Explain how the human circulatory system is adapted to supply oxygen to the tissues and remove waste products from the tissues.

Firstly, humans have a double circulatory system, which increases blood pressure and flow of blood to the tissues. To allow the heart to pump blood around the body, the heart is made of cardiac muscle. These muscle cells have long protein filaments, which have the ability to slide past each other, shortening the cell and leading to contraction. In order to supply oxygen to cells the heart first pumps the blood into the lungs through the pulmonary artery. Here oxygen diffuses from the air in the alveoli into the blood. Blood then returns to heart via the pulmonary vein. It is then pumped out again, towards the rest of the body, through the aorta. Oxygen is then carried around the body by red blood cells. They are specialised cells, which have no nucleus and contain haemoglobin so that there is maximum space for oxygen. The circulatory system is made up of three types of blood vessels. The first type, arteries, carry the oxygenated blood to the tissues. The second type, capillaries, which have very thin wills and a large surface area to allow for easy and maximum diffusion, deliver the oxygen to the cells for respiration and therefore energy release. Waste products such as CO2 are then removed from the tissues, as they diffuse through the capillary walls and into the blood plasma. The third type of blood vessel, veins, then carry this blood back to the heart. Veins contain valves to keep blood flowing in the right direction and prevent back flow.