Capillary Definition
A capillary is a minute, thin-walled blood vessel that connects arterioles with venules. Capillaries are the smallest type of blood vessels in the body and form a network throughout the body. They are typically 5 to 10 micrometers in diameter and are part of the microcirculation system.
Capillary Anatomy
Capillaries are the smallest and most numerous blood vessels in the body, forming a connection between arteries and veins. They are thin-walled vessels composed of a single endothelial layer, which allows for the exchange of nutrients and metabolites primarily via diffusion.
The flow of blood through capillaries is regulated by the arteriolar lumen. Smooth muscle cells in the arterioles where they branch to form capillaries regulate blood flow from the arterioles into the capillaries.
Capillaries have three different shapes: continuous fenestrated, continuous nonfenestrated, and sinusoidal capillaries. Continuous fenestrated capillaries have small openings (fenestrae) that enable rapid exchange of substances.
This type of capillary is found in the kidneys, small intestine, and endocrine glands. Continuous nonfenestrated capillaries have a lining through which only small molecules can pass. This type of capillary exists in the nervous system as well as fat and muscle tissue.
Sinusoidal capillaries have small fenestrae that allow certain substances to pass through. This type of capillary is found in the liver and spleen.
Capillaries play an essential role in transporting blood, nutrients, and oxygen to cells throughout the body. They are also involved in exchanging oxygen, nutrients, and wastes between blood and tissues. Capillary networks are present throughout bodily tissues, making them an important part of human physiology.
Endothelium
The endothelium is a thin layer of single flat cells called endothelial cells that line the interior surface of blood vessels and lymphatic vessels. The endothelium is one of the largest organs in the body and is considered an endocrine organ because it actively performs many jobs to support blood flow and keep the body in a stable state.
Endothelial cells are involved in many aspects of vessel function, including blood clotting, inflammation, and immune responses.
The capillary endothelium is a type of epithelium that forms the inner lining of the entire blood vascular system. In capillaries, the outer tunics of smooth muscle cells are absent, and only the endothelium is present.
Capillaries are invested in pericapillary contractile cells called pericytes. The lumenal (blood compartment) and ablumenal (tissue compartment) surface of endothelial cells contain numerous flask-shaped invaginations called caveolae (little caves).
The pulmonary capillary endothelium is a continuous highly attenuated cell layer that forms a barrier that prevents leakage of excess water and solutes into alveolar spaces. Endothelial cells retain a capacity for cell division and movement throughout the vascular system of the adult body.
If part of the wall of an artery or vein is damaged and denuded of endothelial cells, neighboring endothelial cells proliferate and migrate to cover the exposed surface.
Basement Membrane
The basement membrane, also known as the basal lamina, is a thin, pliable sheet-like type of extracellular matrix that provides cell and tissue support and acts as a barrier between adjacent tissues. It is composed of two layers: the basal lamina and the reticular lamina.
The basal lamina is closer to the epithelial cells and is composed of laminin, collagen IV, entactin/nidogen, and perlecan. The reticular lamina is closer to the underlying connective tissue and contains collagen III, collagen VII, fibronectin, and elastin.
The basement membrane plays an essential role in angiogenesis (the development of new blood vessels) by accelerating the differentiation of endothelial cells. It also contributes to the integrity of the blood-brain barrier (BBB), which is formed by brain capillary endothelial cells (BCECs).
BCECs receive support from pericytes embedded in the vascular basement membrane and from astrocyte endfeet. Thickening of the capillary basement membrane may contribute to the development of abnormal blood vessel function.
The distinct types of vessels denoting the cerebral vascular tree vary in terms of vascular wall thickness and composition of basement membrane proteins.
Especially, capillaries have no clear separation between endothelial and parenchymal basement membranes; thus, they appear as a single entity that envelops pericytes.
In postcapillary venules, both parenchymal and endothelial basement membranes line the border of virtual perivascular space (PVS).
Connective Tissue
Connective tissue is an essential component of blood vessels. Blood vessels are comprised of three layers: the tunica intima, tunica media, and tunica externa. The innermost layer, the tunica intima, is a thin layer composed of a simple squamous epithelium known as endothelium and a small amount of connective tissue.
