 Dear students, in this topic we shall discuss the transport of oxygen in blood. Oxygen is absorbed from the respiratory epithelium into the blood. About 97% of oxygen absorbed binds with hemoglobin and is transported as oxyhemoglobin in the blood. Only 3% of oxygen dissolves in the plasma and is carried through various plasma proteins and its water content. The ability of hemoglobin to bind with oxygen is known as the oxygen capacity of hemoglobin. It is directly proportional to the partial pressure of oxygen. One hemoglobin molecule can bind 4 oxygen molecules at a time because it has 4 hemogrups. Each hemogrups can bind 1 oxygen molecule. This way, a total capacity of 1 hemoglobin molecule is 4 molecules of oxygen. If all sites on hemoglobin molecule are occupied by oxygen, the blood is 100% saturated. The oxygen content at 100% saturation level in blood is equal to the oxygen capacity and it is about 20 ml per 100 ml of blood at sea level at 1 atmosphere with a partial pressure of oxygen at about 115 mmHg. Dear students, oxyhemoglobin is unstable at low oxygen concentration and low partial pressure of oxygen. It dissociates rapidly into hemoglobin and oxygen at low partial pressure of oxygen. In tissues, oxygen is sharply liberated because in tissues, oxygen's partial pressure is much more than 60 mmHg and the bound oxygen is liberated. Red blood cells have a enzyme called carbonic anhydrase that facilitates dissociation activity. Dear students, now we shall discuss the factors which affect the oxygen carrying capacity of hemoglobin and blood. Hemoglobin's affinity with oxygen is labile. It depends on 3 factors, temperature, carbon dioxide concentration and the pH level. A rising temperature results in decreased oxygen carrying capacity of the blood. When carbon dioxide pressure increases, then oxygen tension decreases. So the capacity of hemoglobin to hold oxygen becomes less. The reason is that in tissues where the concentration of carbon dioxide increases, then oxygen tension decreases. So as a result, oxygen is released from the hemoglobin and liberated. That is the basis of how oxygen separates from the hemoglobin in tissues. Dear students, the affinity of oxygen for hemoglobin also declines with declining pH level of the blood. If pH level is decreasing, then the acidity in the blood increases, then oxygen affinity decreases from the hemoglobin. This phenomenon is termed as Bohr's effect or Bohr's shift. Bohr's effect we shall discuss in detail. Low pH means increased hydrogen ion concentrations. When hydrogen ion concentration increases, then hydrogen ions combine with the hemoglobin protein part. The result is that the oxygen carrying capacity of the hemoglobin decreases. Where oxygen is bound, then hydrogen ions are bound. If pH level increases, then the hemoglobin's ability to bind oxygen increases. That again causes problems. That is why maintenance of blood pH is very necessary.