Table of ContentsView AllTable of ContentsStructureFunctionMedical SignificanceAssociated Conditions
Table of ContentsView All
View All
Table of Contents
Structure
Function
Medical Significance
Associated Conditions
Capillariesare the smallest blood vessels in the body, connecting the smallestarteriesto the smallest veins. They deliver oxygen and nutrients to cells while removing carbon dioxide to be eliminated from the lungs.
There are different types of capillaries with specific functions in the body. When these vessels malfunction become injured, or are diseased, they can break or deprive cells of the oxygen they need to survive. This can lead to conditions ranging frompetechiae(“blood spots”) tomacular degeneration(a degenerative eye disease).
This article describes the structure and function of capillaries. It also explains the effects of capillary dysfunction and the diseases and conditions that cause them.
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Capillary Structure
Capillaries form the connection between the body’s smallest arteries (calledarterioles) and the body’s smallest veins (calledvenules). There are about 40 billion capillaries in the average human body.
Capillaries are extremely thin, measuring approximately 5 micrometers in diameter. They are so small thatred blood cellsneed to flow through them single file.
Where Capillaries Are Found
Capillaries are found throughout the body, although the distribution of them in tissues can vary widely.
The lungs are rich with capillaries that exchange oxygen and carbon dioxide via tiny air sacs in the lungs calledalveoli.Outside of the lungs, capillaries are most abundant in metabolically active tissues (meaning those like the liver, brain, heart, and kidneys that convert nutrients to energy at a faster rate).
Capillaries in the central nervous system are also involved in the blood-brain barrier. This barrier limits the ability of toxins (including certain medications) to pass through into the brain. Drugs that act on the brain need to be designed to circumvent this barrier.
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Types of Capillaries
There are three types of capillaries in the body that have specific functions:
Anatomy of the Lymphatic Vessels
Capillary Function
Capillaries are responsible for exchanging gases, fluids, and nutrients in the body. While arteries deliver these to the capillaries, the capillaries are where the actual exchange takes place in cells.
What the Alveoli Do
Role in Blood Pressure and Circulation
When you exercise, more capillary beds are recruited to prepare for an increased need for oxygen in tissues. The same mechanisms are involved in the regulation ofblood pressure.
Why You Get Low Blood Pressure
Capillary Microcirculation
Physiological Effects of Capillaries (and What They Mean)
Capillaries are involved in many physiological effects in the human body, some of which indicate a medical concern.
Examples include:
Capillaries are involved in several health concerns. Some are relatively common while others are rare.
Certain Birthmarks
Capillary Malformation
Capillary malformation may occur as part of an inherited syndrome present in roughly one in 100,000 people of European ancestry. In this syndrome, there is more blood flow than normal through the capillaries near the skin, which results in pink and red dots on the skin.
These may occur alone, or people may have other complications of this syndrome such asarteriovenous malformations(abnormal connections between arteries and veins) which, when in the brain, can cause headaches and seizures.
Systemic Capillary Leak Syndrome
A rare disorder known as capillary leak syndrome involves leaky capillaries, which may result in constant nasal congestion and episodes of fainting due to rapid drops in blood pressure.
This syndrome also may occur due to adverse drug reactions during cancer treatment and can lead to severe and potentially life-threatening complications.
It’s also been noted in people diagnosed withCOVID-19 infections, as well as some people who received COVID-19 vaccinations.
Macular Degeneration
Macular degeneration, now the leading cause of blindness in the United States, occurs secondary to damage in the capillaries of theretina(the nerve-filled tissue at the back of the eye).
Summary
Capillaries are the smallest blood vessels in the circulatory system. These are the vessels that bring oxygen and nutrients to cells in exchange for carbon dioxide and waste that they take from cells.
Capillaries are involved when your skin blanches or you get a rash or hives. They can break or get inflamed, causing petechiae, rosacea, or spider veins. They are also involved in potential serious medical concerns like arteriovenous malformation, capillary leak syndrome, and macular degeneration.
