biology4alevel600

 1 The human heart, like that of all mammals, has two atria and two ventricles. Blood enters the heart by the atria and leaves from the ventricles. A septum separates the right side of the heart, which contains deoxygenated blood, from the left side, which contains oxygenated blood.  2 Semilunar valves at the entrances to the blood vessels that leave the heart (aorta and pulmonary artery) prevent back flow of blood into the heart, and atrioventricular valves prevent backflow of blood from ventricles into the atria.  3 The heart is made of cardiac muscle and is myogenic (the muscle is self-stimulating). 4 The sinoatrial node (SAN) sets the pace of contraction for the muscle in the heart. Excitation waves spread from the SAN across the atria, causing their walls to contract. A non-conducting barrier prevents these excitation waves from spreading directly into the ventricles, thus delaying their contraction. Th e excitation wave travels to the ventricles via the atrioventric...

 1. Blood is carried away from the heart in arteries, passes through tissues in capillaries, and is returned to the heart in veins. Blood pressure drops gradually as it passes along this system. 2. Arteries have thick, elastic walls, to allow them to withstand high blood pressures and to smooth out the pulsed blood flow. Capillaries are only just wide enough to allow the passage of red blood cells, and have very thin walls to allow effi cient and rapid transfer of materials between blood and cells. Veins have thinner walls than arteries and possess valves to help blood at low pressure flow back to the heart.  3. Blood plasma leaks from capillaries to form tissue fluid. This is collected into lymphatics as lymph, and returned to the blood in the subclavian veins. Tissue fluid and lymph are almost identical in composition; both of them contain fewer plasma protein molecules than blood plasma, as these are too large to pass through the pores in the capillary walls.  4. Red b...