PHYSIOLOGY LECTURE THERMOREGULATION (2024)

THIS LECTURE TALKS ABOUT THERMOREGULATION

1. FACULTY OF MEDICINE FOUNDATION MEDICAL SCIENCE PHYSIOLOGY LECTURE # 1 THERMOREGULATION

2. LEARNING OUTCOMES At the end of this session the student would be able to: Describe the basic organization of the body Discuss and explain homeostasis with suitable examples Classify body fluid compartments Tabulate the composition of ECF and ICF. Describe normal variations in body temperature Discuss the role of hypothalamus as thermostat in regulating body temperature Describe the homeostatic mechanisms activated by cold Describe the homeostatic mechanisms activated by heat Describe the mechanisms involved in development of fever Briefly mention the detrimental effects of fever 27-Oct-21 2

3. What is Physiology? A study of how the body works is known as ‘PHYSIOLOGY ’ It describes the vital functions of living organisms and their organs, cells, and molecules

4. Basic Organization A CELL (Latin: Cella = a store room)- is a single Basic Unit of Life A TISSUE (Latin: Tissu= woven material): made of similar cells from the same origin An ORGAN (Latin: Organum = to work) - formed by multiple types of tissues arranged together to form a specific function An Organ System -Consists of organs arranged together to perform specific function/s 27-Oct-21 4

5. BASIC ORGANIZATION : TISSUES TISSUES are of 4 types : Epithelial tissue: It covers the body surface and forms the lining for most internal cavities.  Function: protection, secretion, absorption, filtration. Connective tissue : - composed of cells fibres and ground materials. Functions: provide support and protection.  Example: Loose connective tissue, fat tissue, dense fibrous tissue, cartilage, bone, blood, and lymph 27-Oct-21 5

6. BASIC ORGANIZATION : TISSUES Nervous tissue: Composed of neurons.  Function: rapid conduction of impulses Example: Brain, spinal cord, peripheral nerves Muscle tissue : - 3 types: skeletal, cardiac & smooth muscle Functions: locomotion, movements of internal organs, production of tension.  27-Oct-21 6

7. 27-Oct-21 7 ORGANS ARE MADE OF TISSUES

8. The human body consists of a number of systems 1. Musculoskeletal system - Locomotion 2. Vegetative systems - Digestive System - Respiratory System - Cardiovascular system - Urinary system 3. Defense Systems - Reticulo-endothelial system - Lymphatic System - Blood - Skin 4. Reproductive system 5. Regulatory systems - Maintenance of a species - Nervous system - Endocrine system

9. Homeostasis HOMEOSTASIS: A term used to describe the maintenance of nearly constant internal environment . Internal environment is made of the fluid in which the cells are immersed (interstitial fluid) & the circulating fluid in the blood (plasma) that is in constantly equilibrium with it. Together they form the extra cellular fluid (ECF) Internal environment = ECF = Interstitial fluid + plasma ADVANTAGE OF INTERNAL ENVIRONMENT OVER EXTERNAL ENVIRONMENT: can be controlled at optimum level by fast and slow control systems Examples: Maintaining constant pH, Temperature and Glucose level 27-Oct-21 9

10. 27-Oct-21 10 Total body water & its variation Variation due to age- Neonates: more water than adults 75 to 80% and by the age of 12 months 60% in male young adults Gradually decreases to 50% by the age of 60 years mostly due to increase in fat tissue Variation in sexes --- Females contain less body water due to higher fat content Variation between tissues Most of the tissues contain 70 – 80% of water Three major exceptions are- Plasma 93% Fat 10 – 15% Bone 20% of water

11. 27-Oct-21 11 Body fluid compartments Distribution: 2/3 rd on ICF, 1/3 rd in ECF Compartments of ECF Subdivided in 2 parts: 1. Plasma : fluid portion of blood 2. Interstitial / tissue fluid bathes the cells within the organs Plasma & Interstitial fluid communicate freely in most parts of the body The electrolyte changes in one gets reflected in the other

12. 27-Oct-21 12 Volumes of body fluid compartments In a healthy adult male weighing 70 Kg Total body water (60% of 70 Kg): 42 L ICF (2/3 rd ): 28 L ECF (1/3 rd ) : 14 L Interstitial fluid (80%): 11 L Plasma (20%): 3 L (Blood volume: 5 L ) Transcellular fluid It is a part of ECF that is located inside few spaces lined by epithelial cell (0.5L) Examples: Synovial (joint) fluid, Cerebrospinal fluid, Pleural fluid, Pericardial fluid, Intraocular fluid, Bladder urine etc.

