488 



ORGAN SYSTEMS OF MAN 



the rest is dissolved in the plasma. The 

 molecules unite in a one-to-one ratio, 

 that is: 



Hb + O. 



Hemoglobin 

 (purple) 



(lungs) 



HbO. 



(tissues) Oxyhemoglobin 

 (scarlet) 



Obviously the reaction must be a reversible 

 one, because the oxygen must be as readily 

 released to the tissue cells as it is taken up 

 in the alveoli in the lungs. Hence blood 

 leaving the tissues contains largely hemo- 

 globin, whereas that leaving the alveoli 

 contains primarily oxyhemoglobin. The 

 color difference in these two explains why 

 systemic veins are purplish in color while 

 arteries are bright red. 



Hemoglobin is no less remarkable in its 

 ability to facilitate the carrying of large 

 quantities of carbon dioxide. This gas is 

 immediately converted to carbonic acid 

 (H2CO3), which would become a serious 

 problem if it were not transformed to a 

 harmless carbonate during its journey to the 

 lungs where it is eliminated from the body. 

 Carbonic acid would render the blood very 

 acid and this we know does not happen, for 

 the blood has a relatively constant number 

 of hydrogen and hydroxyl ions (pH 7.45) 

 at all times. If, however, the alveoli become 

 unable to eliminate carbon dioxide as in 

 disease (pneumonia), the carbonic acid 

 will build up in the blood which will then 

 become more acid than usual (it will still 

 be alkaline), resulting in a condition called 

 acidosis. Tissues cannot tolerate this acid 

 condition for long and soon will die. Car- 

 bonic acid in the blood is immediately con- 

 verted to a carbonate by uniting with so- 

 dium or potassium ions that are furnished by 

 oxyhemoglobin when it is converted to 

 hemoglobin in the tissues. This is a complex 

 and involved process but one that again em- 

 phasizes the remarkable nature of this one 

 compound, hemoglobin, which is spread so 

 abundantly throughout the animal world. 



For all chemical purposes, nitrogen plays 

 no part in respiratory exchange in man. 



However, it does produce some effects 

 which become important only under un- 

 usual conditions. In probing into the vari- 

 ous corners of his environment man has 

 gotten into trouble with this abundant and 

 omnipresent gas. Trouble starts when he 

 leaves the earth's surface very far, either 

 up into rarefied air or down into the depths 

 of the sea with its tremendous water pres- 

 sures. Gases under pressure remain in solu- 

 tion as long as the pressure is maintained, 

 but the moment the pressure is decreased 

 they will come out of solution in the form 

 of bubbles. This familiar fact is observed 

 when the cap is removed from a bottle of 

 "coke" — bubbles of carbon dioxide rise 

 rapidly from within the fluid because of 

 the release of pressure. 



At sea level there is an atmospheric pres- 

 sure of 760 mm. of mercury exerted on all 

 parts of the body, inside and outside. Minor 

 differences in pressure, such as those due 

 to altitude changes encountered in driving 

 across the country, have little or no effect, 

 because the change is gradual and the gases 

 in the blood and tissues have a chance to 

 adjust themselves in an undisturbing man- 

 ner. However, if the change is sudden, that 

 is, if one suddenly ascends to 20,000 feet in 

 the matter of a minute or two, these dis- 

 solved gases begin to form bubbles in the 

 blood which float about and become lodged 

 in the blood vessels, plugging them and 

 thus cutting off the circulation in parts of 

 the body. This results in violent pain which 

 sometimes causes the person to bend over, 

 hence the name "bends." This discomfort is 

 also experienced by deep sea divers when 

 they begin to surface after being sub- 

 merged to a considerable depth in the sea. 

 Divers must surface slowly to avoid the 

 "bends," although there are considerable 

 differences in individuals, some suffering 

 more acutely than others. With the advent 

 of high speed rocket planes in recent years 

 the necessity for preventing "decompres- 

 sion sickness," as it is referred to now, has 

 become more important than ever. Re- 



