The respiratory system is composed primarily of the nose, oropharynx, larynx, trachea, bronchi, bronchioles, and lungs. The lungs further divide into individual lobes, which ultimately subdivide into over 300 million alveoli. The alveoli are the primary location for gas exchange. The diaphragm is the primary respiratory muscle and receives innervation by the nerve roots of C3, C4, and C5 via the phrenic nerve. The external intercostals are inspiratory muscles used primarily during exercise and respiratory distress. The significant lung volumes/capacities and their definitions are listed below[rx][rx][rx][rx]:
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Inspiratory reserve volume (IRV): Volume that can be breathed after a normal inspiration
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Tidal volume (TV): Volume inspired and expired with each breath
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Expiratory reserve volume (ERV): Volume that can be expired after a normal breath
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Residual volume (RV): Volume remaining in lung after maximal expiration (cannot be measured by spirometry)
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Inspiratory capacity (IC): Volume that can be breathed after normal exhalation
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Functional residual capacity (FRC): Volume remaining in the lungs after normal expiration
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Vital capacity (VC): Maximum volume able to be expired after maximal inspiration
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Total lung capacity (TLC): Volume of air in the lungs after maximal inspiration
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Forced expiratory volume (FEV1): Volume that can be expired in 1 second of maximum forced expiration
The lung is a primary location for a large proportion of human diseases. Lung disease further classifies into obstructive and restrictive diseases.
Obstructive Disease
The definition of obstructive disease is lung disease with impaired expiration. It presents with decreased FVC, decreased FEV1, and most notably, a dramatic decrease in FEV1/FVC. In obstructive disease, the air that should be expired is not, which leads to air trapping and an increased FRC. The two major examples of obstructive disease are listed below:
Asthma: a multifactorial disease characterized by chronic bronchial inflammation leading to eventual air trapping. Several key characteristics are as follows.
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Airway disease is mostly reversible (i.e., with the administration of a beta-agonist).
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Can cause chronic cough, wheeze, tachypnea, and dyspnea.
Chronic obstructive pulmonary disorder (COPD): a constellation of clinical symptoms that share features of both emphysema and chronic bronchitis leading to expiratory airflow limitation.
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Chronic bronchitis demonstrates long-term airway inflammation causing excessive cough and sputum production.
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Emphysema characteristically shows enlarged airspaces (loss of alveolar elasticity) leading to chronic dyspnea. The overly-distended airspaces prevent the lungs from adequately emptying.
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Smoking is the primary cause of the disease and is directly related to the severity of the disease course.
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Cigarettes induce inflammation in the lungs.
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Airways show small airway disease and parenchymal destruction.
Restrictive Disease
Restrictive lung disease is lung disease in which restricted lung expansion causes decreased lung volumes. Its characteristics include both a decreased FVC and decreased FEV1; however, the FEV1 is more reduced than FVC, causing FEV1/FVC to increase. Several examples of restrictive lung disease are listed below[rx][r][rx][rx][rx]
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Idiopathic pulmonary fibrosis
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Pneumoconiosis
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Sarcoidosis
The respiratory tract describes the organs of the respiratory tract that allow airflow during ventilation. [rx][rx][rx]They reach from the nares and buccal opening to the blind end of the alveolar sacs. They are subdivided into different regions with various organs and tissues to perform specific functions. The airway can be subdivided into the upper and lower airway, each of which has numerous subdivisions as follows.
Upper Airway
The pharynx is the mucous membrane-lined portion of the airway between the base of the skull and the esophagus and is subdivided as follows:
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The nasopharynx, also known as the rhino-pharynx, post-nasal space, is the muscular tube from the nares, including the posterior nasal cavity, divide from the oropharynx by the palate and lining the skull base superiorly
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The oro-pharynx connects the naso and hypopharynx. It is the region between the palate and the hyoid bone, anteriorly divided from the oral cavity by the tonsillar arch
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The hypopharynx connects the oropharynx to the esophagus and the larynx, the region of the pharynx below the hyoid bone.
