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Science

Circulation

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Diffusion in the circulatory system – Higher tier

The shape of red blood cells

Red blood cells are a biconcave shape. This kind of shape (which is disc like) gives them an increased surface area for oxygen exchange.

The biconcave shape of red blood cells

The biconcave shape of red blood cells

Tissue fluid

By the time blood reaches the capillary beds [capillary bed: The network of very thin blood vessels that runs through tissues. This is where oxygen, carbon dioxide, glucose, and other chemicals pass between the blood and the cells. from an artery [artery: Thick-walled muscular tube, which carries blood away from the heart., it is at high pressure and this forces blood plasma out. The plasma leaves the capillary and becomes tissue fluid [tissue fluid: Fluid which is derived from blood plasma that passes through the walls of capillaries.. As the blood plasma moves through the capillary bed towards the vein [vein: Thin-walled, valved tubes which carry blood back to the heart., pressure drops and stops plasma being squeezed out.

The heartbeat can be felt as a pulse in arteries which lie just under the skin

Arteries, veins and capillaries

Tissue fluid acts as a bridge in the diffusion [diffusion: The movement of particles (molecules or ions) from an area of higher concentration to an area of lower concentration. of chemicals between the capillaries and the cells of the tissue. Oxygen and glucose  [glucose: A simple sugar made by the body from food, which is used by cells to make energy in respiration. diffuse from the blood into the tissue fluid and then into the cells. Carbon dioxide and urea [urea: A nitrogenous waste product resulting from the breakdown of proteins. It is excreted in urine. diffuse from the cells into the tissue fluid and then into the blood.

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