The correct answer is A. Oxygen.
Blood leaving the lungs is rich in oxygen because it has just taken up oxygen from the air in the lungs. The blood is then transported to the rest of the body, where it delivers oxygen to the tissues.
Option B is incorrect because blood leaving the lungs is low in carbon dioxide. Carbon dioxide is a waste product that is produced by the body’s cells. The blood carries carbon dioxide from the tissues to the lungs, where it is exhaled.
Option C is incorrect because haemoglobin is a protein in red blood cells that carries oxygen. Haemoglobin is found in all blood, not just blood leaving the lungs.
Option D is incorrect because the number of red blood cells in the blood does not change significantly between the lungs and other parts of the body. The number of red blood cells is regulated by the body to maintain a constant level of oxygen in the blood.
In summary, blood leaving the lungs is rich in oxygen because it has just taken up oxygen from the air in the lungs. The blood is then transported to the rest of the body, where it delivers oxygen to the tissues.