Circulatory system in mammals

• Mammals have a double circulatory system. Blood is pumped to the pulmonary circulation from the right ventricle and returns from the lungs to the left atrium. It is pumped to the systematic circulation from the left ventricle and returns from the body to the right atrium.

• There are three stages to the cardiac cycle:

- diastole, when all heart muscle is relaxed with blood returning to the atria and flowing into the ventricle

- atrial systole, when the atrial muscle contracts to 'top-up' the ventricles

- ventricular systole, when ventricular muscle contracts to force blood into the major arteries (pulmonary artery from the right ventricle and aorta from the left ventricle)

• The heart is myogenic. Excitation originates at the SAN (the pacemaker) and spreads to the AVN before passing down the Bundle of His and up the walls of the ventricles via Purkinje fibres.

• The atrioventricular valves are forced open when pressure in the atria exceeds that in the ventricles and forced closed when the pressure in the ventricles exceeds that in the atria. The semilunar valves are forced closed when arterial pressure exceeds that in the ventricles and forced open when ventricular pressure exceeds that in the major arteries.

• Arteries are adapted to carry blood away from the heart, under high pressure, to the organs. Veins are adapted to return blood, under low pressure, to the heart.

• Exchange takes place in the capillaries within organs, with tissue fluid carrying substances (e.g. oxygen and glucose) to the cells.

• Blood consists of plasma and cells:

- red blood cells to carry haemoglobin

- lymphocytes to form B and T cells in the immune response to antigens

- polymorphs and macrophages (formed from monocytes) to carry out phagocytosis

• Haemoglobin is adapted to 'pick-up' oxygen at high pO2 (in lungs) and release it at low pO2 (in respiring tissue). Further dissociation of oxygen takes place if the pCO2 is increased as a result of increased respiration.

• Blood clotting seals damage to vessels. It involves platelets and the plasma proteins, prothrombin and fibrinogen.

• Atherosclerosis (disease resulting in the thickening of arterial walls) and coronary thrombosis (clotting within the coronary arteries) are examples of cardiovascular disease.