Red blood cells contain haemoglobin, Haemoglobin is a large protein with a quaternary structure that contains a haem group which contains iron and gives haemoglobin its red colour. Haemoglobin has a high affinity for oxygen, meaning it readily picks up oxygen, and each molecule can carry four oxygen molecules. In the lungs, oxygen joins to the iron in the haemoglobin to form oxyhaemoglobin. This is a reversible reaction so when the oxygen dissociates from the oxyhaemoglobin, turning it back to haemoglobin.
Haemoglobin saturation depends on the partial pressure of oxygen. The partial pressure is a measure of the oxygen concentration in the blood. The greater the concentration, the higher the partial pressure. Haemoglobins affinity for oxygen varies depending on the partial pressure of oxygen. Oxygen enters blood capillaries at the alveoli in the lungs. Alveoli have a high partial pressure so the oxygen in the alveoil loads onto haemoglobin to form oxyhaemoglobin easily. When cells respire, they use up oxygen and this lowers the partial pressure of oxygen. Red blood cells deliver oxyhaemoglobin to respiring tissues, where it unloads its oxygen, the haemoglobin then returns to the lungs to pick up more oxygen.
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