The Heart - Blood Vessels and Fluid.

Arteries carry blood from the heart to the rest of the body. The lumen is narrow, surrounded by folded endothelium to allow the artery to expand, it is smooth to reduce friction and ease blood flow, surrounding this is a thick layer of muscle which contracts to prevent blood flow to organs. Surrounding this are collagen fibres that are strong to withstand the high pressure.

Capillaries are the smallest of the blood vessels. Glucose and Oxygen are exchanges between capillaries and cells, adapted for efficient diffusion with only one cell thick walls.

Veins return blood to the heart. They have a wider lumen due to the low pressure, there is very little muscle. They contain valves to prevent to the heart. The blood flow is aided by the contraction of the muscles surrounding them.

Tissue fluid is formed from blood, it is fluid that surrounds the cells in tissues, its formed from substances that leave the blood through the 'leaky' capillaries for example Oxygen, water, glucose. Cells take in the nutrients and release waste products into it.

At the beginning…