Functional Morphology of Blood Vessels
- Elastin, it allows storage of energy.
- Connective tissue, provides structural integrity to the vessel walls.
- Smooth muscle, determines vascular radius and in doing so effects vascular resistance and blood flow.
- Endothelium:
Maintains the fluid characteristics of blood.
Produces a large number physiologically active agents.
E.g. of products Nitric Oxide (NO) which works as a vasodilator.
Endothelin-1 (ET-1) is a vasoconstrictor.
Prostacyclin (PGI2) prevents platelet activation and is a vasodilator.
Essential for filtration and reabsorption.
- Arteries: These vessels carry oxygenated blood away from the heart. They possess thick walls composed of elastic tissue, smooth muscle, and connective tissue, allowing them to withstand the high pressure exerted by the heart's pumping action. The volume of blood within the arteries is referred to as the stressed volume because it's under high pressure.
- Aorta: The largest artery in the systemic circulation, arising from the left ventricle.
- Medium and Small Arteries: Branches off the aorta, distributing blood to various organs.
- Arterioles: The smallest branches of the arteries, arterioles play a critical role in regulating blood flow and blood pressure. They have a well-developed layer of smooth muscle that allows them to constrict or dilate, changing their resistance to blood flow. They are the site of highest resistance in the vasculature. This resistance is crucial for regulating blood flow to specific organs and maintaining overall blood pressure.
- Resistance Changes: Arteriolar resistance can be altered by:
- Sympathetic Nerve Activity: Increased sympathetic activity generally leads to vasoconstriction, increasing resistance and decreasing blood flow.
- Circulating Catecholamines: Epinephrine and norepinephrine, released from the adrenal medulla, can cause either vasoconstriction or vasodilation, depending on the receptor subtype they bind to.
- Vasoactive Metabolites: Substances produced locally in tissues, such as CO2, H+, K+, lactate, and adenosine, can cause vasodilation, increasing blood flow to meet metabolic demands.
- Capillaries: Microscopic vessels with thin walls consisting of a single layer of endothelial cells. This structure facilitates the exchange of gases, nutrients, wastes, and fluids between the blood and surrounding tissues. The low velocity of blood flow in capillaries (due to their large total cross-sectional area) enhances the efficiency of this exchange.
- Selective Perfusion: Not all capillaries are open at all times. The degree of arteriolar and precapillary sphincter dilation or constriction determines which capillary beds are perfused, directing blood flow to areas with higher metabolic demands.