The structure and function of veins

• Blood is pumped from the heart in the arteries. It is returned to the heart in the veins.
• The capillaries connect the two types of blood vessel and molecules are exchanged between the blood and the cells across their walls.
• arteries Always carry blood away from the heart
• arteries Carry oxygenated blood, except for the pulmonary artery
• arteries Carry blood under high pressure
• arteries Have thick muscular and elastic walls to pump and accommodate blood
• arteries have A type of supporting tissue called connective tissue provides strength
• arteries have a channel in the blood vessel that carries blood - the lumen - is narrow
• veins Always carry blood to the heart
• veins Always carry deoxygenated blood, except for the pulmonary vein
• veins Carry blood under low or negative pressure
• veins Have thin walls - have less muscular tissue than arteries
• veins Have less connective tissue than arteries
• veins Have a wide lumen
• Veins contain valves which prevent the backflow of blood.
• In order to control blood flow through the vessels, the smooth muscle surrounding the arteries can constrict which causes vasoconstriction or they can relax which causes vasodilation
• Capillaries connect the smallest branches of arteries and veins
• The walls of capillaries are just one cell thick.
• Capillaries therefore allow the exchange of molecules between the blood and the body's cells - molecules can diffuse across their walls. This exchange of molecules is not possible across the walls of other types of blood vessel.
• • Oxygen diffuses through the capillary wall, into the tissue fluid, and the cells.
• • Carbon dioxide diffuses from the cells into the tissue fluid, then across the capillary walls into the blood plasma.
• Glucose diffuses from the blood plasma, across the capillary walls to the tissue fluid, and then to the cells
• • The waste product urea diffuses from the cells of the liver, to the tissue fluid, and then across the capillary walls into the blood plasma.
• Capillaries allow exchange of substances with body tissues through their thin walls
• As blood travels at high pressure in the arteries towards the capillaries, pressure filtration occurs which results in plasma passing through the capillary wall into the tissue fluid which surrounds the cell.
• Tissue fluid provides cells with useful substances such as glucose and oxygen and waste products are passed out of the cells into the tissue fluid to be removed.
• Most of the tissue fluid is returned to the blood. Any excess tissue fluid is absorbed by lymphatic vessels which return it to the circulatory system as lymph.
• Plasma and tissue fluid are composed of similar substances except for plasma proteins which are only found in plasma. They are too large to be filtered through the capillary walls.