A venule is a small blood vessel in the microcirculation that allows deoxygenated blood to return from capillary beds to larger blood vessels called veins. Venules range from 8 to 100μm in diameter and are formed when capillaries come together. Many venules unite to form a vein.
A venule is a very small blood vessel in the microcirculation that allows blood to return from the capillary beds to drain into the larger blood vessels, the veins. Venules range from 7μm to 1mm in diameter. Veins contain approximately 70% of total blood volume, 25% of which is contained in the venules.[rx] Many venules unite to form a vein.
Venules in the upper and mid dermis usually run in a horizontal orientation. The diameter of the postcapillary venule ranges from 12 to 35 nm. Collecting venules range from 40 to 60 nm in the upper and mid dermis and enlarge to 100 to 400 nm in diameter in the deeper tissues.126 One-way valves are found at the subcutis (dermis)–adipose junction on the venous side of the circulation.25 Valves are found usually in the area of anastomosis of small to large venules and also within larger venules unassociated with branching points. The free edges of the valves are always directed away from the smaller vessel and toward the larger, and serve to direct blood flow toward the deeper venous system.
Microscopic Anatomy
Venules
- The transition from capillaries somewhat arbitrary, based on size
- Pericytes still present
- More subendothelial connective tissue than capillaries
- At most, a single smooth muscle medial layer (often absent)
- High endothelial venules: Specialized venular segment within lymph nodes; site of leukocyte migration
Veins
- Intima – Endothelium and connective tissue; absent internal elastic lamina
- Media – Variable thickness; greatest in lower extremities, mesentery, uterus, umbilicus, and nearly absent in CNS, retina, medullary bone, penis
- Adventitia – Most prominent layer; predominantly longitudinally oriented bundles of dense collagen &/or smooth muscle with coarse elastic fibers
- Valves are present in most veins, composed of paired infoldings from the intimal layer
Venules Structure
Venule walls have three layers: An inner endothelium composed of squamous endothelial cells that act as a membrane, a middle layer of muscle and elastic tissue, and an outer layer of fibrous connective tissue. The middle layer is poorly developed so that venules have thinner walls than arterioles. 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.[rx] Specifically within and between the pituitary lobes is anatomical evidence for confluent interlope 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.[rx]
In contrast to regular venules, high endothelial venules are a special type of venue where the endothelium is made up of simple cuboidal cells. Lymphocytes exit the bloodstream and enter the lymph nodes via these specialized venules when an infection is detected. Compared with arterioles, the venules are larger with a much weaker muscular coat. They are the smallest united common branch in the human body. Venules are small blood vessels in the microcirculation that connect capillary beds to veins.
Vein Classification
Veins are classified in a number of ways, including superficial vs. deep, pulmonary vs. systemic, and large vs. small:
- Superficial veins: Superficial veins are close to the surface of the body and have no corresponding arteries.
- Deep veins: Deep veins are deeper in the body and have corresponding arteries.
- Communicating veins: Communicating veins (or perforator veins) directly connect superficial veins to deep veins.
- Pulmonary veins: The pulmonary veins deliver oxygenated blood from the lungs to the heart.
- Systemic veins: Systemic veins drain the tissues of the body and deliver deoxygenated blood to the heart.
Key Points
Many venules unite to form a vein.
Venule walls have three layers: an inner endothelium composed of squamous endothelial cells that act as a membrane, a middle layer of muscle and elastic tissue, and an outer layer of fibrous connective tissue.
High-endothelial venules are specialized post-capillary venous swellings characterized by plump endothelial cells, in contrast with the thinner endothelial cells found in regular venules.
Key Terms
- high endothelial venule: A specialized post-capillary venous swelling of the lymphatic system that allows lymphocytes (white blood cells) to easily exit the circulatory system.
- venule: A small blood vessel in the microcirculation that allows deoxygenated blood to return from capillary beds to veins.
A venule is a small blood vessel in the microcirculation that allows deoxygenated blood to return from capillary beds to larger blood vessels called veins. Venules range from 8 to 100μm in diameter and are formed when capillaries come together. Many venules unite to form a vein.
Venule: Venules form when capillaries come together and converging venules form a vein.
Venule walls have three layers: an inner endothelium composed of squamous endothelial cells that act as a membrane, a middle layer of muscle and elastic tissue, and an outer layer of fibrous connective tissue. The middle layer is poorly developed so that venules have thinner walls than arterioles. Venules are extremely porous so that fluid and blood cells can move easily from the bloodstream through their walls.
In contrast to regular venules, high-endothelial venules (HEV) are specialized post-capillary venous swellings. They are characterized by plump endothelial cells as opposed to the usual thinner endothelial cells found in regular venules. HEVs enable lymphocytes (white blood cells) circulating in the blood to directly enter a lymph node by crossing through the HEV.
Veins
Veins are blood vessels that carry blood from tissues and organs back to the heart; they have thin walls and one-way valves.
Key Points
The difference between veins and arteries is the direction of blood flow (out of the heart through arteries, returning to the heart through veins).
