Arrangement of capillaries.
a smallest artery.
b smallest vein.
c network of capillaries.
86. Capillaries.—The smallest arteries divide into a fine network of small tubes. These tubes are the capillaries. They lie around every cell of the body. Their sides are very thin. As the blood flows through them, some of it soaks through the sides of the tubes. Blood contains all kinds of food for the cells. Each cell is always wet with food and can eat it at any time. The cells are like the tiny animal, the ameba, and can take in the food by any part of their bodies. The cells are better off than the ameba, for their food is brought to them. They pay the body for their food by working for it.
87. Veins.—The capillaries come together again to form large tubes. These tubes are called veins. Only a little of the blood goes through the sides of a capillary. The rest flows on into the veins. The veins unite to form two large tubes. These two tubes open into the heart.
88. How the blood flows.—The blood is pumped out of the heart, through the arteries to the capillaries. There some goes out to the cells. The rest flows into the veins and goes back to the heart. All the blood in the body passes through the heart every two minutes. It takes only twenty seconds for a drop of blood to go from the heart to the toes and back again. The arteries are deep in the flesh, but some of the large veins can be seen upon the back of the hands.
89. Bleeding.—If a large artery or vein is cut there is a great deal of bleeding. You can always stop a cut from bleeding by holding it fast between the hands. Do not be afraid of the blood when you see any one bleeding, but hold the sides of the cut tightly with both of your hands. This will stop any bleeding until help comes. You may keep a person from bleeding to death by doing this when other persons are afraid of the blood.
90. Healing cuts.—When your flesh is cut it soon grows together again. The work of the little white cells in the blood is to help heal cuts and wounds and bruises. These cells are like little amebas in the blood. They keep moving around with the blood, and now and then burrow outside the capillaries to see if all is well. If they find a cut, hundreds and thousands rush to the spot at once. Some eat up any specks of dirt on the cut. Others fit themselves into the sides of the cut and grow long and slender, like strings, and so bind the two edges of the cut together. In this way all cuts are healed.
Bacteria growing in a kidney and producing an abscess (×300).
a kidney tube.
b white blood cell attacking bacteria.
c bacteria.
d blood vessel of the kidney.