The heart and blood GapFill

The heart and lungs are closely related and the lungs are well adapted for their role in  transpirationphotosynthesiscirculationgas exchange. The heart is adapted to pump blood around the body. It is a  muscularfibroustotipotentcrenelated pump, consisting of  fourtwoacidicdigestive chambers. As blood passes through the heart twice for every heartbeat, the mammalian heart is described as being part of a  double pump systemtwin pump systemtwin circulatory systemdouble circulatory system.

Blood is pumped from the heart by the ventricles, the  vena cavaventriclesmusclesright to the lungs, while the  valveatriumleftred blood cell pumps blood to the body. Blood returns to the heart through veins - blood from the body enters the heart through the  capillariesvena cavaaortavalves, whilst blood from the lungs enters the heart through the  pulmonary veinvalvespulmonary arteryplasma.

Arteries carry blood from the heart and normally contain oxygenated blood. They deal with blood at a much higher pressure, and therefore have  large nucleivalvespacemakersmuscular walls.  VeinsVentriclesCapillariesAtria are the smallest blood vessels. They are about the width of a  haemoglobin moleculemitochondrionred blood cellvalve, which are adapted to carry oxygen. Blood also contains white blood cells and platelets, all suspended in a liquid called  bilelymphplasmastarch, which is mostly water but also contains salts, amino acids and glucose.