Oxygen is carried in the blood mainly bound to hemoglobin; a minor fraction is dissolved in the plasma. Hemoglobin releases the oxygen that sustains every body cell.
There are about 2.5 billion red blood cells circulating at any one time. With some 280 million molecules of hemoglobin packed inside them – each of them capable of carrying four oxygen molecules.
The maximum oxygen that can be bound to the hemoglobin in the blood is the oxygen capacity. One gram of hemoglobin theoretically can carry up to 1.39 mL of oxygen. More commonly an oxygen binding capacity of 1.34 to 1.36 mL oxygen per gram hemoglobin is accepted.
Disturbances in oxygen uptake in the lungs, or in supply of adequate amounts of oxygen to the tissues, are common features of many congenital heart lesions.
Red blood cells can carry much larger quantities of carbon dioxide than they can carry oxygen. During rest 100 mL of blood carries an average of 4 mL of carbon dioxide from the tissue to the lungs.
While the oxygen-carrying hemoglobin imparts a red color to the blood, on its return to the lungs hemoglobin loaded with carbon dioxide loses its bright color and assumes a dark red-dish shade, akin to purple.
Hemoglobin as a carrier