How the Circulatory System Works
14 SourcesVerywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read oureditorial processto learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.Ramasamy SK.Structure and functions of blood vessels and vascular niches in bone.Stem Cells Int. 2017;2017:5046953. doi:10.1155/2017/5046953Maynard RL, Downes N.Histology of the vascular system.Anat Histol Lab Rat Toxicol Biomed Res. 2019:91-95. doi:10.1016/b978-0-12-811837-5.00008-3Cao Y.Blood vessels in fat tissues and vasculature-derived signals in controlling lipid metabolism and metabolic disease.Chin Med J (Engl).2022 Nov 20;135(22):2647–2652. oi:10.1097/CM9.0000000000002406Chen HH.Core concept: circumventing the blood-brain barrier.Proc Natl Acad Sci USA. 2017;114(43):11261-11263. doi:10.1073/pnas.1716187114Dzialowski E, Crossley DA.Chapter 11 - the cardiovascular system. In:Sturkie’s Avian Physiology (Sixth Edition).Cambridge, MA: Academic Press; 2015. doi:10.1016/B978-0-12-407160-5.00011-7Wigley F., and N. Flavahan.Raynaud’s phenomenon.New Engl J Med. 2016;375(6):556-65. doi:10.1056/NEJMra1507638Thomas AE, Baird SF, Anderson J.Purpuric and petechial rashes in adults and children: initial assessment.BMJ. 2016;352:i1285. doi:10.1136/bmj.i1285MedlinePlus.Telangiectasia.Buddenkotte J, Steinhoff M.Recent advances in understanding and managing rosacea.F1000Res.2018;7,December 3. doi:10.12688/f1000research.16537.1MedlinePlus.Red birthmarks.Sibley CD, Ramien ML.Capillary malformation–arteriovenous malformation Syndrome.JAMA Dermatology. 2019;155(6):733. doi:10.1001/jamadermatol.2019.0319Izzedine H, Mathian A, Amoura Z, Ng JH, Jhaveri KD.Anticancer drug-induced capillary leak syndrome.Kidney Int Rep. 2022 Feb 25;7(5):945-953. doi:10.1016/j.ekir.2022.02.014.Pineton de Chambrun M, Moyon Q, Faguer S, Urbanski G, Mathian A, Zucman N,et al.The consequences of COVID-19 pandemic on patients with monoclonal gammopathy-associated systemic capillary leak syndrome (Clarkson disease).J Allergy Clin Immunol Pract. 2022 Feb;10(2):626-629. doi:10.1016/j.jaip.2021.11.023.Mathenge W.Age-related macular degeneration.Community Eye Health. 2014;27(87):49-50.Additional ReadingGenetics Home Reference.Capillary malformation—arteriovenous malformation syndrome.
14 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read oureditorial processto learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.Ramasamy SK.Structure and functions of blood vessels and vascular niches in bone.Stem Cells Int. 2017;2017:5046953. doi:10.1155/2017/5046953Maynard RL, Downes N.Histology of the vascular system.Anat Histol Lab Rat Toxicol Biomed Res. 2019:91-95. doi:10.1016/b978-0-12-811837-5.00008-3Cao Y.Blood vessels in fat tissues and vasculature-derived signals in controlling lipid metabolism and metabolic disease.Chin Med J (Engl).2022 Nov 20;135(22):2647–2652. oi:10.1097/CM9.0000000000002406Chen HH.Core concept: circumventing the blood-brain barrier.Proc Natl Acad Sci USA. 2017;114(43):11261-11263. doi:10.1073/pnas.1716187114Dzialowski E, Crossley DA.Chapter 11 - the cardiovascular system. In:Sturkie’s Avian Physiology (Sixth Edition).Cambridge, MA: Academic Press; 2015. doi:10.1016/B978-0-12-407160-5.00011-7Wigley F., and N. Flavahan.Raynaud’s phenomenon.New Engl J Med. 2016;375(6):556-65. doi:10.1056/NEJMra1507638Thomas AE, Baird SF, Anderson J.Purpuric and petechial rashes in adults and children: initial assessment.BMJ. 2016;352:i1285. doi:10.1136/bmj.i1285MedlinePlus.Telangiectasia.Buddenkotte J, Steinhoff M.Recent advances in understanding and managing rosacea.F1000Res.2018;7,December 3. doi:10.12688/f1000research.16537.1MedlinePlus.Red birthmarks.Sibley CD, Ramien ML.Capillary malformation–arteriovenous malformation Syndrome.JAMA Dermatology. 2019;155(6):733. doi:10.1001/jamadermatol.2019.0319Izzedine H, Mathian A, Amoura Z, Ng JH, Jhaveri KD.Anticancer drug-induced capillary leak syndrome.Kidney Int Rep. 2022 Feb 25;7(5):945-953. doi:10.1016/j.ekir.2022.02.014.Pineton de Chambrun M, Moyon Q, Faguer S, Urbanski G, Mathian A, Zucman N,et al.The consequences of COVID-19 pandemic on patients with monoclonal gammopathy-associated systemic capillary leak syndrome (Clarkson disease).J Allergy Clin Immunol Pract. 2022 Feb;10(2):626-629. doi:10.1016/j.jaip.2021.11.023.Mathenge W.Age-related macular degeneration.Community Eye Health. 2014;27(87):49-50.Additional ReadingGenetics Home Reference.Capillary malformation—arteriovenous malformation syndrome.
