Physiology

The Principle of Maximal Activity.

The most elementary consideration of the architecture of any structure demands some discussion of the sizes of the various components. This question has received little attention from the physiological point of view. The anatomist has tabulated the average dimensions of the vari ous organs but the physiologist has little to say about the reasons which determine these sizes. What little there is on this subject in the literature of physiology is not always to the point. The human body at rest may be compared to a battleship with her fires lit, her ammunition on board and her guns ready for action. Any description of such a ship is, however, given in terms of the energy she puts forth, not when at rest, but when in the fullest possible activity; she has a speed of so many knots, her hitting power is so and so. Only expressed in those terms the dimensions of her various parts become intelligible. So it is with the body, but the difficulties of studying it, or any of its parts, when the maximum of energy is being developed is very great. Some de cision must be come to as to the units in which the energy is to be expressed. In the case of a motor car it is expressed in terms of fuel consumed—so many miles to the gallon or to the litre. In the case of man it may also be expressed in litres—litres of oxygen used in consuming the fuel. A man may use as much as four litres of oxygen per minute. This oxygen is of course used in the tissues themselves and therefore must be carried to them. To carry four litres of oxygen efficiently requires probably about 30 litres of blood, therefore 30 litres of blood must pass out of the heart per minute. That fact fixes the size of the aorta and presumably the dimensions of the heart itself. Moreover the blood has to be oxygenated and therefore the dimensions of the lungs are also fixed. Thus the limiting factor in deciding the size of any organ has nothing to do with the events taking place in the resting body but with the maximum activity with which the organ is concerned.

A striking instance is furnished by the splenic vein. Many ob servers have noted that the splenic vein is large out of propor tion to the quantity of blood which usually passes along it. Only recently has it been appreciated that the spleen may hold at a time perhaps one-fifth of the total quantity of blood in the body and that it may contract suddenly expelling possibly 11 of a litre of blood in a few seconds.

What is true of the body as a whole is true of the individual organs.

The following table gives the quantity of oxygen which they are capable of using in the dog or cat :— Thirty years ago physiologists were exercised with the study of the extent to which organs could be cut away without fatal results. Bradford found that the whole of one kidney and the half of the second could be removed in animals without fatal results; it was observed also that the removal of one lung did not appear to be very serious. Men marvelled at the fact that the organs were so unnecessarily large; the standard which they applied was that of rest and not the conditions under which the kidney or the lung was required to function on the greatest scale of which it is capable.

Again it is well known that but a small portion of many of the endocrine organs, if left in the body, will apparently prevent the obvious sequelae of removal of the whole. Why then should the body contain so much more of the organ than appears necessary and why in many cases will the part left grow perhaps to the original size? Presumably because there are moments when the whole organ is required. It is in relation to such that its size

is regulated.

The Principle of Mobilized Units.

When an organ be comes active it may be conceived of as doing so in one of two ways (of course the two may be combined). The first is that each unit of which the organ is composed may become heightened in activity; the second is that a single unit is capable of only two states, the active and the resting and that the degree of activity of the organ depends simply on the relative number of active and of resting units which it contains.

For many years the view has been held with regard to the heart, that that organ cannot give a partial contraction; if it con tracts at all it gives the full contraction of which it is capable in the condition in which it may be at the moment. This was known as the "all or none" principle. It seemed to indicate that not only did each fibre of the heart muscle contract to its full extent but that when contraction took place, all the fibres in the heart contracted. These are two separate points. For the moment the applicability of the "all or none" principle to the cell will be considered. More recently the principle has been applied both to striated muscle and to nerve. As now conceived each fibre of which a skeletal muscle is composed, contracts to its full extent if it contracts at all, but, unlike the heart, not all the fibres need necessarily contract. Thus the degree of shortening of the muscle will depend upon the number of fibres which are thrown into con traction and not upon the degree of contraction of each.

Very beautiful records have recently been obtained of the im pulses which pass up sensory nerve fibres; the same principle holds good. There is a standard impulse which can pass up, that is a property of the nerve. The gradation of the sensation depends upon the number of such which pass along the nerve in a given time, and on the number of nerve fibres which are stimulated simultaneously.

With regard to the applicability of the "all or none" law to secreting cells there is nothing known.

Passing from the consideration of cells to that of organs, one finds that there is frequently a unit between the cell and the organ; the lung consists of alveoli, the same is true of the sali vary gland, the kidney consists of tubules, and so forth. Till recently it was assumed that in any one organ all the units are at work all the time, the activity of each unit being graded in the same degree as that of its fellow. With regard to the kidney Richards and Wearn have shown it to be otherwise, the degree of activity of the organ being the expression of the number of tubules which are in action at any time ; for the tubules appear to work in shifts. Haldane has found reasons for suspecting the same principle to operate in the case of the lung. In normal quiet breathing he regards only a portion of the lung as being involved, while when the respiration is more laboured a larger number of alveoli become ventilated. The application of the principle of mobilized units to the lung rests, however, on a less sure basis than its application to the kidney. In the latter case the glomeruli have been seen to go in and out of action, the blood flow through them being at times suspended; in the lung it is a matter of inference.