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.
