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

by Xavier Bichat

Observations on the motion of the Black Blood in the Veins.

From what has just been said, it appears, that the blood is beyond the influence of the heart when it arrives in the veins. It is evident, then, that the veins can have no pulse. 1st. This phenomenon depends upon a single impulse, suddenly received by the contraction of the left ventricle; now, the blood is poured from all parts by the capillary system into the veins, this agent of impulse is wanting; the cause of the pulse does not exist in the veins. 2d. The necessary conditions for its production in the texture of the vessels in which it takes place, are elasticity and resistance, which are also wanting in the veins. They are only susceptible, then, either of a pulsation which occasions the reflux of the blood in the derangement of the lungs, or in the irregular motions of the heart, or of an undulation of which they are the seat, when arterial blood accidentally circulates in them; now, in either, the heart is the principle of motion, and it could not exist without it.

This is what takes place in the venous motion. The capillary system, by its insensible contractility, pours continually into the venous system a certain quantity of blood. This fluid, added to what is already there, communicates a general motion to it. Now, as the whole venous system is constantly full, it is necessary that while the fluid enters at one side it should go out at the other; if not, the venous parietes would dilate; but, as they have a resistance by which they can act to a certain point upon the blood, this fluid not being able to dilate the veins, flows towards the heart.

The impulse given by the insensible contraction of the capillary system, is too weak, however, to extend instantaneously from one extremity of the veins to the other, especially where the blood rises against its weight. As this fluid enters these vessels, the weight of that which is before it not being overcome, it would produce a general dilatation, and the blood would not reach the heart; but the valves counteract this, by supporting at short distances the column of blood. Weakness of the venous parietes and the existence of valves are necessarily connected. If the veins were as strong as the arteries, unable to dilate when the blood enters them, they would necessarily transmit the surplus to the heart, if they were destitute of valves; but on the other hand, their circulation would be every instant embarrassed.

It appears that it is not only the insensible contraction of the capillary system which propels the blood in the veins; but that the ramifications of these vessels have a kind of absorbent power, by which they draw blood into this system. Now the insensible motion produced by this power tends evidently from the ramifications towards the trunks, as in the lymphatics; then, when, on the one hand, the blood is propelled in the veins, and, on the other, as it were, attracted by them, it is evident that the primitive source of motion that it obeys, is in the capillary system.

This impulse communicated to the blood, exceeds but very little the resistance which this fluid experiences in its motion; so that the least resistance deranges this motion. Hence, as we have seen, the necessity of anastomoses. Hence also the necessity of other assistance to aid this motion, such as, 1st, the muscular action, the influence of which we cannot doubt, when we see the flow of blood in venesection accelerated by the motion of the muscles of the fore-arm, the palpitations of the heart, produced by the blood that flows there after a rapid circulation; when we observe that varices are as rare in the veins situated among the muscles, as they are common in the sub-cutaneous ones, &c.; 2d, the pulsation of those arteries which are in many places joined to the veins, and which communicate to them a kind of motion; 3d, the motion of certain parts, like that of the brain, which continually rising and falling, accelerates the circulation of the blood of the sinuses in an evident manner; so also the constant locomotion of the gastric viscera, propels it in the veins of the abdomen, and that of the pectoral viscera, in those of the thorax. It is so true that the veins derive assistance to their circulation from external motions, that if a limb is a long time immoveably fixed when fractured, these vessels often dilate. 4th. External frictions, if they are not so violent as to embarrass the venous circulation, evidently facilitate it; this is one of the advantages of dry frictions. 5th. A slight compress, not sufficient to check the venous blood, often promotes its circulation, when the external organs are weakened. We know, since the time of Theden and Desault, the advantage of tight bandages, for varicose ulcers, even for varices, &c.

Since the principle of the motion of the venous blood is generally spread throughout the whole general capillary system, instead of being concentrated, like that of the arteries, in a single organ, it is evident, that this motion cannot be uniform, that it must vary according to the state of the capillary system in the different parts; that it can be more rapid in some veins, and slower in others. This is in fact what we see, especially externally where the veins are more or less swelled, according as the blood circulates there more or less rapidly. In the arteries on the contrary, the motion is every where the same; it is a general and sudden shock, an impulse, which, every where felt at the same time, is necessarily every where uniform; so you never see some arteries more full, others more empty, as it happens in the veins.

There are numerous researches to be made on the motion of the blood in the veins. Notwithstanding all that authors have written upon this question, there is an obscurity in it in which we perceive but few rays of light. These difficulties arise from this, that we do not know precisely what is the kind and form of motion communicated to the blood in the capillary system, what is the influence of the vascular parietes upon this fluid, &c. &c. Our knowledge upon this point is reduced to certain views which I have just presented, and which are particularly relative to the parallel between the motion of the blood in the veins and the arteries. I believe that this parallel carried further at some future day, will throw much light upon the venous circulation; in fact, as the first motion is much more easily understood than the second, we must proceed from what is known to what is unknown, and place in opposition what we are acquainted with in one, with that which we seek to know in the other. This is the summary of this parallel, though imperfect; 1st, General pulsation in the arteries, absence of this general pulsation in the veins. 2d. Rapidity of the course of the blood in the arteries; slowness of the same course in the veins. 3d. Greater capacity and thinner parietes in the veins; less capacity and greater thickness in the parietes of the arteries. 4th. Necessity for accessory assistance for the venous circulation; the inutility of this assistance for the arterial circulation. 5th. The blood flowing per saltem, from the second, the uniform flow from the first. 6th. The susceptibility of the blood in the veins, to be influenced by its gravity and other accessory causes; there is some of this influence in the arterial motion. The following are the phenomena, which, from what we have just said, evidently depend upon the existence of an agent of impulse at the origin of the arteries, and of the absence of this agent at that of the veins.

1st. Constant uniformity of the motion in the arteries; variety of motion in every part of the venous system; 2d, dilatation and contraction generally the same in all the arteries of dead bodies; extreme variety in this respect in the veins of the different parts; these are the other phenomena which arise from the unity of impulse in the first, and from the varieties of the principle of the motion of the blood in the second, &c.

Some authors have insisted much, in explaining the causes of the difference of the arterial and venous motion, upon this, that in the arteries the blood is propelled in decreasing vessels, to the capillary system that resists; in the veins on the contrary it flows in vessels always increasing till it arrives at the right auricle, which offers no resistance. But the black abdominal blood is also carried without the agent of impulse, in a series of decreasing tubes to the capillary system of the liver, and yet the motion is analogous to that of the veins.