24.7:
Veins
A vein is a blood vessel that carries deoxygenated blood to the heart.
Compared to an artery, a vein has a wider lumen that can measure from 0.5 mm to 3 cm in diameter.
Additionally, its tunica interna is thinner, consisting of a thin endothelium. The tunica media is poorly developed, with fewer smooth muscle and elastic fibers. The tunica externa is well-developed and comprises longitudinally arranged collagen and elastic fibers.
Veins also lack internal and external elastic laminae.
Many veins, particularly those in the limbs, are equipped with valves — thin folds of tunica interna that form flaplike cusps. These valves project into the lumen and are oriented to prevent the backflow of blood.
A venous or vascular sinus is a specialized vein that appears flattened as it has only an extremely thin layer of endothelium, lacking all other tunics.
Examples include the dural venous sinuses, which transport deoxygenated blood from the brain to the heart, and the coronary sinus of the heart.
24.7:
Veins
Veins are an integral part of our circulatory system, serving as the blood vessels that transport blood from all body regions to the heart. They are a network of hollow tubes that carry blood low in oxygen from the body's cells back to the heart for reoxygenation. Veins are crucial for maintaining the body's overall fluid balance and the continuous circulation of blood.
Structure of Veins:
The structure of veins is specifically designed to assist in the low-pressure transportation of blood toward the heart. They are composed of three primary layers or 'tunics.'
Tunica Intima (Interna): This is the innermost layer of the vein made up of endothelial cells. It is in direct contact with the blood flowing through the vein. The smooth surface of this layer reduces friction, allowing blood to flow smoothly.
Tunica Media: This middle layer primarily comprises smooth muscle and elastic fibers. The thin layer of muscle helps in controlling the diameter and capacity of the vein, while the presence of elastic fibers allows the vein to stretch and accommodate varying volumes of blood.
Tunica Adventitia (Externa): This is the outermost layer of the vein, which is composed mainly of collagen fibers. Its primary role is to provide structural support and protection to the vein.
Unlike arteries, veins have thinner walls and larger lumens (interior cavities). This difference is because veins operate under much lower pressure than arteries. Another distinctive feature of veins is the presence of one-way valves. These valves prevent the backflow of blood, ensuring that it only moves in one direction—toward the heart.
Prominent Veins in the human body include the superior and inferior vena cava, jugular veins, subclavian veins, and renal veins.
Functions of Veins:
Except for the pulmonary veins, all veins carry deoxygenated blood from the various tissues and organs of the body back to the heart. However, they also serve other vital functions, including:
Blood Reservoir: Veins act as a blood reservoir. At rest, about 60% of the body's total blood volume is found within the venous system.
Nutrient Supply: Although veins primarily carry deoxygenated blood, some veins, such as the hepatic portal vein, carry nutrient-rich blood from the digestive organs to the liver for processing.
Temperature Regulation: Veins contribute to regulating body temperature. When the body is overheated, the veins dilate (expand) to allow more blood to flow near the skin surface, where the heat is released.