516 



ORGAN SYSTEMS OF MAN 



through the cell membrane into the blood 

 where the buffers absorb the extra hydro- 

 gen ions, thus preventing an excess and 

 hence an increase in acidity. Carbon di- 

 oxide coming into the blood from all the 

 cells promptly forms carbonic acid and then 

 carbonate, both of which would make the 

 blood intolerably acid if it were not for 

 the buffers. The carbon dioxide is lost in the 

 lungs, an important factor in removing acid 

 conditions from the blood. Likewise, acidic 

 substances are removed from the circula- 

 tion in the kidneys. All of these factors work 

 together in order that the blood can remain 

 constant as far as the acid-base balance is 

 concerned. 



Water balance. Water is constantly being 

 added to and withdrawn from the blood be- 

 cause it is the only way that this important 

 compound can be delivered to and taken 

 away from the body cells. This is extremely 

 important, since all life processes are main- 

 tained in a water medium within the cells. 

 Therefore, they must have the proper 

 amount at all times. Water is taken into the 

 blood from the digestive tract and lost 

 through the skin as sweat or tlirough the 

 kidneys as urine. Excess water is normally 

 lost through the urine, so that a delicate 

 water balance is maintained in every cell of 

 the body at all times. 



Temperature regulation. Because the 

 body is exposed to widely varying external 

 temperatures, several different mechanisms 

 have been provided to regulate the body 

 temperature. The added muscular contrac- 

 tion brought about by shivering in mam- 

 mals causes a greater burning of the sugar, 

 thus raising the temperature of the entire 

 body. When the body is exposed to low 

 temperatures, the skin becomes pale and 

 cold, owing to the contraction of the capil- 

 laries (vasoconstriction), thus preventing 

 blood from coming close to the external sur- 

 face where it would be unduly cooled. The 

 opposite effect, flushed skin, is noted fol- 

 lowing violent exercise or during particu- 

 larly hot weather. This is because the 



dilated capillaries (vasodilated) in the skin 

 allow the warmer than normal blood to 

 come near the surface where it can lose its 

 excess heat. This control of the size of the 

 skin capillaries is very important in the tem- 

 perature regulating mechanism. In addition, 

 the sweat glands pour out water which, by 

 evaporation, provides an important cooling 

 device. 



Certain special provisions have been 

 made in some animals for increasing sur- 

 face area to make the cooling process more 

 effective. For example, elephants and rab- 

 bits are thought to employ their large ears 

 for this purpose as well as for collecting 

 sound waves. Bats are thought to rely on 

 the circulation in tlie skin of their wings for 

 heat regulation. Man, of course, covers his 

 body with various fabrics, the color and 

 texture of which is changed depending on 

 the temperature. Hair and feathers are 

 excellent insulators against temperature 

 change. Thus the bodies of the warm- 

 blooded birds and mammals are reasonably 

 well suited to withstand the varying tem- 

 peratures they encounter in their particular 

 environments. 



The lymphatic system 



A swelling following a blow on any por- 

 tion of the body is gorged with a fluid 

 called lymph, which is much like plasma 

 except that it does not contain so much pro- 

 tein material. It does, however, contain 

 some white cells, principally lymphocytes. 

 Lymph fills all of the spaces between and 

 around cells and thus bathes every cell of 

 the body. It functions as a medium between 

 the capillary and the cell, a continuum 

 through which food, oxygen, and wastes 

 can enter and leave the cells. 



Lymph passes out through the capillary 

 walls around the cells, and from there 

 moves slowly on into a system of vessels 

 which coalesce and eventually reach the 

 circulatory system in the neck region where 

 the large lymph vessels join the large neck 

 veins. This is as intricate as the venous sys- 



