Quantitative Features of the Oxygen-Hemoglobin Dissociation Curve The Oxygen-Hemoglobin dissociation curve is derived by quantifying the saturation of hemoglobin in blood as the partial pressure of oxygen in the blood is slowly raised. As seen, the curve is not linear, reflecting the unique biochemistry of hemoglobin, to which oxygen molecules bind cooperatively.
Normal Oxygen Transport The Oxygen-Hemoglobin Dissociation Curve allows for a quantitative appreciation of why oxygen loading and unloading occur at their respective locations.
In scenarios of intense exercise when cellular metabolism is greatly increased, the peripheral partial pressure of oxygen may fall to 20 mm Hg, resulting in even more significant quantities of oxygen unloading. In reality, the hemoglobin saturation falls even further in the peripheral tissues than described above due shifts in the oxygen-hemoglobin dissociation curve caused by the environment present in metabolically-active tissues.
Once blood returns to the higher oxygen tension environment of the pulmonary capillaries, oxygen is reloaded onto hemoglobin for another cycle of transport. It should be pointed out that the the total amount of oxygen transported depends not only on the changes in hemoglobin saturation but also on the amount of hemoglobin present in blood.
If the amount of hemoglobin in the blood is low, as might occur in anemia , even normal changes in its saturation profile between pulmonary and peripheral capillaries may not provide sufficient transport of oxygen. Buffering of Oxygen Transport A special feature of the oxygen-hemoglobin dissociation curve is its tendency to buffer oxygen transport against significant drops in the pulmonary capillary oxygen tension.
This is an important feature given that reduced pulmonary capillary oxygen tension is a common consequence of a large variety of pathologies along with breathing at High Altitude.
The basis for this buffering is the flattening of the dissociation curve beyond oxygen partial pressures of 80 mm Hg. Because of this plateau, there is little significant difference in hemoglobin saturation even if pulmonary capillary oxygen tension falls from its normal mm Hg to 80 mm Hg. Consequently, hypoxemia is clinically defined as arterial oxygen levels below that of 80 mm Hg, the threshold at which hemoglobin saturation truly begins to decline. This same plateau also explains why delivering high oxygen tension air to a healthy individual does little to improve oxygen transport.
Buffering of Hemoglobin Saturation The sigmoid shape of the oxygen-hemoglobin saturation curve allows for a natural buffering mechanism against hypoxemia and aids in oxygen delivery to peripheral tissues. As seen, a 10 mm Hg drop in oxygen tension at the right of the curve results in a negligible change in hemoglobin saturation.
Consequently, small drops in lung function do not yield major declines in tissue oxygenation. However, the same 10 mm Hg drop in oxygen tension toward the middle of the curve yields a large decline in hemoglobin oxygen saturation. This allows for a large amount of oxygen unloading when blood reaches peripheral cells. It should be noted that these factors do not change the basic sigmoid shape of the curve but rather shift the curve to the left and right.
Consequently, these factors will change the hemoglobin saturation of blood for the same partial pressure of oxygen. In general, modulation of the dissociation curve occurs in such a way that oxygen unloading by hemoglobin is enhanced in metabolically active tissues.
An easy way to remember these factors and their effect on the dissociation curve is to note that metabolically-active peripheral tissues typically display higher temperatures, higher carbon dioxide tensions, and lower pH. The presence of HbF and carbon monoxide CO shift the curve to the left, increasing the oxygen affinity of hemoglobin. Copyright by Pathway Medicine Terms of Use. Oxygen is loaded in blood in the pulmonary capillaries where the oxygen tension is mm Hg as a result of alveolar ventilation.
The Oxygen-Hemoglobin dissociation curve is derived by quantifying the saturation of hemoglobin in blood as the partial pressure of oxygen in the blood is slowly raised. Buffering of Hemoglobin Saturation.
The sigmoid shape of the oxygen-hemoglobin saturation curve allows for a natural buffering mechanism against hypoxemia and aids in oxygen delivery to peripheral tissues.