The larynx is the portion of the airway between the pharynx and the trachea, contains the organs for the production of speech. Formed of a cartilaginous skeleton of nine cartilages, it includes the important organs of the epiglottis and the vocal folds (vocal cords) which are the opening to the glottis.
Lower Airway
The trachea is a ciliated pseudostratified columnar epithelium-lined tubular structure supported by C-shaped rings of hyaline cartilage. The flat open surface of these C rings opposes the esophagus to allow its expansion during swallowing. The trachea bifurcates and therefore terminates, superior to the heart at the level of the sternal angle.
The bronchi, the main bifurcation of the trachea, are similar in structure but have complete circular cartilage rings.
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Main bronchi: There are two supplying ventilation to each lung. The right main bronchus has a larger diameter and is aligned more vertically than the left
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Lobar bronchi: Two on the left and three on the right supply each of the main lobes of the lung
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Segmental bronchi supply individual bronchopulmonary segments of the lungs.
Bronchioles lack supporting cartilage skeletons and have a diameter of around 1 mm. They are initially ciliated and graduate to the simple columnar epithelium and their lining cells no longer contain mucous producing cells.
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Conducting bronchioles conduct airflow but do not contain any mucous glands or seromucous glands
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Terminal bronchioles are the last division of the airway without respiratory surfaces
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Respiratory bronchioles contain occasional alveoli and have surface surfactant-producing They each give rise to between two and 11 alveolar ducts.
The alveolar is the final portion of the airway and is lined with a single-cell layer of pneumocytes and in proximity to capillaries. They contain surfactant-producing type II pneumocytes and Clara cells.
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Alveolar ducts are tubular portions with respiratory surfaces from which the alveolar sacs bud.
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Alveolar sacs are the blind-ended spaces from which the alveoli clusters are formed and to where they connect. These are connected by pores which allow air pressure to equalize between them. Together, with the capillaries, they form the air-blood barrier.
To allow this and to maintain homeostasis and adequate protection from the external environment they must also perform other barrier functions.
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A moisture barrier is the mucous lining of the airway that provides a barrier to prevent loss of excessive moisture during ventilation by increasing the humidity of the air in the upper airway
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Temperature barrier is relative to body temperature as the external environment is nearly always colder, and the increased vasculature and structures such as nasal turbinates warm air as it enters the airways
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A barrier to infection as the airways are lined with a rich lymphatic system including mucosa-associated lymphoid tissue (MALT) that prevents early access to any invading pathogens. Macrophages also patrol the respiratory surfaces providing an important component of the “air-blood barrier.”
Cellular
Oxygen transport is the primary means by which the circulatory system perfuses tissue. Oxygen gets carried in the body in two major forms: bound to hemoglobin and dissolved. Hemoglobin is the major oxygen carrier in the body. The formula for the oxygen content of blood is as follows:
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CaO2 = 1.34 x [Hgb] x (SaO2 / 100) + 0.003 x PaO2
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CaO2 = oxygen content in blood
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[Hb] = hemoglobin concentration
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SaO2 = percentage of heme groups that are bound to oxygen
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PaO2 = Partial pressure of oxygen
Four subunits comprise hemoglobin, each containing a heme-moiety that binds to iron. One molecule of O2 can bind to each iron atom of the heme group; therefore, each hemoglobin group can bind to four molecules of O2.[rx][rx]
Structure
The conduction portion of the lung begins at the trachea and extends to the terminal bronchioles. Outside the lungs, the conduction system consists of the nasal cavities, nasopharynx, larynx, and trachea. Within the lungs, the conducting portion spits into paired main bronchi. The bronchi begin as a branching pattern, splitting next into lobar (secondary) bronchial branches and then again into segmental (tertiary) bronchi. The tertiary bronchi continue to divide into small bronchioles where the first change in histology takes place as cartilage is no longer present in the bronchioles. The end of the conduction portion of the lungs is at the final segment called the terminal bronchioles. The terminal bronchioles open into the respiratory bronchioles [rx]. This is the start of the respiration portion of the lung.
