Oxygen Uptake Along the Pulmonary Capillary

Let us take a closer look at the uptake of Oj by blood as it moves through a pulmonary capillary. Figure 3-3A shows that the Pq2 in a red blood cell entering the capillary is normally about 40 mm Hg. Across the blood-gas barrier, only 0.3 p.111 away, is the alveolar Pq. of 100 mm Hg. Oxygen floods down this large pressure gradient, and the Po, in the red cell rapidly rises indeed, as we have seen, it very nearly reaches the Pq2 of alveolar gas by the time the red cell is only one-third of t-...

Acid Base Status

Because the concentration of carbonic acid is proportional to the concentrar 3 of dissolved carbon dioxide, we can change the constant and write Because pH is the negative logarithm Because CO2 obeys Henry's law, rhe CO2 concentration (in mmol liter) can be replaced by (Pco2 x 0.03). The equation then becomes The value of pK is 6.1, and the normal HCO1 concentration in arterial blo c 24 mmol Uter. Substituting gives Note that as long as the ratio of bicarbonate concentration to (Pco5 X C l...

High Altitude

The barometric pressure decreases with distance above the earth's surface in an approximately exponential manner (Figure 9-2). The pressure at 5800 m (19,000 ft) is only one-half the normal 760 mm Hg, so the Po2 of moist inspired gas is (380 - 47) X 0.2093 70 mm Hg (47 mm Hg is the partial pressure of water vapor at body temperature). At the summit of Mount Everest (altitude 8848 m or 29,028 ft), the inspired Po2 is only 43 mm Hg. At 19,200 m (63,000 ft), the barometric pressure is 47 mm Hg so...

Cause of Regional Differences in Ventilation

We saw in Figure 2-7 that the lower regions of the lung ventilate more than do the upper zones, and this is a convenient place to discuss the cause of these topographical differences. It has been shown that the intrapleural pressure is less negative at the bottom than the top of the lung (Figure 7-8). The reason for this is the weight of the lung. Anything that is supported requires a larger pressure below it than above it to balance the downward-acting weight forces, and the lung, which is...

Toxicity

The usual problem is getting enough Ot into the txxly, but it is possible to have too much. When high concentrations of O2 are breathed for many hours, damage to the lung may occur, if guinea pigs are placed in 100 O2 at atmospheric pressure for 48 hours, they develop pulmonary edema. The first pathological changes are seen in the endothelial cells of the pulmonary capillaries (see Figure 1-1). It is (perhaps fortunately) difficult to administer very high concentrations of Cb to patients, but...

Other Functions of the Pulmonary Circulation

The chief function of the pulmonary circulation is to move blood to and from the blood-gas barrier so that gas exchange can occur. However, it has other important functions. One is to act as a reservoir for blood. We saw that the lung has a remarkable ability to reduce its pulmonary vascular resistance as its vascular pressures are raised through the mechanisms of recruitment and distension (Figure 4-5). The same mechanisms allow the lung to increase its blood volume with relatively small rises...

Causes of Uneven Ventilation

The cause of the regional differences in ventilation in the lung was discussed on p. 103. Apart from these topographical differences, there is some additional inequality of ventilation at any given vertical level in the normal lung, and this is exaggerated in many diseases. One mechanism of uneven ventilation is shown in Figure 7-19. If we regard a lung unit (Figure 2-1) as an elastic chamber connected to the atmosphere by a tube, the amount of ventilation depends on the compliance of the...

Perspective on Tests of Pulmonary Function

In this chapter, we have touched on some of the lung function tests that are presently available. In conclusion, it should be emphasized that not all these tests are commonly used in a hospital pulmonary function laboratory. Only a few can be used in a doctor's office or on an epidemiological survey. The most useful and simple test in the clinical setting is the forced expiration. It does not matter much which indices are derived from this test, bur the FEV1.3 and FVC are very frequently...

