Capillary Exchange – Anatomy, Structure, Types, Functions

Capillary Exchange – Anatomy, Structure, Types, Functions

Capillary exchange refers to the exchange of material from the blood into the tissues in the capillary. … Hydrostatic pressure is a force generated by the pressure of the fluid on the capillary walls either by the blood plasma or interstitial fluid.

Capillary exchange includes all exchanges that happen at the microcirculatory or capillary level. When capillaries penetrate the tissues, they branch or arborize out to maximize the surface area for the exchange of material that includes gases, nutrients, ions, and waste products. This also minimizes the distance between the capillaries and interstitial regions where such exchanges will occur. Altogether, capillaries contain about 7% of the blood in the body, and they are continuously exchanging material between the interstitial fluid.

Substances are exchanged between capillaries and interstitial fluid via three mechanisms:

  • Diffusion
  • Bulk flow
  • Transcytosis or vesicular transport

The only things excluded from passing through the capillary wall are plasma proteins and whole cells. Other properties that regulate capillary exchange include:

  • Close proximity of a capillary to an interstitial fluid region, which decreases the capillary diffusion rate distance.
  • A large surface area due to capillary branching within the tissue maximizes the surface area available for capillary exchange.
  • The blood flow in the capillaries, however, is relatively slow.

Diffusion

Diffusion is the primary mechanism by which small molecules flow across capillaries and into the interstitial fluid, and vice versa, from the interstitial fluid into the capillaries. In such cases. molecules diffuse across their natural gradient in that they will move from high concentrated areas to low concentrated ones. For instance, glucose, amino acids, and oxygen are in high concentrations, or partial pressure in the case of oxygen, within the capillaries compared to the interstitial fluid. Thus, they will diffuse across the capillaries and into the interstitial fluid.

In contrast, carbon dioxide and other waste products have greater partial pressure or concentration in the interstitial tissue than the capillaries. Thus, these enter the capillaries by diffusion. Properties of the capillaries that regulate what may diffuse across the capillary wall include:

  • Permeability of endothelial cells that line the capillary walls that can be continuous, discontinuous, or fenestrated, see below for more details.
  • The starling equation details the contributions of hydrostatic and osmotic pressures, as further discussed below.

Bulk Flow

Bulk flow is used for the exchange of small lipid-insoluble substances. This exchange is regulated by the architecture of the capillaries with continuous capillaries that have a tight structure reducing bulk flow. Fenestrated capillaries have a perforated structure and increase bulk flow relative to continuous capillaries. Discontinuous capillaries have large intercellular gaps, and thus, allow for the greatest amount of bulk flow.

Pressure gradients determine the exchange of materials. Filtration is where substances are transferred from the capillary to the interstitial space, which is induced by blood hydrostatic pressure (BHP) and interstitial fluid osmotic pressure (IFOP). In contrast, the movement of substances from interstitial tissue to the blood in the capillaries is via a process called reabsorption. This type of movement occurs due to blood colloid osmotic pressure (BCOP) and interstitial fluid hydrostatic pressure (IFHP). Net filtration pressure (NFP) determines whether a substance is filtered or reabsorbed. The formula for NFP is:

NFP = (BHP- IFHP) + (IFOP – BCOP)

Collectively, these pressures are known as Starling forces. If NFP is a positive integer, then filtration of that substance will occur, whereas a negative integer will result in reabsorption.

Transcytosis

Transcytosis, or vesicular transport, is when substances in the blood move across the endothelial cell lining, but exit these cells via exocytosis. This involves the transport of such stances via vesicles that move across the plasma membrane of the endothelial cells and then into the interstitial tissue. This type of capillary exchange is used for lipid-insoluble molecules, such as insulin. Once in the interstitial tissue, vesicles can combine with other vesicles resulting in mixed contents draining into the interstitial region.

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Gas Exchange

A vital example of gas exchange occurs between the terminal portions of the lungs and pulmonary capillaries. Therefore, pulmonary capillaries possess characteristics that allow for rapid and efficient diffusion. The capillaries optimize the diffusion rate by receiving a constant blood supply. They also have an average membrane thickness of only 0.6 micrometers and form a network of capillaries over the alveoli. Furthermore, the alveoli themselves have an extremely large surface area of seventy square meters to further increase the surface area available for diffusion.

