• Fitness & Health
• Sport & Exercise Science
• Physical Education
• Strength & Conditioning
• Sports Medicine
• Sport Management
• Dance

The oxyhemoglobin dissociation curve in pulmonary ventilation

This is an excerpt from Exercise Physiology-6th Edition by George A. Brooks,Thomas D. Fahey,Kenneth M. Baldwin.

At sea level altitude we ventilate to keep the partial pressure of O₂ in the arterial blood (PaO₂) at about 100 mmHg. This means that we need to ventilate to keep the partial pressure of O₂ in the alveoli (PAO₂) at 110 to 150 mmHg. From the alveoli, the diffusion distance for O₂ into erythrocytes (red blood cells) in the blood perfusing the alveolar walls is relatively short and almost impossible to draw at the scale used (figure 11.1). The short distance is necessary because the solubility of O₂ in body water at 37 °C is low; only 0.3 mL O₂ · dL⁻¹ blood is physically dissolved. Fortunately, erythrocytes contain the heme iron compound hemoglobin, which can bind O₂ according to its partial pressure (figure 11.2). At an O₂ partial pressure of 100 mmHg, which exists in alveolar capillaries at sea level, hemoglobin is nearly 100% saturated with O₂. Because a very small percentage of blood passing through the lungs passes through alveoli that are not ventilated, the saturation (S) of blood with oxygen returning to the left heart from the lungs is about 96% to 98%. This impressive saturation is maintained not only in the individual resting at a sea level altitude but also during maximal exercise.

Figures 11.2 and 11.3 are typically described as the oxyhemoglobin dissociation curves. Again, what it tells us is that the saturation of hemoglobin depends mainly on the partial pressure of oxygen. From that realization, we might more correctly refer to figures 11.2 and 11.3 as the oxyhemoglobin association-dissociation curves. Data points to develop the curve were acquired under conditions where oxyhemoglobin O₂ saturation was measured at specific partial pressures of oxygen (PO₂) ranging from the arterial regulatory PO₂ value of 100 torr (PaO₂), down to zero PO₂. Perhaps the best way to think about the curve is to imagine an erythrocyte (red blood cell, RBC) in a pulmonary capillary of a well-ventilated alveolus at sea level, where PAO₂ approximates 110 to 115 mmHg and PaO₂ approximates 100 torr. There in the lung, hemoglobin and O₂ associate, the result being fully, or near fully, O₂-saturated oxyhemoglobin for systemic O₂ delivery. Then imagine the erythrocyte transported via blood flow and entering the capillary bed of a working muscle where PO₂ is much lower and temperature, H⁺, and CO₂ concentrations are higher. There, oxygen and hemoglobin need to disassociate, making O₂ available for cell respiration in the mitochondrial reticulum while at the same time participating in CO₂ transport and H⁺ buffering as blood returns full circuit to the pulmonary capillary bed.

Quantitatively, normal human hemoglobin can bind 1.34 mL of O₂ · g⁻¹. In the average male, blood hemoglobin is about 15 g · dL⁻¹ blood (150 g · L blood). With an arterial saturation (SaO₂) of close to 100%, the arterial O₂ content (CaO₂) is then equal to the sum of the dissolved O₂ plus that combined with hemoglobin as seen in equation 11.2. By convention, this figure can also be referred to as 20.4 vol %, or 20.4 mL O₂ · 100 mL⁻¹. Note that 1 dL = 100 mL, and vol % = vol · 100 mL⁻¹ blood, or 200 to 204 mL O₂ · L blood.

In women, where the blood hemoglobin concentration is less than that in men (about 13 g · dL⁻¹ in females), the CaO₂ approximates 17.7 vol %.

To summarize the relationships in pulmonary ventilation, we state the following for healthy young people at sea level conditions. Appropriate ventilation of the alveoli allows the alveolar partial pressure of oxygen (i.e., the PAO₂) to remain at resting levels (about 100 mmHg), or to increase during exercise. The partial pressure of oxygen in the pulmonary vein (and, hence, in arterial blood, PaO₂) is thus about 100 mmHg. Assuming normal hemoglobin and blood hemoglobin content, PaO₂ of 100 mmHg results in an arterial saturation (SaO₂) of close to 100%. At close to complete saturation, arterial blood has an oxygen content (CaO₂) of about 20 mL · dL⁻¹, 200 mL · L⁻¹ blood, or one-fifth of a liter of O₂ in a liter of blood. Consequently, it takes a cardiac output of 5 L · min⁻¹ to transport 1.0 L of O₂ [(200 mL O₂ · L⁻¹) (5 L blood) = 1,000 mL O₂ · min⁻¹]. Because at sea level PaO₂ is maintained at about 100 mmHg up to maximum effort, it is generally believed that in healthy young people ventilation does not limit oxygen transport during exercise at sea level altitudes.

More Excerpts From Exercise Physiology-6th Edition

---