Mechanisms of Homeostasis

1. Mechanisms of Homeostasis • Variables such as blood pH, carbon dioxide concentration, blood glucose levels, body temperature and water balance are typically maintained within certain limits by homeostatic mechanisms

2. Nervous system and endocrine system work together to maintain homeostasis • Many homeostatic mechanisms are controlled by the autonomic nervous system

3. A homeostatic control system has three functional components • A receptor, a control center, and an effector

4. Most homeostatic control systems function by negative feedback

5. A second type of homeostatic control system is positive feedback • Which involves a change in some variable that triggers mechanisms that amplify the change

6. The internal environment of vertebrates is called the interstitial fluid, and is very different from the external environment • Homeostasis is a balance between externalchanges and the animal’s internal control mechanisms that oppose the changes

7. Regulating and Conforming • Regulating and conforming • Are two extremes in how animals cope with environmental fluctuations

8. An animal is said to be a regulator • If it uses internal control mechanisms to moderate internal change in the face of external, environmental fluctuation • An animal is said to be a conformer

9. Thermoregulation contributes to homeostasis and involves anatomy, physiology, and behavior • Thermoregulation

10. Ectotherms and Endotherms • Ectotherms • Include most invertebrates, fishes, amphibians, and non-bird reptiles • Endotherms • Include birds and mammals

11. Endotherms • Birds and mammals are mainly endothermic, meaning that • Their bodies are warmed mostly by heat generated by metabolism • They typically have higher metabolic rates • Also called homeotherms which means that they keep their body temp relatively constant

12. Ectotherms • Amphibians and reptiles other than birds are ectothermic, meaning that • They gain their heat mostly from external sources • They have lower metabolic rates • Body temp fluctuates with environmental temperatures • Also called poikilotherms

13. In general, ectotherms • Tolerate greater variation in internal temperature than endotherms

14. Endothermy ismore energetically expensive than ectothermy • But buffers animals’ internal temperatures against external fluctuations

15. Modes of Heat Exchange • Organisms exchange heat by four physical processes

16. Balancing Heat Loss and Gain • Thermoregulation involves physiological and behavioral adjustments that balance heat gain and loss

17. Endotherms and some ectotherms maintain a constant internal temperature as the external temperature fluctuates by changing the rate of heat production, or by changing the rate of heat gain or loss

18. Animals adjust rate of heat gain or loss through several adaptations • Hairs trap layer of air next to skin and insulate animal, also thick coats in winter and thinner coats in summer • Countercurrent heat exchanger in some sharks

19. Insulation • Insulation, which is a major thermoregulatory adaptation in mammals and birds • Reduces the flow of heat between an animal and its environment

20. In mammals, the integumentary system • Acts as insulating material

21. Circulatory Adaptations • Many endotherms and some ectotherms • Can alter the amount of blood flowing between the body core and the skin

22. In vasodilation • Blood flow in the skin increases, facilitating heat loss • In vasoconstriction

23. Many marine mammals and birds • Have arrangements of blood vessels called countercurrent heat exchangers that are important for reducing heat loss

24. Some specializedbony fishes and sharks • Also possess countercurrent heat exchangers

25. Many endothermic insects • Have countercurrent heat exchangers that help maintain a high temperature in the thorax

26. Cooling by Evaporative Heat Loss • Many types of animals • Lose heat through the evaporation of water in sweat

27. Bathing moistens the skin • Which helps to cool an animal down

28. Behavioral Responses • Both endotherms and ectotherms use a variety of behavioral responses to control body temperature • Animals may increase or decrease body heat by relocating-migration to a more suitable climate, basking in sun, huddling together • Reptiles move between warmer or cooler locations to keep body temperature fairly stable

29. Torpor and Energy Conservation • Torpor • Is an adaptation that enables animals to save energy while avoiding difficult and dangerous conditions • Is a physiological state in which activity is low and metabolism decreases (heart and respiratory rate slow down • Animals active in day undergo torpor at night

30. Hibernation is long-term torpor • That is an adaptation to winter cold and food scarcity during which the animal’s body temperature declines

31. Estivation, or summer torpor • Enables animals to survive long periods of high temperatures and scarce water supplies • Daily torpor • Is exhibited by many small mammals and birds and seems to be adapted to their feeding patterns

32. They must lower metabolic rate and hibernate, which allows them to burn very little energy all winter • Endotherms can remain active in severe weather, but must use a lot more energy on heat production

33. Insulin and Glucagon: Control of Blood Glucose • Two types of cells in the pancreas secrete insulin and glucagon, antagonistic hormones that help maintain glucose homeostasis and are found in clusters in the islets of Langerhans (pancreatic islets)

34. Glucagon • Is produced by alpha cells of the islets of Langerhans • Insulin

35. Maintenance of glucose homeostasis

36. Target Tissues for Insulin and Glucagon • Insulin reduces blood glucose levels by • Promoting the cellular uptake of glucose • Liver and muscle cells take up glucose from blood and convert to glycogen for storage

37. Glucagon increases blood glucose levels by • Stimulating the conversion of glycogen to glucose in the liver • Stimulating the breakdown of fat and protein into glucose • Blood glucose levels are maintained around 90 mg/100 ml

38. Diabetes Mellitus • Diabetesmellitus, perhaps the best-known endocrine disorder • Is caused by a deficiency of insulin or a decreased response to insulin in target tissues • Is marked by elevated blood glucose levels

39. Type I diabetes mellitus (insulin-dependent diabetes) is an autoimmune disorder in which the immune system destroys the beta cells of the pancreas (develops before age 15, called juvenile onset diabetes) • Type II diabetes mellitus (non-insulin-dependent diabetes) is characterized either by a deficiency of insulin or, more commonly, by reduced responsiveness of target cells due to some change in insulin receptors (pancreas produces enough insulin, but cells fail to respond normally) • 90 % of U.S. cases

40. Hypoglycemia or low blood glucose levels result from hyperactive beta cells producing too much insulin