However, common diseases can interfere with this optimization. A useful way of thinking about these diseases is to frame them with respect to the variables of Fick’s law. For example, some pulmonary diseases cause fibrosis or edema. This increases the diffusion distance that the molecule has to travel, thus decreasing the diffusion rate. Other diseases, such as emphysema, result in damage to the walls of the alveoli causing them to rupture. This consequently forms one larger air space and decreases the surface area available for gas exchange.

Finally, if the lungs are unable to ventilate correctly, such as in restrictive lung diseases, a shallower concentration gradient is established, and the diffusion rate is impaired.

Capillary Dynamics

Hydrostatic and osmotic pressure are opposing factors that drive capillary dynamics.

Key Points

Capillary exchange refers to the exchange of material from the blood into the tissues in the capillary.

There are three mechanisms that facilitate capillary exchange: diffusion, transcytosis, and bulk flow.

Capillary dynamics are controlled by the four Starling forces.

The oncotic pressure is a form of osmotic pressure exerted by proteins either in the blood plasma or interstitial fluid.

Hydrostatic pressure is a force generated by the pressure of the fluid on the capillary walls either by the blood plasma or interstitial fluid.

The net filtration pressure is the balance of the four Starling forces and determines the net flow of fluid across the capillary membrane.

Key Terms

  • proteinuria: Excessive protein in the urine, a condition which can alter the net filtration pressure altering flow of fluid across the capillary wall.
  • hydrostatic pressure: A pressure generated by fluid on the walls of the capillary, usually forcing water out of the circulatory system.
  • net filtration pressure: The balance of the four Starling forces that determines the net flow of fluid across the capillary membrane.
  • oncotic pressure: A form of osmotic pressure exerted by proteins in a fluid that usually tends to pull water into the circulatory system.

Capillary exchange refers to the exchange of material between the blood and tissues in the capillaries. There are three mechanisms that facilitate capillary exchange: diffusion, transcytosis, and bulk flow.

Capillary Exchange Mechanisms

Diffusion, the most widely-used mechanism, allows the flow of small molecules across capillaries such as glucose and oxygen from the blood into the tissues and carbon dioxide from the tissue into the blood. The process depends on the difference of gradients between the interstitium and blood, with molecules moving to low-concentrated spaces from high-concentrated ones.

Transcytosis is the mechanism whereby large, lipid-insoluble substances cross the capillary membranes. The substance to be transported is endocytosed by the endothelial cell into a lipid vesicle which moves through the cell and is then exocytosed to the other side.

Bulk flow is used by small, lipid-insoluble solutes in water to cross the capillary wall. The movement of materials across the wall is dependent on pressure and is bi-directional depending on the net filtration pressure derived from the four Starling forces that modulate capillary dynamics.

Capillary Dynamics

The four Starling forces modulate capillary dynamics.

  • Oncotic or colloid osmotic pressure is a form of osmotic pressure exerted by proteins in the blood plasma or interstitial fluid.
  • Hydrostatic pressure is the force generated by the pressure of fluid within or outside of the capillary on the capillary wall.

The net filtration pressure derived from the sum of the four forces described above determines the fluid flow into or out of the capillary. Movement from the bloodstream into the interstitium is favored by blood hydrostatic pressure and interstitial fluid oncotic pressure. Alternatively, movement from the interstitium into the bloodstream is favored by blood oncotic pressure and interstitial fluid hydrostatic pressure.

This diagram of capillary microcirculation indicates the blood flow, capillary, venous end, osmotic pressure, hydrostatic pressure, and interstitial fluid.

Capillary Dynamics: Oncotic pressure exerted by proteins in blood plasma tends to pull water into the circulatory system.

Due to the pressure of the blood in the capillaries, blood hydrostatic pressure is greater than interstitial fluid hydrostatic pressure, promoting a net flow of fluid from the blood vessels into the interstitium. However, because large plasma proteins, especially albumin, cannot easily cross through the capillary walls, their effect on the osmotic pressure of the capillary interiors will to some extent balance the tendency for fluid to leak from the capillaries.In conditions where plasma proteins are reduced (e.g. from being lost in the urine or from malnutrition), or blood pressure is significantly increased, a change in net filtration pressure and an increase in fluid movement across the capillary result in excess fluid build-up in the tissues (edema).

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Transcytosis

Transcytosis is a process by which molecules are transported into the capillaries.

