Oxyhemoglobin is a bright red substance in red blood cells, formed when the protein hemoglobin binds with oxygen. Its primary function is to transport oxygen from the lungs to the body’s cells. This transport is required for cellular respiration, where cells use oxygen to produce the energy necessary for life.
The Formation of Oxyhemoglobin
Oxyhemoglobin forms within the high-oxygen environment of the lungs’ alveoli. Each hemoglobin molecule is comprised of four protein chains, and each chain holds an iron-containing structure called a heme group. The iron atom within each heme group binds to one oxygen molecule, meaning a single hemoglobin molecule can carry up to four oxygen molecules.
This process is known as “cooperative binding.” When the first oxygen molecule attaches to a heme group, it changes the hemoglobin molecule’s shape. This structural shift, from a “tense” (T) state to a “relaxed” (R) state, increases the affinity of the other three heme groups for oxygen. This makes it progressively easier for subsequent oxygen molecules to bind, allowing hemoglobin to become fully saturated before leaving the lungs.
Oxygen Release to the Tissues
When oxygen-rich blood reaches the body’s tissues, environmental changes prompt the release of oxygen from hemoglobin. In these tissues, cells consume oxygen for metabolism and produce carbon dioxide as a waste product. This results in lower oxygen and higher carbon dioxide concentrations compared to the lungs.
These conditions trigger the dissociation of oxygen from oxyhemoglobin. The increase in carbon dioxide in the blood leads to the formation of carbonic acid, which lowers the blood’s pH. This change in acidity weakens the bond between hemoglobin and oxygen, a phenomenon known as the Bohr effect. This effect facilitates the unloading of oxygen where it is most needed by active cells.
During intense physical activity, muscle cells may produce lactic acid, further increasing the blood’s acidity and enhancing the release of oxygen. The higher temperature in active tissues also contributes to weakening the oxygen-hemoglobin bond. This mechanism ensures that tissues receive a greater supply of oxygen in direct response to their metabolic demands.
The Oxyhemoglobin Dissociation Curve
The relationship between hemoglobin’s oxygen affinity and surrounding oxygen levels is shown by the oxyhemoglobin dissociation curve. This graph plots the percentage of hemoglobin saturated with oxygen against the partial pressure of oxygen (a measure of oxygen concentration). The curve’s S-shape is a direct result of cooperative binding.
The upper, flatter portion of the curve represents conditions in the lungs, where oxygen pressure is high. In this range, hemoglobin has a high affinity for oxygen, and large fluctuations in oxygen pressure cause only small changes in saturation. This plateau signifies that hemoglobin is nearly 100% saturated when it leaves the lungs.
Conversely, the steep section of the curve corresponds to the body’s tissues. Here, a small decrease in oxygen’s partial pressure leads to a large release of oxygen from hemoglobin. Factors like increased carbon dioxide, higher temperature, and lower pH cause the curve to shift to the right. This shift indicates a lower oxygen affinity, promoting greater oxygen release to active tissues.
Measuring Oxyhemoglobin Levels
Oxyhemoglobin levels can be measured non-invasively with a pulse oximeter. This small, clip-like device is typically attached to a fingertip or earlobe and shines two wavelengths of light, red and infrared, through the tissue. The measurement works because oxyhemoglobin and deoxyhemoglobin (hemoglobin without oxygen) absorb these light types differently.
Oxyhemoglobin absorbs more infrared light and allows more red light to pass through, while deoxyhemoglobin does the opposite. A sensor detects the light that passes through the tissue and calculates the ratio of absorption. This calculation provides the percentage of hemoglobin saturated with oxygen (SpO2), a value used to detect hypoxia, or low blood oxygen levels.
Other molecules can interfere with this process. Carbon monoxide binds to hemoglobin with an affinity 200 to 250 times greater than oxygen. When inhaled, it forms carboxyhemoglobin, reducing the blood’s oxygen-carrying capacity and leading to severe tissue hypoxia. A standard pulse oximeter cannot distinguish between oxyhemoglobin and carboxyhemoglobin, which causes falsely high readings in cases of carbon monoxide poisoning.