By D. B'. PEARCE
(.t
pt from the 1939 winning thesis "Psysiolopu Aspects of Gravity")(a) EFFECT ON THE COMPONENTS OF THE SYSTEM.
1. The lungs, together, weigh about 1 kilogram. They are distended by atmos- - pheric and pulmonary artery pressure to completely fill the pleural sacs of the thorax.
The lower position of the diaphragm in the erect posture increases their capacity.
2. The thoracic cage, hinged back and front, tends to drop lower in the erect posture by reason of its weight. This necessitates a general increase in the tone of the elevators of the thoracic cage and a decrease of the depressors. In the normal respiratory movements in the erect posture the force of gravity assists the depres- sors and antagonises the elevators.
3. Vital Capacity.—The early workers showed this to be greater in the erect sitting than the recumbent position. Bohr (38) found the complemental air greater in the sitting or erect posture. Wilson (39) concluded that in a man of average vital capacity the reserve air may be reduced from 1550 cc. in the erect position to as little as 250 cc. in the recumbent posture. The loss of vital capacity in the recum- bent posture is due to two main factors.
(i) X-ray photographs show the upward displacement of the diaphragm by the weight of the abdominal viscera in the recumbent position and downward displace- ment in the erect position.
(ii) Hamilton and Morgan (40) considered the accumulation of blood in the lungs in the recumbent posture as important, and in cases of disease of the left ven- tricle pulmonary congestion on lying down often gives rise to breathlessness from this cause.
4. Paranasal Sinuses.—These open into the nasal cavity. Whereas in grazing quadruped animals the position of the opening is ideal for complete drainage, in man, with the head in the erect position, drainage of the maxillary and frontal sinuses is extremely difficult. This may partly account for the relatively frequent occurrence of diseases of the sinuses in man.
(b) EFFECT ON THE WHOLE SYSTEM.
1. Ventilation.—Tilting experiments by Hamilton Lichty and Pitts (5 ) showed a steady increase in ventilation, i.e., minute volume, as the .subject is tilted from the horizontal to the vertical position as shown by the spirometer. This is supported by recent measurements by Main (41 ) of the pH of the blood and alveolar CO, of students in the recumbent and standing posture. On standing there was a constant rise of 0.03 to 0.06 pH units, i.e., a tendency to alkalaemia. After standing for about twenty minutes partial return to the lying value had been effected. The alveolar CO ,
in standing showed a decrease of 0.5 per cent. to 1 per cent., which is not due to increased acidity of the blood since the acidity actually decreases. These pH change ,:
of the blood and changes in alveolar CO, indicate the increase of ventilation in the erect posture over that in the recumbent posture.
The nervous mechanism involved in this respiratory stimulation on standing is not yet clear. Main shows that it was not due to a rise of body temperature. He
suggests cerebral anoxaemia to be a causal factor. While the respiratory centre is certainly sensitive to changes in CO2 tension, it is much less sensitive to small changes in the oxygen tension of its own blood supply. In normal individuals the small decrease in cardiac output on standing and the degree of circulatory compen- sation normally attained would most probably be insufficient to cause a direct anoxaemia of the respiratory centre of sufficient degree to account for this rise of ventilation. Higher degrees of anoxaemia of the respiratory centre cause depression of the centre.
A much more feasible explanation involves the experimental observation by Hey- mans and Bouckaert (42) that in animal perfusion experiments the respiration was stimulated by a fall of pressure in the carotid sinus even if the cerebral circulation was unaltered. Conversely, a rise in the carotid sinus pressure reflexly inhibits the respiration. Section of the nerves abolishes this effect. As shown above, the arterial pressure at the brachial artery generally falls slightly, but may rise. At the level of the carotid sinus, therefore, the general rule will be fall of pressure in the erect posture with consequent respiratory stimulation. Although the carotid sinus is very sensitive to oxygen lack, and gives great respiratory stimulation in this condition, this chemical control is not involved in the increased ventilation since decreased cardiac output and increased ventilation would, if anything, give greater oxygen saturation of the blood in the erect posture.