The conducting portion provides the pathway for the movement and conditioning of the air entering the lung. Specialized cells collaborate to warm, moisturize, and remove particles that enter. These cells are the respiratory epithelium and comprise the entire respiratory tree. Most of the respiratory epithelium is the ciliated pseudostratified columnar epithelium. The following five types of cells are in this region:
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Ciliated cells
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Goblet cells
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Basal cells
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Brush cells
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Neuroendocrine cells
The ciliated cells are the most abundant. They control the actions of the mucociliary escalator [rx], a primary defense mechanism of the lungs that removes debris. While the mucus provided by the goblet cells traps inhaled particles, the cilia beat to move the material towards the pharynx to swallow or cough out.
Goblet cells, so named for their goblet-shaped appearance, are filled with mucin granules at their apical surface with the nucleus remaining towards the basilar layer. Goblet cells decrease in number as the respiratory tree gets progressively smaller and are eventually replaced by club cells (previously Clara cells) when they reach the respiratory bronchioles.
The basal cells connect to the basement membrane and provide the attachment layer of the ciliated cells and goblet cells. They may be thought of as the stem cells of the respiratory epithelium as they maintain the ability to potentiate ciliated cells and goblet cells [rx].
Brush cells, occasionally referred to as type III pneumocyte cells are sparsely distributed in all areas of respiratory mucosa. Brush cells may be columnar, or flask-like and are identified by their short microvilli-covered apical layer–resembling a push broom or appropriately, a brush. No function has been officially assigned to the brush cells though there are many proposed mechanisms. One popular proposal suggests they have a chemoreceptor function, monitoring air quality, due to their association with unmyelinated nerve endings. [rx]
The bronchial mucosa also contains a small cluster of neuroendocrine cells, also known as Kulchitsky cells [rx]. They have neurosecretory type granules and can secrete several factors. This includes catecholamine and polypeptide hormones, such as serotonin, calcitonin, and gastrin-releasing factors (bombesin). Like brush cells, these neuroendocrine cells make up only a small portion of mucosal epithelium, around 3%.
Within the bronchial submucosa are submucosal glands. These glands are composed of a mixture of serous and mucinous cells, similar to salivary gland tissue. The secretions are emptied into ducts and then on the bronchial mucosa. Older individuals may show oncocytic metaplasia of these glands. Smooth muscle bundles are present at all levels of the airway to allow for regulation of airflow. There are progressively fewer smooth muscle fibers progressing from bronchi to alveoli.
The Reason for Breathing
Breathing allows for the delivery of oxygen to internal tissues and cells where it is needed and allows for the removal of CO2.
Key Points
Breathing is the process that moves air in and out of the lungs of terrestrial vertebrates, to take in oxygen and remove carbon dioxide.
Aerobic organisms require oxygen to release energy via respiration, in the form of metabolized, energy-rich molecules such as glucose.
Another key role of respiration is to maintain proper blood pH —too much carbon dioxide causes acidosis, and too little carbon dioxide causes alkalosis.
Once these dissolved gases are in the blood, the circulatory system transports them around the body, thereby bringing oxygen to the tissues, and carbon dioxide to the lungs.
Key Terms
- passive diffusion: Net movement of material from an area of high concentration to an area of lower concentration without any energy input.
- alkalosis: When blood pH becomes alkaline due to too few hydrogen ions and too little carbon dioxides.
- acidosis: When blood pH becomes acidic due to too many hydrogen ions and too much carbon dioxide.
The Purpose of Breathing
Breathing is the physiological process that moves air in and out of the lungs in terrestrial vertebrates. Respiration is often referred to as breathing, but it can also mean cellular respiration, which is the main reason why breathing is important.
