Capillary

Diagram of a capillary

Blood flows from the heart through arteries, which branch and narrow into arterioles, and then branch further into capillaries where nutrients and wastes are exchanged. The capillaries then join and widen to become venules, which in turn widen and converge to become veins, which then return blood back to the heart through the venae cavae. In the mesentery, metarterioles form an additional stage between arterioles and capillaries.

Individual capillaries are part of the capillary bed, an interweaving network of capillaries supplying tissues and organs. The more metabolically active a tissue is, the more capillaries are required to supply nutrients and carry away products of metabolism. There are two types of capillaries: true capillaries, which branch from arterioles and provide exchange between tissue and the capillary blood, and sinusoids, a type of open-pore capillary found in the liver, bone marrow, anterior pituitary gland, and brain circumventricular organs. Capillaries and sinusoids are short vessels that directly connect the arterioles and venules at opposite ends of the beds. Metarterioles are found primarily in the mesenteric microcirculation.[1]

Lymphatic capillaries are slightly larger in diameter than blood capillaries, and have closed ends (unlike the blood capillaries open at one end to the arterioles and open at the other end to the venules). This structure permits interstitial fluid to flow into them but not out. Lymph capillaries have a greater internal oncotic pressure than blood capillaries, due to the greater concentration of plasma proteins in the lymph.[7]

Types

There are three types of blood capillaries:

Depiction of the three types of capillaries. The fenestrated type in the center shows small pores called fenestrations; the sinusoidal type on the right shows intercellular gaps and an incomplete basement membrane and is also known as a discontinuous capillary.

Sinusoidal

Scanning electron micrograph of a liver sinusoid with fenestrated endothelial cells. Fenestrae are approximately 100 nm in diameter.

Sinusoidal capillaries or discontinuous capillaries are a special type of open-pore capillary, also known as a sinusoid,[14] that have wider 30–40 μm diameters, and wider openings in the endothelium.[15] Fenestrated capillaries have diaphragms that cover the pores whereas sinusoids lack a diaphragm and just have an open pore. These types of blood vessels allow red and white blood cells (7.5 μm – 25 μm diameter) and various serum proteins to pass, aided by a discontinuous basal lamina. These capillaries lack pinocytotic vesicles, and therefore utilize gaps present in cell junctions to permit transfer between endothelial cells, and hence across the membrane. Sinusoids are irregular spaces filled with blood and are mainly found in the liver, bone marrow, spleen, and brain circumventricular organs.[15][16]

Simplified image showing blood-flow through the body, passing through capillary networks in its path.

The capillary wall performs an important function by allowing nutrients and waste substances to pass across it. Molecules larger than 3 nm such as albumin and other large proteins pass through transcellular transport carried inside vesicles, a process which requires them to go through the cells that form the wall. Molecules smaller than 3 nm such as water and gases cross the capillary wall through the space between cells in a process known as paracellular transport.[17] These transport mechanisms allow bidirectional exchange of substances depending on osmotic gradients and can be further quantified by the Starling equation.[18] Capillaries that form part of the blood–brain barrier however only allow for transcellular transport as tight junctions between endothelial cells seal the paracellular space.[19]

Capillary beds may control their blood flow via autoregulation. This allows an organ to maintain constant flow despite a change in central blood pressure. This is achieved by myogenic response, and in the kidney by tubuloglomerular feedback. When blood pressure increases, arterioles are stretched and subsequently constrict (a phenomenon known as the Bayliss effect) to counteract the increased tendency for high pressure to increase blood flow.[20]

In the lungs special mechanisms have been adapted to meet the needs of increased necessity of blood flow during exercise. When the heart rate increases and more blood must flow through the lungs, capillaries are recruited and are also distended to make room for increased blood flow. This allows blood flow to increase while resistance decreases.

Capillary permeability can be increased by the release of certain cytokines, anaphylatoxins, or other mediators (such as leukotrienes, prostaglandins, histamine, bradykinin, etc.) highly influenced by the immune system.

Depiction of the filtration and reabsorption present in capillaries.

The Starling equation defines the forces across a semipermeable membrane and allows calculation of the net flux:

${\displaystyle \ J_{v}=K_{f}([P_{c}-P_{i}]-\sigma [\pi _{c}-\pi _{i}])}$

where:

• ${\displaystyle ([P_{c}-P_{i}]-\sigma [\pi _{c}-\pi _{i}])}$ is the net driving force,
• ${\displaystyle K_{f}}$ is the proportionality constant, and
• ${\displaystyle J_{v}}$ is the net fluid movement between compartments.

By convention, outward force is defined as positive, and inward force is defined as negative. The solution to the equation is known as the net filtration or net fluid movement (J_v). If positive, fluid will tend to leave the capillary (filtration). If negative, fluid will tend to enter the capillary (absorption). This equation has a number of important physiologic implications, especially when pathologic processes grossly alter one or more of the variables.

Disorders of capillary formation as a developmental defect or acquired disorder are a feature in many common and serious disorders. Within a wide range of cellular factors and cytokines, issues with normal genetic expression and bioactivity of the vascular growth and permeability factor vascular endothelial growth factor (VEGF) appear to play a major role in many of the disorders. Cellular factors include reduced number and function of bone-marrow derived endothelial progenitor cells.[21] and reduced ability of those cells to form blood vessels.[22]

• Formation of additional capillaries and larger blood vessels (angiogenesis) is a major mechanism by which a cancer may help to enhance its own growth. Disorders of retinal capillaries contribute to the pathogenesis of age-related macular degeneration.
• Reduced capillary density (capillary rarefaction) occurs in association with cardiovascular risk factors[23] and in patients with coronary heart disease.[22]

Contrary to a popular misconception, William Harvey did not explicitly predict the existence of capillaries, but he clearly saw the need for some sort of connection between the arterial and venous systems. In 1653, he wrote, "...the blood doth enter into every member through the arteries, and does return by the veins, and that the veins are the vessels and ways by which the blood is returned to the heart itself; and that the blood in the members and extremities does pass from the arteries into the veins (either mediately by an anastomosis, or immediately through the porosities of the flesh, or both ways) as before it did in the heart and thorax out of the veins, into the arteries..."[28]

Marcello Malpighi was the first to observe directly and correctly describe capillaries, discovering them in a frog's lung 8 years later, in 1661.[29]

• Alveolar–capillary barrier
• Hagen–Poiseuille equation
• Surface chemistry of microvasculature

Look up capillary in Wiktionary, the free dictionary.

Wikimedia Commons has media related to Capillaries.

• Histology image: 00903loa – Histology Learning System at Boston University
• The Microcirculatory Society, Inc.
• The Histology Guide – Capillaries