Gaseous Exchange

GASEOUS EXCHANGE The extent of gaseous exchange in man varies greatly with the size and age of the person, the degree of activity, etc.

The absolute minimum of oxygen consumption for any person is known as the basal metabolism and is that which takes place when the person is at rest in bed some ten hours after a meal. The basal metabolism in persons of different size, but otherwise comparable, varies not proportionately to the weight but to the body surface (see ANIMAL HEAT) and is therefore expressed in calories (i.e., in units of heat produced) or in oxygen consumed per square metre of body surface per hour.

The basal metabolism varies with

age, thus: blood into the alveolar air.

Transport of the

Oxygen.—Oxygen constitutes about of the moist air which fills the pulmonary alveoli and therefore exerts a pressure of a little over ioo mm. of mercury. In order that blood exposed to so low a pressure should carry away any considerable quantity of the gas, it must needs be a fluid possessing very special qualities. These qualities blood owes to the red pigment in the corpuscles—haemoglobin. This material can unite with large quantities of oxygen at a pressure not exceeding ioo mm. of mercury and so leaves the lung charged with oxygen. But what is more remarkable—haemoglobin gives up this oxygen again when exposed to lower oxygen pressures. Thus, at about 3o mm. of mercury the blood surrenders one-half of the oxygen united to it; at 20 mm. it gives up about three-quarters and so on. The curve which represents the relation between the quantity of oxygen united with the haemoglobin in blood, and the pressure of oxygen to which the blood is exposed, is called the oxygen dissociation curve.

There is one synthetic substance which attaches oxygen to itself if exposed to more than a certain critical pressure of that gas and from which the oxygen escapes if the oxygen pressure drops below the critical point—that substance which has recently been discovered by Prof. Moureu is rubrine, a complicated hydro carbon. Though rubrine and haemoglobin seem to unite with oxygen after a manner not wholly dissimilar and, so far as other oxides are concerned, unique, they are otherwise not at all alike. Rubrine consists of three benzine rings united in a way not at present ascertained. Haemoglobin consists of a protein united with haematin, a substance which contains iron on the one hand and four pyrrol rings on the other. Moreover, it is clear from spectroscopic and other evidence, that the attachment of the oxygen to the haemoglobin has some relation both to the iron and to the pyrrol constitution of the substance. The constitution and properties of haemoglobin are of great theoretical importance because, but for it, the warm blooded animal could never have developed the high degree of vitality which he possesses. Oxy The reason is as follows : The capacity for heat loss depends upon the superficial area of the animal. The heat production naturally

must equal the heat loss; therefore the heat production must also vary with the superficial area, i.e., in some way proportionally to the square of the linear dimensions on the animal. The weight, however, bears in proportion not to the square but to the cube; therefore, as the animal gets larger its area becomes relatively less proportionally to its weight, i.e., the heat production, and there fore the degree of oxidation per gram of animal diminishes as the size increases. If the amount of oxygen required per gram of animal diminishes, the whole mechanism for its supply and there fore the total ventilation will diminish correspondingly.

The exchange of gases in the lung is regarded by almost all authorities as being a process of diffusion, the oxygen diffusing from the alveolar air through the pulmonary epithelium into the blood which circulates through the capillaries in the alveolar wall. (The most notable opponents of this view are Dr. J. S. Haldane, F.R.S., and some of those who have been associated with him. His views are fully set forth in his book Respiration.) The carbon dioxide likewise is regarded as passing by diffusion out of the gen is so insoluble in water that, apart from haemoglobin, blood could only carry to the tissues about one-sixtieth part of the quantity of oxygen which it does ; therefore to main tain oxidation in the tissues at its normal level 6o times as much blood would have to circulate as at present. As the blood already forms one-fifteenth of the weight of the body, without haemo globin it must needs form four times the weight of the rest of the body—an impossible burden. There are in the lower forms of life some respiratory pigments not altogether dissimilar from haemo globin. In the blood of some worms, for instance, is found a ma terial chlorocronorin, which is really a form of haemoglobin, but possessed of a somewhat different scaffold of porphyrin. In some of the molluscs is found haemocyanin, also a protein body containing not iron but copper, and which, moreover, contains no porphyrin. These bodies like haemoglobin possess the power of condensing, transporting and yielding oxygen under suitable con ditions. The exact affinity of haemoglobin for oxygen is an ex ample of nice adjustment of the conditions under which the haemoglobin is found to the needs of the body. The affinity varies according to the saline concentration of the medium in which the haemoglobin is dissolved, according to the hydrogen ion concen tration and according to the temperature. Moreover, there ap pears to be an assortment of haemoglobins specific to the forms of life in which they are found, and which have affinities for oxygen suitable to those forms of life.