Layers of the Blood Vessels

LAYERS OF THE BLOOD VESSELS • • • Arteries: carry blood away from heart to body tissues Arteries branch (bifurcate) into smaller vessels → feed into capillaries (gas + nutrient exchange) From the capillaries, veins return blood to the heart → become progressively larger as they merge closer to h Tunica Intima/Interna • • • Innermost layer of a blood vessel wall 2 major components: endothelium, subendothelium o Endothelium: simple squamous → smooth surface for passage of blood ▪ Secrete pro/anticoagulant factors: endolthelin, vWF, NO ▪ Secrete basement membrane components: collagen, laminin ▪ Membrane bound enzymes e.g. ACE ▪ Cell adhesion proteins o Subendothelium ▪ Loose connective tissue ▪ Dispersed smooth muscle cells Internal elastic lamina: v developed in muscular arteries Tunica media • • • • • Middle, thickest layer of vessel wall Composed of helically arranged layers of smooth muscle cells Vasoconstriction (narrowing of blood vessel lumen) → sympathetic function Vasodilation → parasympathetic function Absent in capillaries and post-capillary venules Tunica Externa/Adventitia • • • • • Outermost layer of blood vessel wall Composed of an areolar connective tissue that contains elastic and collagen fibres Helps anchor the vessel to other tissues Vasa vasorum: small arteries that enter the vessel wall Blends into surrounding connective tissue Basement membrane • • Acellular region between epithelium and connective tissue Composed of: o Basal lamina (glycoproteins & collagens) → lamina lucida and lamina densa o Lamina reticularis (collagens) • Basal lamina is secreted by epithelium, endothelium and mesothelium while lamina reticularis is secreted by connective tissue Arteries and veins Arteries • • • Systemic circulation: carry oxygenated blood to body tissues Pulmonary arteries carry deoxygenated blood to lungs 3 basic types: elastic, muscular, arterioles → as diameter decreases there is a decrease in elastic fibres (relatively more smooth muscle Elastic artery e.g. aorta Muscular artery Arterioles Tunic intima Tunica intima Tunica intima • Endothelium axes parallel to • Thinner than • Sub-endothelial layer virtually longitudinal axis of vessel elastic arteries, absent but with • Internal elastic lamina • Endothelial nuclei protrudes pronounced into lumen • Inclusions containing von internal elastic Tunica media Willebrand factor lamina Tunica media • 1-3 layers of smooth muscle Tunica media cells • Fenestrated elastic membrane • 4-40 layers of Tunica adventitia • Organised as lamellae and smooth muscle • Scant, loose fibroelastic alternate with smooth muscle cells connective tissue cells Tunica adventitia • Larger arterioles have the 3 • Number of lamellae increases • Relatively thin tunics, smaller arterioles with age (40 in newborn, 70 in layer of limited to endothelium + layers adult) fibroelastic of smooth muscle fibres • External elastic lamina tissue Tunica adventitia • • Relatively thin layer of fibroelastic tissue Vasa vasorum • Metarterioles function as sphincters for capillary beds Capillaries (functional unit of cardiovascular system) • • • • • • Veins • • • • • • • Only have the tunica intima, which consists of a basal lamina and endothelium Numerous pinocytotic vesicles Allow gas and nutrient exchange between the blood and the body tissues to occur rapidly Smallest blood vessels, connect arterioles to venules A group of capillaries (10–100) functions together and forms a capillary bed Pericytes are contractile cells that wrap around the endothelial cells that line the capillaries and venules throughout the body, regulating blood flow Drain capillaries and return the blood to the heart Walls are relatively thin and the vein lumen is larger Systemic veins carry deoxygenated blood to the right atrium of the heart Pulmonary veins carry oxygenated blood to the left atrium of the heart Blood pressure is substantially reduced by the time blood reaches the veins Hold about 60% of the body’s blood at rest Veins also function as blood reservoirs