It originates in the left ventricle, arches up over the heart, and passes down behind it through the chest and abdomen. It gives off many smaller arteries which conduct blood to all body organs and tissues. Atrium One of the two upper chambers of the heart. Each receives blood through veins from the body or lungs. The tiniest blood vessels.
Capillary networks connect the arterioles and venules.
Their thin walls permit the exchange of gases and of nutrients or waste materials between the blood and the body tissues. Carbon Dioxide A gaseous waste product of chemical reactions in the cells. It passes from the cells to the blood which eventually releases it in the lungs to be breathed out. Oxygen A gas which is the most important component of the air we breathe. It is vital to energy-producing chemical reactions in the living cells of the body.