526 



COLLEGE ZOOLOGY 



Human erythrocytes are anuclear circular 

 disks, biconcave in profile. They measure 

 approximately 0.0077 mm. in diameter. The 

 framework of the erythrocyte is colorless 



and sufficiently elastic so that it can squeeze 

 through openings smaller than its diameter. 

 Erythrocytes contain hemoglobin, which 

 consists of a protein (globin) and a nonpro- 





^ 



I 



Blood 

 platelets 



Surface 



I Side 

 Erythro^e Lympnocyte Monocyte Eosinophil Basophil 



Figure 382. Human blood platelets and corpuscles. 



Neutrophil 



tein pigment (hematin). Hemoglobin com- 

 bines easily with oxygen to form oxyhemo- 

 globin and gives up the oxygen in the 

 tissues, becoming thereby reduced hemo- 

 globin. The total surface area of the red 

 blood corpusles in a man is approximately 

 ¥4 acre. 



If hemoglobin were merely dissolved in 

 the blood fluid, as it is in invertebrates, the 

 blood would be so thick that the heart 

 would have to work much harder to pump it 

 through the body. 



Blood platelets are very minute disk- 

 shaped bodies varying in number but aver- 

 aging about 270,000 per cubic millimeter of 

 blood. They release a substance that aids in 

 the clotting of the blood. Leukocytes are 

 capable of amoeboid movements; they wan- 

 der about in the body and pass back and 

 forth through capillary walls (Fig. 383). In 

 man there are about 5 million erythrocytes 

 and about 9000 white blood cells per cubic 

 millimeter of blood. 



The erythrocytes carry oxygen, which in 

 combination with hemoglobin gives them 

 their red color. The white blood cells devour 

 foreign bodies in the blood such as bacteria 

 and broken-down tissue and produce anti- 

 bodies. The neutrophils are the first line of 

 defense against invading organisms. In gen- 

 eral, the chief functions of the blood are to 

 carry oxygen from the lungs to the tissues, 

 to carry waste products to the excretory 

 organs, to carry food to the tissues, to trans- 



port hormones and other secretions, to help 

 maintain a normal temperature, and to de- 

 fend the body against infection. 



Blood clotting 



When a blood vessel is damaged so that 

 blood escapes from it, the flow of blood is 

 stopped by a clot which forms on the sur- 

 face of the wound. This blood clot is com- 

 posed of an interlacing network of fibers in 

 which are enmeshed numerous red blood 

 corpuscles. The chemical changes in the 

 formation of the blood clot are complex and 

 not very well understood. However, the 

 fibers are not present in normal circulating 

 blood; therefore it appears that the fibrin 

 of which the fibers are composed is in a 

 soluble form. The name fibrinogen has been 

 given to this soluble form of fibrin. The sub- 

 stance that converts fibrinogen into the in- 

 soluble fibrin is thrombin. If thrombin were 

 normally present in the blood, clotting 

 would result, but instead it is in the inactive 

 form of prothrombin. Prothrombin, in the 

 presence of calcium and other plasma fac- 

 tors, is converted into thrombin. One of 

 these factors, thrombokinase complex, is not 

 normally present in the blood; therefore, 

 before the chain of clotting reactions can 

 take place, the thrombokinase complex must 

 be formed from the plasma factors, plate- 

 lets, and an extract from the injured tissue 

 cells. Briefly summarized, the steps are as 

 follows-. 



