Faster and Slower^ Organisms are continually generating and losing 

 heat. When the internal temperature rises in our own body, the blood 

 vessels of the skin dilate. More warm blood flows to the body surface, and 

 more heat is lost by radiation. If the rise in temperature continues, sweating 

 and panting cool the body by evaporation. 



On the contrary, if the surface is chilled, the blood vessels of the skin 

 become constricted. If cooling continues, a secretion from a ductless gland 

 (the adrenal) is discharged into the blood; and this induces more rapid 

 oxidation and so increases the heat. 



The so-called goose-flesh that results from chilling the skin corresponds 

 to the "hair-raising" sometimes observed in dogs and cats and other mam- 

 mals, and to the fluffing out of feathers in birds. This reaction increases the 

 air insulation between die body surface and the cold environment. 



Vigorous muscular activity increases the oxygen consumption of cells. 

 At the same time the pumplike movements of the limb muscles make the 

 blood return to the heart more quickly. The heartbeat is quickened, and 

 with an increased quantity of blood in the heart each contraction delivers 

 more blood. As muscular activity increases, the active cells yield more lactic 

 acid and carbonic acid. This slight increase in the acidity of the blood stim- 

 ulates a nerve center and accelerates breathing. Chemical changes similarly 

 stimulate the secretion of epinephrine (see page 313), which in turn brings 

 more sugar into the blood. As activity ceases, the composition of the blood 

 returns to normal. If there is still an excess of acid dissolved in the blood, 

 it is temporarily neutralized by the so-called "buffer salts" — some of the 

 sodium compounds. If the condition of the blood swings toward the alka- 

 line side, respiration becomes slower, and alkaline, or basic, salts are ex- 

 creted through the kidneys until neutrality is re-established. 



We see, then, that the blood maintains its balance both as to materials 

 and as to processes. It draws upon reserves and eliminates or stores sur- 

 pluses. It changes the rates of continuous processes. In almost every emer- 

 gency changes within the body and the action of the "sympathetic" part 

 of the nervous system maintain homeostasis, or the constancy of the in- 

 ternal environment. 



Flying and Circulation There are situations in which the organism 

 cannot adjust its blood system. When a dive-bomber plunges down rapidly 

 and then suddenly turns his plane to fly upward, the blood in his vessels 

 continues down toward his feet and leaves his brain depleted. That condi- 

 tion may last only a few seconds, but that is enough for a complete "black- 

 out" or loss of consciousness. In those circumstances being unconscious for 

 only a short time may be disastrous. 



Even in ordinary flying, a rapidly moving plane making a turn banks 



iSee No. 5, p. 200. 

 196 



