英文科學(xué)讀本 第四冊(cè)·Lesson 18 Circulation of the Blood

Lesson 18 Circulation of the Blood

We have studied the body as a living machine. This machine requires to be nourished, or it cannot do its work. It is the great work of the blood to feed the tissues in every part of the body. This explains why blood is found in all parts of the body. We cannot prick ourselves even with the finest needle without drawing blood.

One of the first things to learn about this blood is the fact that it does not rest stagnant as water does in a bottle; it is always on the move. It flows through the body in a continuous stream, round and round, returning to the same spot again and again. This incessant onward flow of the blood through the body is known as the circulation of the blood.

All the blood in the body is contained in pipes or tubes, which we usually call blood-vessels. Taking the arm and hand as an illustration, it is clear that there must be a flow of blood down the arm to the fingers, and a return flow up the arm again. Those vessels which convey blood down the arm towards the hand are called arteries; those which carry it back are veins. But what causes the blood to keep up this incessant flow? It is pumped through the body by the heart.

The heart is a pear-shaped organ, about the size of one's fist. It is situated in the middle of the chest, is made entirely of muscle, and contains four chambers of equal size. The two upper chambers are called auricles, the two lower ones are named ventricles; so that there is a right and a left auricle, and a right and a left ventricle. Each auricle communicates with the ventricle below it by means of a hole in the partition which separates them, but there is no communication of any kind between the right and left sides of the heart.

It is the work of the veins to carry back the blood from all parts of the body to the heart. These veins, beginning at first as mere hair-tubes, unite again and again as they get nearer to the heart. All the blood from the upper parts of the body is brought back to the heart at last by one great vein; all that from the lower parts by another.

These two great veins open into the right auricle of the heart. I told you the heart is a muscular organ, and you know it is the nature of a muscle to contract when it is interfered with. There are certain nerves whose business it is to act as overseers of the heart, and see that it does its work. How vigilant these nerve overseers must be! The heart, under their control, begins its work at the very commencement of life, and never rests for a single moment till death comes.

This work is always done in one way. Auricles and ventricles do not work together. Both auricles begin to contract at the same moment, but while this is going on the ventricles, left to themselves, expand. The instant the auricles cease working, the ventricles begin, but while the ventricles contract, the auricles are expanding. Hence, when the auricles are contracted to their smallest size, the ventricles are stretched out to their fullest, and when the ventricles, in their turn, finish their contraction, the auricles are expanded to their utmost.

The auricles, at the moment they commence to contract, are stretched to their fullest extent, and have been filled with blood from the two great veins. The act of contracting must squeeze the blood out of the auricle. It cannot return, because of the great mass of blood behind in those veins. It flows onward into the right ventricle, which has expanded to receive it.

The ventricle is no sooner filled than it begins, in its turn, to contract, and so force out the blood. As it contracts it closes the orifice between it and the auricle above, so that the blood is prevented from returning. It finds an easy passage along a great vessel which opens out from the ventricle, and leads away from the heart to the lungs, and that is the course it now takes.

This great vessel is called the pulmonary artery. It conveys the blood from the right ventricle of the heart to the lungs. The blood, after passing through the lungs, is collected up and brought back again by the pulmonary veins. These veins open into the left auricle, and the rest of the work is just a repetition of what took place in the right side of the heart. The auricle expands to receive the blood, and when full begins to contract. As it contracts it drives the blood from the left auricle into the left ventricle, through the hole between them.

When the ventricle next contracts, this hole closes, and the blood cannot return. There is a great artery—the largest artery in the body—opening out from the left ventricle, and the way through it is open. Hence the blood flows along in that direction.

This artery is the aorta. It branches out into smaller arteries, and so carries the blood through the body. The smallest of these branching arteries break up at last into still finer tubes—so fine, indeed, that they are the merest hair-tubes. They are called capillaries, from a Latin word which means "a hair." These unite again and form the commencement of the veins, so that the capillaries form the link between the smallest arteries and the smallest veins.