body, many kinds of material are constantly diffusing into and out of the 

 stream. A cell absorbing food is moment by moment reducing the supply 

 for itself, as well as for its neighbors. It is at the same time poisoning the 

 lymph with its wastes and other products of its metabolism. The environ- 

 ment must be a constant source of needed supplies, if life is to continue. 

 But if the environment remains constant, life cannot continue. 



How Does the Blood Maintain Its Stability? 



The Steadfast Blood^ In spite of the physical and chemical changes 

 going on in it all the time, the blood of animals, especially of warm-blooded 

 ones, is remarkably stable. This constancy of the blood has been called 

 homeostasis — standing or remaining the same. Homeostasis is not, how- 

 ever, a static fact or a fixed condition. It is rather a complex process; indeed, 

 it is a living process, remaining "the same" only because it is constantly 

 changing. 



Homeostasis is attained not by preventing changes, or by insulating the 

 blood against all happenings, inside and outside the body. It is attained by 

 making adjustments that neutralize alterations or compensate for them. 

 Chemical changes in the blood, for example, mean an increase in the pro- 

 portions of some substances and a decrease in the proportions of others. Or 

 they mean greater acidity or less, or the appearance of new substances. The 

 blood meets such changes, in general, by removing surpluses and by re- 

 plenishing deficits. 



The circulation itself is a factor in bringing about uniformity, since it 

 stirs up and so redistributes the contents. In addition, however, the struc- 

 ture of the blood, the nervous system, and special "glands" interact in ways 

 that bring about compensations and adjustments from moment to moment. 



Excesses and Deficiencies We are familiar with many adaptive proc- 

 esses that help to keep the blood stable. It is not always clear, however, just 

 how the adjustments are brought about. What is the connection, for exam- 

 ple, between sweating and getting warm ? Or between feeling hunger and 

 running short of nutrition? How does running make one out of breath.? 



When the quantity of a particular substance increases in the blood, some 

 of it diffuses into the tissue spaces by osmosis (see page 87). If the propor- 

 tion of this substance diminishes, some of the relative excess in various 

 tissues diffuses back into the blood. Through osmosis relative excess or 

 shortage becomes equalized. Surpluses removed from the blood-stream may 

 remain temporarily in the spongy network of connective tissue under the 

 skin, and around muscle fibers. Such "temporary storage" in tissue spaces 

 has been compared to the merchant's practice of displaying on his shelves 



iSee No. 4, p. 199. 

 193 