Veins differ from arteries in structure and function. For example, arteries are more muscular than veins, veins are often closer to the skin, and veins contain valves to help keep blood flowing toward the heart, while arteries do not have valves and carry blood away from the heart.
Veins are also called capacitance vessels because they contain 60% of the body’s blood volume.
The return of blood to the heart is assisted by the action of the skeletal- muscle pump. As muscles move, they squeeze the veins running through them. Veins contain a series of one-way valves, and they are squeezed, blood is pushed through the valves, which then close to prevent backflow.
Key Terms
- venous pooling: When blood accumulates in the lower extremities, resulting in low venous return to the heart which can result in fainting.
- skeletal-muscle pump: Rhythmic contraction of limb muscles that occurs during normal locomotory activity (walking, running, swimming), which promotes venous return by the pumping action on veins within muscles.
- portal vein: A short, wide vein that carries blood to the liver from the organs of the digestive system.
Veins are blood vessels that carry blood towards the heart. Most carry deoxygenated blood from the tissues back to the heart, but the pulmonary and umbilical veins both carry oxygenated blood to the heart. The difference between veins and arteries is the direction of blood flow (out of the heart through arteries, back to the heart through veins), not their oxygen content. Veins differ from arteries in structure and function. For example, arteries are more muscular than veins, veins are often closer to the skin, and veins contain valves to help keep blood flowing toward the heart, while arteries do not have valves and carry blood away from the heart. The precise location of veins is much more variable than that of arteries, since veins often display anatomical variation from person to person.
Veins are also called capacitance vessels because they contain 60% of the body’s blood volume. In systemic circulation, oxygenated blood is pumped by the left ventricle through the arteries to the muscles and organs of the body, where its nutrients and gases are exchanged at capillaries. The blood then enters venules, then veins filled with cellular waste and carbon dioxide. The deoxygenated blood is taken by veins to the right atrium of the heart, which transfers the blood to the right ventricle, where it is then pumped through the pulmonary arteries to the lungs. In pulmonary circulation the veins return oxygenated blood from the lungs to the left atrium, which empties into the left ventricle, completing the cycle of blood circulation.
Mechanisms to Return Blood
The return of blood to the heart is assisted by the action of the skeletal-muscle pump and by the thoracic pump action of breathing during respiration. As muscles move, they squeeze the veins that run through them. Veins contain a series of one-way valves. As the vein is squeezed, it pushes blood through the valves, which then close to prevent backflow. Standing or sitting for prolonged periods can cause low venous return from venous pooling. In venous pooling, the smooth muscles surrounding the veins become slack and the veins fill with the majority of the blood in the body, keeping blood away from the brain, which can cause unconsciousness.
Venous valve: Venous valves prevent backflow and ensure that blood flows in one direction.
Although most veins take the blood back to the heart, portal veins carry blood between capillary beds. For example, the hepatic portal vein takes blood from the capillary beds in the digestive tract and transports it to the capillary beds in the liver. The blood is then drained in the gastrointestinal tract and spleen, where it is taken up by the hepatic veins and blood is taken back into the heart. Since this is an important function in mammals, damage to the hepatic portal vein can be dangerous. Blood clotting in the hepatic portal vein can cause portal hypertension, which results in a decrease of blood fluid to the liver.
Blood Supply and Lymphatics
The walls of large blood vessels, like the aorta and the vena cava, are supplied with blood by vasa vasorum. This term translates to mean “vessel of a vessel.”
Three types of vasa vasorum exist (1) vasa vasorum internae, (2) vasa vasorum externae, and (3) venous vasa vasorae. Vasa vasorum internal originates from the lumen of a vessel and penetrates the vessel wall to supply oxygen and nutrients. Vasa vasorum external originates from a nearby branching vessel and feedback into the larger vessel wall[rx]. Some infections, such as late-stage manifestations of tertiary syphilis may lead to endarteritis of the vasa vasorum of the ascending aorta.[rx] Venous vasa vasorae originate within the vessel wall and drain into a nearby vein to provide venous drainage for vessel walls.
Nerves
The sympathetic nervous system primarily innervates blood vessels. The smooth muscles of vasculature contain alpha-1, alpha-2, and beta-2 receptors.[rx] A delicate balance between the influence of the sympathetic and parasympathetic nervous systems is responsible for the underlying physiological vascular tone. Specialized receptors located in the aortic arch and the carotid arteries acquire information regarding blood pressure (baroreceptors) and oxygen content (chemoreceptors) from passing blood. This information is then relayed to the nucleus of the solitary tract via the vagus nerve.[rx] Blood vessel constriction or relaxation then ensues accordingly, determined by the body’s sympathetic response.
Muscles
Blood vessels contain only smooth muscle cells. These muscle cells reside within the tunica media along with elastic fibers and connective tissue. Although vessels only contain smooth muscles, the contraction of skeletal muscle plays an important role in the movement of blood from the periphery towards the heart in the venous system.
References