Key Points

Transcytosis is the process by which various macromolecules are transported across the endothelium of the capillaries.

Due to this function, transcytosis can be a convenient mechanism for pathogens to invade a tissue.

transcytosis: The process whereby macromolecules are transported across the interior of a cell via vesicles.

Transcytosis, or vesicle transport, is one of three mechanisms that facilitate capillary exchange, along with diffusion and bulk flow.

Substances are transported through the endothelial cells themselves within vesicles. This mechanism is mainly used by large molecules, typically lipid-insoluble preventing the use of other transport mechanisms. The substance to be transported is endocytosed by the endothelial cell into a lipid vesicle which moves through the cell and is then exocytosed to the other side. Vesicles are capable of merging, allowing for their contents to mix, and can be transported directly to specific organs or tissues.

Pathology

Due to the function of transcytosis, it can be a convenient mechanism by which pathogens can invade a tissue. Transcytosis has been shown to be critical to the entry of Cronobacter sakazakii across the intestinal epithelium and the blood-brain barrier.

Listeria monocytogenes has been shown to enter the intestinal lumen via transcytosis across goblet cells. Shiga toxin secreted by entero-hemorrhagic E. coli has been shown to be transcytosed into the intestinal lumen. These examples illustrate that transcytosis is vital to the process of pathogenesis for a variety of infectious agents.

Transcytosis in Pharmaceuticals

Pharmaceutical companies are currently exploring the use of transcytosis as a mechanism for transporting therapeutic drugs across the human blood-brain barrier. Exploiting the body’s own transport mechanism can help to overcome the high selectivity of this barrier, which blocks the uptake of most therapeutic antibodies into the brain and central nervous system.

Bulk Flow: Filtration and Reabsorption

Capillary fluid movement occurs as a result of diffusion (colloid osmotic pressure), transcytosis, and filtration.

Key Points

Bulk flow is a process used by small lipid-insoluble proteins to cross the capillary wall.

Capillary structure plays a large role in the rate of bulk flow, with continuous capillaries limiting flow and discontinuous capillaries facilitating the greatest amount of flow.

When moving from the blood to the interstitium, bulk flow is termed filtration.

When moving from the interstitium to the blood, bulk flow is termed re-absorption.

The kidney is a major site of bulk flow where waste products are filtered from the blood.

Key Terms

  • filtration: In bulk flow, this refers to the movement of proteins or other large molecules from the blood into the interstitium.
  • reabsorption: In bulk flow, this refers to the movement of proteins or other large molecules from the interstitium into the blood.
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Bulk flow is one of three mechanisms that facilitate capillary exchange, along with diffusion and transcytosis.

Bulk Flow Process

Bulk flow is used by small, lipid-insoluble solutes in water to cross the the capillary wall and is dependent on the physical characteristics of the capillary. Continuous capillaries have a tight structure reducing bulk flow. Fenestrated capillaries permit a larger amount of flow and discontinuous capillaries allow the largest amount of flow.

The movement of materials across the capillary wall is dependent on pressure and is bidirectional depending on the net filtration pressure derived from the four Starling forces.

When moving from the bloodstream into the interstitium, bulk flow is termed filtration, which is favored by blood hydrostatic pressure and interstitial fluid oncotic pressure. When moving from the interstitium into the bloodstream, the process is termed reabsorption and is favored by blood oncotic pressure and interstitial fluid hydrostatic pressure.

Modern evidence shows that in most cases, venular blood pressure exceeds the opposing pressure, thus maintaining a positive outward force. This indicates that capillaries are normally in a state of filtration along their entire length.

The Kidneys and Bulk Flow

The kidney is a major site for bulk flow transport. Blood that enters the kidneys is filtered by nephrons, the functional unit of the kidney. Each nephron begins in a renal corpuscle composed of a glomerulus containing numerous capillaries enclosed in a Bowman’s capsule. Proteins and other large molecules are filtered out of the oxygenated blood in the glomerulus and pass into Bowman’s capsule and the tubular fluid contained within. Blood continues to flow around the nephron until it reaches another capillary-rich region the peritubular capillaries, where the previously filtered molecules are reabsorbed from the tubule of the nephron.

Tubular reabsorption is the process by which solutes and water are removed from the tubular fluid and transported into the blood. Reabsorption is a two-step process beginning with the active or passive extraction of substances from the tubule fluid into the renal interstitium, and then the transport of these substances from the interstitium into the bloodstream

This diagram of the urinary excretion process indicates afferent and efferent arterioles, glomerular capillaries, Bowman's capsule, renal veins, and peritubular capillaries.

