A venule is a very small

blood vessel The blood vessels are the components of the circulatory system that transport blood throughout the human body. These vessels transport blood cells, nutrients, and oxygen to the tissues of the body. They also take waste and carbon dioxide awa ...

in the

microcirculation The microcirculation is the circulation of the blood in the smallest blood vessels, the microvessels of the microvasculature present within organ tissues. The microvessels include terminal arterioles, metarterioles, capillaries, and venules. ...

that allows

blood Blood is a body fluid in the circulatory system of humans and other vertebrates that delivers necessary substances such as nutrients and oxygen to the cells, and transports metabolic waste products away from those same cells. Blood in the cir ...

to return from the

capillary A capillary is a small blood vessel from 5 to 10 micrometres (μm) in diameter. Capillaries are composed of only the tunica intima, consisting of a thin wall of simple squamous endothelial cells. They are the smallest blood vessels in the bod ...

beds to drain into the larger blood vessels, the

vein Veins are blood vessels in humans and most other animals that carry blood towards the heart. Most veins carry deoxygenated blood from the tissues back to the heart; exceptions are the pulmonary and umbilical veins, both of which carry oxygenated ...

s. Venules range from 7μm to 1mm in diameter. Veins contain approximately 70% of total blood volume, while about 25% is contained in the venules. Many venules unite to form a vein.

Structure

Venule walls have three layers: An inner

endothelium The endothelium is a single layer of squamous endothelial cells that line the interior surface of blood vessels and lymphatic vessels. The endothelium forms an interface between circulating blood or lymph in the lumen and the rest of the ve ...

composed of squamous

endothelial cell The endothelium is a single layer of squamous endothelial cells that line the interior surface of blood vessels and lymphatic vessels. The endothelium forms an interface between circulating blood or lymph in the lumen and the rest of the vesse ...

s that act as a

membrane A membrane is a selective barrier; it allows some things to pass through but stops others. Such things may be molecules, ions, or other small particles. Membranes can be generally classified into synthetic membranes and biological membranes. ...

, a middle layer of muscle and elastic tissue and an outer layer of

fibrous connective tissue Connective tissue is one of the four primary types of animal tissue, along with epithelial tissue, muscle tissue, and nervous tissue. It develops from the mesenchyme derived from the mesoderm the middle embryonic germ layer. Connective tissue is ...

. The middle layer is poorly developed so that venules have thinner walls than

arterioles An arteriole is a small-diameter blood vessel in the microcirculation that extends and branches out from an artery and leads to capillaries. Arterioles have muscular walls (usually only one to two layers of smooth muscle cells) and are the pri ...

. They are porous so that fluid and blood cells can move easily from the bloodstream through their walls. Short portal venules between the neural and anterior pituitary lobes provide an avenue for rapid hormonal exchange via the blood. Specifically within and between the pituitary lobes is anatomical evidence for confluent interlobe venules providing blood from the anterior to the neural lobe that would facilitate moment-to-moment sharing of information between lobes of the pituitary gland. In contrast to regular venules, high endothelial venules are a special type of venule where the endothelium is made up of

simple cuboidal Simple cuboidal epithelium is a type of epithelium that consists of a single layer of cuboidal (cube-like) cells which have large, spherical and central nuclei. Simple cuboidal epithelium is found on the surface of ovaries, the lining of nephro ...

cells.

Lymphocyte A lymphocyte is a type of white blood cell (leukocyte) in the immune system of most vertebrates. Lymphocytes include natural killer cells (which function in cell-mediated, cytotoxic innate immunity), T cells (for cell-mediated, cytotoxic a ...

s exit the blood stream and enter the lymph nodes via these specialized venules when an infection is detected. Compared with arterioles, the venules are larger with much weaker muscular coat. They are the smallest united common branch in the human body.

References

External links

{{Authority control Veins

Structure

See also

References

External links