The Structure and Function of Arteries, Capillaries and Veins U2

This is the widening of blood vessels Caused by smooth muscle cells relaxing This results in the lumen becoming wider and blood flow increasing Occurs during exercise to body parts e.g. muscle

This is the narrowing of blood vessels Caused by smooth muscle cells contracting This results in the lumen becoming narrower and blood flow decreasing Occurs during exercise to body parts e.g. small intestine

A central lumen lined with endothelial cells A middle layer containing smooth muscle and elastic fibres An outer layer of connective tissue containing elastic fibres The elastic walls of the arteries stretch and recoil to accommodate the surge of blood after each contraction of the heart Thickest muscular wall Carry blood away from the heart at a higher pressure

A central lumen lined with endothelial cells A middle layer containing smooth muscle and elastic fibres An outer layer of connective tissue containing elastic fibres A thinner muscular wall than arteries Valves to prevent the back-flow of blood Carry blood into the heart at a lower pressure

A central lumen lined with endothelial cells Endothelium only one cell thick Allow exchange of substances with tissues due to their thin walls e.g. oxygen, carbon dioxide and glucose

Red blood cells, white blood cells and platelets bathed in plasma Plasma is a watery yellow fluid that contains many dissolved substances such as glucose, amino acids, respiratory gases, plasma proteins and ions

• Blood is transported from the arteries at high pressure through an arteriole, and enters the narrow capillaries surrounding the tissues. • Pressure filtration squeezes the blood plasma though the capillary walls and into the surrounding cells. This is now called tissue fluid. The plasma proteins are too large to leave the capillary and will not be found in the tissue fluid. • Glucose/oxygen diffuse out of the tissue fluid and into the surrounding cells. At the same time, CO2/ other metabolic wastes diffuse out of the surrounding cells and into the tissue fluids to be excreted. • Most of this tissue is then absorbed back into the capillaries (by osmosis) and leave via the venule. • Excess tissue fluid does not return to the capillaries. Instead, the excess fluid is absorbed by lymph vessels. • The absorbed tissue fluid is called lymph, and it taken away by lymph vessels, where it is transported around the body by the lymphatic system. The pressure of the lymph fluid is even lower than that of the blood in the veins. Lymph vessels contain valves to prevent the back flow of lymph.

So dense that every cell in the body is located close to a blood capillary and is constantly surrounded in tissue fluid Much of the tissue fluid returns to the capillaries at the venus end of the capillary bed This process is brought about by diffusion Carbon dioxide and metabolic wastes enter the bloodstream by diffusion