Cells require oxygen from the air to extract energy from glucose through respiration, which produces carbon dioxide and water as waste products. Therefore, oxygen is vital for every part of normal cellular function, and oxygen deficiency can have severe pathological consequences.
The respiratory system facilitates breathing. In the alveoli tissue of the lungs, the exchange of oxygen and carbon dioxide molecules between the air and the bloodstream occur by passive transport, so that oxygen is taken in and carbon dioxide and water are removed.
Passive diffusion (also called bulk flow) is the term for the movement of these gases between the air and bloodstream based on their relative concentration, with the gas with the greater concentration moving across to the area with the lower concentration. This process consumes no energy.
The circulatory system is deeply connected with the respiratory system because it distributes the dissolved oxygen to the tissues of the body and the waste carbon dioxide to the lungs.
Breathing Controls Blood Chemistry
Another key role of respiration is maintaining proper blood pH. The concentration of hydrogen ions in blood is partially determined by the amount of dissolved carbon dioxide in the blood, so that more carbon dioxide results in more hydrogen, causing the blood to have a lower pH and be more acidic.
When the blood becomes acidic, respiratory acidosis occurs, which can cause tissue damage if too severe. Acidosis can be caused by hypoventilation (too little breathing), which reduces the removal rate of carbon dioxide, causing it to build up in the bloodstream along with hydrogen. There are many symptoms of acidosis, such as headache, confusion, increased heart rate, and muscle weakness.
Respiratory alkalosis happens when the opposite effect occurs. When the blood pH becomes too high, from too few hydrogen ions because of too little carbon dioxide, the blood will become alkaline, which is also harmful to the body. Alkalosis can happen from hyperventilation (too much breathing) which removes too much carbon dioxide from the bloodstream.
Thankfully, negative feedback mechanisms exist so that hyperventilation and hypoventilation can be corrected. These feedback mechanisms can fail in people with chronic respiratory diseases like emphysema and bronchitis, or from the side effects of certain drugs, in which acidosis and alkalosis will occur regardless.
Some Symptoms of Acidosis and Alkalosis: One of the primary reasons for breathing is to regulate blood pH so that respiratory acidosis and alkalosis don’t occur.
Functional Anatomy of the Respiratory System
The respiratory system includes the lungs, airways, and respiratory muscles. Ventilation is the rate at which gas enters or leaves the lung.
Key Points
- Ventilation occurs under the control of the autonomic nervous system from parts of the brain stem—the medulla oblongata and the pons —that together form the respiration regulatory center.
- The three types of ventilation are minute ventilation, alveolar ventilation, and dead space ventilation.
- Inhalation is initiated by the diaphragm and supported by the external intercostal muscles. Additional accessory muscles include sternocleidomastoid, platysma, the scalene muscles of the neck, pectoral muscles, and the latissimus dorsi.
- When the diaphragm contracts, the ribcage expands and the contents of the abdomen are moved downward, resulting in a larger thoracic volume and negative pressure (with respect to atmospheric pressure) inside the chest.
- Exhalation is generally a passive process since the lungs have a natural elasticity; they recoil from the stretch of inhalation and air flows back out until the pressures in the chest and the atmosphere reach equilibrium.
- Gas exchange occurs at the alveoli, the tiny sacs that are the basic functional component of the lungs. The alveoli are interwoven with capillaries that connect to the larger bloodstream.
Key Terms
- elastic recoil: The lungs’ rebound from the stretch of inhalation that passively removes air from the lungs during exhalation.
- Dead space: Any space in the airways that is not involved in alveolar gas exchange, such as the conducting zones.
- ventilation: The bodily process of breathing, the inhalation of air to provide oxygen, and the exhalation of spent air to remove carbon dioxide.