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read oureditorial processto learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
Ramasamy SK.Structure and functions of blood vessels and vascular niches in bone.Stem Cells Int. 2017;2017:5046953. doi:10.1155/2017/5046953Maynard RL, Downes N.Histology of the vascular system.Anat Histol Lab Rat Toxicol Biomed Res. 2019:91-95. doi:10.1016/b978-0-12-811837-5.00008-3Cao Y.Blood vessels in fat tissues and vasculature-derived signals in controlling lipid metabolism and metabolic disease.Chin Med J (Engl).2022 Nov 20;135(22):2647–2652. oi:10.1097/CM9.0000000000002406Chen HH.Core concept: circumventing the blood-brain barrier.Proc Natl Acad Sci USA. 2017;114(43):11261-11263. doi:10.1073/pnas.1716187114Dzialowski E, Crossley DA.Chapter 11 - the cardiovascular system. In:Sturkie’s Avian Physiology (Sixth Edition).Cambridge, MA: Academic Press; 2015. doi:10.1016/B978-0-12-407160-5.00011-7Wigley F., and N. Flavahan.Raynaud’s phenomenon.New Engl J Med. 2016;375(6):556-65. doi:10.1056/NEJMra1507638Thomas AE, Baird SF, Anderson J.Purpuric and petechial rashes in adults and children: initial assessment.BMJ. 2016;352:i1285. doi:10.1136/bmj.i1285MedlinePlus.Telangiectasia.Buddenkotte J, Steinhoff M.Recent advances in understanding and managing rosacea.F1000Res.2018;7,December 3. doi:10.12688/f1000research.16537.1MedlinePlus.Red birthmarks.Sibley CD, Ramien ML.Capillary malformation–arteriovenous malformation Syndrome.JAMA Dermatology. 2019;155(6):733. doi:10.1001/jamadermatol.2019.0319Izzedine H, Mathian A, Amoura Z, Ng JH, Jhaveri KD.Anticancer drug-induced capillary leak syndrome.Kidney Int Rep. 2022 Feb 25;7(5):945-953. doi:10.1016/j.ekir.2022.02.014.Pineton de Chambrun M, Moyon Q, Faguer S, Urbanski G, Mathian A, Zucman N,et al.The consequences of COVID-19 pandemic on patients with monoclonal gammopathy-associated systemic capillary leak syndrome (Clarkson disease).J Allergy Clin Immunol Pract. 2022 Feb;10(2):626-629. doi:10.1016/j.jaip.2021.11.023.Mathenge W.Age-related macular degeneration.Community Eye Health. 2014;27(87):49-50.
Ramasamy SK.Structure and functions of blood vessels and vascular niches in bone.Stem Cells Int. 2017;2017:5046953. doi:10.1155/2017/5046953
Maynard RL, Downes N.Histology of the vascular system.Anat Histol Lab Rat Toxicol Biomed Res. 2019:91-95. doi:10.1016/b978-0-12-811837-5.00008-3
Cao Y.Blood vessels in fat tissues and vasculature-derived signals in controlling lipid metabolism and metabolic disease.Chin Med J (Engl).2022 Nov 20;135(22):2647–2652. oi:10.1097/CM9.0000000000002406
Chen HH.Core concept: circumventing the blood-brain barrier.Proc Natl Acad Sci USA. 2017;114(43):11261-11263. doi:10.1073/pnas.1716187114
Dzialowski E, Crossley DA.Chapter 11 - the cardiovascular system. In:Sturkie’s Avian Physiology (Sixth Edition).Cambridge, MA: Academic Press; 2015. doi:10.1016/B978-0-12-407160-5.00011-7
Wigley F., and N. Flavahan.Raynaud’s phenomenon.New Engl J Med. 2016;375(6):556-65. doi:10.1056/NEJMra1507638
Thomas AE, Baird SF, Anderson J.Purpuric and petechial rashes in adults and children: initial assessment.BMJ. 2016;352:i1285. doi:10.1136/bmj.i1285
MedlinePlus.Telangiectasia.
Buddenkotte J, Steinhoff M.Recent advances in understanding and managing rosacea.F1000Res.2018;7,December 3. doi:10.12688/f1000research.16537.1
MedlinePlus.Red birthmarks.
Sibley CD, Ramien ML.Capillary malformation–arteriovenous malformation Syndrome.JAMA Dermatology. 2019;155(6):733. doi:10.1001/jamadermatol.2019.0319
Izzedine H, Mathian A, Amoura Z, Ng JH, Jhaveri KD.Anticancer drug-induced capillary leak syndrome.Kidney Int Rep. 2022 Feb 25;7(5):945-953. doi:10.1016/j.ekir.2022.02.014.
Pineton de Chambrun M, Moyon Q, Faguer S, Urbanski G, Mathian A, Zucman N,et al.The consequences of COVID-19 pandemic on patients with monoclonal gammopathy-associated systemic capillary leak syndrome (Clarkson disease).J Allergy Clin Immunol Pract. 2022 Feb;10(2):626-629. doi:10.1016/j.jaip.2021.11.023.
Mathenge W.Age-related macular degeneration.Community Eye Health. 2014;27(87):49-50.
Genetics Home Reference.Capillary malformation—arteriovenous malformation syndrome.
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