The middle layer, the tunica media, is primarily smooth muscle and contains elastic fibers and connective tissue arranged circularly around the vessel. The outermost layer, the tunica externa or adventitia, attaches the vessel to surrounding tissues and is made up of connective tissue.
Capillaries have only a single layer called the tunica intima consisting of endothelium and associated connective tissue. Arteries have thicker walls than veins due to their thicker tunica media which contains more contractile tissue than veins. Veins have thinner walls than arteries because they contain less smooth muscle and connective tissue in their walls.
Connective tissue plays an important role in maintaining the structure and function of blood vessels. It provides support for blood vessels by attaching them to surrounding tissues. It also helps regulate blood flow by controlling the contraction and relaxation of smooth muscle cells in the vessel wall.
Capillary Types
There are three types of capillaries in the body: continuous, fenestrated, and sinusoidal. Continuous capillaries have a lining that contains pores (intracellular clefts) that let only small molecules pass through. These molecules include water, glucose, hormones, and gases.
Continuous nonfenestrated capillaries make up the blood-brain barrier where they control the movement of water, oxygen, and other essential substances between the blood and the brain. They prevent toxins from entering the brain, protecting it from injury and disease.
Fenestrated capillaries have small pores in their lining called fenestrae. The basement membrane of the epithelial cells in the lining remains unbroken by these fenestrae. Fenestrated capillaries are found in organs that require a rapid exchange of materials such as kidneys and endocrine glands.
Sinusoidal capillaries have endothelial linings with multiple fenestrations (openings) around 30 to 40 nm in diameter. They have no diaphragm and either a discontinuous or non-existent basal lamina. This allows blood cells and serum proteins to pass through the capillary wall as if it were a colander.
Sinusoidal capillaries are mainly found in the liver between epithelial cells and hepatocytes. They can also be found in the sinusoids of the spleen where they are involved in filtering blood to remove antigens, defective red blood cells, and microorganisms.
Capillary Function
Capillaries are the smallest and most numerous blood vessels in the body, connecting arteries to veins and facilitating the exchange of materials between blood and tissue cells.
Capillaries transport blood, nutrients, and oxygen to cells in organs. There are three types of capillaries: continuous fenestrated capillaries, continuous nonfenestrated capillaries, and sinusoidal capillaries.
Continuous fenestrated capillaries have small openings that enable rapid substance exchange. This type of capillary is found in the kidneys, small intestines, and endocrine glands. Continuous nonfenestrated capillaries have a lining through which only small molecules can pass.
This type of capillary exists in the nervous system as well as fat and muscle tissue. Sinusoidal capillaries have small fenestrae that allow certain substances to pass through. This type of capillary is found in the liver and spleen.
The smooth muscle cells in arterioles regulate blood flow from arterioles into the capillaries. After passing through the capillaries, blood enters venules before flowing into progressively larger veins until it reaches the heart. Arteriovenous malformation (AVM) is a condition where there is a tangle of arteries and veins that may bypass the capillaries.
Capillaries play an essential role in facilitating the exchange of various substances between your bloodstream and tissues. They are responsible for transporting gases, fluids, nutrients, etc., throughout your body.
Capillary Damage
Capillaries are the smallest blood vessels in the body that connect arteries and veins. Capillary damage can occur due to various reasons, including capillary leak syndrome (CLS) and SARS-CoV-2 infection.
CLS is a rare disorder that causes fluid leakage from the capillaries into surrounding tissues, leading to hypovolemia, low blood pressure, thickened blood, and peripheral edema.
The condition can be triggered by other diseases, conditions, or medications. There are two types of CLS: systemic capillary leak syndrome (SCLS) and secondary CLS. SCLS is a rare disorder that affects less than 500 people worldwide.
The symptoms of SCLS include low blood pressure, loss of albumin from the blood, thickened blood, general swelling, peripheral edema, diarrhea, and hypercoagulability due to elevated red blood cell count or elevated white blood count. The condition is probably underrecognized because of its nonspecific symptoms and signs and high mortality rate.
Treatment for SCLS includes intravenous fluids to increase blood flow to organs such as kidneys and steroids to reduce capillary leaks. During an episode of SCLS, patients are usually hospitalized.
SARS-CoV-2 infection can also cause microvascular damage in the form of capillary COVID-19 effects. This damage may trigger symptoms and tissue injury by critically reducing local tissue oxygenation.
