Transport of Oxygen Flashcards
Transport of oxygen
Oxygen molecules diffusing into the blood plasma from the alveoli enter red blood cells and associate (bind reversibly) with the haemoglobin as the partial pressure of oxygen is high. This takes O2 molecules out of solution maintaining a steep concentration gradient, allowing further diffusion of O2 molecules into the blood plasma.
The blood carries the oxygen around the body, where cells need oxygen for aerobic respiration. Dissociation occurs, where the oxygen is released from the oxyhaemoglobin, as the pO2 is lower in the respiring tissue than in the lungs and blood, as O2 has been used for aerobic respiration. Dissociation occurs when partial pressure of oxygen is low. Association occurs when pO2 is high (capillaries near alveoli).
The ability of haemoglobin to associate with and release oxygen depends on the concentration of oxygen in the surrounding tissues.
Partial pressure of oxygen (pO2): The O2 concentration is measured by the relative pressure that it contributes to a mixture of gases (kPa) [also called oxygen tension].
Note: A graph of % saturation with O2 against pO2 is a straight line for a normal fluid.
However, haemoglobin can associate with O2 is a way that produces an S- shaped curve – haemoglobin dissociation curve. At low pO2, haemoglobin doesn’t readily associate with O2 molecules as the haem groups that attract the O2 molecules are in the centre of the haemoglobin molecule, making it difficult for oxygen molecule to reach the haem group and associate with it. This difficulty in combining accounts for low saturation level of haemoglobin at low pO2.
As pO2 rises, the conc gradient of O2 into the haemoglobin molecule increases. Eventually an O2 molecule enters and associates with a haem group. This causes a conformational change in the haemoglobin molecule, allowing more O2 molecules to enter the haemoglobin and bind with other haem groups relatively easily. This accounts for the steepness of the curve as partial pO2 rises. When the curve is steep a small change in pO2 causes a big change in % saturation.
As haemoglobin approaches 100% saturation the curve levels off. Mammalian haemoglobin is well adapted to transporting oxygen to the tissues. The oxygen tension in the lungs to close to 100%.
