The blood does even more than carry food and oxygen to every part of our bodies. Its work is so important that the heart has to keep pumping it day in and day out without ever stopping to rest.
A person who weighs 100 pounds has about four quarts of blood. Bigger people have more. Smaller people have less.
A drop of blood looks like a drop of red ink. But it is much, much more complicated. The liquid part of our blood is not red. Blood looks red because it has many tiny red cells in it. Red cells are far too small to be seen without a microscope. It would take thousands of thern to make a row an inch long. Everyone has billions of red cells in his blood.
The chief work of the red cells is to carry oxygen. They gather it in the lungs and take it to all the other cells of the body.
Red blood cells are made inside some of our bones. Millions of new ones are made every second of our lives. But millions of wornout red cells are destroyed every second, too. They are destroyed in the liver and in the spleen.
There are also white cells of several kinds in our blood. Those of one kind make up an army to fight disease germs. They really eat the disease germs up. Those of another kind help repair parts of the body that have been damaged by germs. There are usually one or two white cells for every 1,000 red cells.
The liquid part of the blood is called plasma. It is mostly water. But the water has many important chemicals dissolved in it. It has, for instance, one chemical that makes blood grow thick, or clot. It has another that keeps it from clotting when it shouldn't. It has chemicals that help keep our bodies working as they should. There are other chemicals that help the white cells fight germs.
It is the plasma that carries food to all parts of the body. The plasma also gathers up waste materials and carries them to the lungs or the skin or the kidneys so that they can be got rid of.
It is easy to see that a drop of blood as it rushes through our bodies is always changing. Here it picks up oxygen. There it picks up food. Here it leaves some food. Here it leaves some oxygen. Here it gathers up some waste. Here it leaves a company of white cells to fight some germs that have got into a cut. Here it picks up a chemical. There it leaves it. Round and round it travels, like a delivery boy that works 24 hours a day every day.
People are not the only animals with blood. All animals with backbones have blood. So do many animals without backbones. Only simple animals can get along without it.
