The commonwealth of cells

by H. G. F. Spurrell

The tubes through which the lymph is brought back to the blood-stream have thin walls, and no muscle of their own. They are subjected, however, to a constantly varying pressure by the movements of the limbs and trunk, and as, owing to valves inside them, the lymph can only escape in one direction, there is a constant flow towards the junction with the bloodvessels.

The bloodvessels are quite different. A far more certain and expeditious current is necessary—hence the steady circulation through a system of closed tubes.

In order to understand this passage of the blood, it is necessary to keep in mind the great principle with which hydrostatics supplies us, viz., that a liquid always flows from a region of high to a region of lower pressure. The problem of the vascular system is, therefore: How can the pressure within a ring of tube be so arranged as to maintain a regular flow always in the same direction?

Let us begin with the structure of the system. The tube through which the blood first passes on leaving the heart is composed of four distinct and essential elements: A lining of endothelial cells, which we need not discuss at length; a main substance of tough white fibrous connective tissue; elastic fibres and muscle fibres, the two last arranged in the substance of the connective tissue. All these parts are present in the main arteries which leave the heart, but in the fine meshwork of capillaries to which the arteries give rise by repeated branchings there is nothing left of the outer coats, only the lining of endothelial cells separating the blood from the organ traversed. In the veins which these capillaries unite to form, the connective-tissue sheath reappears, and also some muscle; but the elastic coat is quite absent. The heart is really a double coil of the tube ([see Diagram 12]), in which the muscular coat is predominant, and is divided into four chambers by the valves, which insure the blood flowing in the right direction when it contracts. ([See Diagram 33.])

Diagram 33.—Scheme of Circulation

The way in which these structures work is as follows: Two of the chambers of the heart (the auricles) receive blood from the veins, and when full suddenly contract, driving their contents into the other two chambers (the ventricles). The blood does not run back into the veins, although the pressure in them is very low and there are no valves to prevent it, because there is still less pressure in the ventricles, and also because the veins enter the auricles obliquely, and the tendency of the increasing pressure is to close their orifices. Having discharged the blood into the ventricles, the auricles relax, and the pressure within being a minus quantity, they are speedily filled with blood from the veins, blood not being able to return after entering the ventricles, as valves close automatically to prevent it.

Stimulated by the blood distending them, the ventricles then contract simultaneously like the auricles, only with much greater force: for the right ventricle has to drive the blood all through the vessels pervading the lungs back to the left auricle; whilst the left ventricle, which is proportionately stronger than the right, has to send its contents to the furthest extremities of the body. They then relax, in order that conditions of their internal pressure may favour another inflow from the auricles, return of blood from the arteries being, as in the preceding case, prevented by valves.

The pressure in the arteries during life is always fairly high; indeed, the ventricles have to get up a considerable force before the valves leading from them will open. The result of this is not only that the blood is driven along them with a rush, but also that they are slightly distended at each beat; and so, owing to the elasticity of their walls, the blood continues to flow forwards even between the beats of the heart. The rest of the journey is quite simple; the pressure in the capillaries is lower than in the arteries, and the pressure in the veins lower than in the capillaries, and lower in the veins, too, as they approach the heart, till, where they join the auricle, it is actually minus, and the blood has no other course open to it but to return to the auricle. It looks as though accidents might happen in the veins owing to there being so low a pressure there to direct the current, but this is prevented by the presence of valves at intervals, to stop any return.

The rate at which the blood travels is another point which has an important bearing on the nutrition. It does its work—i.e., gives out nutriment and picks up refuse—whilst flowing through the capillaries; so here one finds that it moves slowly. On the other hand, the sooner it reaches them the better, so it races fast through the arteries. Finally, its return to the heart need not be delayed, so it is quickened up again through the veins. The principle by which this variation in the rate of flow is obtained is simple and inevitable. If a tube through which liquid is flowing is not the same size all the way along, the liquid will be found to flow faster in the narrow parts than in the wider ones. Now, in branching, the arteries do not keep becoming smaller in regular proportion, and the result is that the capillaries have collectively a diameter five hundred times larger than the aorta; hence the blood flows through them only one-five-hundredth of the pace at which it leaves the heart. But in uniting again to form the veins their cross-section is reduced once more, so that that of the large veins near the heart is only two and a half times larger than that of the aorta, and hence a flow only two and a half times slower results.