That is, oxygen partial pressure will be somewhat higher in the alveolus than in the blood, and carbon dioxide pressure will be slightly higher in the blood than in the air in the alveolus. This blood is not oxygenated, and forms a physiologic shunt. Because of this blood that bypasses the alveoli, arterial blood will always contain less oxygen pressure than blood that has equilibrated with the oxygen in the lung alveoli. This shunt becomes part of the calculated Aa gradient. As blood circulates through the body, an opposite change occurs in the capillaries of the systemic circulation.
Oxygen diffuses from the area of higher pressure the blood into the lower pressure of the cells.
Carbon dioxide diffuses from the cells into the blood. Back to Index. Oxygen Transport Hemoglobin is a molecule composed of four subunits. Each subunit is a protein chain attached to a porphyrin ring containing one iron atom. Normal blood contains about 15-16 grams hemoglobin per 100 ml.