Capillary

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Sinusoid (blood vessel)
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Capillary
Transmission electron microscope image of a cross-section of a capillary occupied by a red blood cell
A simplified illustration of a capillary network
Details
PronunciationUS: /ˈkæpəlɛri/, UK: /kəˈpɪləri/
SystemCirculatory system
Identifiers
Latinvas capillare[1]
MeSHD002196
TA98A12.0.00.025
TA23901
THH3.09.02.0.02001
FMA63194
Anatomical terminology]

A capillary is a small

lymph vessels to drain lymphatic
fluid collected in microcirculation.

Etymology

Capillary comes from the Latin word capillaris, meaning "of or resembling hair", with use in English beginning in the mid-17th century.[4] The meaning stems from the tiny, hairlike diameter of a capillary.[4] While capillary is usually used as a noun, the word also is used as an adjective, as in "capillary action", in which a liquid flows without influence of external forces, such as gravity.

Structure

Diagram of capillary bed

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

Types

Blood capillaries are categorized into three types: continuous, fenestrated, and sinusoidal (also known as discontinuous).

Types of capillaries: (left) continuous with no big gaps, (center) fenestrated with small pores, and (right) sinusoidal (or 'discontinuous') with intercellular gaps

Continuous

Diagram of a continuous capillary

Continuous capillaries are continuous in the sense that the endothelial cells provide an uninterrupted lining, and they only allow smaller molecules, such as water and ions, to pass through their intercellular clefts.[7][8] Lipid-soluble molecules can passively diffuse through the endothelial cell membranes along concentration gradients.[9] Continuous capillaries can be further divided into two subtypes:

  1. Those with numerous transport vesicles, which are found primarily in skeletal muscles, fingers, gonads, and skin.[10]
  2. Those with few vesicles, which are primarily found in the central nervous system. These capillaries are a constituent of the blood–brain barrier.[8]

Fenestrated

Fenestrated capillaries have pores known as

glomeruli of the kidney
.

Sinusoidal

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,

pinocytotic vesicles, and therefore use 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.[14][15]

Annotated diagram of the exchange between capillary and body tissue through the exchange of materials between cells and fluid

Development

During early

endothelial cells that then form vascular tubes.[16] The term angiogenesis denotes the formation of new capillaries from pre-existing blood vessels and already-present endothelium which divides.[17] The small capillaries lengthen and interconnect to establish a network of vessels, a primitive vascular network that vascularises the entire yolk sac, connecting stalk, and chorionic villi.[18]

Function

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.[19] These transport mechanisms allow bidirectional exchange of substances depending on osmotic gradients.[20] Capillaries that form part of the blood–brain barrier only allow for transcellular transport as tight junctions between endothelial cells seal the paracellular space.[21]

Capillary beds may control their blood flow via

Bayliss effect) to counteract the increased tendency for high pressure to increase blood flow.[22]

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.[citation needed] Extreme exercise can make capillaries vulnerable, with a breaking point similar to that of collagen.[23]

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.[citation needed]

Diagram of the filtration and reabsorption in capillaries

Starling equation

The transport mechanisms can be further quantified by the Starling equation.[20] The Starling equation defines the forces across a semipermeable membrane and allows calculation of the net flux:

where:

is the net driving force,
is the proportionality constant, and
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 (Jv). 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.[citation needed]

According to Starling's equation, the movement of fluid depends on six variables:

  1. Capillary
    hydrostatic pressure
    (Pc)
  2. Interstitial hydrostatic pressure (Pi)
  3. Capillary oncotic pressure (πc)
  4. Interstitial oncotic pressure (πi)
  5. Filtration coefficient (Kf)
  6. Reflection coefficient (σ)

Clinical significance

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.[24] and reduced ability of those cells to form blood vessels.[25]

Therapeutics

Major diseases where altering capillary formation could be helpful include conditions where there is excessive or abnormal capillary formation such as cancer and disorders harming eyesight; and medical conditions in which there is reduced capillary formation either for familial or genetic reasons, or as an acquired problem.

Blood sampling

Capillary blood sampling can be used to test for

sexually transmitted infections that are present in the blood stream, such as HIV, syphilis, and hepatitis B and C, where a finger is lanced and a small amount of blood is sampled into a test tube.[32]

History

William Harvey did not explicitly predict the existence of capillaries, but he 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..."[33]

Marcello Malpighi was the first to observe directly and correctly describe capillaries, discovering them in a frog's lung 8 years later, in 1661.[34]

August Krogh discovered how capillaries provide nutrients to animal tissue. For his work he was awarded the 1920 Nobel Prize in Physiology or Medicine.[35]

See also

References

External links