B7.2 Peak performance – circulation
The HEART is a muscular organ in the circulatory system. It beats automatically, pumping blood around the body to provide cells with oxygen and dissolved food for RESPIRATION.
Humans – and all other mammals – have a double circulatory system which comprises two separate circuits.
Blood pumps through the heart twice during a complete journey around the body – some blood is sent to the lungs and some blood is sent to the rest of the body each time the heart beats.
The blood in each circuit is kept separate – this is called a double circulation and is a more efficient way of delivering oxygen to the tissues than single circulation.
The blood carries glucose molecules and oxygen TO molecules – this is because all cells respire and need a supply of dissolved food and oxygen for this to take place.
The blood then carries AWAY from the cells waste products such as carbon dioxide (taking it back to the lungs so that it can leave the body).
Blood is a mixture of different components:
- Plasma – the liquid part of the blood – it transports:
- Platelets – are tiny particles found in blood plasma – when a blood vessel is damaged, platelets are triggered to clump together to form a clot and prevent blood leaving the body.
- Red blood cells – transport oxygen from the lungs to the body – they have no nucleus which means they have more space to be packed full with the red pigment haemoglobin, which binds to oxygen to form oxyhaemoglobin
Their biconcave shape increases the surface area for oxygen exchange
The heart is mainly made from muscle with the following features
- The artia (atriums) are the top chambers of the heart – they collect blood as it flows back to the heart through the veins
- Two ventricles which are the larger, more muscular lower chambers
- The vena cava is a larger vein that returns blood from the body into the right atrium
- The pulmonary arteries (one for each lung transport blood to the lungs. Blood travels from the right ventricle to the lungs – the pulmonary arteries to carry deoxygenated blood
- The pulmonary vein returns blood from the lungs to the left atrium – it is the only vein that transports oxygenated blood
- The aorta is the largest artery in the body – taking oxygenated blood at high pressure to the whole body from the left ventricle
- The coronary artery is a branch of the aorta which transports blood to the heart muscle itself.
The pressures that build up inside the atria and ventricles are very high – valves between each chamber and the arteries leaving the heart, prevent the back flow of blood. Valves are also found in veins in the rest of the body. Blood must only travel in one direction – if it moves back it causes the valve to close.
Blood flows at high pressure from the heart in the artery – the blood reaches its destination via arterioles that branch off the artery and into the capillary beds that surround cells
By the time blood reaches the capillary beds from an artery, it is at high pressure and this forces blood plasma out – the plasma leaves the capillary and becomes tissue fluid.
As the blood plasma moves through the capillary bed towards the vein pressure drops and stops plasma being squeezed out.
Tissue fluid acts as a bridge in the diffusion of chemicals between the capillaries and the cells of the tissue.
Oxygen and glucose diffuse from the blood into the tissue fluid and then into the cells.
Carbon dioxide and urea diffuse from the cells into the tissue fluid and then into the blood.