Transport in animals

• Open circulatory systems


  Blood does not move around the body in blood vessels. Instead, cells are bathed by blood or a fluid called haemolymph in a fluid filled space around the organs called a haemocoel, which returns slowly to the dorsal, tube shaped-heart, e.g. insects
• Open circulatory systems


  • There is no need for a respiratory pigment as oxygen is supplied directly to tissues via the tracheal system
  • They are relatively inefficient
  • In insects, they are responsible for the distribution of respiratory gases
• Closed circulatory systems


  Blood can be transported more quickly under a higher pressure, to all parts of the animal's body
• Types of circulatory systems

  • Single circulatory system
  • Double circulatory system
• Single circulatory system

  1. Blood passing through the heart once during its passage around the body

  2. Found in fish where blood is pumped to the gills and onto the body organs before returning to the heart

  3. Also found in the earthworm where five pairs of 'pseudohearts' (which are thickened muscular blood vessels) pump blood from the dorsal vessel to the ventral vessel
• Double circulatory system


  1. Blood passing through the heart twice

  2. In mammals involves one circuit which supplies the lungs where blood is oxygenated (pulmonary circulation), and a second circuit which supplies the body with oxygenated blood (systemic circulation)
• Advantages of double circulation over single circulation

  • Allows a higher blood pressure and faster circulation to be sustained in systemic circulation
  • Oxygenated and deoxygenated blood are kept separate, which improves oxygen distribution
• Artery

  • Carry blood AWAY from the heart
  • Have thick walls to resist the high blood pressure
  • Elastic fibres stretch to allow the arteries to accommodate blood and the elastic recoil of the fibres pushes blood along the artery
  • The pressure in these arteries shows a rhythmical rise and fall, corresponding to ventricular systole
  • As the blood flows along the artery, friction with vessel walls causes the blood pressure and the rate of blood flow to decrease
• Arteriole


  • The main arteries continually branch to form smaller arteries and eventually arterioles
  • Arterioles have a large total surface area and relatively narrow lumen causing a further reduction in pressure and rate of blood flow
  • The important structure of an arteriole is the smooth muscle tissue, which can widen or narrow the lumen to increase or decrease blood flow
• Capillary

  • Millions of capillaries form dense networks in tissues
  • They have a narrow lumen (8-10μm in diameter), but their total cross-sectional area is very large
  • As blood flows through the capillaries both blood pressure and the rate of blood flow decrease
  • Their function is to supply oxygen and nutrients and absorb carbon dioxide and waste
• Venule

  • Small veins that converge forming larger venules and eventually veins
  • They have a similar structure to veins and as they widen, the resistance to blood flow decreases allowing blood rate of flow to increase again
• Vein

  • Carry blood back to the heart
  • The important structures are the semi-lunar valves, which prevent the backflow of blood ensuring that blood travels in one direction only
  • Although the pressure in veins is low, blood is returned to the heart due to the effects of surrounding skeletal muscle contracting squeezing the vein, which reduces the volume and increases the pressure inside the vein; this forces blood through the valve
• Pressure changes in blood vessels

  • Pressure decreases from aorta to capillaries, then increases again in veins
  • Rate of blood flow decreases from aorta to capillaries, then increases again in veins
  • Total cross-sectional area increases from aorta to capillaries, then decreases again in veins
• Blood flow through the heart

  1. Blood enters the heart from the head and body via the vena cava into the right atrium

  2. The right atrium contracts, forcing blood through the right atrio-ventricular valve into the right ventricle

  3. The right ventricle contracts, forcing blood out of the heart through the right semi-lunar valve to the lungs via the pulmonary artery

  4. Oxygenated blood returns from the lungs to the heart via the pulmonary vein and enters the left atrium

  5. The left atrium contracts, forcing blood through the left atrio-ventricular valve into the left ventricle

  6. The left ventricle contracts, forcing blood out through the left semi-lunar valve into the aorta and then to the rest of the body
• Cardiac cycle


  1. Left atrium contracts, decreasing volume and increasing pressure

  2. When blood pressure in left atrium exceeds that in left ventricle, blood flows into the left ventricle

  3. The ventricle then contracts (ventricular systole) and pressure rises in left ventricle as volume decreases

  4. As the ventricle contracts blood is pushed against the atrioventricular valves closing them and preventing blood flow back to the atria

  5. When pressure in left ventricle exceeds that in the aorta, the left semi-lunar valve opens and blood flows out into the aorta