13. Composition of ECF and ICF There is a profound difference of macromolecular and ionic composition of ECF and ICF. The ECF has higher conc. of Na + , Ca 2+ , Cl - and HCO 3 - while K + , PO 4 - and Protein conc. is higher in ICF. This has a direct implication about the excitability, membrane transport and other normal functions of cells.

14. ECF vs ICF composition Cations ECF ICF Anions ECF ICF Na + 142 mEq/L 10 mEq/L Cl - 110 mEq/L 4 mEq/L K + 4 mEq/L 140 mEq/L HCO 3 - 28 mEq/L 10 mEq/L Ca ++ 2.5 mEq/L .0001 mEq/L PO 4 - 4 mEq/L 75 mEq/L Mg ++ 3 mEq/L 28 mEq/L Protein 5 mEq/L (2g%) 40 (16g%)

15. 27-Oct-21 15 Homeostasis: Feedback control In a feedback system an output signal has an effect , negative or positive, on the input signal 2 Types of feedback: Positive feedback control (uncommon) – accelerates a process Negative feedback control (most common) -- helps in the maintenance of homeostasis In negative feedback control the output signal has an inhibitory control over input signal Negative feedback control prevents deviations from a set goal

16. Principle of Negative Feedback Loops Mechanism: Deviations from a given normal set point are detected by a sensor Signals from the sensor trigger Compensatory changes that continue until the set point is again reached Examples: Blood pressure regulation Body temperature regulation Blood glucose regulation Maintenance of ECF volume Maintenance of Sodium ion in ECF Maintain plasma levels of hormones in the normal range 16

17. Negative & Positive Feedback 27-Oct-21 17

18. Positive Feedback Control In positive feedback system the output signal reinforces the deviation The change is gradually increased Examples: Clotting cascade Labor (childbirth) Generation of Action Potential

19. Failure of Homeostasis: Disease Failure of homeostasis results in a condition called disease 🡪 a state of disturbed body function Diabetes 🡪 failure of glucose homeostasis Fever 🡪Failure of temperature homeostasis Acidosis or alkalosis 🡪 failure of acid-base homeostasis Renal failure, respiratory failure, heart failure, liver failure are all examples of failure of homeostatic mechanisms

20. Normal Body Temperature Normal body temperature – 97°F(36°C) to 99.5°F (37.5°C) Average body temp: 37°C or 98.6°F Core temperature (temp of deep tissues of the body) – is maintained constant Shell temperature (Skin temperature) varies with temperature of surroundings 27-Oct-21 20

21. C = (F- 32) X 5/9 F = ( C X 9/5) + 32 25 ° C = (25 X 9/5) +32 = 45 + 32 = 77 ° F

22. TEMPERATURE AT VARIOUS PARTS OF BODY Rectal temperature represents core temperature Oral: 0.5°C lower than rectal temperature Axillary temperature: 0.5°C lower than oral temp Extremities cooler than other parts Scrotal temp: 32°C ( 5°C lower than core body temp) 27-Oct-21 22

23. BODY HEAT PRODUCTION AND HEAT LOSS HEAT PRODUCTION Metabolism including BMR Food intake (specific dynamic action) Muscular activity HEAT LOSS Radiation Conduction Convection Evaporation Respiration Micturition & defecation 27-Oct-21 24

24. RADIATION Heat is transferred from objects of higher temp to objects of lower temp in the form of infra red rays --- does not need a medium Body can gain or lose heat depending on temp of objects surrounding it. Usually substantial amount of body heat is lost by this mechanism when the atmosphere is cooler 27-Oct-21 25

25. CONDUCTION Heat is transferred from between two objects that are in direct contact. If the body is in contact with a cooler surface it will lose heat Example: from muscle to skin 🡪clothes 🡪 any object in contact/air 27-Oct-21 26

26. CONVECTION As the air is constantly moving around us, the colder molecules in the air sweep away the warm molecules next to our skin Convection loss is proportional to wind speed Conduction and convection together will account for 10-20% of heat loss This also occurs when skin is submerged in water. Convection is 25 times more in water 27-Oct-21 27

27. EVAPORATION Heat is lost from skin by evaporation: Insensible perspiration (constant) Sweating (variable) Body secretes watery sweat from sweat glands – evaporation of water 🡪 removes substantial amount of heat from the body In presence of high moisture in the environment, the sweat can not evaporate easily 🡪 person feels discomfort 🡪 sweat factor 27-Oct-21 28

