The science of life : fully illustrated in tone and line and including many diagrams

BOOK 1

be watched by followimg the oval red corpuscles as they float along.

The blood can be seen hurrying apparently at headlong speed! along certain larger vessels—highways—which are minute arteries. The arteries branch and divide and their branches divide again, so that the blood passes into smaller and smaller canals. At each division the cross area of the two branches taken together somewhat exceeds that of the original trunk, so that the blood travels more and more slowly as it finds its way into narrower and narrower passages. ‘The final, minutest blood-vessels are the capillaries ; their diameter is only very slightly greater than that of the red blood-cells themselves, and in these ultimate canals the cells no longer bustle—they crawl. It is through the invisibly thin walls of the capillaries that there takes place that interchange of substances between blood and flesh for which purpose the circulation exists. The whole appearance suggests women coming to a marketthe swift stream pours into the busy marketplace, it breaks and divides into slower and slower subsidiary streams, until, finally, the housewives are hovering and bargaining in the narrow spaces among the crowded stalls.

But the blood does not stay long in the capillaries. As it loiters, its dissolved food and oxygen are diffusing into the cells, and the unwanted residue of their activity is passing back in exchange. Diffusion, over distances measured in thousandths of a millimetre, is a process of lightning speed, and the blood must not hang about after its function is performed. Hence, after a brief passage through one of the myriad capillaries, the blood reaches a point where the vessels instead of subdividing, come together again. ‘The capillaries join, and the resulting trunks join each other, and so veins are formed. But now the previous conditions are reversed, for each trunk has an area slightly smaller than that of the two branches which unite to make it, so that as the blood finds its way into larger and larger vessels it travels at a greater and greater speed. ‘Thus, in one of the little veins it hurries out of the picture with the same effect of headlong haste that characterized its arrival. We cannot watch the blood as it travels, ever more swiftly, towards the heart ; the frog is too opaque for our microscope to penetrate. ‘The rest of the

1 Velocities always look deceptively high through the microscope, because the instrument magnifies distance without magnifying time.

36

THE SCIENCE OF LIFE

GHAPTER 2

story has been pieced together by dissection and by experiment.

When it leaves the field of view, the blood continues in the same way. The veins from the web are joined by veins from the muscles of the toes, and later by veins from the muscles and skin of the calves, from the bone and cartilage, and other tissues in the leg. The stream becomes greater and swifter. Still later, in the region of the hips, two great vessels from the legs join each other to form a venous trunk, the inferior vena cava. This, as it runs up the back of the abdomen, receives branches from the muscles of the back, from the ladneys, from the intestines, from the liver. In the chest it combines with two other main trunks, the superior vene cave, which carry blood down from the head, neck, and arms. Finally, the stream of used blood collected from every part of the body by the union of these three vessels pours into the heart.

When the blood from the tissues reaches the heart through the vene cave it is depleted blood. While passing through the capillaries it has parted with the substances required by the flesh and with its oxygen, and it has encumbered itself with the waste products there produced. It is unfit to be driven round again until it has undergone a process of restoration and purification. Now the various needs of living cells vary in their urgency. The food-supply is necessary, but it need not be constant. Cells are able to accumulate little stores of sugar, fat, or a very important nitrogenous material, protein, in their own bodies, to be used when required, and it is sufficient for the blood to bring them food once in a while, after a meal, for them to replenish their stores. Moreover, the removal of waste products need not be very rapid; they are formed only in small quantities and slowly. But with the oxygen supply things are different. The need for oxygen is imperative—a man cannot live without oxygen for more than three minutes —and the cells have no way of laymg up stores of oxygen. It must be brought to them constantly and abundantly. A fresh charge of oxygen is therefore a primary requirement of this returning blood before it can go back to the tissues.

The circulatory system in a mammal works first and foremost to meet this need for oxygen. The removal of other waste substances and the taking up of more food-stuff is a less urgent matter. Venous blood, returning from the tissues, is driven at once