The Respiratory System
The primary function of the respiratory system is gas exchange between the external environment and an organism’s circulatory system. In humans and other mammals, this exchange balances oxygenation of the blood with the removal of carbon dioxide and other metabolic wastes from circulation.
Bronchial anatomy: The pulmonary alveoli are the terminal ends of the respiratory tree, outcropping from either alveolar sacs or alveolar ducts, which are both sites of gas exchange with the blood.
As gas exchange occurs, the acid-base balance of the body is maintained as part of homeostasis. If proper ventilation is not maintained, two opposing conditions could occur respiratory acidosis (a life-threatening condition) and respiratory alkalosis.
At the molecular level, gas exchange occurs in the alveoli—tiny sacs which are the basic functional component of the lungs. The alveolar epithelial tissue is extremely thin and permeable, allowing for gas exchange between the air inside the lungs and the capillaries of the bloodstream. Air moves according to pressure differences, in which air flows from areas of high pressure to areas of low pressure.
The Ventilation Rate
In respiratory physiology, the ventilation rate is the rate at which gas enters or leaves the lung. There are several different terms used to describe the nuances of the ventilation rate.
- Minute Ventilation (VE): The amount of air entering the lungs per minute. It can be defined as tidal volume (the volume of air inhaled in a single breath) times the number of breaths in a minute.
- Alveolar Ventilation (VA): The amount of gas per unit of time that reaches the alveoli (the functional part of the lungs where gas exchange occurs). It is defined as tidal volume minus dead space (the space in the lungs where gas exchange does not occur) times the respiratory rate.
- Dead Space Ventilation (VD): The amount of air per unit of time that doesn’t reach the alveoli. It is defined as the volume of dead spacetimes the respiratory rate.
Dead space is any space that isn’t involved in the alveolar gas exchange itself, and it typically refers to parts of the lungs that are conducting zones for air, such as the trachea and bronchioles.
If someone breathes through a snorkeling mask, the length of their conducting zones increases, which increases dead space and reduces alveolar ventilation. Feedback mechanisms increase the ventilation rate in such a case, but if dead space becomes too great, they won’t be able to counteract the effect.
The ventilation rate is controlled by several centers of the autonomic nervous system in the brain, primarily the medulla and the pons.
The human respiratory system: A complete, schematic view of the human respiratory system with its parts and functions.
Mechanisms of Inhalation
Inhalation is initiated by the activity of the diaphragm and supported by the external intercostal muscles. A normal human respiratory rate is 10 to 18 breaths per minute.
During vigorous inhalation (at rates exceeding 35 breaths per minute), or in approaching respiratory failure, accessory muscles—such as the sternocleidomastoid, platysma, and the scalene muscles of the neck—are recruited to help sustain the increased respiratory rate. Pectoral muscles and latissimus dorsi are also accessory muscles for the activity of the lungs.
Under normal conditions, the diaphragm is the primary driver of inhalation. When the diaphragm contracts, the rib cage expands and the contents of the abdomen are moved downward, resulting in a larger thoracic volume and negative pressure (with respect to atmospheric pressure) inside the thorax.
As air moves from zones of high pressure to zones of low pressure, the contraction of the diaphragm allows the air to enter the conducting zone (such as the trachea, bronchioles, etc.), where it is filtered, warmed, and humidified as it flows to the lungs.
Mechanisms of Exhalation
Exhalation is generally a passive process. The lungs have a high degree of elastic recoil, so they rebound from the stretch of inhalation and air flows out until the pressures in the lungs and the atmosphere reach equilibrium.
The reason for the elastic recoil of the lung is the surface tension from water molecules on the epithelium of the lungs. A molecule called surfactant (secreted by the alveoli) prevents the surface tension from becoming too great and collapsing the lungs.
Active or forced exhalation is achieved by the abdominal and internal intercostal muscles. During this process, the air is forced or exhaled out. During forced exhalation, as when blowing out a candle, the expiratory muscles, including the abdominal muscles and internal intercostal muscles, generate abdominal and thoracic pressure that forces air out of the lungs.
