The main transport system of human is the circulatory system, a system of tubes (blood vessels) with a pump (the heart) and valves to ensure one-way flow of blood. The blood pumped by the heart, travels all around the body in blood vessels. It leaves the heart in arteries and returns in the veins. Valves, present in the heart and veins, ensure a one-way flow for the blood. As blood enters an organ, the arteries divide into smaller arterioles, which supply capillaries. In these vessels the blood moves much more slowly, allowing the exchange of materials such as oxygen and glucose, carbon dioxide and other wastes. Blood leaving an organ is collected in venules which transfer it on to larger veins.

The main vessels of the human circulatory system are:

  • Pulmonary arteries that supply oxygen-poor blood from the heart to the lungs
  • Pulmonary veins that bring oxygen-rich blood from the lungs to the heart.
  • Aorta that supplies oxygen-rich blood from the heart to the rest of the body. The aorta branches into: coronary arteries which supply cardiac tissue, an anterior branch leading to the head and arms and a posterior branch (dorsal aorta) leading to abdominal organs and legs.
  • Branches of the dorsal aorta include:
    • Hepatic artery from the heart to the liver
    • Arteries to the alimentary canal
    • Renal arteries from the heart to the kidneys
  • Vena cava consists of an anterior branch which returns blood from the head and arms to the heart and a posterior branch.
  • Posterior vena cava collects blood from the posterior parts of the body such as from:
    • Hepatic veins from the liver to the heart
    • Renal veins from the kidneys to the heart
  • Hepatic portal vein transports blood from the alimentary canal to the liver. Blood from the liver is returned to the heart via the hepatic vein.
The human circulatory system

Single circulation of fish

Fish have the simplest circulatory system of all the vertebrates. A heart, consisting of one blood-collecting chamber (the atrium) and one blood-ejection chamber (the ventricle), sends blood to the gills where it is oxygenated. The blood then flows to all the parts of the body before returning to the heart. This is known as single circulation because the blood goes through the heart once for each complete circulation of the body. However as the blood passes through capillaries in the gills, blood pressure is lost, but the blood still needs to circulate through other organs of the body before returning to the heart to increase blood pressure. This makes the fish circulatory system inefficient.

The double circulation

Beginning at the lungs, blood flows into the left-hand side of the heart, and then out to the rest of the body. It is brought back to the right-side of the heart, before going back to the lungs again.

This is called a double circulation system, because the blood travels through the heart twice on one complete journey around the body:

  • one circuit links the heart and lungs (low pressure circulation)
  • the other circuit links the heart with the rest of the body (high pressure circulation)

The importance of a double circulation

  • Oxygenated blood is kept separate from deoxygenated blood. The septum in the heart ensures this complete separation. Oxygenated blood flows through the left side of the heart while deoxygenated blood flows through the right.
  • The blood pressure in the systemic circulation is kept higher than that in the pulmonary circulation. The left ventricle, with a thicker wall, pumps oxygenated blood effectively to all parts of the body. The right ventricle has a thinner wall and pumps blood to the lungs under lower pressure, thereby avoiding any lung damage.