The heart and blood GapFill

The heart is adapted to pump blood around the body. It is a  globularfibroustotipotentmuscular pump, consisting of  twodigestivevariablefour chambers. As blood passes through the heart twice for every heartbeat, the mammalian heart is described as being part of a  double circulatory systemtwin circulatory systemtwin heartbeat systemdouble pump system.

Blood is pumped from the heart by the ventricles, the  vena cavaventriclesleftright to the lungs, while the  valveatriumrightleft pumps blood to the body. Blood returns to the heart through veins - blood from the body enters the heart through the  aortavalvesalveolivena cava, whilst blood from the lungs enters the heart through the  pulmonary arterypulmonary veincapillariesplasma.

Arteries carry blood from the heart and normally contain oxygenated blood. They deal with blood at a much higher pressure than in veins, and therefore have   large nucleithick, muscular wallscapillariesvalves.  VeinsAtriaAlveoliCapillaries are the smallest blood vessels. They are about the width of a  red blood cellvalvehaemoglobin moleculemitochondrion, which are adapted to carry oxygen. Blood also contains white blood cells and platelets, all suspended in a liquid called  plasmabilestarchlymph, which is mostly water but also contains salts, amino acids and glucose.