Blood Vessels and their Functions Blood Vessels Arteries

Blood Vessels and their Functions

Blood Vessels Arteries: Carry blood away from the heart Arterioles: Control blood flow from arteries to capillaries Capillaries: Link arterioles to venules Venules: Link capillaries to veins Veins: Carry blood towards the heart

Guide to blood vessels 3 of 33 © Boardworks Ltd 2008

Structure of Blood Vessels • Tough outer layer – resists pressure • Muscle layer – can contract and control the flow of blood • Elastic layer – can stretch and recoil to maintain blood pressure • Endothelium – smooth layer to prevent friction • Lumen – not a layer – a cavity

Structure of Arteries • Thick muscle layer – control the flow of blood • Thick elastic layer – maintains blood pressure by stretching and recoiling • Narrow lumen

Structure of Veins • • Thin muscle layer Thin elastic layer Wide lumen Valves to prevent backflow of blood

Blood flow in veins 7 of 33 © Boardworks Ltd 2008

Varicose veins If a vein wall becomes weakened, valves may no longer close properly. This allows backflow of blood, causing the vein to become enlarged and bumpy, and become varicose. This usually happens in superficial veins, near the skin surface in the lower legs, as opposed to deep veins, which lie underneath muscles. Varicose veins can be surgically removed without affecting blood flow, as most blood is returned to the heart by deep veins. 8 of 33 © Boardworks Ltd 2008

Structure of Capillaries • No muscle • No elastic • Thin layer of cells only

Capillary Structure and Function • Thin layer of cells – short diffusion distance. • Numerous and highly branched – large SA for diffusion. • Narrow diameter – keep all cells close by. • Narrow lumen – bring red blood cells close to cells = short diffusion distance. • Spaces between cells – allow white blood cells to escape.

Structure of Arterioles Structure of Venules • Thicker muscle layer than arteries • Thinner elastic layer than arteries • Can constrict to control blood flow through capillaries • Muscle and elastic layers • Similar to veins but lumen is smaller • Can constrict to control blood flow through capillaries

What is blood? Blood is a specialized transport medium that is also considered a special type of connective tissue. An average adult has 4– 6 litres of blood. Blood has a range of functions such as: l transport l defence l thermoregulation l maintaining p. H of body fluids. 15 of 33 © Boardworks Ltd 2008

The composition of blood 16 of 33 © Boardworks Ltd 2008

Features of erythrocytes What are the specialized features of an erythrocyte? flattened, biconcave disc shape: ensures large surface area to volume ratio for efficient gas exchange large amount of haemoglobin: for transporting oxygen no nucleus or organelles: maximises space for haemoglobin, so more diameter (6– 8 µm) larger than oxygen can be capillary diameter: slows blood transported flow to enable diffusion of oxygen 17 of 33 © Boardworks Ltd 2008