Circulatory system

• Fish have a single circulatory system where deoxygenated blood is pumped from the heart to the gills, becomes oxygenated, and then passes straight to the organs
• The problem with a single circulatory system is that the blood loses a lot of pressure as it passes through the gills before reaching the organs, so the blood travels to the organs relatively slowly and cannot deliver a great deal of oxygen
• Humans have a double circulatory system where deoxygenated blood is pumped from the heart to the lungs, becomes oxygenated, and then returns to the heart to be pumped to the organs
• The benefit of the double circulatory system is that the blood passes through the heart twice, allowing it to travel rapidly to the body cells and deliver the oxygen they need
• Heart


  • Organ consisting mainly of muscle tissue
  • Job is to pump blood around the body
• Chambers of the heart

  • Left atrium
  • Right atrium
  • Left ventricle
  • Right ventricle
• The atria are separated from the ventricles by valves
• Blood vessels entering and leaving the heart

  • Vena cava (brings in oxygenated blood from the body)
  • Pulmonary artery (blood passes from heart to lungs)
  • Pulmonary vein (oxygenated blood passes from lungs to heart)
  • Aorta (blood pumped from heart to body)
• Pattern of blood flow through the heart

  1. Blood enters left and right atria

  2. Atria contract, forcing blood into ventricles

  3. Ventricles contract, forcing blood out of heart

  4. Valves prevent backflow into atria
• Left ventricle

  • Has thicker muscular wall than right ventricle
  • Pumps blood around entire body, so needs greater force
• Right ventricle


  • Only pumps blood to the lungs
• Coronary arteries

  • Branch out of the aorta and spread into the heart muscle
  • Purpose is to supply oxygen to the muscle cells of the heart
• Pacemaker


  Group of cells in the right atrium that control the natural resting heart rate
• If the pacemaker stops working correctly, doctors can implant an artificial pacemaker to correct irregularities in the heart rate
• The heart is located between the lungs, behind the sternum (breastbone), and to the left side.
• The heart has four chambers - two atria and two ventricles.
• Blood flows from the right atrium into the right ventricle through the tricuspid valve.