# 2.7 Blood Vessels This section outlines the structure and function of the three primary types of blood vessels in the circulatory system: arteries, capillaries, and veins. Each is uniquely adapted for its specific role in transporting blood throughout the body. ### 2.7.1 Arteries Arteries are blood vessels that carry blood away from the heart. They are typically located deep within muscles. * **Structure:** The wall of an artery is thick, muscular, and elastic, consisting of three layers (tunics): 1. **Tunica Adventitia (Outer Layer):** Composed of white fibrous connective tissue, providing structural support. 2. **Tunica Media (Middle Layer):** The thickest layer, containing multiple layers of circular smooth muscle cells and a significant amount of elastic fibres. This allows the artery to withstand and maintain high blood pressure. 3. **Tunica Intima (Inner Layer):** Composed of a smooth lining of simple squamous epithelium (endothelium) and elastic fibres. * **Function:** To transport blood under high pressure from the heart to the rest of the body. * **Branching:** Large arteries (e.g., Aorta, 23 mm diameter) branch into smaller arterioles (0.2 mm diameter), which then lead into capillary beds. * **Valves:** Arteries do not have valves, as the high pressure from the heart's pumping action prevents the backflow of blood. ### 2.7.2 Capillaries Capillaries are the smallest blood vessels and are the site of material exchange with body tissues. * **Structure:** The wall consists of a single layer of endothelial cells. Their diameter is extremely small (7-9 μm), forcing red blood cells to flow in a single file. * **Function:** The thin walls and slow blood flow facilitate the efficient exchange of gases (O₂, CO₂), nutrients, and waste products between the blood and surrounding cells. * **Network:** Capillaries form extensive networks, called capillary beds, that connect arterioles and venules. ### 2.7.3 Veins Veins are blood vessels that bring blood back to the heart. They are often visible as blue vessels under the skin. * **Structure:** Like arteries, veins have three layers, but they are thinner and less muscular. 1. **Tunica Adventitia:** Composed of collagenous connective tissue. 2. **Tunica Media:** A thin layer of smooth muscle cells, collagen, and a few elastic fibres. It is much thinner than in arteries. 3. **Tunica Intima:** A smooth lining of endothelial cells with a thin layer of elastic fibres. * **Function:** To collect deoxygenated blood from the tissues (via venules) and transport it back to the heart under low pressure. * **Branching:** Capillaries drain into small venules (40-50 μm diameter), which merge to form larger veins. #### Valves in Veins * **Function:** Veins (with diameters greater than 2 mm) contain semilunar valves which are flaps of tissue that allow blood to flow toward the heart but close to prevent it from flowing backward. * **Necessity:** These are crucial because blood in veins is under very low pressure and often has to move against gravity, especially in the limbs. * **Location:** Valves are primarily found in the lower parts of the body (abdomen and limbs). They are absent in veins above the heart, where gravity assists blood flow. * **Blood Flow Assistance:** The movement of blood in veins is aided by: * Gravity (from the upper body) * Semilunar valves * Muscular contraction (skeletal muscles squeeze the veins, pushing blood along) ### Comparison of Blood Vessels | Feature | Arteries | Capillaries | Veins | | :--- | :--- | :--- | :--- | | **Direction of Flow** | Away from the heart | Connects arteries and veins | Towards the heart | | **Wall Thickness** | Thick, muscular, elastic | Extremely thin (one cell) | Thin, less muscular | | **Lumen Diameter** | Narrow (relative to wall) | Very narrow (single RBC) | Wide | | **Blood Pressure** | High | Low | Very Low | | **Presence of Valves** | No | No | Yes (in limbs/abdomen) | | **Function** | Transport under high pressure | Material exchange | Transport under low pressure | ### Biological Significance The specialized structures of arteries, veins, and capillaries ensure the efficient and directed transport of blood, which is essential for delivering oxygen and nutrients and removing waste products, thereby maintaining homeostasis throughout the body.