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Unit 1:
Ch. 1
Ch. 2
Ch. 3
Interlude A
Unit 2:
Ch. 4
Ch. 5
Ch. 6
Ch. 7
Ch. 8
Ch. 9
Interlude B
Unit 3:
Ch. 10
Ch. 11
Ch. 12
Ch. 13
Ch. 14
Ch. 15
Interlude C
Unit 4:
Ch. 16
Ch. 17
Ch. 18
Ch. 19
Interlude D
Unit 5:
Ch. 20
Ch. 21
Ch. 22
Ch. 23
Ch. 24
Ch. 25
Ch. 26
Ch. 27
Ch. 28
Ch. 29
Ch. 30
Interlude E
Unit 6:
Ch. 31
Ch. 32
Interlude F
Unit 7:
Ch. 33
Ch. 34
Ch. 35
Ch. 36
Ch. 37
Ch. 38
Interlude G

» Getting Started » A Guide to the Reading » Tying it all together

Getting Started

Below are a few questions to consider prior to reading Chapter 20. These questions will help guide your exploration and assist you in identifying some of the key concepts presented in this chapter.

  1. What is homeostasis and why is it important for the normal function of an animal?
  2. What things do animals regulate when maintaining homeostasis?
  3. What roles do the circulatory system, metabolism, and the kidneys play in regulating salt and water balance?
  4. How does the regulation of salts and water differ between freshwater and marine organisms?  between large and small animals?
  5. How does the body temperature of an endotherm differ from an ectotherm?  What does this mean for the physiology of each animal?
  6. How do organisms interact with their environment to regulate body temperature?
  7. What happens to organisms if the environment is too stressful and they are unable to regulate their internal environment?

A Guide to the Reading

When exploring the content in Chapter 20 for the first time, the following concepts typically give students the most difficulty. For each concept, one or more references have been identified which may help you gain a better understanding of these potentially problematic areas.


Maintenance of a constant and appropriate internal environment by an organism is known as homeostasis. Most organisms function best within a narrow range of conditions and must regulate their internal environment while living in a fluctuating external environment. Internal conditions are kept constant through the regulation of water content, solute concentration, and body temperature.  Maintenance of homeostasis requires that all of an organism’s gains equal the losses and many times this requires the expenditure of energy.  Realize that not all organisms maintain homeostasis, but allow the internal environment to change with their surroundings and that maintenance of homeostasis differs between small and large organisms. 

For more information on this concept, be sure to focus on:

  • In Section 20.2, Gains must equal losses
  • In Section 20.2, Homeostasis requires energy
  • Figure 20.2, Size Affects Homeostasis

Solute and Water Regulation and the Function of the Kidney

Water and solute balance is crucial for the survival of animals.  While some organisms live in stable environments and do not need to control water exchange, most organisms must actively regulate their water and solute balance.  This is especially true for aquatic fish that maintain their internal solute concentrations at different levels from their marine or freshwater environments.  The major site for the regulation of solute concentrations, water, and wastes in terrestrial vertebrates is the kidney.  The kidney uses three basic mechanisms — filtration, reabsorption, and secretion  — to accomplish this task.  Human kidneys filter water and a variety of small molecules from the blood, reabsorb much of that water and some of the molecules from the filtrate, secrete other molecules and toxins into the filtrate, and finally produce a highly concentrated fluid called urine.  The kidneys are also important for the loss of nitrogenous waste such as urea or uric acid in terrestrial animals. 

For more information on this concept, be sure to focus on:

  • In Section 20.3, The environment affects the regulation of body fluid composition
  • In Section 20.3,  Human kidneys regulate body water and solutes
  • Figure 20.5, How an Animal Regulates Its Water and Solute Concentrations Depends on Its Environment
  • Figure 20.7, The Human Kidney Regulates Internal Water Content and Removes Toxic Wastes

Ectotherm vs. Endotherm

Endotherms are organisms such as mammals that can generate and trap sufficient metabolic heat to maintain an elevated and constant body temperature independent of environmental conditions. Ectotherms, such as snakes, cannot produce and store metabolic heat as effectively and are thus dependent on the external environment for their heat gain.  Both types of animals live in a thermally complex environment and exchange heat with this environment in three ways: conduction, radiation, and evaporation.  Conduction occurs by direct contact between the organism and its environment.  Radiant heat exchange involves the transfer of energy through light.  Convection is an additional means by which the movement of heat in a gas or liquid transfers heat and it increases the rate of heat exchange by conduction and radiation.  Additional cooling of a surface can occur through evaporation that uses heat to change water from liquid to vapor.  Just as with water and solute regulation all inputs must equal the outputs.  Additionally endotherms maintain their body temperature under cold conditions by increasing their metabolic heat production, which is a by-product of the biochemistry of life.

For more information on this concept, be sure to focus on:

  • In Section 20.4, Animals have two basic ways of regulating their internal temperature
  • Figure 20.8, Temperature Regulation in Endotherms versus Ectotherms
  • Figure 20.9,  Heat Exchange on Land and in Water
  • Figure 20.10, Some Strategies for Dealing with Different Kinds of Heat Exchanges

Tying it all together

Several concepts presented in this chapter build upon concepts presented in previous chapters and are also revisited and discussed in greater detail in subsequent chapters, including:

Basic Rules Governing Homeostasis

  • Chapter 24 – Section 24.2, Hormones Regulate Our Internal Environment

Regulation of Water and Solute Concentrations

  • Chapter 6 - Section  6.2,  Water Requires a Cellular Balancing Act

How Animals Gain and Lose Heat

  • Chapter 8 - Section 8.3,  Catabolism: Breaking Down Molecules for Energy

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