Question

Blood at a pH of \(7.20\) has what percent oxyhemoglobin saturation at a \(\mathrm{P}_{\mathrm{O}_{2}}\) of (a) \(70 \mathrm{mmHg}\); and (b) \(20 \mathrm{mmHg}\) ?

Short answer

Using the standard oxyhemoglobin dissociation curve, we first estimate the percent saturation for the given \(P_{O_2}\) values of (a) 70 mmHg and (b) 20 mmHg. Next, we account for the Bohr effect by adjusting the curve for the given blood pH of 7.20. Calculating the adjusted P50 value for each \(P_{O_2}\) and using this new P50 value, we find the adjusted percent saturation of oxyhemoglobin. The percent oxyhemoglobin saturation for (a) 70 mmHg and (b) 20 mmHg will be lower than the values estimated from the standard curve, due to the lower blood pH. The precise numerical values will depend on the specific dissociation curve, Bohr constant, and reference pH provided in the problem statement or textbook.

Step-by-step solution

Understand the Oxyhemoglobin Dissociation Curve

The oxyhemoglobin dissociation curve is the relationship between the partial pressure of oxygen (PO2) and the percent saturation of hemoglobin with oxygen. The curve is sigmoidal in shape, reflecting the cooperative binding of oxygen to hemoglobin. At high PO2 levels, hemoglobin is almost fully saturated with oxygen, while at low PO2 levels, it releases a significant amount of bound oxygen.

Estimate Oxyhemoglobin Saturation Percentage

For each given PO2 value, find the corresponding percent saturation on the oxyhemoglobin dissociation curve. Since the curve is not linear, you may need to use an oxyhemoglobin dissociation curve chart or a mathematical equation derived from the curve. In this case, PO2 values are: (a) 70 mmHg and (b) 20 mmHg.

Consider the Bohr Effect

The Bohr effect states that a decrease in blood pH (increase in acidity) causes a decrease in the affinity of hemoglobin for oxygen and a shift of the oxyhemoglobin dissociation curve to the right. Since the given pH value is 7.20, which is lower than the normal pH value of 7.40, the percent saturation of oxyhemoglobin will be lower than estimated from the standard curve. To account for the pH effect, use the following relationship: \[ \Delta{}Log_{10}P_{50} = K \times \Delta{}pH \] where P50 is the oxygen partial pressure at which hemoglobin is 50% saturated, \( \Delta{}Log_{10}P_{50} \) is the change in LogP50, K is the Bohr constant, and \( \Delta{}pH \) is the difference between actual pH and reference pH.

Calculate the Adjusted Percent Saturation

Using the relationship from the Bohr effect, calculate the adjusted P50 value for each PO2 and the given pH value. Then, use the new P50 value to find the adjusted percent saturation of oxyhemoglobin. Report the percent saturation for both (a) 70 mmHg and (b) 20 mmHg. Note: The numerical values required to solve this problem like the standard oxyhemoglobin dissociation curve, exact constant, and the reference pH are generally given in specific textbooks or provided in the problem statement.

Key concepts

Bohr Effect

The Bohr Effect is a physiological phenomenon where changes in blood pH levels influence the oxygen-binding affinity of hemoglobin. When the body's environment becomes more acidic, as indicated by a lower pH, it decreases hemoglobin's ability to bind oxygen. This results in the oxyhemoglobin dissociation curve shifting to the right. This rightward shift shows that hemoglobin is more likely to release oxygen to the tissues, which is crucial in conditions where active tissues require more oxygen.

This effect is primarily due to changes in the shape of hemoglobin molecules as pH alters. Lower pH increases hydrogen ions which bind to hemoglobin, changing its structure and reducing its affinity for oxygen. So, understanding the Bohr Effect aids in analyzing how efficiently oxygen is released from blood cells to tissues under different pH conditions.

pH Impact on Oxygen Saturation

pH levels significantly impact oxygen saturation in hemoglobin. Blood pH can influence how readily hemoglobin picks up and releases oxygen. At a normal body pH of around 7.4, hemoglobin tends to have optimal oxygen binding. But as pH drops below this level, indicating an acidic environment (pH 7.2 for instance, as given in the problem), hemoglobin's affinity for oxygen decreases, causing more oxygen to be released to the tissues.

This decrease in affinity, associated with a rightward shift in the saturation curve, is crucial when considering the body's response to various physiological conditions, such as exercise or acidosis. During such activities, tissues might demand more oxygen, and a lower pH assists by facilitating oxygen release. Therefore, pH levels can act as an adjustment tool for the body to manage oxygen delivery.

Hemoglobin Affinity for Oxygen

Hemoglobin affinity for oxygen represents the strength of the bond between oxygen and hemoglobin. This affinity is crucial for oxygen transport and delivery throughout the body. Stronger affinity means hemoglobin holds onto oxygen more tightly, whereas weaker affinity allows oxygen to be released more readily. Various factors, including pH (Bohr Effect) and temperature, influence this affinity.

A normal scenario would have hemoglobin efficiently taking up oxygen in the lungs where oxygen partial pressure is high and releasing it in the tissues where oxygen levels are lower. However, during conditions such as a decrease in pH, hemoglobin changes its affinity. This ensures that at sites requiring more oxygen, hemoglobin efficiently transfers oxygen to the tissues. Thus, hemoglobin's adaptability in changing conditions underscores its role in maintaining physiological balance and responding to the body's needs.