A
venule is a small
blood vesselThe blood vessels are the part of the circulatory system that transport blood throughout the body. There are three major types of blood vessels: the arteries, which carry the blood away from the heart, the capillaries, which enable the actual exchange of water and chemicals between the blood and...
in the
microcirculationThe microcirculation is a term used to describe the small vessels in the vasculature which are embedded within organs and are responsible for the distribution of blood within tissues; as opposed to larger vessels in the macrocirculation which transport blood to and from the organs...
that allows deoxygenated
bloodBlood is a specialized bodily fluid that delivers necessary substances to the body's cells — such as nutrients and oxygen — and transports waste products away from those same cells....
to return from the
capillaryCapillaries are the smallest of a body's blood vessels and are part of the microcirculation. They are only 1 cell thick. These microvessels, measuring 5-10 μm in diameter, connect arterioles and venules, and enable the exchange of water, oxygen, carbon dioxide, and many other nutrient and waste...
beds to the larger blood vessels called
veinIn 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 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
endotheliumThe endothelium is the thin layer of cells that line the interior surface of blood vessels, forming an interface between circulating blood in the lumen and the rest of the vessel wall. Endothelial cells line the entire circulatory system, from the heart to the smallest capillary...
composed of squamous
endothelial cellThe endothelium is the thin layer of cells that line the interior surface of blood vessels, forming an interface between circulating blood in the lumen and the rest of the vessel wall. Endothelial cells line the entire circulatory system, from the heart to the smallest capillary...
s that act as a
membraneA 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 molecules and proteins that may constitute close to 50% of membrane...
, a middle layer of muscle and elastic tissue and an outer layer of
fibrous connective tissueIn zootomy, fibrous connective tissue is a type of connective tissue which has relatively high tensile strength.It has a collagenous and elastic fibers...
. The middle layer is poorly developed so that venules have thinner walls than arterioles.
In contrast to regular venules,
high endothelial venulesHigh 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.
They form from anastomosis of capillaries.