113 The Venous System
Venules
Venules are small blood vessels in the microcirculation that connect capillary beds to veins.
Learning Objectives
Describe the venules of the venous system
Key Takeaways
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.
Learning Objectives
Distinguish the venous system from the arterial system
Key Takeaways
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 back flow and ensure that blood flows in one direction.
Although most veins take 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.
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.
