animals

• Types of circulatory systems

  • Open
  • Closed
  • Single
  • Double
• Open circulatory system


  • Blood is pumped into a haemocoel where it bathes organs and returns slowly to the heart with little control over direction of flow. Blood is not contained in blood vessels.
• Closed circulatory system


  • Blood is pumped into a series of vessels; blood flow is rapid and direction is controlled. Organs are not bathed by blood but by tissue fluid that leaks from capillaries.
• Single circulatory system


  • Blood passes through the heart once in each circulation.
• Double circulatory system


  • Blood passes through the heart twice in each circulation – once in the pulmonary (lung) circulation and then again through the systemic (body) circulation.
• Comparison of circulatory systems

  • Transport medium
  • System of vessels
  • Pump
  • Valves
  • Respiratory pigment to carry oxygen
• Circulatory systems


  • Insects
  • Earthworms
  • Fish
  • Mammals
• Insect circulatory system


  • Open circulatory system. Dorsal tube-shaped heart. No respiratory pigment in blood as lack of respiratory gases in blood due to tracheal gas exchange system.
• Earthworm circulatory system


  • Closed circulatory. 5 pseudohearts. Respiratory pigment haemoglobin carries respiratory gases in blood.
• Fish circulatory system


  • Closed, single circulatory system. Blood pumped to and oxygenated in the gills continues around body tissues. This means a lower pressure and slower flow around the body.
• Mammalian circulatory system


  • Closed, double circulatory system. High blood pressure to body delivers oxygen quickly. Lower pressure to lungs prevents hydrostatic pressure forcing tissue fluid into and reducing efficiency of alveoli.
• Structure of arteries, veins and capillaries

  • Artery
  • Vein
  • Capillary
• Artery


  • Thick layer of smooth muscle that contracts and relaxes to alter blood flow to different organs. Thick layer of elastic tissue recoils to propel blood forward and even out flow. Tough collagen outer coat to prevent overstretching. Small lumen surrounded by smooth endothelium to prevent friction.
• Vein


  • Larger lumen as blood is under lower pressure. This gives less resistance to blood flow. Less muscle and elastic fibres. Instead, veins contain semilunar valves to prevent backflow of blood.
• Capillary


  • A single layer of endothelium giving a short diffusion path.
• The cardiac cycle


  1. Atrial systole

  2. Ventricular systole

  3. Ventricular diastole

  4. Diastole
• Atrial systole


  Atrial contract. Pressure opens atrio-ventricular valves. Blood flows into ventricles.
• Ventricular systole


  Ventricles contract. Atrio-ventricular valves close due to pressure in ventricles being higher than that in the atria. Semilunar valves in aorta and pulmonary artery open. Blood flows into arteries.
• Ventricular diastole


  Ventricle muscle relaxes. Semilunar valves close to prevent backflow of blood into the ventricles.
• Diastole


  Heart muscle relaxes and atria begin to fill from vena cava and pulmonary veins.
• Initiating the heartbeat


  1. The heartbeat is myogenic; initiation comes from the heart itself.

  2. The sinoatrial node acts as a pacemaker sending waves of excitation across the atria causing them to contract simultaneously.

  3. A layer of connective tissue prevents the wave of excitation passing down to the ventricles. The wave of excitation passes to the atrio-ventricular node where there is a delay to allow the atria to complete contraction.

  4. The atrio-ventricular node transmits impulses down the bundle of His to the apex of the heart.

  5. The impulse then travels up the branched Purkinje fibres, simulating ventricles to contract from the bottom up. This ensures all the blood is pumped out.
• Parts of the mammalian heart

  • Superior vena cava
  • Aorta
  • Pulmonary artery
  • Pulmonary (semilunar) valve
  • Pulmonary Veins
  • Bicuspid (mitral) valve
  • Left ventricle
  • Right ventricle
  • Tricuspid valve
  • Right atrium
  • Septum
  • Apex
  • Inferior vena cava
• Chloride shift


  Some CO2 is carried in the blood dissolved in plasma, while some is carried in the blood as carbaminohaemoglobin. However, most is carried as hydrogen carbonate ions.
• Chloride shift


  1. CO2 diffuses into a red blood cell (RBC).

  2. CO2 combines with H2O catalysed by the enzyme carbonic anhydrase, forming carbonic acid.

  3. Carbonic acid dissociates into hydrogen ions (H+) and hydrogen carbonate ions (HCO3-) diffuse out of the RBC into the plasma.

  4. Chloride ions (Cl-) diffuse (facilitated diffusion) into the RBC to maintain electrochemical neutrality – the chloride shift.

  5. H+ bind to oxyhaemoglobin, reducing its affinity for oxygen. This is the Bohr effect.

  6. Oxygen is released from the haemoglobin.

  7. Oxygen diffuses from the RBC into the plasma and body cells.
• Formation of tissue fluid

  1. At the arterial end of the capillary bed, hydrostatic pressure is higher than osmotic pressure. Water and small soluble molecules are forced through the capillary walls, forming tissue fluid between the cells. Proteins and cells in the plasma are too large to be forced out.

  2. Due to reduced volume of blood and friction, blood pressure falls and it moves through the capillary.

  3. At the venous end of the capillary bed, osmotic pressure of the blood is higher than the hydrostatic pressure. Most of the water from tissue fluid moves back into blood capillaries (down its water potential gradient). The remainder of the tissue fluid is returned to the blood via lymph vessels.
• Pressure changes in the vessels

  Pressure is high in the aorta due to contraction of the left ventricle, but falls substantially in the arterioles and capillaries due to their large surface area and narrow diameter. Low pressure in the veins requires valves and muscle action to aid blood flow back to the heart.
• The electrical activity that spreads through the heart during the cardiac cycle can be detected using electrodes placed on the skin and shown on a cathode ray oscilloscope. This is called an electrocardiogram (ECG).
• P wave


  Depolarisation of the atria corresponding to atrial systole.
• QRS wave


  Spread of depolarisation through the ventricles resulting in ventricular systole.
• T wave


  Repolarisation of the ventricles resulting in ventricular diastole.
• Oxygen dissociation curves


  Haemoglobin has a high affinity for oxygen at high partial pressures (in the lungs) but releases it readily at lower partial pressures (in respiring tissues). The Bohr shift causes haemoglobin to have a lower affinity for oxygen in the presence of CO2, helping oxygen release in respiring tissues. Myoglobin and foetal haemoglobin have different oxygen dissociation curves.
• Red blood cells transport oxygen. Haemoglobin has a high affinity for oxygen. Each molecule of haemoglobin can carry four oxygen molecules, forming oxyhaemoglobin. This reaction is reversible.
• Pressure changes in the heart

  Contraction of the ventricle increases pressure, forcing the semilunar valve open and blood into the aorta. Relaxation of the ventricle decreases pressure, closing the semilunar valve and allowing the atrio-ventricular valve to open as the atrium contracts, refilling the ventricle.