Partial pressures of oxygen and carbon dioxide

As explained in the previous section, the PO2 of the atmosphere is 160 mmHg. The partial pressure of carbon dioxide (PCO2) is negligible (see Table 17.1). As air is inspired, it is warmed and humidified as it flows through the conducting airways. Therefore, water vapor is added to the gas mixture. This is accounted for in the calculation of PO2 in the conducting airways:

PO2 inspired air = 0.21 x (760 mmHg - 47 mmHg) = 150 mmHg

The partial pressure of the water vapor is 47 mmHg and, as a result, the PO2 is slightly decreased to 150 mmHg. The PCO2 remains at 0 mmHg. By the time the air reaches the alveoli, the PO2 has decreased to about 100 mmHg. The PO2 of the alveolar gas is determined by two processes:

• Rate of replenishment of oxygen by ventilation

• Rate of removal of oxygen by the pulmonary capillary blood

The primary determinant of alveolar PO2 is the rate of replenishment of oxygen by ventilation. As mentioned previously, hypoventilation causes a decrease in alveolar PO2. The rate of removal of oxygen by the pulmonary capillary blood is determined largely by the oxygen consumption of the tissues. As metabolic activity and oxygen consumption increase, the PO2 of the mixed venous blood decreases. As a result, the partial pressure gradient for oxygen between the alveoli and the blood is increased and the diffusion of oxygen is enhanced.

The PCO2 of the alveoli is about 40 mmHg and is also determined by two processes:

Rate of delivery of carbon dioxide to the lungs from the tissues Rate of elimination of carbon dioxide by ventilation

As cellular metabolism increases, the rate of production of carbon dioxide also increases. Typically, increased activity is associated with an increase in ventilation so that the increased amounts of carbon dioxide delivered to the lungs are eliminated. Hypoventilation impairs the elimination of carbon dioxide and causes an increase in alveolar PCO2.

Assuming that oxygen diffuses down its partial pressure gradient from the alveoli into the pulmonary capillary blood until equilibration is reached, the PO2 of this blood reaches 100 mmHg. This blood flows back to the left side of the heart and into the systemic circulation. Therefore, the PO2 of the arterial blood is 100 mmHg. Likewise, assuming that carbon dioxide diffuses down its partial pressure gradient from the pulmonary capillary blood into the alveoli until equilibration is reached, the PCO2 of the blood leaving these capillaries should be 40 mmHg. Therefore, the PCO2 of the arterial blood is 40 mmHg.

The arterial blood, which is high in oxygen and low in carbon dioxide, is delivered to the tissues. Within the tissues, oxygen is consumed by metabolism and carbon dioxide is produced. Under typical resting conditions, the PO2 of the tissues is 40 mmHg. Therefore, oxygen diffuses down its concentration gradient from the systemic capillary blood into the cells of the tissues until equilibration is reached. The PO2 of the venous blood leaving the tissues is also 40 mmHg. The PCO2 of the tissues is 45 mmHg. Therefore, carbon dioxide diffuses down its concentration gradient from the tissues into the blood until equilibration is reached. The PCO2 of the venous blood leaving the tissues is 45 mmHg.

The mixed venous blood, which is low in oxygen and high in carbon dioxide, flows back to the lungs to obtain oxygen and eliminate carbon dioxide. Note that the partial pressure gradient for oxygen between the alveoli (100 mmHg) and the mixed venous blood (40 mmHg) is 60 mmHg. The partial pressure gradient for carbon dioxide between the mixed venous blood (45 mmHg) and the alveoli (40 mmHg) is 5 mmHg. According to Fick's law of diffusion, the small partial pressure gradient for carbon dioxide would tend to reduce the exchange of this gas; however, its relatively high solubility and diffusion constant allow it to diffuse quite readily across the blood-gas interface.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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