Capillary comes from the Latin wordcapillaris, 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] Whilecapillary 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 asgravity.
Blood flows from the heart througharteries, which branch and narrow intoarterioles, and then branch further into capillaries where nutrients and wastes are exchanged. The capillaries then join and widen to becomevenules, which in turn widen and converge to becomeveins, which then return blood back to the heart through thevenae cavae. In themesentery,metarterioles form an additional stage between arterioles and capillaries.
Individual capillaries are part of thecapillary bed, an interweaving network of capillaries supplyingtissues andorgans. The moremetabolically 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, andsinusoids, a type of open-pore capillary found in theliver,bone marrow,anterior pituitary gland, and braincircumventricular 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 themesentericmicrocirculation.[5]
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 permitsinterstitial fluid to flow into them but not out. Lymph capillaries have a greater internaloncotic pressure than blood capillaries, due to the greater concentration ofplasma proteins in thelymph.[6]
Types of capillaries: (left) continuous with no big gaps, (center) fenestrated with small pores, and (right) sinusoidal (or 'discontinuous') with intercellular gaps
Blood capillaries are categorized into three types: continuous, fenestrated, and sinusoidal (also known as discontinuous).
Continuous capillaries are continuous in the sense that the endothelial cells provide an uninterrupted lining, and they only allow smallermolecules, such as water andions, to pass through theirintercellular 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 inskeletal muscles, fingers,gonads, and skin.[10]
Fenestrated capillaries have pores known asfenestrae (Latin for "windows") in the endothelial cells that are 60–80 nanometres (nm) in diameter. They are spanned by a diaphragm of radially orientedfibrils that allows small molecules and limited amounts of protein to diffuse.[11][12] In therenal glomerulus the capillaries are wrapped inpodocyte foot processes or pedicels, which have slit pores with a function analogous to the diaphragm of the capillaries. Both of these types of blood vessels have continuousbasal laminae and are primarily located in theendocrine glands,intestines,pancreas, and theglomeruli of thekidney.
Sinusoidal capillaries or discontinuous capillaries are a special type of open-pore capillary, also known as asinusoid,[13] that have wider fenestrations that are 30–40 micrometres (μm) in diameter, with wider openings in the endothelium.[14] Fenestrated capillaries have diaphragms that cover the pores whereas sinusoids lack a diaphragm and just have an open pore. These types of blood vessels allowred andwhite blood cells (7.5 μm – 25 μm diameter) and variousserum proteins to pass, aided by a discontinuous basal lamina. These capillaries lackpinocytotic 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 theliver,bone marrow,spleen, and braincircumventricular organs.[14][15]
During earlyembryonic development, new capillaries are formed throughvasculogenesis, the process ofblood vessel formation that occurs through a novel production ofendothelial cells that then form vascular tubes.[16] The termangiogenesis 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 entireyolk sac,connecting stalk, andchorionic villi.[18]
Annotated diagram of the exchange between capillary and body tissue through the exchange of materials between cells and fluid
The capillary wall performs an important function by allowing nutrients and waste substances to pass across it. Molecules larger than 3 nm such asalbumin and other large proteins pass throughtranscellular transport carried insidevesicles, 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 asparacellular transport.[19] These transport mechanisms allow bidirectional exchange of substances depending onosmotic gradients.[20] Capillaries that form part of theblood–brain barrier only allow for transcellular transport astight junctions between endothelial cells seal the paracellular space.[21]
Capillary beds may control their blood flow viaautoregulation. This allows an organ to maintain constant flow despite a change in central blood pressure. This is achieved bymyogenic response, and in thekidney bytubuloglomerular feedback. When blood pressure increases, arterioles are stretched and subsequently constrict (a phenomenon known as theBayliss effect) to counteract the increased tendency for high pressure to increase blood flow.[22]
In thelungs, special mechanisms have been adapted to meet the needs of increased necessity of blood flow during exercise. When theheart 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 ofcollagen.[23]
Capillarypermeability can be increased by the release of certaincytokines,anaphylatoxins, or other mediators (such as leukotrienes, prostaglandins, histamine, bradykinin, etc.) highly influenced by theimmune system.[24]
Diagram of the filtration and reabsorption in capillaries
The transport mechanisms can be further quantified by theStarling 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 toleave the capillary (filtration). If negative, fluid will tend toenter 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:
Disorders of capillary formation as adevelopmental 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 factorvascular 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 derivedendothelial progenitor cells.[25] and reduced ability of those cells to form blood vessels.[26]
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-relatedmacular degeneration.
