506 



ORGAN SYSTEMS OF MAN 



The more we learn about blood, the 

 greater its importance becomes. Let us re- 

 view its function in nutrition and metabo- 

 lism by following it on a routine trip 

 through the body ( Fig. 19-13 ) . Blood leav- 

 ing the left ventricle goes to the liver, di- 

 gestive tract, kidney, and body tissues. The 

 blood coming from the liver contains more 

 urea, that from the kidney less urea, that 

 from the intestines more amino acids and 

 glucose, and that from the general body 

 tissues less food and more waste products. 

 Moreover, all of the blood leaving these 

 organs is low in Oo, which must be replen- 

 ished in the lungs at the same time that 

 COo is lost. The urea which is formed in 

 the liver must make a complete circuit 

 through the lungs before it reaches the 

 kidneys, where it is extracted from the 

 blood. Fats are absorbed into the lymphatic 

 system which eventually joins the blood 

 system, making possible the distribution of 

 this food. As the blood passes through the 

 endocrine glands, food products are ab- 

 sorbed and converted into hormones which 

 are then secreted into the blood. The per- 

 formance of all of these functions and many 

 more makes blood truly a most remarkable 

 fluid. 



Closer examination shows that blood is 

 a tissue like muscle, nerve, or bone, even 

 though it exists in a fluid state. The fluid 

 portion of the blood is called the plasma. 

 Floating in the plasma are certain formed 

 elements consisting of erythrocytes (red 

 blood cells ) , leucocytes ( white blood cells ) 

 and platelets. In addition, the plasma car- 

 ries a load of a large variety of substances, 

 some of which still are not well understood. 

 Like all tissues, blood is mostly water, about 

 80 per cent; the 20 per cent of solids consists 

 of approximately 18 per cent protein and 2 

 per cent other chemical substances. When 

 the formed elements are separated from 

 the plasma, they are found to make up 

 nearly one-half of the volume ( 45 per cent ) . 

 The total amount of blood in a normal per- 

 son is 5-6 liters, or approximately 8-10 per 



cent of his body weight. One can lose some- 

 what less than half of this amount and sur- 

 vive, but a special mechanism for coagula- 

 tion or clotting is present to prevent blood 

 loss. 



Blood clotting 



The survival value of any mechanism 

 that prevents the loss of blood is obvious. 

 Blood coagulates even more rapidly in the 

 earthworm, for example, than in man. The 

 blob of jelly-like substance on the wind- 

 shield of a speeding car demonstrates the 

 rapidity with which the insect's blood co- 

 agulates. Animals have very short clotting 

 times when compared to the three minutes 

 required for man's blood to clot, for the 

 speed with which the clot forms often spells 

 the difference between life and death. 



The exact series of chemical reactions 

 that takes place in forming a blood clot 

 are not completely understood, although a 

 great deal of work has been done on the 

 problem. The clot is made up of a mass of 

 threads of a protein called fibrin, which en- 

 meshes red blood cells so that a semi-solid 

 plug is formed. It starts as a small clot but 

 grows rapidly until it is of sufficient size to 

 fill the opening in the vessel. If the vessel 

 is too large or the pressure too great from 

 behind, as in the large arteries, the clot fails 

 to stop the blood flow and death of the ani- 

 mal results. Ragged injuries produce better 

 and faster-forming clots than do clean cuts, 

 the explanation of which will follow shortly. 



There are several observations on blood 

 that need to be known before the clotting 

 mechanism can be understood. If blood is 

 collected in a vessel containing sodium 

 citrate or oxalate, it fails to clot; and if it is 

 allowed to flow into a paraffin-lined vessel, 

 it clots only very slowly. In the first in- 

 stance, the chemicals must have blocked 

 the clotting action of the blood and in the 

 second, the nature of the paraffin surface 

 must have been involved. It is now known 

 that the sodium citrate or oxalate combines 

 with the calcium in the blood, removing it 



