Medulla

^V-MIUAMMÎI/Vr

Irregular rate and depth of respiration

Figure 6-4

mechanisms to come into play and return the pH toward normal.

5) Poor pulmonary blood flow. It is relatively uncommon for a low arterial pC>2 to be purely the result of a primary failure of oxygen to diffuse across the alveolar membrane. Rather, a mismatched (air ventila-tion)/(blood perfusion) ratio, whether low (e.g. due to poor ventilation, pneumonia) or high (due to poor blood perfusion) are the most common causes of arterial hypoxemia.

CO2 diffuses much better than O2 between air and the blood because it is far more soluble than O2. Therefore, patients may develop oxygen diffusion problems, with anoxia, before they develop CO2 retention.

6) Inefficient cardiovascular perfusion of the tissues. This may be a primary pathological problem, as occurs in blood vessel disease or cardiac insufficiency, or a temporary manifestation of marked exercise. When oxygen supply cannot meet the tissue energy needs during marked exercise, the body switches to anaerobic metabolism, rather than the oxygen-dependent Krebs cycle. This is energy inefficient, however, and the individual is also limited by the accumulation of lactate and hydrogen ions in the muscle, with compensatory hyperventilation.

7) Problem at cell level of gas exchange (e.g., anemia, carbon monoxide poisoning).

Evaluation Of Pulmonary Function

Arterial blood gas analysis is an important tool in the evaluation of pulmonary function (see Fig. 3-6 and related text for discussion of metabolic versus respiratory acidosis and alkalosis). Proper interpretation of blood gases requires a knowledge of normal lab values for arterial pH, HCO3 , pC>2 and pC02. Normal arterial pH is about 7.35-7.45. Normal serum HC03 is about 24-28meq/L. Normal arterial pC>2 is about 95mm Hg (normal range 80-100mm Hg at sea level) and normal arterial pC02 is about 40mm Hg (normal range about 35—45mm Hg).

Fig. 6-5. The pC>2 and pC02 levels in the atmosphere and body. The pC>2 averages about 23mm Hg at the peripheral intracellular level, but not to worry—the cell needs a pC>2 of only about 2mm Hg for adequate functioning.

The pulmonary capillary pC>2 and pCOi equilibrate with alveolar pC>2 and pCOj after passing about 1/3 of the way through the pulmonary capillaries. Thus, even in exercise, where blood flow is increased, there is enough time for capillary O2 and CO2 to equilibrate with the alveolar air.

Although the pressure differences for CO2 diffusion (only 45/40 between veins and alveoli, and 46/45 between cells and capillaries) are much less than that for

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