cardiovascular

Structure and Function of Blood Vessels

• Arterioles are small blood vessels with a diameter of 1 mm and have a thin subendothelial layer, circularly arranged smooth muscle cells in the media, and a thin adventitia.
• Capillaries are the smallest blood vessels and function in networks called capillary beds to permit and regulate metabolic exchange between blood and surrounding tissues.
• The density of the capillary bed is related to the metabolic activity of the tissues, with high metabolic rates requiring more capillary beds.
• Capillaries consist of a simple layer of endothelial cells rolled up as a tube surrounded by a basement membrane. They have an average diameter of 4-10 micrometers and are the major site of water and solute exchange between blood and tissues.
• There are three histologic types of capillaries: continuous, fenestrated, and discontinuous/sinusoids, each with different levels of metabolic exchange.
• Continuous capillaries have tight occluding junctions between endothelial cells and are found in muscle, connective tissue, lungs, exocrine glands, and nervous tissue.
• Fenestrated capillaries have sieve-like structures that allow more extensive molecular exchange and are found in organs with rapid interchange of substances, such as the kidneys, intestine, choroid plexus, and endocrine glands.
• Discontinuous capillaries/sinusoids have large perforations without diaphragms and are found in the liver, spleen, some endocrine organs, and bone marrow, allowing maximal exchange of macromolecules and easier movement of cells.
• Venules are small veins that resemble capillaries and are the primary site for white blood cells to adhere to the endothelium and leave the circulation at sites of infection or tissue damage.
• Veins carry blood back to the heart and have valves to prevent backflow. They have different sizes, with small and medium veins located close to corresponding muscular arteries, and large veins being big venous trunks close to the heart.
• The lymphatic vascular system returns fluid from tissue spaces to the blood. Lymphatic capillaries collect excess interstitial fluid as lymph, which merges to form larger lymphatic vessels that connect with the blood vascular system.
• Lymphatic vessels have thin walls, increasing amounts of connective tissue and smooth muscle, and valves. Lymph nodes are interposed in the path of larger lymphatic vessels, and large lymphatic trunks, such as the thoracic duct and right lymphatic duct, empty into the large veins.