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Capillaries are the smallest blood vessels in the body, with a diameter of 5-10 micrometres. They are a part of the microcirculation system and are composed of only the tunica intima, the innermost layer of an artery or vein. The wall of a capillary consists of an endothelial layer surrounded by a basement membrane with occasional smooth muscle fibres. The capillaries that wrap around each muscle fibre are located within the endomysium, the innermost layer of connective tissue surrounding each muscle fibre. This layer is crucial for providing nutrients and removing waste, ensuring proper muscle function.
| Characteristics | Values |
|---|---|
| Capillary definition | Smallest blood vessels in the vascular system |
| Capillary size | 5-10 micrometres in diameter |
| Capillary wall thickness | 1 micrometer |
| Capillary function | Exchange of gases, nutrients, and waste between blood and cells |
| Capillary types | Continuous, fenestrated, and sinusoidal |
| Continuous capillaries | Found in muscle, skin, fat, nerve tissue, heart, lungs, and central nervous system |
| Fenestrated capillaries | Have small pores allowing exchange of small molecules and limited proteins |
| Sinusoidal capillaries | Open-pore capillaries with wider fenestrations; found in liver, spleen, and bone marrow |
| Capillary structure | Endothelial layer surrounded by a basement membrane with occasional smooth muscle fibers |
| Capillary wall permeability | Varies with the complexity of the tight junction architecture |
| Capillary exchange regulation | Endothelial cells release endothelins affecting smooth muscle constriction and blood pressure |
| Capillary blood flow | Slowed by arterioles to cause a drop in blood pressure |
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What You'll Learn
Capillaries are the smallest blood vessels in the body
Capillaries are part of the microcirculation system and are essential for the exchange of substances between the blood and surrounding cells. They connect arteries and veins, completing the circulatory system, and are often referred to as "exchange centres". They supply tissues and organs with nutrients and oxygen and remove carbon dioxide and other waste products.
There are three main types of capillaries: continuous, fenestrated, and sinusoidal. Continuous capillaries are found in the heart, lungs, skeletal muscles, and central nervous system. They have a continuous basal lamina and strong occluding junctions between endothelial cells. Fenestrated capillaries have small openings (fenestrae) that enable rapid exchange. These are found in the kidneys, small intestine, and endocrine glands. Sinusoidal capillaries are a type of open-pore capillary with wider openings in the endothelium, allowing red and white blood cells and serum proteins to pass through. They are found in the liver, spleen, bone marrow, anterior pituitary gland, and brain circumventricular organs.
Capillaries can develop issues such as capillary leak syndrome, which causes a sudden drop in blood pressure, and hereditary hemorrhagic telangiectasia, an inherited disorder that causes abnormal growths that can burst. Keeping the vascular system healthy through physical activity and a nutritious diet can help maintain capillary health.
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They are composed of a thin wall of endothelial cells
Capillaries are the smallest blood vessels in the body. They are part of the microcirculation system and are extremely thin, measuring about 5 micrometers in diameter. They are so small that red blood cells need to flow through them in single file.
Capillaries are composed of a thin wall of endothelial cells. The wall of a capillary is made up of the endothelial layer surrounded by a basement membrane with occasional smooth muscle fibres. The endothelial cells within the capillaries are connected by strong occluding junctions, and their extracellular surface is lined with a continuous basal lamina. The surface structures of the continuous capillary endothelium are present underlying the endothelial glycocalyx. The capillaries are involved in the exchange of gases and other substances between the blood and the surrounding cells and their tissue fluid (interstitial fluid).
There are three types of capillaries: continuous, fenestrated, and sinusoidal. Continuous capillaries are enclosed and have no perforations in them. They are present in muscle, skin, fat, and nerve tissue. They only allow smaller molecules, such as water and ions, to pass through their intercellular clefts. Lipid-soluble molecules can also passively diffuse through the endothelial cell membranes along concentration gradients. Fenestrated capillaries have small pores that allow small molecules (like water) to be passed back and forth between tissues. Sinusoidal capillaries or discontinuous capillaries are a special type of open-pore capillary, also known as a sinusoid, that have wider openings in the endothelium.
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Capillaries are involved in the exchange of gases and nutrients
Capillaries are the smallest blood vessels in the body, forming a connection between arteries and veins. They are so small that red blood cells have to pass through them in single file. The walls of capillaries are thin, composed of a single layer of endothelial cells, and are permeable, allowing nutrients, fluids, and gases to pass through.
The exchange of gases and nutrients between blood and tissues is a major function of capillaries. Capillaries deliver nutrients and oxygen to cells and remove carbon dioxide and other waste products. This exchange occurs across capillary walls through processes such as diffusion, filtration, and osmosis. The thin walls of capillaries facilitate the efficient diffusion of oxygen and carbon dioxide, as well as the movement of liquids and ions through pores in the capillary walls.
