PROBLEM 3. How Materials Are Moved to and from Cells 



stop in its course to load and unload. As 

 it moves through the capillaries there is 

 a constant passage of substances in and 

 out. 



Blood is more than just plasma. In the 

 plasma there are three kinds of cells: red 

 corpuscles (core''pus-ls), white corpus- 

 cles, and tiny platelets (plate'lets). The 

 red corpuscles are very numerous and 

 give the red color to the blood. If you 

 follow directions in Exercise 2 you can 

 study a drop of blood with a microscope 

 and see the two kinds of corpuscles. 



Almost half of the volume of the blood 

 is cells. For this reason blood is thought 

 of as a tissue. Some of the other tissues 

 which you think of as "solid" tissues have 

 almost as much liquid around their cells. 

 If your school has an instrument known 

 as a centrifuge (sen'tre-fewj) you can 

 easily separate the blood plasma from 

 the mass of cells. If you can get blood 

 from a slaughter house, do Exercise 3. 



Red corpuscles. A red corpuscle is 

 shaped like a coin which is much thinner 

 in the center than around its edge. The 

 red corpuscles contain a special protein 

 substance, rich in iron, known as hemo- 

 globin (he^mo-globe'ln). Hemoglobin is 

 unlike other proteins in that it unites 

 with oxygen very easily and releases it 

 just as easily. It is because of this that 

 the red corpuscles can be the transport- 

 ers of oxygen. When hemoglobin unites 

 with oxygen it forms a new compound 

 (oxyhemoglobifi) which is bright red in 

 color. If, later, this red compound is in 

 surroundings where there is little oxygen, 

 it again separates into oxygen and hemo- 

 globin. When blood flows from a cut it 

 is at once exposed to oxygen and there- 



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fore takes on the color you think of a? 

 blood red. When examined under the 

 microscope, however, red corpuscles are 

 disappointing, for each single cell is quite 

 pale even when in contact with oxygen. 

 It is only when there are large numbers 

 of red corpuscles close together that we 

 can see the brilliant red color of fresh 

 blood. 



Red corpuscles are much smaller than 

 most other body cells. One drop of blood 

 normally contains more than 5,000,000 

 of them. Since there are more than five 

 quarts of blood in the average man he has 

 about twenty-five trillion (25,000,000,- 

 000,000) red blood cells, a number too 

 large to hold any meaning for most of us. 

 It may mean more to learn that if all the 

 red corpuscles of a normal person, small 

 as they are, were laid out flat next to one 

 another they would cover an area as 

 large as a baseball diamond. 



Red corpuscles are made in the red 

 marrow of the bones. Before they enter 

 the blood they lose their nuclei. They 

 live, on the average, only about a month. 

 In healthy people about a million cells 

 may be destroyed every second. If they 

 are destroyed too rapidly, or are not 

 manufactured fast enough, or if a large 

 amount of blood is lost, a person may 

 have too few red corpuscles. He then 

 has too little hemoglobin, a condition 

 called ajiemia (an-ee'me-a). Since iron is 

 an important part of hemoglobin, an in- 

 sufficient amount of iron in the diet can 

 also cause anemia. The organ known as 

 the spleen is a reservoir of blood and 

 particularly a storage chamber of red 

 corpuscles. During muscular exercise and 

 in people living at high altitudes the 



