In cases of drowning, when the lungs are filled with water, the resistance to the passage of blood through their capillary vessels is greater than it is when they are still filled with air. The heart is sooner beaten in its effort to drive the blood through them. Usually it stops in about four minutes. Yet it is difficult to say for how long after a person has been immersed in water it may be still possible to resuscitate him. Reports vary, owing in large measure to uncertainty as to the exact time at which the immersed person sank and his lungs filled with water. It is a wise precept that artificial respiration should be tried in every case, without waiting a single instant to ascertain whether the heart still beats. The first thing to do is to empty the chest of water. Then place the subject on his back. Kneel on the ground behind his head. Grasp an arm just below the elbow, in each hand. Draw the arms up above the patient’s head, so that the pectoral and other muscles drag on the ribs, enlarging the chest; then lower them, and press them into the sides. This must be done with the natural rhythm of respiration, and not more frequently than twenty times in a minute. It is well if an assistant draws the tongue forward, to give free admission to air. Presumably the slight exchange of air brought about by mechanical expansion and compression of the chest favours the passage of blood through the capillaries of the lungs; but the real object of artificial respiration is to stretch the endings of the vagus nerve, and in this way to originate impulses which will call the respiratory centre into action. Perhaps it may not be superfluous to point out that the failure of the pulse must not be taken as an indication that the heart has ceased to beat. Owing to the obstruction to the circulation through the lungs, the left side of the heart is almost empty. Very little blood is pumped into the aorta. None reaches the wrist.
=Exchange of Gases in the Lungs.=—In the lungs each red corpuscle takes from the air a charge of oxygen which it carries to the tissues. In the tissues the plasma of the blood receives carbonic acid, which escapes from it when it reaches the lungs. Water dissolves oxygen and carbonic acid. Towards animals and plants which live in it, water plays the same rôle as the atmosphere towards dwellers on land. The quantity of a gas which will dissolve in water is proportional to the pressure to which it is subjected. If water were the circulating fluid, some oxygen would enter it in the lungs; some carbonic acid would be taken up in the tissues and liberated in the lungs. But it is clear that the small quantity of fluid which the vascular system will hold would be incapable of serving as an efficient medium of exchange between the tissues and the lungs. When a given quantity of venous blood is agitated with air, five times as much oxygen is taken up as the blood could carry if the gas were simply dissolved. Both oxygen and carbonic acid are held by the blood in chemical combination.