2. Energy for respiration is greater in the erect than the recumbent posture.
In respiration the thoracic cage must be raised against gravity. In expiration, while gravity assists the descent of the thoracic cage, the abdominal muscles must by contracting raise the abdominal viscera as they follow the rising diaphram.
Since the gravitational attraction is the direct cause of variation of atmospheric pressure with height above sea level or increase of pressure in the sea with depth, we may now consider the physiological effects of low and high atmospheric pressures.
3. Low Atmospheric Pressures.—The symptoms produced differ according to whether the change to low pressures is rapid as in aeroplane ascents, or very gradual, as in mountain climbing.
In judging the physiological effects of these low pressures one must be careful to eliminate the complications produced by solar radiation and by cold.
(i) Rapid fall in atmospheric pressure. By the study of twenty healthy young men, first in a compression chamber, and secondly in a metabolism apparatus for oxygen experiments, Anthony, Atmer and Heits (43) found that up to 6000-7000 metres there were no essential differences as indicated by writing tests or hand or eye muscles. Up to this height oxygen lack is the only factor in mountain sickness.
When a person gets abol7e 12,000 feet the air pressure falls below 500 mm. Hg., and the symptoms begin to appear.
(a) The breathing is stimulated by oxygen lack, especially through the carotid sinus. The effect of oxygen lack is mainly to increase the rate of respira- tion. This increased ventilation leads to lowering of the alveolar and arterial CO2 levels giving alkalemia. This decreased CO2 level depresses breathing so that the respiration tends to be depressed. This gives rise to periodic breathing—first stimulation from oxygen lack followed by depres- sion from low CO2 level. In rapid ascents there is frequently breathless- ness and cyanosis.
(b) Nervous System.—Slight oxygen lack may produce apparent exhilaration
probably from paralysis or decreased activity of the higher association areas. Lower oxygen tensions rapidly lead to unconsciousness, and the breathing ceases from paralysis of the respiratory centre.
(c) Circulation.—Effect on blood pressure is a question of balance between the stimulation and depressant effects. Vaso-constrictor centre is stimulated when the oxygen saturation is decreased to about 75 per cent. This rise of pressure may be replaced by a fall. The diastolic pressure fall, the accumu- lation of metabolites in anoxaemic tissues, and the dilatation of muscle vessels by adrenalin have been suggested as the causes of the peripheral dilatation. The action on the heart is at first stimulation due to increased activity of cardiac accelerator centre from oxygen lack. The rate and force of the beat improved. Secretion of adrenalin may further augment the stimulation. Secondly, at higher altitudes there is depression partly due to increased vagus activity and partly due to the direct action on the heart. Unconsciousness has occurred long before this. Mateeff and Schwarz (44) found that symptoms of mountain sickness disappeared when the subjects lay down or had their legs bandaged. This lack of circulatory compensation is probably due to the peripheral dilatation mentioned above, as the vasoconstrictor centre is fairly resistant to oxygen lack.
(d) The digestive system may be upset, due to anoxaemia of the C.N.S. with the level of reflexes altered. Vomiting or mountain sickness may occur.
The administration of oxygen alleviates the symptoms up to the point where, with pure oxygen, the partial pressure is insufficient.
(ii) A gradual but more prolonged fall in atmospheric pressure allows of some compensation 'or acclimatisation.
(a) After some time the breathing is maintained at a higher level as the respiratory centre adapts itself to the lower CO2 level. Christensen (45)
1937 has pointed out that for each individual a constant relation is main- tained between the work done and the oxygen consumed, and he found that the ventilation, reduced to volume of dry air at N.T P., does not alter even at great heights provided the work done was the same.
(b) The alkalaemia is compensated by the excretion from the kidney of a more alkaline urine and the retention of non-volatile acids in the blood since the decreased carbonate ion concentration necessitates an increase of the other anions such as chloride and phosphate to maintain neutrality. The lower CO.4 level reduces the tendency of the haemoglobin to dissociate this, reducing the efficiency of the red blood cells.
