Blood circulates from the heart through the arteries to the capillaries then to the veins and back to the heart. There is a decrease in blood pressure as blood moves away from the heart.
Arteries have an outer layer of connective tissue containing elastic fibres and a middle layer containing smooth muscle with more elastic fibres.
The elastic walls of the arteries stretch and recoil to accommodate the surge of blood after each contraction of the heart.
To control blood flow, the smooth muscle surrounding arteries can contract causing vasoconstriction or relax causing vasodilation.
Capillaries allow exchange of substances with tissues through their thin walls.
Veins have an outer layer of connective tissue containing elastic fibres, but a much thinner muscular wall than arteries.
They contain valves to prevent the backflow of blood.
Pressure filtration causes plasma to pass through capillary walls into the tissue fluid surrounding the cells
Tissue fluid supplies cells with glucose, oxygen and other substances.
Carbon dioxide and other metabolic wastesdiffuse out of the cells and into the tissue fluid to be excreted
Much of the tissue fluid returns to the blood.
Lymphatic vessels absorb excesstissue fluid and return it as lymph to the circulatory system.
Tissue fluid and blood plasma are similar in composition, with the exception of plasma proteins, which are too large to be filtered through the capillary walls.