Forced exhalation is often used as an indicator to measure airway health, as people with obstructive lung diseases (such as emphysema, asthma, and bronchitis) will not be able to actively exhale as much as a healthy person because of obstruction in the conducting zones from inhalation
, or from a loss of elastic recoil of the lungs.
Blood Supply and Lymphatics
The main distinction is between the pulmonary artery and bronchial arteries. The pulmonary artery takes deoxygenated blood from the heart to be oxygenated by the lung parenchyma. However, the bronchial arteries provide oxygen for survival to the lung parenchyma.
The main pulmonary artery emerges from the right ventricle and bifurcates into the left main and right main pulmonary arteries. The pulmonary artery branches usually trail and expand along the branches of the bronchial tree and eventually become capillaries around the alveoli. The pulmonary veins receive oxygenated blood from the alveoli capillaries and deoxygenated blood from the bronchial arteries and visceral pleura. Four pulmonary veins come together at the right atrium.
Bronchial circulation is part of the systemic circulation. The left bronchial artery arises as two (superior and inferior) from the thoracic aorta. The right brachial artery usually comes from one of the following three: the right posterior intercostal artery, with the left superior bronchial artery off the aorta or directly from the aorta. The bronchial veins collect the deoxygenated blood and empty it into the azygos vein.
The superficial and deep lymphatic plexuses drain the lung. The lymph flow from the lung parenchyma first drains into the intraparenchymal nodes and then to the peribronchial nodes. Subsequently, the lymphatics will drain to the tracheobronchial, paratracheal lymph nodes, the bronchomediastinal trunk, and then into the thoracic duct.
Nerves
The phrenic nerve comes from C3,4,5 cervical nerve roots. It innervates the fibrous pericardium, portions of the visceral pleura, and the diaphragm.
The lung receives innervation from two main sources: the pulmonary plexus (a combination of parasympathetic and sympathetic innervation) and the phrenic nerve. The pulmonary plexus is at the root of the lung and consists of efferent and afferent autonomic nerve fibers. It consists of branches of the vagus nerve (parasympathetic) and sympathetic fibers—the plexus branches around the pulmonary vasculature and bronchi. The parasympathetic innervation causes constriction of the bronchi, dilation of the pulmonary vessels, and increase gland secretion. The sympathetic innervation causes dilation of the bronchi and constriction of the pulmonary vessels.
Function of respiratory system
Respiratory functions of lungs
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Respiration – The lungs are the primary organs of respiration, where the exchange of gases takes place. In the alveoli, which are the functional units of the lungs, the oxygen is taken up, and carbon dioxide is removed from the bloodstream through the alveolar-capillary bed [rx].
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Air conditioning – The function of conducting part is not only to lead the air but also acts as an efficient air conditioner. This conditioning is done by warming/cooling the inhaled air to bring it to the level of body temperature, humidifying the air, and also removal of all the foreign particles present in it. The removal of foreign particles like dust, bacteria, virus, etc., is done by mucous secretion, which traps the suspended particles and beating of the cilia, which clears the mucus from the respiratory passage [rx].
Non-respiratory functions of lungs
Even though the lungs are primarily for respiration, studies indicate that they have many non-respiratory functions. Some of the few important ones are mentioned below.
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Converting an inactive chemical precursor into its active form, like turning angiotensin-I into angiotensin-II, help in raising of blood pressure [rx].
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It is also an essential site for degrading/inactivating important vasoactive chemical mediators like bradykinin, serotonin, and norepinephrine
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The bronchial mucosa also contains a small cluster of neuroendocrine cells [rx], also known as Kulchitsky cells that can secrete several factors, including catecholamine and polypeptide hormones, such as calcitonin, serotonin, and gastrin-releasing factors (bombesin)
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Pulmonary epithelium acts as the first line of defense for the inspired air [rx].
References
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