THE BLOOD 



345 



sues where the food is oxidized, the plasma brings back with it 

 to the lungs the carbon dioxide liberated from the tissues of the 

 body where oxidation has taken place. Blood returning from the 

 tissues of the body has from 45 to 50 cubic centimeters of carbon 

 dioxide to every 100 cubic centimeters. (See Respiration, page 

 380.) Some waste products, to be spoken of later, are also found 

 in the plasma. 



Demonstration. — Get some fresh beef blood. Let it stand overnight in a 

 jar. In the morning it will be found to have separated into two parts, a 

 dark red clot and a thin straw-colored liquid, called serum. Serum is found 

 to be made up of about 90 per cent water, 8 to 9 per cent proteid, and from 

 1 to 2 per cent sugars, fats, and mineral matter. In these respects it rather 

 closely resembles the fluid food that is absorbed from the intestines. 



Clotting of Blood. — Pour another jar of fresh beef blood into a pan and 

 briskly whip it with a bundle of little rods (or with an egg beater). A 

 stringy substance will be found to stick to the rods. This, if washed care- 

 fully, is seen to be almost colorless. Test with nitric acid and ammonia. 

 Note the deep orange color. It is a proteid substance called fibrin. 



Blood plasma, then, is made up of serum, a fluid portion, and 

 fibrin, which, although in a fluid state in the blood vessels within 

 the body, coagulates when removed from the body. 



It is this coagulation which aids in the formation of a blood 

 clot. A clot is simply a mass of fibrin with a large number of 

 corpuscles tangled within. The clotting of blood is of great physi- 

 ological importance, for otherwise we might bleed to death from 

 the smallest wound. 



In blood within the circulatory system of the body the fibrin 

 is held in a fluid state called fibrinogen. It is believed that an 

 enzyme, acting upon this fibrinogen, 

 causes the change to take place in 

 the blood. 



The Red Blood Corpuscle; its 

 Structure and Functions. — In the 

 blood of the frog we have seen that 

 the red corpuscle is a true cell of disk- 

 like form. The red corpuscle of 

 man, however, lacks a nucleus. Its 

 form is that of a biconcave disk. 

 So small and so numerous are these 

 corpuscles that over five million are found in a drop of normal blood. 







Human blood as seen under the com- 

 pound microscope: at the extreme 

 right is a colorless corpuscle. 



