Our lesson on the circulation showed us the blood flowing out from the heart, along the arteries, through the hair-like capillaries, and back again to the heart by theveins. The blood which the heart pumps out is a bright scarlet colour, and fit for its proper work in the body. We call it arterial blood. That which is returned to the heart by the veins is of a dark purple color, and loaded with impurities from the worn-out tissues. We call it venous blood. This is the blood which enters the right auricle of the heart, and is sent out from the right ventricle to the lungs. When it returns from the lungs it is bright scarletarterial blood. It has lost its impurities there. We want now to find out what happens to it in the lungs, and to do so we must examine the lungs themselves.
The lungs are two large organs situated in the chest, one on each side of the heart. They occupy the whole of the space in the chest which is not taken up by the heart and its great blood-vessels. They are of a pinkish color, soft and spongy, and very elastic. They are attached to the walls ofthe chest and the diaphragm by a membrane-the pleura.
If you run the finger down the front of the throat, you will feel a hard gristly pipe, which passes down into the chest from the back of the mouth and nasal passages. This is the wind-pipe. On entering the chest it splits into two-one pipe going to each lung. Each of these branching pipes is called a bronchus.
In the lung the bronchus splits up again and again into smaller and still smaller pipes-the bronchial tubes-the smallest ending in little bags or cells. These are known as air-cells. The whole substance of the lung is simply a mass of these air-cells.
Let us see why they are called air-cells. They are indirect communication with the air all round us. When we breathe, the air passes down the wind-pipe, the bronchus, the bronchial tubes, and so into the air-cells. Now think for a moment of the great blood-vessel, which conveys the dark impure blood from the right ventricle of the heart to the lungs. We call it the pulmonary artery. This artery, likeall others, splits up in the lung into smaller and smaller vessels, ending at last in capillaries.
The capillaries spread themselves all round the walls of the little air-cells. The air-cells are full of air, the capillariesall round them are full of this
bad blood. Two things then take place. The blood gives up its impurities to the air in the air- cells, and takes something back from the air in exchange. You remember that when we breathe into lime-water, the water becomes cloudy and milky-looking, because carbonic acid has been put into it. Whencedid this carbonic acid come? It came from the breath.
The blood is purified in the lungs by giving up to the air-cells the carbonic acid which it has absorbed from the worn-out tissues. This carbonic acid is sent out every time we breathe. But that is only part of the purifying work that takes place. The air in the air-cells is a mixture of the two gases, oxygen and nitrogen. The blood absorbs this oxygen in exchange for the carbonic acid which it gives up.
Thus the difference between the blood which leaves the lungs and that which enters them is, that it has lost its carbonic acid there, and taken up oxygen in its place.
Lesson 40