General Anatomy, Applied to Physiology and Medicine, Vol. 1 (of 3)

by Xavier Bichat

III. Proportion of the capacity of the two systems with Black and Red Blood.

After the observation I have just made upon the origin and course of the veins, it is evident that their sum total has a capacity much greater than that of the arteries. This assertion it is easy to prove in detail, wherever there is an artery and vein united, as in the kidneys, the spleen, the extremities, &c.; where the arteries are separate from the veins, as in the brain, the liver, &c. it is not less sensible. Finally, there is, as I have just said, a division of sub-cutaneous veins, which is evidently one more than the arteries have.

Many physiologists have endeavoured to calculate the relation of capacity between the two systems with black and red blood; but this relation varies too much ever to be the subject of any calculation. Is it upon the dead body that the attempt is made? But the veins will be more or less dilated according to the kind of death; in apoplexy, asphyxia, drowning, &c. they have a diameter almost double that which they exhibit when the subject has died of hemorrhage, because the first kind of death accumulates much blood in the veins, and the second deprives them of it. We can give a greater or less capacity to the veins of an animal, according to the manner in which we kill him, as we can enlarge or contract the right cavities of the heart by similar means. You can never find the veins exactly equal in any two subjects, though there may be a great resemblance as to size, age, &c. Is it upon a living animal that the attempt is made? But, besides its being very difficult, you will not then have a result uniformly applicable, because the veins vary in diameter as they are more or less full. Examine these vessels in subjects in whom you can see them by the transparency of the integuments; sometimes they are more, sometimes less apparent; their size sometimes appears double, at others, hardly distinguishable. Certainly after drinking copiously, by which the black blood has received a great augmentation of its fluid, the vessels are more dilated than in an opposite state. The veins are remarkably contracted after death from hunger. I have often observed the same phenomenon in dropsies, phthisis, marasmus, &c. Always when the mass of blood is diminished, the veins contract by their contractility of texture. The arteries are infinitely less subject to variations of diameter, on account of their firm and compact texture, though, however, they show much of it.

Let us reject, then, every kind of calculation upon the proportions of capacity of organized canals. We can only calculate what is fixed and invariable; but that which varies at every instant can only be the object of general assertion. Besides, of what importance are the rigorous proportions that some physicians have endeavoured to establish between our parts? They are nothing in the explanation of the phenomena of health and disease. Let us, then, be content with this general assertion, that the venous capacity surpasses the arterial. It may be said, that in a given time there is more blood in one than the other.

The same observation applies to the two sides of the heart, one of which belongs to the same system with the veins, and the other to the one with the arteries. The right has commonly a greater capacity than the left, not precisely under the relation of the fleshy texture, but under that of the fluid that distends it; this is so true, that if in an animal whose thorax is opened, the blood is made to accumulate in the left side by ligatures, and the right is emptied by puncturing it, the last will be less in size than the first. Always when we find it larger than the other in the dead body, except in diseases of the heart, it is because it contained more blood at the moment of death; in fact, as this fluid ordinarily stops first in the lungs, it flows back to this side of the heart, which is almost always the largest.

This is the great difference between inert cavities and those that possess life, viz. that these last can change their capacity every moment, whilst the others remain always the same. In the living animal, the right side of the heart has almost always a greater capacity than the left, because the quantity of blood it contains is greater.

There are, then, two things generally true, viz. 1st, that the great tree that terminates the system with red blood is in general of less capacity than the great tree that commences the system with black blood; 2d, that the same observation is applicable to the two sides of the heart, which correspond with these two trees.

As to the tree that terminates the system with black blood, compared with that which commences the system with red blood, the same thing does not hold true. The pulmonary artery and the veins of the same name exhibit a disproportion of capacity, less it is true, than in the other parts, a disproportion which is real, however, and which, notwithstanding what many authors have said, is in favour of the veins. How does this happen? it would seem that since the one is continuous with the veins and propels the same fluids, it ought to have the same proportion of diameter; and that since the others are continuous with the arteries, that they also should be in proportion to them. This arises from the difference of the velocity of the blood; in fact, this fluid circulates quicker in the pulmonary artery than in the veins of the same name, since it has the impulse of the heart, which these last want; then, in a given time, as great a quantity of blood passes through it, though the diameter of this artery is smaller; what do I say? if it was equal, the circulation could not go on. In the same way, if the aorta equalled in capacity the two venæ cavæ and the coronaries united, and the blood had the same velocity there, the circulation would cease.

The four pulmonary veins united are a little larger than the aorta, which, however, transmits all the blood received from them. Why? Because the impulse that the left ventricle communicates, makes, in a given time, more blood pass by the aorta than by the four pulmonary veins. These two things, 1st, the velocity of the fluid; 2d, the capacity of the cavities in which it circulates, are then in an inverse order in the two opposite trees that form each vascular system. In that with red blood, there is less velocity and greater capacity from the pulmonary capillary system to the agent of impulse; and from this agent to the general capillary system, there is, on the contrary, greater velocity and less capacity. In the vascular system with black blood, there is less velocity and greater capacity from the general capillary system to the agent of impulse; and from this agent to the pulmonary capillary system, there is more velocity and less capacity. Without this double opposite arrangement, it is evident that the circulation could not take place.

There is, however, a remark to be made upon this subject; it is, that the capacity of the four pulmonary veins united, is not so much larger than the aorta as that of the venæ cavæ and the coronary is than the pulmonary artery; and this is the reason of it; as the pulmonary veins run a very short course, the impulse, on the one hand, that the red blood has received from the pulmonary capillary system, is preserved there more; on the other hand, this fluid is there free from numerous causes of delay that the blood experiences in the venæ cavæ and coronaries; then the velocity is greater there, and the capacity should be therefore less. If the lungs were situated in the pelvis, the pulmonary veins would certainly have a greater capacity, because having a greater extent to go over, the velocity of the blood would be more retarded.