum, which is almost 2/5th of the whole small intestine, extends till the ileum which terminates at the ileocaecal valve.

The small intestine also has a characteristic large collection of lymphoid tissue that is known as Peyer patches. The jejunum is the main absorptive site of the digestive tract and has a complex villous system.

The ileum is characterized by the presence of GALT (i.e. the gut-associated lymphoid tissue). The lymphoid cells further combine to form large nodules i.e. Peyer’s patches. The ileum is present in the lowermost part of the abdomen and has a limited blood supply.

Peristaltic movement in the small intestine forces the food material to move through different segments of the small intestine and eventually moves to the large intestine.

The ileum is involved in the absorption of vitamin B12, bile salts, and all the unabsorbed digestion products from the duodenum and jejunum.

Water and electrolytes are absorbed throughout the small intestine. The pH of the intestine is alkaline (Table 1).

C. Large Intestine

The large intestine or large bowel or colon is the last part of the human alimentary canal. The primary function of the large intestine is to absorb water from the indigestible residue of food before it is eliminated from the body as feces.

The large intestine is the place in the alimentary canal wherein the liquid content received from the small intestine is converted into solid indigestible waste material, feces.

The large intestine specializes in the absorption of water, salts, ionic contents from the residual food. Extensive mucus production occurs in the large intestine. This is to facilitate the bowel movement to eliminate the feces from the body.

As compared to the small intestine, the large intestine is wider and shorter. It is approximately 1.5 meters, or 5 feet, in length and has smooth inner walls.

As for the pH, the level varies depending on the part of the human intestines. The overall pH of the large intestine is alkaline (in the range of 6.5 to 7.5).

Good bacteria or probiotic bacteria reside in the colon (more than 700 species of bacterial flora exist in the colon). These probiotic bacteria are essential for a healthy human body. These bacteria are also involved in the synthesis of Vitamins such as vitamin K. Vitamin K is essential for the normal clotting of the blood. The large intestine is further divided into the following parts: (1) caecum, (2) ascending, transverse and descending colons, (3) sigmoid colon, and (4) rectum.

Partly digested food enters into the caecum from the small intestine and further moves through to enter the colon, wherein the residual water, nutrients, and electrolytes are reabsorbed. The residual solid waste further moves through the colon and is stored in the rectum for some time, and is eventually expelled from the body through the anal canal and anus. A small appendage or blind-ending tubular diverticulum that arises from the caecum is known as an appendix. The appendix is almost 5-10cm in length and has a diameter of approximately 0.8cm. Interestingly, the diameter of the appendix reduces with the increase in the age of human beings. The appendix, which is part of the alimentary canal (as opposed to the appendix of the testis, vermiform appendix, etc.), is a vestigial organ. Its specific function in the human body is yet to be established.

Common Disorders of the Alimentary Canal

• Vomiting: Forceful egestion of the food from the stomach. It can voluntary or involuntary action.
• Diarrhea: Overactivation or stimulation of the colon results in frequent and watery stools. Generally, this is induced by any harmful microbe ingested through food or poison.
• Constipation: Fewer and dry bowel movements or hard stool that is difficult to pass out.
• Bloody stool: Stools accompanied by the presence of blood.
• Heartburn or acidity: Acidic content refluxed into the esophagus.
• Stomach ulcers: Sore in the stomach lining. They are also known as peptic ulcers.
• Gastritis: Inflammation of the stomach lining
• Inflammatory bowel diseases (IBD): Chronic inflammation of the digestive tract particularly the large intestine. Crohn’s disease and ulcerative colitis are the two most common IBDs.
• Hemorrhoids: Swollen and painful blood vessels of the anal canal.

References