This document discusses the human circulatory system. It describes the composition of blood, the structure of the heart, and the cardiac cycle. The cardiac cycle involves atrial and ventricular systole and diastole. During systole, the heart muscles contract to pump blood out of the chambers. During diastole, the muscles relax and blood flows back into the chambers. The document then explains the blood circulation process, where oxygenated blood is pumped from the lungs to the body and deoxygenated blood returns to the heart and lungs.