Ventilation

A very useful and simple test of pulmonary function is the measurement of a single forced expiration. Figure 10-1 shows the spirometer record obtained when a subject inspires maximally and then exhales as hard and as completely as he or she can. The volume exhaled in the first second is called the forced expiratory volume, or FEVj.o, and the total volume exhaled is the forced vital capacity, or FVC (this is often slightly less than the vital capacity measured on a slow exhalation as in Figure...

Elastic Properties of the Chest Wall

Just as the lung is elastic, so is the thoracic cage. This can be illustrated by putting air into the intrapleural space (pneumothorax). Figure 7-10 shows that the normal pressure outside the lung is subatmospheric just as it is in the jar of Figure 7-3. When air is introduced into the intrapleural space, raising the pressure to atmospheric, the lung collapses inward, and the chest wall springs outward. This Figure 7-10. The tendency of the lung to recoil to its deflated volume is balanced by...

Abnormal Patterns of Breathing

Subjects with severe hypoxemia often exhibit a striking pattern of periodic breathing known as Chew-Srokes respiration. This is characterized by periods of apnea of 10 to 20 sec, separated by approximately equal periods of hyperventilation when the tidal volume gradually waxes and then wanes. This pattern is frequently seen at high altitude, especially at night during sleep. It is also found in some patients with severe heart disease or brain damage. The pattern can be reproduced in...

Blood Tissue Gas Exchange

O2 and CO move between the systemic capillary blood and the tissue cells by simple diffusion, just as they move between the capillary blood and alveolar gas in the lung. We saw in Chapter 3 that the rate of transfer of gas through a tissue sheet is proportional to the tissue area and the difference in gas partial pressure between the two sides, and inversely proportional to the thickness. The thickness of the blood-gas barrier is less than 0.5 p,m, but the distance between open capillaries in...

Sensors

A chemoreceptor is a receptor that responds ro a change in the chemical composition of the blood or other fluid around it. The most important receptors involved in the minute-by-minute control of ventilation are those situated near the ventral surface of the medulla in the vicinity of the exit of the 9th and 10th nerves. In animals, local application of H+ or dissolved CO2 to this area stimulates breathing within a few seconds. At one time, it was thought that the medullary respiratory center...

Effectors

The muscles of respiration include the diaphragm, intercostal muscles, abdominal muscles, and accessory muscles such as the sternomastoids. The actions of these were described at the beginning of Chapter 7. In the context of the control of ventilation, it is crucially important that these various muscle groups work in a coordinated manner, and this is the responsibility of the central controller. There is evidence that some newborn children, particularly those that are premature, have...

Central Controller

The normal automatic process of breathing originates in impulses that come from the brainstem. The cortex can override these centers if voluntary control is desired. Additional input from other parts of the brain occurs under certain conditions. The periodic nature of inspiration and expiration is controlled by neurons located in the pons and medulla. These have been designated respiratory centers. However, these should not be thought of as discrete nuclei but rather as somewhat poorly defined...

Work of Breathing

Work is required to move the lung and chest wall. In this context, it is most convenient to measure work as pressure X volume. This can be illustrated on a pressure-volume curve Figure 7-20 . During inspiration, the intrapleural pressure follows the curve ABC, and the work done on the iung is given by the area OABCDO. Of this, the trapezoid OAECDO represents the work required to overcome the elastic forces, and the hatched area ABCEA represents the work overcoming viscous airway and tissue...

Table 41 Fate of Substances in the Pulmonary Circulation

Angiotensin I Angiotensin II Vasopressin Bradykinin Converted to angiotensin II by ACE Unaffected Amines Serotonin Norepinephrine Histamine Dopamine Almost completely removed Up to 30 removed Not affected Not affected Arachidonic acid metabolites Prostaglandin E2 and F2tt Prostaglandin A2 Prostacyclin PGU Leukotrienes Almost completely removed Not affected Not affected The only known example of biological activation by passage through the pulmonary circulation is the conversion of the...