large veins have what to prevent the backflow of blood

Venules

Venules are small blood vessels in the microcirculation that connect capillary beds to veins.

Learning Objectives

Depict the venules of the venous system

Key Takeaways

Primal Points

• Many venules unite to course a vein.
• Venule walls have three layers: an inner endothelium equanimous of squamous endothelial cells that act equally 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 past plump endothelial cells, in contrast with the thinner endothelial cells plant in regular venules.

Key Terms

• loftier endothelial venule: A specialized postal service-capillary venous swelling of the lymphatic organization that allows lymphocytes (white blood cells) to easily exit the circulatory organisation.
• venule: A small claret vessel in the microcirculation that allows deoxygenated blood to render from capillary beds to veins.

A venule is a minor blood vessel in the microcirculation that allows deoxygenated claret to return from capillary beds to larger blood vessels called veins. Venules range from 8 to 100μm in bore 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 equanimous of squamous endothelial cells that act equally a membrane, a eye layer of muscle and elastic tissue, and an outer layer of fibrous connective tissue. The centre layer is poorly adult so that venules have thinner walls than arterioles. Venules are extremely porous so that fluid and blood cells tin can movement 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 claret 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 center; they have thin walls and one-mode valves.

Learning Objectives

Distinguish the venous system from the arterial organisation

Cardinal Takeaways

Key Points

• The difference between veins and arteries is the direction of claret flow (out of the centre through arteries, returning to the heart through veins).
• Veins differ from arteries in structure and office. For example, arteries are more muscular than veins, veins are often closer to the skin, and veins contain valves to help keep claret flowing toward the heart, while arteries exercise non have valves and carry blood abroad from the centre.
• Veins are also chosen capacitance vessels because they contain threescore% of the body's blood volume.
• The render of claret to the heart is assisted by the activity of the skeletal- muscle pump. Every bit muscles move, they squeeze the veins running through them. Veins incorporate a series of one-mode valves, and they are squeezed, blood is pushed through the valves, which and so close to prevent backflow.

Primal Terms

• venous pooling: When blood accumulates in the lower extremities, resulting in low venous render to the middle which tin result in fainting.
• skeletal-musculus pump: Rhythmic wrinkle of limb muscles that occurs during normal locomotory action (walking, running, swimming), which promotes venous return by the pumping action on veins inside muscles.
• portal vein: A brusk, wide vein that carries blood to the liver from the organs of the digestive system.

Veins are blood vessels that behave blood towards the heart. Most behave deoxygenated blood from the tissues back to the center, only the pulmonary and umbilical veins both deport oxygenated blood to the center. The difference betwixt veins and arteries is the direction of blood flow (out of the heart through arteries, dorsum to the heart through veins), non their oxygen content. Veins differ from arteries in structure and function. For example, arteries are more muscular than veins, veins are oftentimes closer to the peel, and veins contain valves to help keep blood flowing toward the heart, while arteries do non take valves and comport blood away from the heart. The precise location of veins is much more than variable than that of arteries, since veins often display anatomical variation from person to person.

Veins are besides called capacitance vessels because they contain threescore% of the torso's blood volume. In systemic apportionment, 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 and so enters venules, then veins filled with cellular waste product 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 information technology 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 claret circulation.

Mechanisms to Return Blood

The render of claret to the centre is assisted past the action of the skeletal-musculus pump and by the thoracic pump action of breathing during respiration. As muscles move, they clasp the veins that run through them. Veins comprise a series of one-way valves. Equally the vein is squeezed, information technology pushes blood through the valves, which then shut to prevent backflow. Standing or sitting for prolonged periods tin crusade depression venous return from venous pooling. In venous pooling, the smoothen muscles surrounding the veins become slack and the veins fill with the majority of the blood in the body, keeping claret away from the brain, which can crusade unconsciousness.

Venous valve: Venous valves foreclose back flow and ensure that claret flows in one direction.

Although near veins take blood back to the heart, portal veins carry claret 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 upward by the hepatic veins and claret is taken back into the heart. Since this is an important function in mammals, damage to the hepatic portal vein tin can be dangerous. Blood clotting in the hepatic portal vein tin crusade portal hypertension, which results in a decrease of claret fluid to the liver.

Vein Nomenclature

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 torso and take no corresponding arteries.
• Deep veins: Deep veins are deeper in the body and have respective arteries.
• Communicating veins: Communicating veins (or perforator veins) directly connect superficial veins to deep veins.
• Pulmonary veins: The pulmonary veins evangelize oxygenated blood from the lungs to the heart.
• Systemic veins: Systemic veins drain the tissues of the body and evangelize deoxygenated blood to the heart.

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Source: https://courses.lumenlearning.com/boundless-ap/chapter/the-venous-system/

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