Capillary
Capillary | |
---|---|
Transmission electron microscope image of a cross-section of a capillary occupied by a red blood cell | |
Details | |
Pronunciation | US: /ˈkæpəlɛri/, UK: /kəˈpɪləri/ |
System | Circulatory system |
Identifiers | |
Latin | vas capillare[1] |
MeSH | D002196 |
TA98 | A12.0.00.025 |
TA2 | 3901 |
TH | H3.09.02.0.02001 |
FMA | 63194 |
Anatomical terminology] |
A capillary is a small
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
Blood flows from the heart through
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).
Continuous
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:
- Those with numerous transport vesicles, which are found primarily in skeletal muscles, fingers, gonads, and skin.[10]
- 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
Sinusoidal
Sinusoidal capillaries or discontinuous capillaries are a special type of open-pore capillary, also known as a sinusoid,
Development
During early
Function
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
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]
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:
- Capillary hydrostatic pressure(Pc)
- Interstitial hydrostatic pressure (Pi)
- Capillary oncotic pressure (πc)
- Interstitial oncotic pressure (πi)
- Filtration coefficient (Kf)
- Reflection coefficient (σ)
Clinical significance
Disorders of capillary formation as a
- 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 coronary heart disease.[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.
- In patients with the retinal disorder, neovascular age-related anti-VEGF therapy to limit the bio-activity of vascular endothelial growth factor has been shown to protect vision by limiting progression.[27] In a wide range of cancers, treatment approaches have been studied, or are in development, aimed at decreasing tumour growth by reducing angiogenesis.[28]
Blood sampling
Capillary blood sampling can be used to test for
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
- Blood–air barrier, also known as alveolar–capillary barrier – Membrane separating alveolar air from blood in lung capillaries
- Capillary refill – Medical term
- Hagen–Poiseuille equation – Law describing the pressure drop in an incompressible and Newtonian fluid
- Surface chemistry of microvasculature
References
- ISBN 9780781766104.
- ^ "Structure and Function of Blood Vessels | Anatomy and Physiology II". courses.lumenlearning.com. Retrieved 19 November 2021.
- ISBN 978-0-13-981176-0.
- ^ a b "Capillary". Online Etymology Dictionary. 2021. Retrieved 14 July 2021.
- PMID 23824465.
- ISBN 9780808923176.
- PMID 27141427.
- ^ PMID 27141424.
- PMID 22163071.
- S2CID 205759387.
- ISBN 978-0195151732.
- ISBN 978-3-211-26392-1.
- ^ "Histology Laboratory Manual". www.columbia.edu.
- ^ ISBN 9780071222075.
- PMID 1410407.
- ISBN 978-1-4020-6779-2. Retrieved 26 June 2010.
- ISBN 0-87893-243-7. Retrieved 1 February 2021.
- ISBN 9781455706846.)
{{cite book}}
: CS1 maint: location missing publisher (link - PMID 25610592.
- ^ PMID 18293091.
- PMID 25520612.
- ISBN 978-1-4557-4377-3.
- PMID 17021008.
- PMID 20646126.
- ^ S2CID 12041051.
- PMID 9109431.
- PMID 20811159.
- PMID 19273326.
- PMID 26524965.
- PMID 28102549.
- ^ "Managing diabetes:Check your blood glucose levels". National Institute of Diabetes and Digestive and Kidney Diseases,US National Institutes of Health. 2021. Retrieved 9 September 2021.
- ^ "Fettle - How to take a blood sample". Archived from the original on 16 March 2023. Retrieved 16 March 2023.
- ^ Harvey, William (1653). On the motion of the Heart and Blood in Animals. pp. 59–60. Archived from the original on 1 December 2011.
- ISBN 9780835773287.
- ^ "August Krogh". July 2023.
External links
- Histology image: 00903loa – Histology Learning System at Boston University
- The Microcirculatory Society, Inc.
- The Histology Guide – Capillaries