28. SKIN RESPONSE IN TEMP VARIATION HOT CONDITIONS sweat glands secrete sweat 🡪evaporation 🡪heat loss Arteriolar dilatation🡪 increases blood flow through superficial capillaries 🡪 skin gets heated 🡪 heat loss by conduction, convection and radiation COLD CONDITIONS No sweat secretion 🡪 no heat loss Hairs on skin stand erect due to contraction of arrector pili muscles🡪 trapping of air 🡪 reduce convection 🡪 conserve heat Arteriolar constriction 🡪 less blood flow into superficial capillaries 27-Oct-21 29

29. HYPOTHALAMUS REGULATES TEMPERATURE Hypothalamus acts as “thermostat” 🡪 thermoregulatory adjustments to the set point, resulting in 37 ° C ± 1 ° C or 98.6 ° F ±1.8 ° F in the core body temperature Anterior hypothalamus responds to increased body temperature Posterior hypothalamus responds to exposure to cold environment Hypothalamus is activated in two ways: Thermal receptors in skin provide input Direct stimulation of hypothalamus through changes in blood temperature perfusing it 27-Oct-21 30

30. RESPONSE OF HYPOTHALAMUS Autonomic : cutaneous vasodilation / vasoconstriction, sweating, shivering (increased muscle tone, micorvibration leading to tremors), non-shivering thermogenesis in infants, piloerction, etc. Hormonal : increase secretion of catecholamines, thyroid hormones causing chemical thermogenesis through altered metabolism 27-Oct-21 31

31. RESPONSE OF SOMATIC SYSTEM Somatic activity: increased or decreased voluntary activity Behavioral : Moving away from heat or cold environment Curling up of body 🡪 reduce body surface area Appropriate clothing Using fans, air conditioners ,heaters Hunger: more food intake 27-Oct-21 32

32. 2.Afferents: Neural:cold receptors in the skin Blood 3.Thermo regulatory Centre Posterior hypothalamus 4.Responses 1.Stimulus Decrease in body temp Increase heat production Shivering Hunger Increased voluntary activity Increased secretion of norepinephrine and epinephrine Decrease heat loss Cutaneous vasoconstriction Curling up Horripilation 5.Body temperature back to normal

33. FEVER Pyrexia /Fever rise in temperature >99°F Occurs in Bacterial infections : upper respiratory tract infection Viral infections: influenza Protozoal infections: malaria Tissue destruction: myocardial infarction Trauma Induced by certain drugs 27-Oct-21 35

34. Pathogenesis of fever Exogenous pyrogens: Bacterial/ viral/ parasitic toxins / antigens / medicines 🡪act on macrophages 🡪 produce interleukins 1 & 6 , Tumor necrosis factor etc. 🡪hypothalamic endothelium 🡪 Arachidonic acid 🡪 Prostaglandin (pyrogens) 🡪 preoptic area of hypothalamus🡪 raises set point 🡪 fever Endogenous pyrogens: Similarly the cytokines released from inflammation of tissues enter the brain and stimulate formation of prostaglandin Aspirin, a prostaglandin inhibitor used as antipyretic 27-Oct-21 36

35. Pyrogenic & Nonpyrogenic Hyperthermia Hyperthermia is divided into two categories: pyrogenic (fever) and nonpyrogenic (heatstroke) Pyrogenic hyperthermia is a result of the hypothalamus being reset to a higher point than normal Nonpyrogenic hyperthermia (heat stroke) is caused by external or environmental factors, and the hypothalamus “set point” remains normal but the ability to dissipate heat and regulate body temperature is lost. 27-Oct-21 37

36. 27-Oct-21 38 Heat Stroke (Hyperpyrexia) Few causes: Being stuck in a hot car Exercise intolerance with high humidity Thick hair coat Hyperthyroidism Seizures Tremors Characteristic features Headache Restlessness Vomiting Mental confusion Delirium Panting Convulsions Weak pulses Cardiovascular collapse Coma

37. 27-Oct-21 39 Detrimental effects of Pyrexia FEVER Dehydration Loss of NaCl in sweat Alkalosis due to hyperventilation Negative nitrogen balance HYPERPYREXIA All the detrimental effects of fever It can cause permanent damage to brain, liver and kidneys

38. Main Reference / Additional Reference Medical Physiology by RK Marya - 4 th Ed Text book of Medical physiology by Guyton and Hall – 11 th Ed Ganong’s Review Of Medical Physiology – 23rd Ed. 27-Oct-21 40

39. THANK YOU