All living animal cells need a supply of oxygen and nutrients to grow and survive. They also need to removewasteproducts so that these do not buildup and become toxic.
Verysmall animals do not need a separatetransport system, because all their cells are surrounded by the environment in which they live.Diffusion will supplyenoughoxygen and nutrients to keep the cellalive.
The blood is at lowpressure and the walls do not need to be thick
The lumen is relatively large, in order to ease the flow of blood
The walls have thinner layers of collagen, smooth muscle and elastictissue than in artery walls
Contain valves that stop the backflow of blood and by contraction of the surrounding skeletal muscleappliespressure to the blood, forcing the blood to movealong in a directiondetermined by the valves
1. At the arterial end of the capillary bed, the blood has high hydrostatic pressurecompared to the hydrostatic pressure in the tissuefluid, forcingfluidout of the capillary and into spaces around the cells
2. As fluid leaves, the hydrostatic pressure reduces in the capillary, allowing some tissuefluid to return to the capillary containing waste products
3. At the venules end of the capillary bed, the water potential in the capillaries is lower than the water potential in tissue fluid due to fluid loss from the capillaries and the highoncotic pressure, allowing some water to re-enter the capillaries from the tissue fluid by osmosis
The net result of the pressure gradients creates a pressure gradient to push fluid out of the capillary at the arterial end and into the capillary at the venule end.
Notall of the tissue fluid re-enters the blood. The excess tissue fluid eventually returns to the blood through the lymphatic system - a kind of drainage system made from lymph vessels.
If a tissue is infected, then the capillaries become moreleaky and morefluid is directed into the lymph system — this helpsdirectbacteriatowards the Lymph nodes.
The mammalian heart is a muscular pump. On both sides, the heart squeezes the blood, putting it under pressure. This pressure forces the blood along the arteries and through the circulatory system.
The heart is divided into two sides. The right sidepumps the deoxygenated blood to the lungs to be oxygenated. The left side pumps oxygenated blood to the rest of the body.
The heart lies just off-centre towards the left of the chest cavity
The main part of the heart consists of firm, dark-redmuscle called cardiac muscle
There are two main pumping chambers - the ventricles. Above the ventricles are two thin-walled chambers - the atria
Lying over the surface of the heart are coronary arteries that supply oxygenated blood to the heart muscle
At the top of the heart are a number of tubular blood vessels - the veins that carry blood into the atria and the arteries that carry blood away from the heart