The Circulatory System

Large and complex animals have circulatory systems that consist of tubes, a transport fluid and a means of pumping the fluid. Blood is the transport fluid which contains dissolved substances and cells. The tubes are blood vessels through which dissolved substances are circulated around the body. The heart is the pumping organ which keeps the blood in circulation.


The types of circulatory system exist in animals: open and closed.

In an open circulatory system;

  • The heart pumps blood into vessels which open into body spaces known as haemocoel.
  • Blood comes into contact with tissues.

A closed circulatory system;

  • Found in vertebrates and annelids where the blood is confined within blood vessels and does not come into direct contact with tissues.


Transport in Insects

  1. In an insect, there is a tubular heart just above the alimentary canal.
  2. This heart is suspended in a pericardial cavity by ligaments.
  3. The heart has five chambers and extends along the thorax and abdomen .
  4. Blood is pumped forwards into the aorta by waves of contractions in the heart.
  5. It enters the haemocoel and flows towards the posterior.
  6. The blood flows back into the heart through openings in each chamber called ostia.
  7. The ostia have valves which prevent the backflow of blood.
  8. Blood is not used as a medium for transport of oxygen in insects.
  9. This is because oxygen is supplied directly to the tissues by the tracheal system.
  10. The main functions of blood in an insect are to transport nutrients, excretory products and hormones.


Mammalian Circulatory System

  1. Mammals have a closed circulatory system where a powerful heart pumps blood into arteries.
  2. The arteries divide into smaller vessels called arterioles.
  3. Each arteriole divides to form a network of capillaries inside the tissues.
  4. The capillaries eventually re-unite to form venules, which form larger vessels called veins.
  5. The veins take the blood back to the heart.
  6. Blood from the heart goes through the pulmonary artery to the lungs and then back to the heart through pulmonary vein.
  7. This circulation is called pulmonary circulation.
  8. Oxygenated blood leaves the heart through the aorta and goes to all the tissues of the body.
  9. From the tissues, deoxygenated blood flows back to the heart through the vena cava.
  10. This circulation is called systemic circulation.
  11. In each complete circulation, the blood flows into the heart twice.
  12. This is called double circulation.
  13. Some other animals like fish have a single circulation.
  14. Blood flows only once through the heart for every complete circuit.


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