Oxygenated blood has two means of ferrying oxygen to the tissues of the body: dissolved in the blood plasma or attached to the hemoglobin within red blood cells. Hemoglobin-oxygen combinations typically account for about 98.5 percent of the oxygen transported from the lungs throughout the body. Hemoglobin saturation refers to the extent to which hemoglobin is loaded with oxygen molecules.
Four polypeptide chains, each bound to an iron-containing heme group, constitute the oxygen-carrying hemoglobin. The iron atoms can bind to oxygen. One hemoglobin can bind with up to four oxygen molecules. This combination of hemoglobin and oxygen is rapid and reversible — meaning, the hemoglobin can unload the oxygen molecules as well as load them.
When all four heme groups have attached to an oxygen molecule, the hemoglobin is fully saturated. If one, two, or three heme groups are bound to oxygen, the hemoglobin is partially saturated. The hemoglobin-oxygen combination is called oxyhemoglobin, while hemoglobin that has released its oxygen molecules is referred to as either reduced hemoglobin or deoxyhemoglobin.
The binding strength of the iron to the oxygen depends upon the level of hemoglobin saturation. Once the first oxygen molecule attaches to the iron, the hemoglobin itself changes shape. As a result, it more easily picks up the subsequent two oxygen molecules. Uptake of the fourth oxygen molecule is even easier. Similarly, as the hemoglobin releases each oxygen molecule, the strength of the bond between the iron and the remaining oxygen molecules grows progressively weaker.
Generally, hemoglobin saturation varies depending on the needs of the body at the time. Factors including temperature, blood pH, and partial pressures of oxygen and carbon dioxide can all influence the rate at which hemoglobin binds or releases oxygen molecules. These factors work together to maintain sufficient delivery of oxygen to the tissues of the body.
Hemoglobin saturation changes as the partial pressure of oxygen (PO2) in the blood changes. The relationship between the partial pressure of oxygen and the saturation of hemoglobin is non-linear; instead, it follows an S-shaped curved. Hemoglobin is almost completely saturated when P02 is at 70 mm Hg.
At typical resting conditions, P02 is at 100 mm Hg, and arterial blood hemoglobin saturation is at about 98 percent. As blood flows from the arteries through the systematic capillaries, hemoglobin releases about 5 ml of oxygen per 100 ml of blood, thereby resulting in a hemoglobin saturation of about 75% percent. P02 can drop to as low as 15 mm Hg during vigorous activities such as exercise. In response, hemoglobin will unload an addition 50 percent of its oxygen, thereby resulting in a saturation of as low as 25 percent.
Temperature, blood pH, and partial pressure of carbon dioxide influence hemoglobin saturation by altering hemoglobin's three-dimensional structure, thus changing its affinity for oxygen. Generally, an increase in any one of these factors will lower hemoglobin's affinity for oxygen, thereby spurring the hemoglobin to release more oxygen into the blood. Conversely, a decrease in one of these factors will usually strengthen the bond between hemoglobin and oxygen, thus decreasing the rate of oxygen unloading. Since heat, declining blood pH, and rising levels of carbon dioxide are all by-products of active tissues hard at work in the body, these factors ensure that oxygen is unloaded where it is needed most.