The Heart

• Blood is pumped around the body by the heart.

  The heart has four chambers - two atria and two ventricles
• blood enters the heart through the atria. Blood from two vena cavae enters the right atrium. Blood from the pulmonary veins enters the left atrium. The atria fill, followed by the ventricles. Blood is prevented from flowing back into the atria by heart valves.

  Blood leaves the heart in the body's main artery - the aorta - from the left side, and the pulmonary artery, from the right.
• Double circulation

  Humans have a double circulatory system. The heart pumps blood through two circuits:

  1. the pulmonary circulation
  2. the systemic circulation
• he pulmonary circulation transports blood to the lungs. At the lungs:

  • oxygendiffuses into the blood from the alveoli - the blood becomes oxygenated
  • carbon dioxide diffuses from the blood into the lungs
• The systemic circulation transports:

  • oxygen and nutrients to the body
  • carbon dioxide and other wastes away from cells
• The systemic circulation is under high pressure - it has to deliver blood to the extremities of the body.

  The pulmonary circulation is under lower pressure, because:

  • blood is delivered to the lungs only, which are very close to the heart
  • in a healthy person, this lower pressure is optimum for the diffusion of gases