Tubular Secretion: Diagram showing the basic physiologic mechanisms of the kidney and the three steps involved in urine formation.

or

The primary purpose of the cardiovascular system is to circulate gases, nutrients, wastes, and other substances to and from the cells of the body. Small molecules, such as gases, lipids, and lipid-soluble molecules, can diffuse directly through the membranes of the endothelial cells of the capillary wall. Glucose, amino acids, and ions—including sodium, potassium, calcium, and chloride—use transporters to move through specific channels in the membrane by facilitated diffusion. Glucose, ions, and larger molecules may also leave the blood through intercellular clefts. Larger molecules can pass through the pores of fenestrated capillaries, and even large plasma proteins can pass through the great gaps in the sinusoids. Some large proteins in blood plasma can move into and out of the endothelial cells packaged within vesicles by endocytosis and exocytosis. Water moves by osmosis.

Hydrostatic Pressure

The primary force driving fluid transport between the capillaries and tissues is hydrostatic pressure, which can be defined as the pressure of any fluid enclosed in a space. Blood hydrostatic pressure is the force exerted by the blood confined within blood vessels or heart chambers. Even more specifically, the pressure exerted by blood against the wall of a capillary is called capillary hydrostatic pressure (CHP), and is the same as capillary blood pressure. CHP is the force that drives fluid out of capillaries and into the tissues.

As fluid exits a capillary and moves into tissues, the hydrostatic pressure in the interstitial fluid correspondingly rises. This opposing hydrostatic pressure is called the interstitial fluid hydrostatic pressure (IFHP). Generally, the CHP originating from the arterial pathways is considerably higher than the IFHP, because lymphatic vessels are continually absorbing excess fluid from the tissues. Thus, fluid generally moves out of the capillary and into the interstitial fluid. This process is called filtration.

Osmotic Pressure

The net pressure that drives reabsorption—the movement of fluid from the interstitial fluid back into the capillaries—is called osmotic pressure (sometimes referred to as oncotic pressure). Whereas hydrostatic pressure forces fluid out of the capillary, osmotic pressure draws fluid back in. Osmotic pressure is determined by osmotic concentration gradients, that is, the difference in the solute-to-water concentrations in the blood and tissue fluid. A region higher in solute concentration (and lower in water concentration) draws water across a semipermeable membrane from a region higher in water concentration (and lower in solute concentration).

As we discuss osmotic pressure in blood and tissue fluid, it is important to recognize that the formed elements of blood do not contribute to osmotic concentration gradients. Rather, it is the plasma proteins that play the key role. Solutes also move across the capillary wall according to their concentration gradient, but overall, the concentrations should be similar and not have a significant impact on osmosis. Because of their large size and chemical structure, plasma proteins are not truly solutes, that is, they do not dissolve but are dispersed or suspended in their fluid medium, forming a colloid rather than a solution.

The pressure created by the concentration of colloidal proteins in the blood is called the blood colloidal osmotic pressure (BCOP). Its effect on capillary exchange accounts for the reabsorption of water. The plasma proteins suspended in the blood cannot move across the semipermeable capillary cell membrane, and so they remain in the plasma. As a result, blood has a higher colloidal concentration and lower water concentration than tissue fluid. It, therefore, attracts water. We can also say that the BCOP is higher than the interstitial fluid colloidal osmotic pressure (IFCOP), which is always very low because interstitial fluid contains few proteins. Thus, water is drawn from the tissue fluid back into the capillary, carrying dissolved molecules with it. This difference in colloidal osmotic pressure accounts for reabsorption.

Interaction of Hydrostatic and Osmotic Pressures

The normal unit used to express pressures within the cardiovascular system is millimeters of mercury (mm Hg). When blood leaving an arteriole first enters a capillary bed, the CHP is quite high—about 35 mm Hg. Gradually, this initial CHP declines as the blood moves through the capillary so that by the time the blood has reached the venous end, the CHP has dropped to approximately 18 mm Hg. In comparison, the plasma proteins remain suspended in the blood, so the BCOP remains fairly constant at about 25 mm Hg throughout the length of the capillary and considerably below the osmotic pressure in the interstitial fluid.

