B7 mass transport

• haemoglobin is a water-soluble globular protein, made up of four polypeptides
• each polypeptide in haemoglobin forms a complex containing a haem group, this binds to oxygen so each haemoglobin can carry four oxygen molecules
• loading is the process of haemoglobin binding with oxygen, it takes place in the lungs
• unloading is the process of haemoglobin releasing its oxygen, it takes place in the tissues
• to most efficiently transport oxygen, haemoglobin needs to readily associate with oxygen at the gas exchange surface, and readily dissociate from oxygen at the respiring tissues, this is possible as haemoglobin changes shape under different conditions, changing its affinity for oxygen molecules
• in the presence of more carbon dioxide and less oxygen, haemoglobin has weaker affinity for oxygen, so releases it
• in the presence of less carbon dioxide and more oxygen, haemoglobin has stronger affinity for oxygen, so binds to it
• oxygen dissociation curves show how the binding of haemoglobin to oxygen changes at different partial pressures, they have partial pressure of oxygen on the x-axis and saturation of haemoglobin on the y-axis
• cooperative binding process:

  • the initial shape of haemoglobin means it is difficult for the first oxygen to bind, so at low partial pressure of oxygen, little oxygen can bind to haemoglobin
  • the binding of the first oxygen molecule changes the quaternary structure of the haemoglobin, the new shape is easier for the next oxygens to bind to
  • a smaller increase in partial pressure is required for the second and third oxygens to bind due to the change in shape of haemoglobin
  • the fourth oxygen is more difficult to bind as there are fewer binding sites available and fewer oxygens available
• the further to the left an oxygen dissociation curve, the greater the affinity of haemoglobin for oxygen, so it loads oxygen easily and unloads oxygen less easily
• the further to the right an oxygen dissociation curve, the lower the affinity of haemoglobin for oxygen, so it unloads oxygen easily and loads oxygen less easily
• in the presence of carbon dioxide, haemoglobin has reduced affinity for oxygen
• the greater the concentration of carbon dioxide, the more readily haemoglobin releases its oxygen, this is called the bohr effect
• the bohr effect at gas exchange surfaces:

  carbon dioxide concentration is low because it diffuses across the surface and exits, affinity of haemoglobin for oxygen increases, oxygen concentration is high, so oxygen is readily loaded by haemoglobin
• the bohr effect at respiring tissues:

  carbon dioxide concentration is high because it is produced in respiration, affinity of haemoglobin for oxygen decreases, oxygen concentration is low, so oxygen is readily unloaded by haemoglobin
• the bohr effect works by:

  • at gas exchange surfaces, pH is high as carbon dioxide concentration is low, this changes the shape of haemoglobin into one which readily loads oxygen, and increases its affinity for oxygen
  • at respiring tissues, pH is low as carbon dioxide concentration is high, this changes the shape of haemoglobin into one which readily unloads oxygen, and decreases its affinity for oxygen
• large organisms cannot use just simple diffusion to transport substances around the body as they have smaller surface area to volume ratio, instead they need a circulatory system
• the common features of the different circulatory systems are:

  • transport medium = in mammals this is the blood
  • way of moving the medium = in mammals this is the heart
  • mechanism to control flow = in mammals this is the valves
  • closed system of vessels = in mammals the blood is confined to the vessels
• mammals have a double circulatory system in which the blood is confined to the vessels and passes through the heart twice in every circuit of the body
• the double circulatory system is required because when blood passes through the lungs, its pressure is decreased, so if it went around the rest of the body, the circulation would be very slow, so blood is returned to the heart to increase its pressure before it passes around the rest of the body
• the heart is made up of two pumps, the left side which deals with oxygenated blood from the lungs, and the right side which deals with deoxygenated blood from the body
• each pump in the heart has two chambers:

  • atrium = thin-walled, elastic, stretches as it collects blood
  • ventricle = thick-walled, muscular, contracts strongly to pump blood around
• between the atria and ventricles there are valves which prevent the backflow of blood into the atria when the ventricles contract, these are called atrioventricular valves
• the left atrioventricular valve is called the bicuspid valve
• the right atrioventricular valve is called the tricuspid valve
• the four blood vessels connected to the heart are the aorta, the vena cava, the pulmonary artery and the pulmonary vein
• the aorta is connected to the left ventricle and carries oxygenated blood from the heart to the body
• the vena cava is connected to the right atrium and carries deoxygenated blood from the body to the heart
• the pulmonary artery is connected to the right ventricle and carries deoxygenated blood from the heart to the lungs
• the pulmonary vein is connected to the left atrium and carries oxygenated blood from the lungs to the heart
• the heart also has coronary arteries which branch off of the aorta just after it leaves the heart and supply it with its own oxygen
• blockage of the coronary arteries can lead to a myocardial infarction which is a heart attack, as the area of the heart will be deprived of blood, so deprived of oxygen, so the cells cannot respire and they die
• the heart is referred to as myogenic because it can start its own contraction
• in the wall of the right atrium there is a region of fibres called the sinoatrial node, SAN, which acts as the pacemaker of the heart by initiating a wave of electrical stimulation which causes the atria to contract together
• the process of the heart beating is the cardiac cycle
• the stages of the cardiac cycle are diastole, atrial systole and ventricular systole
• diastole process:

  atria and ventricles relax, elastic recoil of the heart lowers the pressure inside the chambers, atria fill with blood from the vessels, atrial pressure increases until atrioventricular valves open, ventricles fill with blood, semi-lunar valves stay closed
• atrial systole process:

  atria contract, remaining blood is forced into ventricles
• ventricular systole process:

  ventricles contract, atrioventricular valves close, semi-lunar valves open, blood moves out of the ventricles into the vessels
• atrioventricular valves are between the atria and the ventricles, they prevent backflow of blood when ventricular pressure > atrial pressure