Physiology, in brief, is the study of function as distinguished from structure. When the physiologist looks at a living organism, he asks, "What does it do and how does it do it? What are its various parts for and how do they perform their functions?" The knowledge which has accumulated in regard to function is of two kinds, descriptive and theoretical. The first consists in statements of facts, as, for example, that the heart beats, that stimulation of the vagus nerve causes a slowing of the heart beat, or that muscular exercise causes an increase in the breathing and heart rate. Such observations, if accurately made, are incontrovertible, as they are mere description. They may be simply qualitative or may attempt to describe the phenomena in quantitative terms. Much of the older physiology was necessarily of this type. But after a certain number of facts have been accumulated, it is essential for the progress of science that they be correlated and that explanations for the observed facts be sought. The more recent physiology has, in large part, been of this theoretical or explanatory nature.

There are, in general, two modes of approach in attempting to explain the function of any particular organ, to unravel the processes concerned in the functioning of that part. One of these involves studying its variations in function in the intact normal animal, while the other involves more or less interference with the normal animal by isolating, or partly isolating, the particular organ in question. The former, which has been called the analytic method of experimentation in contrast to the latter, or synthetic, method, has the great advantages that normality is maintained - but at the expense of a control of variables - and furthermore, that it can be applied to the human subject The latter method, while sacrificing normality, admits of a much better control of the variable factors which enter into the experiment.

In the past, a great deal of knowledge has accumulated concerning the function and mode of operation of the separate organs of the animal organism; but, owing probably to the use of the synthetic method, it is difficult to bring all these observations and generalizations into harmony with the physiology of the organism as a whole, of the normal, healthy, everyday man. And it is becoming increasingly evident that the physiology of the organism as a whole is not simply the sum of the functions of the constituent organs or parts. There exists among all the parts a finely balanced regulation, which always operates in such a way that the individual organs or systems cannot work separately in the body, but must be correlated together or integrated into a unity; for the animal must function as a unit - an individual. This phase of our subject is so vital that it will be well to illustrate what is meant by an example.

Let us see how an animal reacts to a change in its environment which demands movement - the seeking of food, fighting, or flight from some serious danger. Let us take an extreme case, such as where maintenance of life demands the most violent muscular movement of which the organism is capable. In the change from rest to violent muscular exercise two things are necessary for the acting muscles: oxygen and a readily oxidizable foodstuff to supply energy for the movement. To supply these, many organs or systems besides those which are directly involved (muscular and nervous) must operate in a different manner from what they do at rest. Some must carry out their function much more rapidly than before; others must almost cease to function for the time being. The rate and depth of breathing may be increased many fold (in man, from six to one hundred liters of air per minute); this supplies the needed oxygen to the blood. But if the blood moved around with no more rapidity than when the body is at rest, the increased supply of oxygen at the lungs would be useless. Hence, the heart pumps the blood much more rapidly and in greater quantity than before; in man, the amount of blood put out in one minute from one ventricle may increase from about four liters to twenty-five liters or more. This is accomplished by an increase in both the number of strokes per minute and the size of each stroke. But at rest, only about one-third, or less, of the total blood pumped by the heart goes to the muscles, although they comprise about one-half of the total weight or volume of the body; so a further adjustment is necessary. This consists of a widening of the blood vessels in the muscles and, in fact, of an opening up of many stagnant blood capillaries so that the number of capillary vessels through which blood is flowing may be increased forty to one hundred times. But the opening of these vessels alone would not supply enough blood carrying oxygen to the muscle; and, what is more important, it would involve a marked lowering of blood pressure which would secondarily decrease the blood and oxygen supply to the brain - the master tissue of the body - a thing which must not happen if the body is to remain active. So what happens, in order that the muscles and brain may receive an adequate blood supply, is that the supply to all the other organs is greatly depleted by a decrease in the caliber of the blood vessels, and we find, under these conditions, that the intestines get so little blood and oxygen that they almost cease to function, while the same thing has been shown for the kidneys, which almost cease to secrete urine.