The blood carries oxygen both physically dissolved in the blood and chemically bound to the hemoglobin in red blood cells. Normally, much more oxygen is transported in combination with hemoglobin than physically dissolved in the blood. Without hemoglobin, the cardiovascular system could not transport sufficient oxygen to meet tissue demands. The blood carries carbon dioxide in physical solution, chemically combined to amino acids in blood proteins, and as bicarbonate ions.
TRANSPORT OF OXYGEN IN THE BLOOD
Transport of Physically Dissolved Oxygen
The solubility of oxygen in plasma is such that at a temperature of 37ºC, 1 ml of plasma contains 0.00003 ml of oxygen per mm Hg PO2. This corresponds to Henry’s law (see Appendix II for the gas laws). Oxygen probably dissolves in the fluid inside the red blood cells in about the same amount, so whole blood contains a similar amount of dissolved oxygen per milliliter.
Blood oxygen content is conventionally expressed in milliliters of oxygen per 100 ml of blood, which is called volume percent, so there is 0.003 ml of oxygen per mm Hg PO2 physically dissolved in 100 ml of whole blood. Thus, at an arterial PO2 of 100 mm Hg, there is only 0.3 ml of oxygen transported physically dissolved in the 100 ml of blood.
Chemically Combined with Hemoglobin
Hemoglobin has a molecular weight of about 65,000 daltons. The protein portion (globin) consists of four linked polypeptide chains, each of which is attached to a protoporphyrin (heme) group. Each heme group consists of four symmetrically arranged pyrrole groups with a ferrous (Fe++) iron atom at its center. The iron atom is bound to each of the pyrrole groups and to one of the four polypeptide chains. A sixth binding site on the ferrous iron atom can bind with oxygen (or carbon monoxide). Therefore, each of the four polypeptide chains is able to bind an oxygen (or carbon monoxide) molecule to the iron atom in its own heme group. Thus, the tetrameric hemoglobin molecule can combine chemically with four oxygen molecules.
Hemoglobin rapidly combines reversibly with oxygen. It is the reversibility of the reaction that allows oxygen to be released to the tissues. The reaction is extremely fast, with a half time of 0.01 s or less. Each gram of hemoglobin can theoretically combine with about 1.39 ml of oxygen under optimal conditions, but normally some hemoglobin may be in forms that cannot bind oxygen, such as methemoglobin (the iron atom is in the ferric [Fe+++] state) or combined with carbon monoxide. For this reason, the oxygen-carrying capacity of hemoglobin is conventionally considered to be 1.34 ml of oxygen per gram of hemoglobin. That is, each gram of hemoglobin, when fully saturated with oxygen, binds 1.34 ml of oxygen.