(c) The reduced oxygen tension in the blood, and the decreased availability serve as a stimulus to the bone marrow to increased erythropoiesis (see later).
The new level of red blood cells and Hb. content depends on the a'titude.
At 17,500 ft. the proportion of unused haemoglobin may be 2/5th owing to the decreased availability although the concentration of oxyhaemoglobin in arterial blood may be greater than at sea level (Dill) (46). There is no change in the osmotic pressure of the blood. There is also an increase in vital capacity. Dill considers the limiting factors in exercise at high alti- tudes have to do with internal respiration and capacity of the heart. He states also that individual variations in adaptability to high altitudcz appear to be dependent on the respiratory centre.
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TO U RN II
Starling doubts whether any of the present human race by bringing into play all the compensatory factors could live permanently at 6000 m.
4. The Effects of High Atmospheric Pressures.—These high pressures occur mainly in deep sea diving and working in diving bells and caissons. For every 33 feet of sea water the pressure increases by on atmosphere owing to the gravitational attraction on the water. The liquids of the body transmit pressure equally in all directions, so that, provided the air breathed is at the same pressure as the water and provided the Eustachian tubes and sinuses have filled to the same pressure, there are no uncomfortable features of pressures of 6 to 7 atmospheres. Although the alveolar CO2 percentage is much lower, the partial pressure is found to be the same as in normal pressures. The partial pressures of oxygen and nitrogen are increased enormously. The increase in solubility of oxygen may enable the subject to do more work, but the increase of the solubility of nitrogen causes relatively larger quantities to dissolve in the tisues of the body, especially the fatty tissues, which for a given blood supply dissolve nitrogen at a greater rate than a like amount of other tissue.
The solubility of nitrogen in lipoids is five times as much as in water. The fatty tissues of the body are, however, relatively avascular, so that actually their rate of saturation is less than that of the other tissues. When the subject is suddenly brought back to normal pressure, the release of pressure liberates rela- tively large volumes of gas consisting of oxygen and nitrogen, especially from the fatty tissues, so that bubbles often form. The oxygen is soon used up by the tissues, but the nitrogen remains for a relatively long time. The bubbles cause blocking of the blood vessels, especially in the C.N.S., at the junction of the white and grey matter of the spinal cord. This gives rise to pain in the joints or other parts, paralysis of the legs, or, in severe cases, may cause death in a few minutes. Having once formed the bubbles in the fatty tissue, the rate of blood flow is not fast enough to dissolve the bubbles away quickly, so that recompression is necessary to prevent serious effects. This redissolves the bubbles. Owing to the slow rate of denatura- tion of fatty tissues, it is found that it is best, after recompression, to quickly reduce the pressure to half. This does not give bubble formation, but hastens the rate of uesaturation. When equilibrium has occurred, the pressure may again be halved rapidly. The change from 11 atmospheres to 1 atmosphere as the final stage gives no symptoms. The appreciation of these factors has greatly reduced the dangers of men working at high pressures under water.
References.
Starling—Physiology, 1936.
Wright—Applied Physiology, 1937.
Best and Taylor—Physiology.
5—Hamilton, Lichty, Pitts; Amer. J. Physiol, 100, 392.
38—Bohr; Deutsch. Arch. F. Klin. Med., 1907, 88, 385.
39—Wilson; J. Physiol; 1927, 64, 54.
40—Hamilton & Morgan; Amer. J. Physiol; .1932, 99, 526.
41—Main; Amer. J. Physiol; 1937, 118, 435.
42—Heymans & Boukaert; J. Physiol; 1930, 69, 8 p.
43—Anthony, Atmer & Heits; K lin. Woch; 1936, 15, 846.
44—Mateeff & Schwartz; Arch. ges. Physiol; 1935, 236, 77.
45—Christensen Skand; Arch. Phys.; 1937, 76, 88.
46—Dill; Life, Heat and Altitude; 1938.