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

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

Mammalian Circulatory System

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

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