The heart and blood GapFill

The heart is adapted to pump blood around the body. It is a  fibrouscrenelatedmusculartotipotent pump, consisting of  digestivetwofouracidic 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 pump systemdouble pump system.

Blood is pumped from the heart by the ventricles, the  musclesvena cavarightventricles 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  aortavalvesvena cavacapillaries, whilst blood from the lungs enters the heart through the  plasmapulmonary veinvalvespulmonary artery.

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   valvespacemakersthick, muscular wallslarge nuclei.  CapillariesVentriclesVeinsAtria are the smallest blood vessels. They are about the width of a  hairmitochondrionred blood cellhaemoglobin molecule, which are adapted to carry oxygen. Blood also contains white blood cells and platelets, all suspended in a liquid called  cell sapsulfuric acidplasmalymph, which is mostly water but also contains salts, amino acids and glucose.