Exercise XVI 



BLOOD AND CIRCULATION 81 



metabolically, dying after an average life of 

 about 120 days. 



Human red blood cells are about 7.5 microns 

 in diameter and have a biconcave disc shape 

 which facilitates gas exchange. They are present 

 in great numbers in the blood; a normal young 

 man may have nearly six million erythrocytes 

 per cubic millimeter of blood. (If the human 

 blood volume is 6 liters, how many new red 

 blood cells must be produced per day to keep the 

 total number constant?) 



For defense, the body depends on both plasma 

 proteins and cells. The plasma contains a special 

 group of proteins, called antibodies, which com- 

 bine with and hence inactivate foreign proteins, 

 viruses, or polysaccharides, and also cause in- 

 vading bacteria to clump together. Each anti- 

 body is specific for the substance or type of cell 

 with which it reacts. Somehow our defense 

 machinery knows the shapes of our own pro- 

 teins and leaves them alone. When foreign pro- 

 teins or polysaccharides called antigens are intro- 

 duced into the circulation, antibodies against 

 them are quickly synthesized. 



The cells of the defense system, the white 

 blood cells or leucocytes, in marked contrast to 

 the red blood cells, are motile and highly active. 

 They can travel about in the blood stream, or by 

 going through the wall of a blood vessel can 

 wander out into the tissues and tissue spaces. 

 They move more or less as does an ameba, by 

 flowing in one direction or another. When in- 

 fection strikes, they quickly travel to the inva- 

 sion site in great numbers. There they destroy 

 large numbers of mvading organisms by in- 

 gesting them, a process called phagocytosis, and 

 also release special substances which help or- 

 ganize the defense. The pus formed in and 

 around an infection consists of dead white 

 blood cells. 



A specialized group of white blood cells, the 

 plasma cells (plasmocytes), produce antibodies. 

 White blood cells can be divided into two groups : 

 the round, smooth-nucleated lymphocytes and 

 the granulocytes, which have irregularly lobed 

 nuclei. White blood cells are slightly larger than 

 red blood cells, and are present in considerably 



smaller numbers (about 8000 per cubic milli- 

 meter of blood). During infection, however, 

 their number increases enormously, and this in- 

 crease provides a sensitive warning that an infec- 

 tion is present. 



A third group of elements in the blood, the 

 platelets (thrombocytes), is involved in clotting. 

 When a blood vessel is cut open, an interlacing 

 network of fibrin forms a clot which eventually 

 closes the wound. This process is complicated, 

 involving the platelets, calcium ions, and the 

 plasma proteins thrombin and fibrinogen (throm- 

 bin is a proteinase which activates fibrinogen by 

 hydrolyzing off part of it, turning it into fibrin). 



In addition to its nutritive and defensive 

 activities, the blood provides a constant internal 

 environment for the cells and tissues of the body. 

 In a mammal the pH, temperature, and sugar 

 concentration of the blood are held within very 

 narrow limits. This relative stability of the in- 

 ternal environment makes it possible for a mam- 

 mal to experience enormous changes in the ex- 

 ternal environment without damage. The great 

 nineteenth century physiologist, Claude Bernard, 

 was thinking of this when he said, "The con- 

 stancy of the internal environment is the condi- 

 tion of a free life." 



During this period you will prepare and ex- 

 amine a stained smear of your own blood, and 

 will determine your blood type. We shall ex- 

 amine also the anatomy of the circulatory sys- 

 tem and the heart, and will observe the absorp- 

 tion spectrum of hemoglobin and its changes on 

 combination with oxygen and carbon monoxide. 



BLOOD CELLS 



Swab the ball of your middle or forefinger with 

 70% alcohol. Using a new, sterile lancet, punc- 

 ture the skin lightly, so that you can squeeze out 

 a drop of blood. Touch this to a microscope 

 slide about 1 cm from the end. 



[At the same time suspend another drop or 

 two of blood in about 5 drops of isotonic saline 

 solution (0.9% sodium chloride in water) in a 

 small test tube. This concentration of salt solu- 



