MY BODY TEMPLELesson 9: CirculationWhile the blood is passing through the small blood vessels in the various parts of the body, each cell takes out just what it needs to build up its own tissues. At the same time, the tissues give out their worn-out or waste matters. The red blood cells (corpuscles) in the capillaries give up their oxygen, and the blood receives in its stead a poisonous substance called carbon-dioxide. While in the arteries the blood is of a bright red color; but while it is passing through the capillaries the color changes to a bluish red, or purple, color. The red blood is called arterial blood, because it is found in the arteries. The purple blood is called venous blood, because it is found in the veins. The loss of oxygen in the cells causes the change of color. Exactly the opposite happens in the blood when it passes through the lungs. The blood, which has been gathered from the various parts of the body, is dark, impure blood. In the lungs this dark blood is spread out in very tiny capillaries and exposed to the air. While passing through the capillaries of the lungs, the blood gives up some of its impurities in exchange for oxygen from the air. The red cells absorb the oxygen and the color of the blood changes from dark purple to bright red again. The purified blood is then carried back to the upper chamber of the left side of the heart through four large veins. The blood is now ready to begin another journey around the body. If you place your finger on your wrist at just the right spot, you can feel a slight beating. This beating is called the pulse. It is caused by the movement of the blood in the artery of the wrist at each beat of the heart. The pulse can be felt at the neck and in other parts of the body wherever an artery comes near to the surface. The heart is a small organ, only about as large as your fist, and yet it does an amazing amount of work. Each time it beats, it does as much work as your arm would do in lifting a large apple from the ground to your mouth. It beats when we are asleep as well as when we are awake. When we run it beats very fast. A ton is 2,000 pounds; in twenty-four hours the heart does as much work as a man would do in lifting stones enough to weigh more than one hundred and twenty tons. While the blood is passing through the capillaries, some of the white cells escape from the blood vessels. They sometimes do this to look for germs or harmful things in the body tissues. What do you suppose becomes of these run-away cells? Nature has provided a way by which they can get back to the heart. In the little spaces among the tissues outside of the blood vessels very tiny channels called lymph channels or lymphatics (lym-phat'-ics) begin. The whole body is filled with these small channels, which run together like a net. In the center of the body the small lymphatics run into large ones, which empty into the veins near the heart. This is how the stray white blood cells get back into the blood. In the lymph channels the white cells float in a colorless fluid called lymph. The lymph is plasma from the blood, which has soaked through the walls of the small vessels. The chief purpose of the lymphatics is to carry the lymph from the tissues back to the heart. Here and there, scattered through the body, are oval structures where many lymphatic vessels connect. These are called lymph nodes. They are like little filters. In these germs and foreign matter are trapped and eaten up by the white blood cells. The heart and blood vessels are among the most wonderful structures in the body. =^..^= Questions: 1. What color is the fresh blood that has oxygen? What color is the impure blood? 2. Can you name the three kinds of tubes that carry the blood?
|