Modulation of the Oxygen-Hemoglobin Dissociation Curve. Oxygen dissociation curve : The oxygen dissociation curve demonstrates that as the partial pressure of oxygen increases, more oxygen binds hemoglobin. However, the affinity of hemoglobin for oxygen may shift to the left or the right depending on environmental conditions. The oxygen-carrying capacity of hemoglobin determines how much oxygen is carried in the blood. In addition, other environmental factors and diseases can also affect oxygen-carrying capacity and delivery; the same is true for carbon dioxide levels, blood pH, and body temperature.
The increase in carbon dioxide and subsequent decrease in pH reduce the affinity of hemoglobin for oxygen. The oxygen dissociates from the Hb molecule, shifting the oxygen dissociation curve to the right. Therefore, more oxygen is needed to reach the same hemoglobin saturation level as when the pH was higher. A similar shift in the curve also results from an increase in body temperature. Increased temperature, such as from increased activity of skeletal muscle, causes the affinity of hemoglobin for oxygen to be reduced.
In sickle cell anemia, the shape of the red blood cell is crescent-shaped, elongated, and stiffened, reducing its ability to deliver oxygen. In this form, red blood cells cannot pass through the capillaries.
This is painful when it occurs. Thalassemia is a rare genetic disease caused by a defect in either the alpha or the beta subunit of Hb. Patients with thalassemia produce a high number of red blood cells, but these cells have lower-than-normal levels of hemoglobin.
Therefore, the oxygen-carrying capacity is diminished. Sickle cell anemia : Individuals with sickle cell anemia have crescent-shaped red blood cells. Diseases such as this one cause a decreased ability in oxygen delivery throughout the body. Dissolution, hemoglobin binding, and the bicarbonate buffer system are ways in which carbon dioxide is transported throughout the body.
Carbon dioxide molecules are transported in the blood from body tissues to the lungs by one of three methods:. Several properties of carbon dioxide in the blood affect its transport.
First, carbon dioxide is more soluble in blood than is oxygen. About 5 to 7 percent of all carbon dioxide is dissolved in the plasma. Second, carbon dioxide can bind to plasma proteins or can enter red blood cells and bind to hemoglobin.
This form transports about 10 percent of the carbon dioxide. When carbon dioxide binds to hemoglobin, a molecule called carbaminohemoglobin is formed. Binding of carbon dioxide to hemoglobin is reversible. Therefore, when it reaches the lungs, the carbon dioxide can freely dissociate from the hemoglobin and be expelled from the body. Third, the majority of carbon dioxide molecules 85 percent are carried as part of the bicarbonate buffer system.
In this system, carbon dioxide diffuses into the red blood cells. Carbonic anhydrase CA within the red blood cells quickly converts the carbon dioxide into carbonic acid H 2 CO 3. Since carbon dioxide is quickly converted into bicarbonate ions, this reaction allows for the continued uptake of carbon dioxide into the blood, down its concentration gradient. The newly-synthesized bicarbonate ion is transported out of the red blood cell into the liquid component of the blood in exchange for a chloride ion Cl- ; this is called the chloride shift.
When the blood reaches the lungs, the bicarbonate ion is transported back into the red blood cell in exchange for the chloride ion. This produces the carbonic acid intermediate, which is converted back into carbon dioxide through the enzymatic action of CA. The carbon dioxide produced is expelled through the lungs during exhalation. This is important because it takes only a small change in the overall pH of the body for severe injury or death to result.
The presence of this bicarbonate buffer system also allows for people to travel and live at high altitudes. When the partial pressure of oxygen and carbon dioxide change at high altitudes, the bicarbonate buffer system adjusts to regulate carbon dioxide while maintaining the correct pH in the body. While carbon dioxide can readily associate and dissociate from hemoglobin, other molecules, such as carbon monoxide CO , cannot. Carbon monoxide has a greater affinity for hemoglobin than does oxygen.
Therefore, when carbon monoxide is present, it binds to hemoglobin preferentially over oxygen. As a result, oxygen cannot bind to hemoglobin, so very little oxygen is transported throughout the body.
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