The capillary wall consists of an endothelial layer surrounded by a basement membrane, with occasional smooth muscle fibres. The structure of the capillary wall varies, with larger capillaries having multiple endothelial cells lining the lumen, while smaller capillaries have a single cell layer. The degree of "leakiness" in capillary walls differs between the three types of capillaries: continuous, fenestrated, and sinusoidal. Continuous capillaries have a lining that only allows small molecules to pass through and are found in the nervous system, as well as in fat and muscle tissue. Fenestrated capillaries have small openings that enable the rapid exchange of substances and are found in the kidneys, small intestine, and endocrine glands. Sinusoidal capillaries have small fenestrae and are present in the liver and spleen.
The exchange of gases and nutrients in capillaries is influenced by pressure differences. The primary force driving fluid transport between capillaries and tissues is hydrostatic pressure, which can be defined as the pressure exerted by a fluid within an enclosed space. The movement of fluid out of capillaries and into tissues is called filtration, while the movement of fluid from tissues back into capillaries is known as reabsorption or osmosis. These processes are regulated by capillary hydrostatic pressure (CHP) and blood colloidal osmotic pressure (BCOP). CHP drives filtration, while BCOP, influenced by the concentration of colloidal proteins in the blood, drives reabsorption.
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There are three types of capillaries with distinct functions
Capillaries are the smallest blood vessels in the vascular system, measuring about 5 micrometres (0.005 mm) in diameter. They are so small that red blood cells must pass through them in single file. They are also delicate, with thin walls that allow fluids, gases, nutrients, and waste products to pass through.
There are three types of capillaries, each with a unique function:
Continuous Capillaries
Continuous capillaries are found in the heart, lungs, skeletal muscles, and central nervous system. They are also present in fat tissue and the endocrine glands. These capillaries have a continuous basal lamina and strong occluding junctions between their endothelial cells. The continuous capillary endothelium in the lungs, for example, has an ultrastructure where the vessel wall is continuous. In the brain, continuous capillaries are part of the blood-brain barrier, which protects the brain by allowing only essential nutrients to cross.
Fenestrated Capillaries
Fenestrated capillaries are "leakier" than continuous capillaries due to their small pores and gaps between endothelial cells. This allows for the exchange of larger molecules. They are found in areas requiring significant exchange between blood and tissues, such as the small intestine for nutrient absorption and the kidneys for filtering waste from the blood. They are also present in the endocrine glands and play a crucial role in kidney function during urine formation.
Sinusoidal Capillaries
Sinusoidal capillaries are the most permeable type, with many large gaps, pores, and an incomplete basement membrane. This allows for the exchange of large molecules and even cells. They are found in the liver, spleen, and bone marrow, where they enable newly produced blood cells to enter the bloodstream.
The capillary bed, an interwoven network of capillaries, supplies organs with blood and ensures the delivery of nutrients and removal of waste. While capillaries have distinct functions, they all contribute to the vital process of facilitating exchange between blood and tissues.
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Capillaries are susceptible to various health conditions
Capillaries are the smallest blood vessels in the human body, with a diameter of around 5 micrometers. They are so small that red blood cells have to pass through them in single file. Capillaries play a crucial role in delivering nutrients and oxygen to cells and removing waste products like carbon dioxide. They connect arteries and veins, completing the circulatory system.
Due to their delicate nature and vital functions, capillaries are susceptible to various health conditions. Broken capillaries, for instance, can result from severe coughing or vomiting. This can lead to petechiae, characterised by small dots of discolouration on the skin caused by bleeding from the broken capillaries. Petechiae usually heal on their own.
Capillaries can also be affected by conditions such as arteriovenous malformation (AVM), which involves a tangle of arteries and veins that may bypass the capillaries. Capillary angiosarcoma, a rare cancer of the endothelial cells, can also impact the capillaries. Additionally, capillary leak syndrome, resulting from a sudden drop in blood pressure, can affect capillary function and sometimes requires emergency treatment.
Furthermore, individuals with hereditary hemorrhagic telangiectasia, also known as Osler-Weber-Rendu syndrome, experience abnormal growths of blood vessels that can burst. Capillaries can also be involved in macular degeneration, a degenerative eye disease caused by capillary leaks. Microcephaly-capillary malformation syndrome is another condition that affects the capillaries, resulting in wide capillaries in individuals with smaller-than-average heads.
Spider angioma, or spider nevus, is a condition where small blood vessels branch out from a central spot, usually on the face, neck, or chest. This can be a cosmetic concern and may cause bleeding if irritated. Rosacea, an inflammatory skin condition, can also cause capillary enlargement, giving the skin a permanent pink or reddish appearance.
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Frequently asked questions
Capillaries are the smallest blood vessels in the body. They are part of the microcirculation system and are composed of only the tunica intima, the innermost layer of an artery or vein.
Capillaries deliver nutrients and oxygen to cells throughout the body. They also remove waste products, such as carbon dioxide, from cells.
Capillaries are not lined with muscle. However, they are surrounded by a basement membrane with occasional smooth muscle fibres. The wall of a capillary consists of an endothelial layer, which is surrounded by the basement membrane.
Capillaries wrap around each muscle fibre and are located within the endomysium, the innermost layer of connective tissue surrounding each muscle fibre.
