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2. The Animal Body and Principles of Regulation Chapter 43

3. Homeostasis As animals have evolved, specialization of body structures has increased For cells to function efficiently and interact properly, internal body conditions must be relatively constant The dynamic constancy of the internal environment is called homeostasis It is essential for life 3

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5. Homeostasis Negative feedback mechanisms –Changing conditions are detected by sensors (cells or membrane receptors) –Information is fed to an integrating center, also called comparator (brain, spinal cord, or endocrine gland) –Compares conditions to a set point –If conditions deviate too far from a set point, biochemical reactions are initiated to change conditions back toward the set point 5

6. Homeostasis Humans have set points for body temperature, blood glucose concentrations, electrolyte (ion) concentration, tendon tension, etc. Integrating center is often a particular region of the brain or spinal cord Effectors (muscles or glands) change the value of the condition in question back toward the set point value 6

7. 7 Homeostasis

8. Mammals and birds are endothermic –Maintain a relatively constant body temperature independent of the environmental temperature –Humans 37 o C or 98.6 o F –Changes in body temperature are detected by the hypothalamus in the brain 8

9. Homeostasis Negative feedback mechanisms often oppose each other to produce finer degree of control Many internal factors are controlled by antagonistic effectors Have “push–pull” action Increasing activity of one effector is accompanied by decrease in the other 9

10. Homeostasis Antagonistic effectors are involved in the control of body temperature If hypothalamus detects high temperature –Promotes heat dissipation via sweating and dilation of blood vessels in skin If hypothalamus detects low temperature –Promotes heat conservation via shivering and constriction of blood vessels in skin 10

11. 11 Homeostasis

12. Positive feedback mechanisms –Enhance a change – not common –These do not in themselves maintain homeostasis –Important components of some physiological mechanisms Blood clotting Contraction of uterus during childbirth 12

13. 13 Homeostasis

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