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1 Chapter 9 Motivation: Temperature & Thirst

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1 1 Chapter 9 Motivation: Temperature & Thirst

2 2 Maintaining Homeostasis Homeostasis an organism’s ability to adjust its physiological processes in order to maintain equilibrium (optimal levels of a certain state)

3 Copyright © 2004 Allyn and Bacon Physiological Regulatory Mechanisms Thermostat Metaphor (55º) 65º

4 4 Motivation a process that activates and directs behavior. Perceived deviations from a set point generates an unpleasant drive state. Resulting behaviors are specific to the drive state (e.g. we do not seek out water when we’re hungry).

5 5 Regulating Body Temperature Ectotherms (amphibians, reptiles and fish) rely on external factors, such as sunlight, in order to maintain body temperature. Endotherms (mammals and birds) have the ability to maintain body temperature through internal activity. Although they use different control methods, ectotherms and endotherms maintain similar body temperatures.

6 6 Body Temperature and Surface-to-Volume Ratios Surface area determines heat loss. Volume determines heat generation. Smaller animals work harder to maintain warmth.

7 7 Animals Adapt to Temperature Niches Adaptations to temperature may include slim vs. stocky bodies and length of limbs, ears and tails. © Royalty-free/CORBIS

8 8 Behavioral Responses to Temperature (Ectothermic) Change location Change body position Change color or weight of fur or clothing

9 9 Endothermic Responses to Cold Shivering Blood vessels constrict Release of thyroid hormone Activation of brown fat Raynaud’s Disease

10 10 Endothermic Responses to Heat Perspiration In species that do not perspire, animals may pant or lick their fur Blood vessels dilate

11 11 Disturbances in Human Core Temperature Fever a controlled increase in the temperature set point. Hyperthermia a failure in mechanisms designed to cope with high core temperatures. Hypothermia occurs when sufficient heat cannot be maintained. Korey Stringer is one of 16 football players to die recently from heatstroke. AP/Wide World

12 12 Two Areas of the Hypothalamus Participate in Temperature Regulation

13 13 The Hypothalamus Responds to Changes in Its Own Temperature The hypothalamus receives input from skin sensors and the spinal cord. Heating or cooling the hypothalamus overrides other input. Brainstem and spinal cord systems provide backup to the hypothalamus.

14 14 Thirst Intracellular fluid (67%) Extracellular fluid (33%) Cerebrospinal fluid (<1%) Blood (7%) Interstitial fluid (26%) Extracellular fluid contains different solutes than intracellular fluid. Both fluids types are isotonic containing equal concentrations of solutes.

15 15 Osmosis: Water Moves to Make Solutions More Isotonic

16 16 Hypertonic Solution has a higher concentration than another solution Hypotonic Solution has a lower concentration than another solution Kidneys excrete excess water and salt

17 17 The Sensation of Thirst Claude Bernard’s fistula experiments provided evidence against Cannon’s dry mouth hypothesis of thirst. Thirst is more likely to be the result of: drops in intercellular fluid levels (osmotic thirst). drops in extracellular fluid (hypovolemic thirst).

18 18 Mechanisms of Osmotic Thirst Blood becomes hypertonic (relative to intercellular fluid). Eating a salty meal Diabetes mellitus Water leaves cells in an effort to regain the isotonic state. Osmoreceptors in the brain detect cell dehydration.

19 19 Mechanisms of Hypovolemic Thirst Usually occurs due to loss of interstitial fluid and/or blood. Drops in blood volume are accompanied by drops in blood pressure. Baroreceptors in the heart and kidney assess blood pressure. If blood pressure is low, thirst is initiated and the kidneys conserve fluid.

20 20 Hormones Help Conserve Fluids Osmotic or hypovolemic thirst is sensed. Posterior pituitary gland releases ADH. Kidneys reduce urine and release renin. Renin is converted into angiotensin II. Blood vessels constrict and the adrenal glands release aldosterone. Kidneys retain sodium.

21 21 The Cessation of Drinking Receptors may be located in the mouth, throat and digestive tract. Lesions of the septal area produce overdrinking (polydipsia). Overdrinking is usually not a problem, as excess water is excreted in urine.


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