The human circulatory system transports oxygen, nutrients, hormones, and other materials to cells throughout the body and removes waste via the blood. Blood is pumped from the heart through arteries and returns to the heart via veins, circulating through a closed double loop system. The heart has four chambers that pump blood through two circuits - pulmonary circulation to the lungs and systemic circulation to the rest of the body. Diseases can develop if blood pressure and cholesterol levels rise due to risk factors like genetics, diet, smoking, and lack of exercise.