Page:Encyclopædia Britannica, Ninth Edition, v. 20.djvu/500

482 RESPIRATION after the first few years of life give off more CO 2 than females. When the external temperature is so low as to depress the body temperature, less C0 2 is given off ; if it is so high as to raise the body temperature, the CO., is increased. If, however, the surrounding medium is cooler than the body but not cold enough to lower the body temperature, more O is taken in, and more CO 2 is given out; and vice versa. Muscular exercise also increases considerably the C0 2 given off ; and more CO 2 is given off a short time after a meal than during fasting, especially when the meal includes substances rich in carbon. Speak- ing generally, alcohols, ethers, tea, &c., diminish the CO 2 ; but the results are not constant. Again, while the number and depth of the respirations do not influence the formation of C0 2 in the body, they affect the removal of that which is already formed. Increased rate of respira- tion and increased depth of respiration both cause an absolute increase in the quantity of CO 2 expired, although with reference to the total amount of air which passes into and out of the lungs during such laboured breathing the C0 2 is relatively diminished. Lastly, when the atmo- spheric pressure is diminished, as in ballooning, respira- tion becomes difficult, CO 2 is imperfectly removed from the body, and the blood contains less O. When pressure is increased, respiration is easy and slow (2-4 per minute), the capacity of the lungs increases, the activities of the tissues are marked, and as a result of this more O is absorbed and more CO 2 is excreted. Tlie Blood in Respiration. Having ascertained the nature and quantity of the materials exchanged in the lungs, we may now ask con- cerning the method of the exchange. There is no difficulty in understanding how the cold air introduced into the lungs is warmed and saturated with moisture by contact with the moist walls of the air passages and bronchi. As to the gaseous substances which appear in traces in the expired air nothing definite is known ; they may in part arise in the decomposition of the solid organic impurities of expired air, and may in part escaps from the blood itself. The origin of the solid poisonous organic substances, which are of such vital interest from a sanitary point of view, is also a matter of great obscurity. Some portions of it doubtless consist of effete particles of tissue from the walls of the bronchial tubes and cavity of the mouth and nose; for example, epithelial scales may be discovered in the condensed moisture of the air expired into an ice-cold globe, and so also may the organized bacilli of tubercle when the subject of experiment happens to be phthisical. Other portions may spring directly from the blood. The chief inquiry, however, centres about the origin of the carbon dioxide and the absorption of the oxygen, for the understanding of which a knowledge of the blood is necessary. The Blood. This, as it circulates in the vessels of man and vertebrates generally, is a viscous and to the naked eye homogeneous liquid of red colour, the blood of the pulmonary veins, of the left side of the heart, and of the systemic arteries being normally of a bright scarlet hue, and the blood of the right side of the heart, of the systemic veins, and of the pulmonary artery being of a brownish-red colour. In other words the blood enters the capillaries of the lungs of a brownish-red colour, and leaves them bright scarlet in hue ; it enters the general capillaries of the body as a bright scarlet fluid and leaves them as a brownish-red fluid. Although homogeneous to the naked eye, the blood is found on microscopic examina- tion to consist of a colourless fluid, called liquor sanguinis or plasma, holding in suspension large numbers of solid bodies, the corpuscles of the blood; the more numerous of these are red, the others are colourless and are commonly spoken of as white. The specific gravity of blood probably varies between 1045 and 1075. Fresh blood is feebly alkaline in reaction. When blood is shed it remains for a minute or two as fluid as it is in the blood-vessels ; but in 2-6 minutes it begins to pass into the state of a soft red jelly, which gradually acquires greater consistence, and by the contrac- tion of one of its constituents at length expresses a fluid ; the contracted jelly is called the clot, or crassamentum ; the expressed fluid is the serum ; and the whole process is denominated coagulation. Contraction of the clot may go on for 10-48 hours. This process of coagulation is due to the separation from the plasma of a body called fibrin, which entangles in its meshes the corpuscles of the blood. When coagulation is delayed for several minutes (as it always is in horse's blood, and as it usually is in the blood of men suffering from inflammatory diseases), the blood corpuscles, being specifically heavier than the plasma, have time to subside a little way before coagulation jcommences. Hence the uppermost layers of such blood become nearly free from coloured corpuscles ; and subsequently, when the blood coagulates, the clot exhibits the phenomenon of the buffy coat, that is, the upper part of the clot is of a yellowish colour. If, instead of allowing blood to coagulate undisturbed, it be stirred or whipped with twigs, the fibrin does not entangle the corpuscles but separates as a stringy mass which adheres to the twigs ; the corpuscles remain in the serum and constitute defibrinated blood. Coagula- tion is promoted (a) by exposure to a temperature slightly higher than that of the living body ; (b) by contact with foreign matter ; (c) by the addition of minute quantities of common salt or other neutral salts. It is delayed or suspended (a) by exposure to an ice-cold temperature ; (b} by contact with the living blood-vessels ; (c) by the addi- tion of a sufficient quantity of sodium chloride, sodium sulphate, or some other neutral salts. Reviewing all the facts which have been ascertained respecting coagulation, it would appear that the process is dependent upon the presence in the liquor sanguinis of a proteid body called fibrinogen, which under favourable- circumstances undergoes conversion, or perhaps decomposi- tion, into fibrin. This conversion, when it occurs outside the body, appears to be connected with the action of a ferment produced in the colourless corpuscles, and pro- bably only set free when they break down. The red corpuscles of man and the Mammalia gener- ally, except the Camelidse, are biconcave disks, possessing neither skin nor nucleus or interior body. In birds, reptiles, and most fishes they are nucleated, elliptical, and biconvex. In the camel the red corpuscles are oval. The average diameter of the disk in man is ^^7 inch and the thickness about Y^XTJTJ inch. The substance of which the red disks is composed is elastic ; the disks may therefore be squeezed through fine chinks smaller than their own diameter and may after- wards regain their original shape. When in the blood- vessels the red corpuscles are hurried along indiscrimately in the blood current, but when the blood stands the red corpuscles cling together in rows like piles of coin, which are technically called rouleaux; this is a physical, pheno- menon entirely due to the shape of the corpuscles. The white or colourless corpuscles are globular masses of granular protoplasm, provided with one or more interior bodies called nuclei, but destitute of a skin. They have the power of independent movement, which the red cells have not, resembling in this respect the amoaba. They can send out processes, change their outline, and move from place to place by laying hold on resistent objects by means of a projected process and dragging their bodies