  6. Left ventricle then relaxes (diastole) so its volume increases and pressure falls

  7. When pressure in ventricle drops below that of the aorta, blood tries to flow back into the ventricle from the aorta, pushing against the left semi-lunar valve and closing it

  8. When pressure in left ventricle drops below that in the left atrium, the left atrio-ventricular valve opens and the cycle begins again
• The heart is myogenic - the heartbeat is initiated within the cardiac muscle itself and is not dependent upon external stimulation
• Control of the heart

  1. The sinoatrial node initiates a wave of excitation across both atria

  2. The wave cannot spread to ventricles due to a layer of connective tissue

  3. The wave spreads via the atrioventricular node, through the Bundle of His to the apex of the ventricle

  4. The Bundle of His branches into Purkinje fibres carrying the wave upwards through the ventricle muscle causing it to contract

  5. Ventricle contraction is therefore delayed and contraction is from base upwards
• Cardiac cycle


  1. Atrial systole

  2. Ventricular systole
• Cardiac muscle


  • It is self-excitable and is not dependent upon external stimulation
  • It can be regulated by the sinoatrial node which initiates a wave of excitation across both atria
• Atrial systole


  1. Wave of excitation spreads out from the sinoatrial node across both atria

  2. Both atria start contracting

  3. Wave cannot spread to ventricles due to layer of connective tissue

  4. Wave spreads via the atrioventricular node (AVN), through the Bundle of His to apex of ventricle

  5. The Bundle of His branches into Purkinje fibres carrying wave upwards through ventricle muscle causing it to contract

  6. Ventricle contraction is therefore delayed and contraction is from base upwards
• These events can be seen in an ECG (electrocardiogram)
• What does P represent

  Associated with contraction of atria
• QRS


  Depolarisation and contraction of ventricles
• T


  Repolarisation of ventricle muscles
• Isoelectric line


  Represents the filling time between T and the next P wave
• Changes in this ECG trace can be used to diagnose problems with the heart, e.g. patient suffering a heart attack (myocardial infarction) may show depression in the S-T segment
• Blood consists of plasma (55%) and cells (45%). By far the majority of the cells (45%) are white cells and platelets
• Red blood cells (erythrocytes)


  Contain haemoglobin for the absorption and release of oxygen, and do not possess a nucleus. This means they have a biconcave shape, which increases their surface area for the absorption
• Red blood cells are continually produced
• Types of white blood cells

  • Phagocytic cells
  • Lymphocytes that develop into cells that produce antibodies
• Plasma


  90% water, and contains dissolved solutes, eg glucose and amino acids, hormones and plasma proteins. It is responsible for the distribution of heat and transport of carbon dioxide as HCD, lons Excretory products such as urea are also transported dissolved in the plasma
• Haemoglobin


  Binds to oxygen in the lungs, and releases it to the respiring tissues. This reversible reaction can be shown by: haemoglobin <-> oxyhaemoglobin
• Haemoglobin


  • Each molecule can accommodate four molecules of oxygen (402), one attached to each of the four haem groups
  • As oxygen molecules bind, the haemoglobin molecule changes slightly, making it easier for the next one to bind. This is known as cooperative binding, and can be seen by the steep part of the curve
  • The fourth and final oxygen molecule is more difficult to bind, and requires a large increase in partial pressure of oxygen to accomplish this: this is shown by the plateau on the graph, giving the curve a sigmoid (S) shape
• Partial pressure


  The pressure exerted by a particular gas in a mixture of gases
• In low oxygen environments, such as high altitudes or muddy burrows, animals have become adapted by evolving haemoglobin with a higher affinity for oxygen than normal haemoglobin
• Animals such as the llama and lugworm have oxygen dissociation curves to the left of normal, meaning that their haemoglobin is more saturated at the same partial pressure of oxygen than normal, i.e. it is more able to pick up oxygen
• Foetal haemoglobin also has a higher affinity for oxygen than normal (maternal blood) and so is also to the left. This means that it is able to absorb oxygen from the mother's blood via the placenta
• When carbon dioxide concentrations in the blood rise during exercise

  The haemoglobin's affinity for oxygen is reduced, so more oxygen is released at the same partial pressure of oxygen. This supplies oxygen more quickly to respiring tissues, where it is needed. This is called the Bohr effect
• Bohr effect


  The reduction in haemoglobin's affinity for oxygen as a result of higher partial pressure of carbon dioxide
• Carbon dioxide is carried in three ways: dissolved in plasma (5%), as HCO3- ions (85%) in the plasma, and bound to haemoglobin as carbamino-haemoglobin (10%)