substance 



which 

 indirectly 



acts on 



form 



_5:!^i£hentangj£ 



PROBLEM 3. Honjo Materials Are Moved to and jro?n Cells 



one of the dissolved proteins of the K^jg|i^V^- 

 plasma, fibrinogen, to harden and form '^^^^:^f^^'^^&-^ 

 threads. These are known as fibrin (fye"- — 



brin) threads. They entangle the red and 

 white corpuscles, and this tangled mass 

 is the clot. 



When a large quantity of blood is 

 allowed to stand in a tumbler a solid mass 

 of fibrin threads and corpuscles forms in 

 the way just described. This mass shrinks 

 and you then see it as a clot floating in 

 a faintly yellow liquid \\'hich looks like 

 plasma. But it is not plasma because it has 

 lost the fibrinogen which hardens into 

 threads. It is sermii, a substance which 

 does not clot. You may have heard that 

 a doctor sometimes injects purified blood 

 serum into a person. 



In most people bleeding from small 

 wounds stops soon because of clotting. 

 Bleeding from larger wounds may often 

 be stopped by various methods used by 

 physicians. Sometimes vitamin K is in- 

 jected to hasten clotting. Some people 

 are known as "bleeders" because their 

 blood clots very slowly. The cause of 

 this condition is not definitely known. 



Transfusions and blood banks. The 

 transfer of blood from one person into 

 the veins of another is practiced when 

 large amounts of blood have been lost, in 

 treating for shock, and under various 

 other circumstances. Great care must be 

 taken to choose the right person to give 

 blood. If the blood of the donor (the 

 person who gives) is not of the right 

 type it clots or coagulates within the 

 body of the patient, causing death. There 

 seem to be four main groups of people, 

 classified according to the chemical com- 

 position of their blood. This has nothing 

 to do with the race to which they belong 



21 1 





fibrinogen 

 a protein 

 in plasma 



Fig. 212 Hoiv blood is supposed to clot. Begin 

 at the top. Cells, mostly platelets probably, start 

 the process. Of what is the clot composed? 

 What surrounds the clot? 



because the same four groups are found 

 in all races. It was once thoutjht that one 

 type of person, called the universal 

 donor, could give blood with safety to 

 any other person. While in general this 

 "universal" blood, called also "O" blood, 

 can be mixed with any of the four types, 

 occasionally there are disastrous results. 

 For this reason tests are made before the 

 transfusion. A second type of blood is 

 "A" blood; this can be used only for a 

 person who also has "A" blood. A third 

 type called "B" blood can also be mixed 

 only with its kind. The fourth kind is 

 called "AB." The person with "AB" 

 blood can receive blood from every 

 other type and is called the "universal 

 recipient." Here again there are occa- 

 sional exceptions. This knowledge of the 

 four kinds of blood is the result of the 



