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A venule is a small blood vessel
Blood vessel

The blood vessels are the part of the circulatory system that transport blood throughout the body. There are three major types of blood vessels: the artery, which carry the blood away from the heart, the capillary, which enable the actual exchange of water and chemicals between the blood and the tissues; and the veins, which carry blood from...
 that allows deoxygenated blood
Blood

Blood is a specialized bodily fluid that delivers necessary substances to the body's Cell s ? such as nutrients and oxygen ? and transports waste products away from those same cells....
 to return from the capillary
Capillary

Capillaries are the smallest of a body's blood vessels, measuring 5-10 micrometre in diameter, which connect arterioles and venules, and enable the interchange of water, oxygen, carbon dioxide, and many other nutrient and waste chemical substances between blood and surrounding tissue s....
 beds to the larger blood vessels called vein
Vein

In the circulatory system, veins are blood vessels that carry blood toward the heart. Most veins carry deoxygenated blood from the tissues back to the heart; exceptions are the pulmonary vein and umbilical veins, both of which carry oxygenated blood....
s.

Venules are blood vessels that drain blood directly from the capillary beds. Many venules unite to form a vein.

le walls have three layers: An inner endothelium
Endothelium

The endothelium is the thin layer of cell that line the interior surface of blood vessels, forming an interface between circulating blood in the lumen and the rest of the vessel wall....
 composed of squamous endothelial cell
Endothelium

The endothelium is the thin layer of cell that line the interior surface of blood vessels, forming an interface between circulating blood in the lumen and the rest of the vessel wall....
s that act as a membrane
Biological membrane

A biological membrane or biomembrane is an enclosing or separating amphipathic layer that acts as a barrier within or around a cell . It is, almost invariably, a lipid bilayer, composed of a double layer of lipid-class molecules, specifically phospholipids and cholesterol, with occasional integral membrane protein intertwined, some o...
, a middle layer of muscle and elastic tissue and an outer layer of fibrous connective tissue
Fibrous connective tissue

In zootomy, fibrous connective tissue is a type of connective tissue which has relatively high tensile strength, due to a relatively high concentration of collagen or elastin fibers....
.






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Illu Capillary
A venule is a small blood vessel
Blood vessel

The blood vessels are the part of the circulatory system that transport blood throughout the body. There are three major types of blood vessels: the artery, which carry the blood away from the heart, the capillary, which enable the actual exchange of water and chemicals between the blood and the tissues; and the veins, which carry blood from...
 that allows deoxygenated blood
Blood

Blood is a specialized bodily fluid that delivers necessary substances to the body's Cell s ? such as nutrients and oxygen ? and transports waste products away from those same cells....
 to return from the capillary
Capillary

Capillaries are the smallest of a body's blood vessels, measuring 5-10 micrometre in diameter, which connect arterioles and venules, and enable the interchange of water, oxygen, carbon dioxide, and many other nutrient and waste chemical substances between blood and surrounding tissue s....
 beds to the larger blood vessels called vein
Vein

In the circulatory system, veins are blood vessels that carry blood toward the heart. Most veins carry deoxygenated blood from the tissues back to the heart; exceptions are the pulmonary vein and umbilical veins, both of which carry oxygenated blood....
s.

Venules are blood vessels that drain blood directly from the capillary beds. Many venules unite to form a vein.

Structure

Venule walls have three layers: An inner endothelium
Endothelium

The endothelium is the thin layer of cell that line the interior surface of blood vessels, forming an interface between circulating blood in the lumen and the rest of the vessel wall....
 composed of squamous endothelial cell
Endothelium

The endothelium is the thin layer of cell that line the interior surface of blood vessels, forming an interface between circulating blood in the lumen and the rest of the vessel wall....
s that act as a membrane
Biological membrane

A biological membrane or biomembrane is an enclosing or separating amphipathic layer that acts as a barrier within or around a cell . It is, almost invariably, a lipid bilayer, composed of a double layer of lipid-class molecules, specifically phospholipids and cholesterol, with occasional integral membrane protein intertwined, some o...
, a middle layer of muscle and elastic tissue and an outer layer of fibrous connective tissue
Fibrous connective tissue

In zootomy, fibrous connective tissue is a type of connective tissue which has relatively high tensile strength, due to a relatively high concentration of collagen or elastin fibers....
. The middle layer is poorly developed so that venules have thinner walls than arterioles. These vessels are pink sometimes due to the high content of nitrogen.

In contrast to regular venules, high endothelial venules
High endothelial venules

High endothelial venules are specialized post-capillary venous swellings characterized by simple cuboidal cells as opposed to simple squamous cells found in regular venules....
 are a special type of venule where the endothelium is made up of simple cuboidal cells. Lymphocytes re-enter the bloodstream through the HEVs.

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