Adaptations for transport in 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

  • 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.
• Artery

  • Tough collagen outer cost to prevent overstretching
  • Small lumen surrounded by smooth endothelum to prevent friction
  • Thick layer of smooth muscle that can stretch as blood is pumped in and recoils to maintain pressure.
• Vein

  • Larger lumen as blood is under lower pressure
  • Less muscle and elastic fibres
  • Contains semilunar valves to prevent backflow of blood.
• Capillary

  • A single layer of endothelium allowing movement of substances between blood and cells.
• The cardiac cycle


  1. Atrial systole

  2. Ventricular systole

  3. Ventricular diastole

  4. Diastole
• Sinoatrial node


  Acts as a pacemaker sending waves of excitation across the atria causing them to contract simultaneously
• Atrioventricular node


  Transmits impulses down the bundle of His to the apex of the heart
• Purkinje fibres


  Transmit the impulse up, stimulating ventricles to contract from the bottom up to ensure all the blood is pumped out
• The heartbeat is myogenic; initiation comes from the heart itself
• The mammalian heart

  • Superior vena cava returns deoxygenated blood to the heart
  • Right atrium contracts and pumps deoxygenated blood into the right ventricle
  • Tricuspid valve opens to allow blood flow and then closes to prevent backflow
  • Right ventricle has a thinner muscular wall compared to the left ventricle
  • Pulmonary artery takes deoxygenated blood to lungs from right ventricle
  • Pulmonary veins return oxygenated blood from lungs to the left atrium
  • Bicuspid (mitral) valve prevents backflow of blood into the left atrium
  • Left ventricle has a comparatively thicker muscular wall to produce a higher pressure to push oxygenated blood rapidly around the body
  • Septum divides oxygenated and deoxygenated blood sides of the heart
• Contraction of the ventricle (systole)


  Increases the pressure in the ventricle, forcing the semilunar valve open and blood enters the aorta, increasing the pressure
• Relaxation of the ventricle (diastole)


  Pressure drops in both the ventricle and aorta, causing the semilunar valve to close and prevent backflow
• Elastic recoil of the aorta walls increases pressure momentarily
• Contraction of the atria


  Increases pressure in the atrium, forcing blood through the atrioventricular valves into the ventricles
• Relaxation of the ventricles

  Causes the atrioventricular valves to open and the ventricles to refill with blood
• Electrocardiogram (ECG) detects the electrical activity that spreads through the heart during the cardiac cycle
• 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
• Chloride shift


  1. CO2 diffuses into red blood cell

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

  3. Carbonic acid dissociates into H+ and HCO3- which diffuse out into plasma

  4. Cl- ions diffuse into red blood cell to maintain electrochemical neutrality

  5. H+ bind to oxyhaemoglobin, reducing its affinity for oxygen (Bohr effect)

  6. Oxygen is released from the haemoglobin and diffuses into plasma and body cells
• Oxygen dissociation curve


  • Sigmoid curve showing haemoglobin has high affinity for oxygen at high partial pressures (lungs) but releases it readily at lower partial pressures (respiring tissues)
  • Bohr shift - curve moves to the right when CO2 is present, reducing haemoglobin's affinity for oxygen and allowing more release
  • Myoglobin curve shifts to the left, holding onto oxygen until partial pressures are very low before rapidly releasing it, acting as an oxygen store in muscle
  • Fetal haemoglobin curve is just left of adult, allowing it to take oxygen more readily from mother's blood
• Formation of tissue fluid


  1. At arterial end of capillary, hydrostatic pressure is higher than osmotic pressure so water and small solutes are forced out

  2. Proteins and cells remain in the blood

  3. At venous end, osmotic pressure is higher so most water moves back into blood capillaries

  4. Remainder of tissue fluid is returned to blood via lymph vessels