The net filtration pressure (NFP) represents the interaction of the hydrostatic and osmotic pressures, driving fluid out of the capillary. It is equal to the difference between the CHP and the BCOP. Since filtration is, by definition, the movement of fluid out of the capillary, when reabsorption is occurring, the NFP is a negative number.

NFP changes at different points in a capillary bed. Close to the arterial end of the capillary, it is approximately 10 mm Hg, because the CHP of 35 mm Hg minus the BCOP of 25 mm Hg equals 10 mm Hg. Recall that the hydrostatic and osmotic pressures of the interstitial fluid are essentially negligible. Thus, the NFP of 10 mm Hg drives a net movement of fluid out of the capillary at the arterial end. At approximately the middle of the capillary, the CHP is about the same as the BCOP of 25 mm Hg, so the NFP drops to zero. At this point, there is no net change of volume: Fluid moves out of the capillary at the same rate as it moves into the capillary. Near the venous end of the capillary, the CHP has dwindled to about 18 mm Hg due to loss of fluid. Because the BCOP remains steady at 25 mm Hg, water is drawn into the capillary, that is, reabsorption occurs. Another way of expressing this is to say that at the venous end of the capillary, there is an NFP of −7 mm Hg.

This diagram shows the process of fluid exchange in a capillary from the arterial end to the venous end. 

Figure 1. Net filtration occurs near the arterial end of the capillary since capillary hydrostatic pressure (CHP) is greater than blood colloidal osmotic pressure (BCOP). There is no net movement of fluid near the midpoint since CHP = BCOP. Net reabsorption occurs near the venous end since BCOP is greater than CHP.

The Role of Lymphatic Capillaries

Since overall CHP is higher than BCOP, it is inevitable that more net fluid will exit the capillary through filtration at the arterial end than enters through reabsorption at the venous end. Considering all capillaries over the course of a day, this can be quite a substantial amount of fluid: Approximately 24 liters per day are filtered, whereas 20.4 liters are reabsorbed. This excess fluid is picked up by capillaries of the lymphatic system. These extremely thin-walled vessels have copious numbers of valves that ensure unidirectional flow through ever-larger lymphatic vessels that eventually drain into the subclavian veins in the neck. An important function of the lymphatic system is to return the fluid (lymph) to the blood. Lymph may be thought of as recycled blood plasma. (Seek additional content for more detail on the lymphatic system.)

Glossary

blood colloidal osmotic pressure (BCOP): pressure exerted by colloids suspended in blood within a vessel; a primary determinant is the presence of plasma proteins

blood hydrostatic pressure: force blood exerts against the walls of a blood vessel or heart chamber

capillary hydrostatic pressure (CHP): force blood exerts against a capillary

filtration: in the cardiovascular system, the movement of material from a capillary into the interstitial fluid, moving from an area of higher pressure to lower pressure

interstitial fluid colloidal osmotic pressure (IFCOP): pressure exerted by the colloids within the interstitial fluid

interstitial fluid hydrostatic pressure (IFHP): force exerted by the fluid in the tissue spaces

net filtration pressure (NFP): force driving fluid out of the capillary and into the tissue spaces; equal to the difference of the capillary hydrostatic pressure and the blood colloidal osmotic pressure

reabsorption: in the cardiovascular system, the movement of material from the interstitial fluid into the capillaries

Lifestyle

  • Quit smoking. Your doctor can recommend programs and products to help.
  • Follow a healthy diet. Eat a variety of fruits, vegetables, and whole grains, plus lean meat, poultry, fish, and low-fat/fat-free milk. Your diet should be low in fat, cholesterol, sodium, and sugar.
  • Watch your weight. A daily record of your weight can help you be aware of rapid weight gain, which may be a sign that your pulmonary hypertension is worsening.
  • Stay active. Incorporate physical activity such as walking into your lifestyle. Discuss the level of activity with your doctor. Avoid straining or lifting heavyweights. Rest when you need to.
  • Avoid sitting in a hot tub or sauna, or taking long baths, which will lower your blood pressure.
  • Be cautious about air travel or high-altitude locales. You may need to travel with extra oxygen.
  • Get support for the anxiety and stress of living with pulmonary hypertension. Talk with your healthcare team, or ask for a referral to a counselor. A support group for people living with pulmonary hypertension can be invaluable in learning how to cope with the illness.

References

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