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  Section: General Zoology » Activity of Life
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Water and Osmotic Regulation 
  - How Marine Invertebrates Meet Problems of Salt and Water Balance
  - Invasion of Fresh Water
  - Return of Fishes to the Sea
  - How Terrestrial Animals Maintain Salt and Water Balance
Invertebrate Excretory Structures 
  - Contractile Vacuole
  - Nephridium
Vertebrate Kidney 
  - Ancestry and Embryology
  - Vertebrate Kidney Function
  - Tubular Reabsorption
  - Tubular Secretion
  - Water Excretion
Temperature Regulation 
  - Ectothermy and Endothermy
  - How Ectotherms Achieve Temperature Independence
  - Temperature Regulation in Endotherms
  - Adaptive Hypothermia in Birds and Mammals

Homeostasis: Birth of a Concept

The tendency toward internal stabilization of the animal body was first recognized by Claude Bernard, great French physiologist of the nineteenth century who, through his studies of blood glucose and liver glycogen, discovered the first internal secretions. Out of a lifetime of study and experimentation gradually grew the principle for which this retiring and lonely man is best remembered, that of constancy of the internal environment, a principle that in time would pervade physiology and medicine. Years later, at Harvard University, American physiologist Walter B. Cannon (Figure 32-1) reshaped and restated Bernard’s idea. Developed out of his studies of the nervous system and reactions to stress, he described the ceaseless balancing and rebalancing of physiological processes that maintain stability and restore the normal state when it has been disturbed. He also gave it a name: homeostasis. The term soon flooded the medical literature of the 1930s. Physicians spoke of getting their patients back into homeostasis. Even politicians and sociologists saw what they considered deep nonphysiological implications. Cannon enjoyed this broadened application of the concept and later suggested that democracy was the form of government that took a homeostatic middle course. Despite the enduring importance of the homeostasis concept, Cannon never received the Nobel Prize—one of several acknowledged oversights of the Nobel Committee. Late in life, Cannon expressed his ideas about scientific research in his autobiography, The Way of an Investigator. This engaging book describes the resourceful career of a homespun man whose life embodied the traits that favor successful research.
Digestion and Nutrition in Animals, Zoology, Biocyclopedia
Figure 32-1 Walter Bradford Cannon
(1871 to 1945), Harvard professor of physiology
who coined the term “homeostasis” and developed
the concept originated by French physiologist
Claude Bernard (Figure 33-2).

The concept of homeostasis, described in the section opening essay, permeates all physiological thinking and is the theme of this and the following section. Although this concept was first developed from studies with mammals, it applies to single-celled organisms as well as to vertebrates. Potential changes in the internal environment arise from two sources. First, metabolic activities require a constant supply of materials, such as oxygen, nutrients, and salts, that cells withdraw from their surroundings and that must be replaced. Cellular activity also produces waste products that must be expelled. Second, the internal environment responds to changes in the organism’s external environment. Changes from either source must be stabilized by the physiological mechanisms of homeostasis.

In more complex metazoans, homeostasis is maintained by the coordinated activities of the circulatory, nervous, and endocrine systems, and especially by organs that serve as sites of exchange with the external environment. These last include kidneys, lungs or gills, digestive tract, and integument. Through these organs oxygen, foodstuffs, minerals, and other constituents of body fluids enter, water is exchanged, heat is lost, and metabolic wastes are eliminated.

We look first at the problems of controlling the internal fluid environment of animals living in aquatic habitats. Next we briefly examine how these problems are solved by terrestrial animals and consider the function of the organs that regulate their internal state. Finally we look at strategies that have evolved for living in a world of changing temperatures.


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