To understand the effects of saline solutions in the body we must know what is meant by the physiologic terms filtration, diffusion, and osmosis, and the nature of hypotonic (hypoisotonic), isotonic, and hypertonic (hyperisotonic) solutions. These are well explained in any modern physiology, such as Schafer, Starling, or Howell.

In infusion, a large quantity of liquid is passed into the circulation; it should, therefore, be practically isotonic with the blood. If a hypertonic liquid is employed, i. e., a liquid containing too large a proportion of salts, the blood abstracts water from the tissues and swells in volume, to become still more dilute than the amount of injected liquid alone would make it; a greatly hypertonic liquid will injure the blood-cells. On the other hand, a hypotonic liquid will tend to lake the blood; outside the body a solution of 0.4 to 0.44 per cent. of sodium chloride will do this normally.

The effects of a saline infusion differ according to whether the volume of blood has been previously decreased or not; therefore must be considered from these two points of view.

1. When The Volume Of The Blood Has Not Been Decreased By Hemorrhage Or Other Cause

In normal animals the tendency of the blood to regain its normal condition is so pronounced that almost as soon as an infusion is begun the mechanisms for regulation are started. As the result of increased pressure in the capillaries there is an immediate outpouring of weak lymph, and this is followed by elimination of liquid through the intestines and kidneys (Starling), so that in half an hour not only will the volume of the blood have returned to normal, but its constituents will have regained their proper relative proportions (Crile).

In experimenting with saline infusions in 61 normal dogs, Crile found that, besides the rapid transudation of lymph, there was a dilatation of the splanchnic arterioles, so that most of the extra volume of blood was received in the splanchnic area without raising the general arterial pressure; thence it was rapidly excreted by the kidneys and intestines. Both on account of this sensitive vasomotor mechanism and of the active capillary transudation, he was unable to get a rise in the arterial pressure of more than 8 mm. of mercury, even from enormous amounts of saline. Indeed, the mechanisms for keeping the blood normal proved so active that after a certain dilution of the blood was reached it was practically impossible to bring about further dilution, and the only result of further infusion was to produce general edema. The limit of safe dosage he ascertained to be 30 c.c. of saline per kilo of body weight, which in the same ratio would be about 2200 c.c. for a 160-pound man. Clinical experience favors smaller amounts for man, and has proved the danger of such large quantities.

So when the volume of blood is already normal, the addition of saline solution has only a transitory mild effect on arterial pressure, and chiefly increases urination and the tendency to edema. It tends also to lessen the viscosity of the blood, but this action is so ephemeral that it probably has very little influence on the blood-stream.

Crile found, further, that the dilution of the blood does not prevent the action of circulatory stimulants; that if vasoconstrictor stimulants were administered at the same time as the saline, the arterial pressure could be raised above normal for a time; but that, when the splanchnic arteries were excluded, the dilution of the blood increased so rapidly with the progress of the infusion that edema set in very quickly, even though the arterial pressure was not essentially raised. This indicates that if, by a strong vasoconstrictor, such as epinephrine, dilatation of the splanchnic arteries is prevented, the chances of edema are increased. Hence in intravenous infusion, since the liquid must pass to the right heart and to the lungs first, pulmonary edema is favored; and especially is this the case if at the same time there is marked back pressure on the left heart from constriction of the peripheral arterioles. Therefore, as might be expected, pulmonary edema is especially readily brought about by a combination of saline infusion and epinephrine.

Summary

When the volume of the blood has not been reduced, saline infusion to raise arterial pressure is almost useless, and by producing edema, may have serious consequences. If used as a medium for the administration of drugs, it should be employed in small quantity, and slowly introduced. By transfusion of blood, on the contrary, it has been found possible to raise arterial pressure away above the normal, and to maintain it there for some little time.

2. When The Volume Of The Blood Is Notably Below Normal, As After A Large Hemorrhage

From 25 to 50 per cent of an animal's blood may be removed and replaced with saline without serious results (Levin). Crile noted that after a moderate hemorrhage a saline infusion would increase the volume of the blood so that normal arterial pressure would be maintained for a considerable period. He found also that the blood has a shorter coagulation time, the saline thus favoring the cessation of the hemorrhage. So saline infusions are valuable to replace lost blood, and may be used with advantage whether the bleeding has stopped or not.

A few further observations of Crile on the effects of infusions are worth mentioning: The temperature of the infusion, if within reasonable limits, makes almost no difference, either in the temperature of the patient or in the heart-beat. The rate of flow makes no difference in the extent of the effect on arterial pressure. The effect on respiration is an increase in frequency and depth; but "from greater than safe amounts the breathing becomes slowed, and there regularly ensue edema of the lungs and death from respiratory failure."