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At the lungs, alveolar ventilation is matched with pulmonary blood flow to create a concentration gradient to exchange O2 and CO2
– Oxygen binds to haemoglobin and is transported in the blood.
– The dissociation curve is sigmoidal, due to the cooperative binding of oxygen molecules.
– This means that it is at first difficult to load the first oxygen molecule onto haemoglobin
– However, once the first one binds, this causes a conformational shift which increases haemoglobin’s affinity for oxygen
It is then easier to bind the next oxygen molecules. This causes the steep rise in the oxygen dissociation curve.
– O2 saturation at 100mmHg = 98% and it is mostly saturated above 60mmHg –> hence not affected much by changes in barometric pressure
Whilst loading oxygen onto haemoglobin is important at the lungs, when we get to the muscle, it is important to offload the oxygen so that our cells can use it.
– This is achieved as the dissociation curve is shifted to right by low pH, increased CO2 and 2,3 DPG levels.
– This means that less oxygen can bind to haemoglobin and instead gets released for use by cells.
Transport of CO2
Carbon dioxide is carried in the blood in 3 ways:
– It is dissolved directly in the plasma (very little)
– It can be carried as carbamino compounds attached to proteins
– Most important method however is being transferred as bicarbonate.
However, this is potentially dangerous as many H+ ions are produced as a by-product and this acidifies the blood
– The pH of the blood can be calculated by the Henderson-Hasselbach equation.
– This shows that the blood pH depends on the amount of bicarbonate and carbon dioxide.
– An increase in CO2 will acidify the blood, as it will react with water to release H+ ions
– An increase in bicarbonate ions will make the blood more alkaline, as it can mop up and buffer any H+ ions.
The ratio of bicarbonate to CO2 is monitored by the kidney and lungs.
– Alveolar ventilation can control CO2 levels by changing the respiratory rate
– Kidneys control excretion of bicarbonate ions
The shape of this relationship can be seen on a Davenport diagram, which shows how the kidneys and lungs work together to maintain a constant pH.
– If one variable is raised, compensatory mechanisms are employed to adjust the level of other variable to stabilise the pH.
– We can see how the lungs and kidneys are interacting by looking at the pH, CO2 and bicarbonate on an arterial or venous blood gas.