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-relatedmacular degeneration, localanti-VEGF therapy to limit the bio-activity ofvascular endothelial growth factor has been shown to protect vision by limiting progression.[28] In a wide range of cancers, treatment approaches have been studied, or are in development, aimed at decreasing tumour growth by reducing angiogenesis.[29]
A 13th centurymanuscript by Ibn Nafis contains the earliest known description of capillaries. The manuscript records Ibn Nafis' prediction of the existence of the capillaries which he described as perceptible passages (manafidh) between pulmonary artery and pulmonary vein. These passages would later be identified byMarcello Malpighi as capillaries. He further states that the heart's two main chambers (right and left ventricles) are separate and that blood cannot pass through the (interventricular) septum.[34][35]
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..."[36]
Marcello Malpighi was the first to observe directly and correctly describe capillaries, discovering them in a frog's lung 8 years later, in 1661.[37]
August Krogh discovered how capillaries provide nutrients to animal tissue. For his work he was awarded the 1920Nobel Prize in Physiology or Medicine.[38] His 1922 estimate that total length of capillaries in a human body is as long as 100,000 km, had been widely adopted by textbooks and other secondary sources. This estimate was based on figures he gathered from "an extraordinarily large person".[39] More recent estimates give a number between 9,000 and 19,000 km.[40][39]
^Federative International Committee on Anatomical Terminology (2008).Terminologia Histologica: International Terms for Human Cytology and Histology. Baltimore: Lippincott Williams & Wilkins. p. 87.ISBN978-0-7817-6610-4.
^Guyton, Arthur C.; Hall, John Edward (2006). "The Microcirculation and the Lymphatic System".Textbook of Medical Physiology (11th ed.). Philadelphia: Elsevier Saunders. pp. 187–188.ISBN978-0-8089-2317-6.
^Pavelka, Margit; Roth, Jürgen (2005). "Fenestrated Capillary".Functional Ultrastructure: An Atlas of Tissue Biology and Pathology. Vienna: Springer. p. 232.doi:10.1007/3-211-26392-6_120.ISBN978-3-211-26392-1.
^abSaladin, Kenneth S. (2011).Human Anatomy. McGraw-Hill. pp. 568–569.ISBN978-0-07-122207-5.
^Gross, P. M (1992). "Chapter 31: Circumventricular organ capillaries".Circumventricular Organs and Brain Fluid Environment - Molecular and Functional Aspects. Progress in Brain Research. Vol. 91. pp. 219–33.doi:10.1016/S0079-6123(08)62338-9.ISBN978-0-444-81419-7.PMID1410407.
^Schoenwolf, Gary C. (2015).Larsen's human embryology (Fifth ed.). Philadelphia, PA. p. 306.ISBN978-1-4557-0684-6.{{cite book}}: CS1 maint: location missing publisher (link)
^Boulpaep, Emile L. (2017). "The Microcirculation". In Boron, Walter F.; Boulpaep, Emile L. (eds.).Medical Physiology (3rd ed.). Philadelphia, PA: Elsevier. p. 481.ISBN978-1-4557-4377-3.
^Yunfei, Chi; Xiangyu, Liu (9 April 2021), Jiake, Chai (ed.), "A narrative review of changes in microvascular permeability after burn",Annals of Translational Medicine,9 (8): 719,doi:10.21037/atm-21-1267,PMC8106041,PMID33987417
