The heart and blood GapFill

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

Blood is pumped from the heart by the ventricles, the  vena cavaleftrightmusclescapillariesventricles to the lungs, while the  lungsred blood cellrightatriumleftvalve pumps blood to the body. Blood returns to the heart through veins - blood from the body enters the heart through the  vena cavaaortavalvesalveolicapillariesventricles, whilst blood from the lungs enters the heart through the  valvespulmonary veinplasmacapillariespulmonary arteryventricles.

Arteries carry blood from the heart and normally contain oxygenated blood. They deal with blood at a much higher pressure, and therefore have  alveolipacemakersmuscular wallslarge nucleivalvescapillaries.  VeinsCapillariesAtriaVentriclesArteriesAlveoli are the smallest blood vessels. They are about the width of a  hairmitochondrioncarbon dioxide moleculered blood cellvalvehaemoglobin molecule, which are adapted to carry oxygen. Blood also contains white blood cells and platelets, all suspended in a liquid called  starchlymphcell sapplasmabilesulfuric acid, which is mostly water but also contains salts, amino acids and glucose.