MED1001 Human Physiology Lecture 01 – Homeostasis PDF

MED1001 Human Physiology Lecture 01 – Homeostasis 4th September, 2024 Dr TSANG Chi Ching (MHS) [email protected] 7  8 Levels of Cellular Organisation Cells – basic unit of life 4 major categori...

MED1001 Human Physiology Lecture 01 – Homeostasis 4th September, 2024 Dr TSANG Chi Ching (MHS) [email protected] 7  8 Levels of Cellular Organisation Cells – basic unit of life 4 major categories of specialised cells Muscle cells Force & movement Nerve cells (neurones) Electrical signals Epithelial cells Secrete/absorb ions & organic molecules Connective-tissue cells Connect, anchor & support  9 Levels of Structural Organisation © Gordon Betts J et al. Anatomy and Physiology 2e. 2022. Houston: OpenStax.  10 11 Organ Systems Organ system ▫ Collection of organs which perform related functions and interact to accomplish a common activity MED1002 W13 © Gordon Betts J et al. Anatomy and Physiology 2e. 2022. Houston: OpenStax.  11 11 Organ Systems © Gordon Betts J et al. Anatomy and Physiology 2e. 2022. Houston: OpenStax. W11 & W12 W02 W05 & W06  12 11 Organ Systems © Gordon Betts J et al. Anatomy and Physiology 2e. 2022. Houston: OpenStax. MED1006 Online recording (W03) & W04 W07 & W08  13 11 Organ Systems © Gordon Betts J et al. Anatomy and Physiology 2e. 2022. Houston: OpenStax. W09 & W10 W14 14  15 Internal environment © Pearson  16 Internal environment Fluids that surround cells (interstitial fluid) & exist in blood (plasma, higher [protein]) -> Extracellular fluids  17 Homeostasis Cells perform basic cellular processes & specialised activities to maintain internal environment relatively stable (dynamic steady state) ▫ So that organism can survive in changing external environment A dynamic process © Sherwood L. Human Physiology: From Cells to Systems. Brooks/Cole.  18 Homeostasis The dynamic mechanisms that detect & respond to deviations in physiological variables from their set point ‘values’ by initiating effector responses which restore variables to optimal physiological range ▫ Most of common physiological variables of body are maintained within optimal ranges  Which are predictable  19 Homeostasis Elements regulated: ▫ [Nutrients] ▫ PO2 & PCO2 ▫ [Wastes] ▫ pH ▫ [Electrolytes] ▫ Temperature ▫ Blood volume & pressure ▫ Etc.  20 Organism in homeostasis Homeostasis External Internal change change When homeostasis is maintained: Internal change PHYSIOLOGY results in loss of homeostasis When it is not: PATHOPHYSIOLOGY Organism attempts to compensate Compensation fails Compensation succeeds Illness or disease Wellness Pathophysiology Physiology  21 Homeostatic Control System Functionally interconnected network of body components that operate to maintain a given factor in internal environment relatively constant around an optimal level 3 components involved: ▫ Receptor / Sensor  Detects changes (stimuli) ▫ Controller / Integrating centre  Analyses changes, compares them with set points (normal, optimal ranges) & determines proper responses to changes ▫ Effector  Exerts responses  22 Homeostatic Control System Adapted. 3 components: (Controller) (Sensor) Disruption of homeostasis  23 Homeostatic Control System Example (Receptor) © Gordon Betts J et al. Anatomy and Physiology 2e. 2022. Houston: OpenStax.  24 Homeostatic Control Mechanisms Feedback systems ▫ Negative feedback  Change in variable being regulated brings about responses that tend to move variable in direction opposite to original change  i.e. Return to original range (set point) ▫ Positive feedback  Accelerate a process, enhance change  Much less common in nature Feedforward systems ▫ Anticipate changes in regulated variables, improve speed of homeostatic responses & minimise fluctuations in level of regulated variables  25 Feedback Systems Adapted. (Controller) − Reverse (−ve feedback) (Sensor) + Reinforce (+ve feedback)  26 Negative feedback Organ level, e.g. Shuts system off once the set point is reached Molecular/cellular level, e.g.  27 © Sherwood L. Human Physiology: Negative feedback From Cells to Systems. Brooks/Cole. EFFECTOR EFFECTOR RESPONSE RESPONSE  28 Positive feedback  29 Positive feedback  30 Feedforward Example: thermoregulation 1. ↓ Environmental temperature 2. Temperature-sensitive neurones in skin detects change & relay this information to brain 3. Then brain then sends out signal to blood vessels & muscles 4. Heat conservation & ↑ heat production 5. Compensatory thermoregulatory responses are activated before internal body temperature drops  31 Homeostatic Control System Not possible to maintain internal environment in complete constancy ▫ Any regulated variable will have a narrow, optimal range of values depending on external environment  But not always possible to achieve this for every variable!  Hierarchy of importance  Certain variables may be altered markedly to maintain others within their normal range (‘clashing demands’) Set points may be reset ▫ e.g. Fever (higher body temp. set point) 32  33 Homeostatic Control System Adapted. 3 components: (Controller) (Sensor) Disruption of homeostasis  34 Chemical Messengers Intercellular communication  35 Adaptation & Acclimatisation Adaptation ▫ Characteristic that favours survival in specific env. ▫ e.g. ↑ RBC number in people living in high attitude region than those living in sea level area Acclimatisation ▫ Improved functioning of an already existing homeostatic system based on an environmental stress ▫ e.g. Lung function improves with regular exercise Acclimatisation is reversible but adaptation is not  36 Day Night Day Biological Rhythms Many body functions are regulated as rhythmical changes ▫ e.g. Circadian rhythm, which cycles approximately once every 24 h  Waking & sleeping  Body temperature  blood [hormone]  Ion excretion into urine  Etc. Rajaratnam SM, Arendt J. Lancet. 2001;358(9286):999–1005.  37 Biological Rhythms They add an anticipatory component to homeostatic control systems and in effect are a feed-forward system operating without need of receptors/sensors  38 Feedback Mechanisms vs Biological Rhythms Feedback mechanisms ▫ Corrective responses ▫ Initiated after the steady state of the individual has been perturbed Biological rhythms ▫ Homeostatic mechanisms immediately & automatically utilised ▫ Activated when a challenge is likely to occur but before it actually occurs ▫ No receptor needed (cf. feedforward mechanisms) 39  40 Body Temperature Internal core temperature ≈ 37°C Normal variations of body temperature: ▫ Different body regions  Rectal temperature ~0.5°C higher than oral ▫ Different activity patterns & changes in external temperature ▫ Circadian fluctuation of ~1°C during a day ▫ Fluctuation in women in different stages of menstrual cycle  41 Circadian Fluctuation of Body Temperature  42 Heat Loss & Heat Gain Heat input must balance heat output to maintain stable core temperature ▫ Heat input (heat gain)  Heat gain from external environment  Internal heat production ▫ Heat output (heat loss)  Heat loss from exposed body surfaces to external environment If core temperature ↓, heat loss ↓ & heat production ↑ If core temperature ↑, heat loss ↑ & heat production ↓  43 Mechanisms of Heat Transfer  44 Mechanisms of Heat Transfer 4 mechanisms of heat transfer: ▫ Radiation  Emission of heat energy from a surface in form of electromagnetic waves (infrared)  Net heat transfer always goes from warmer objects to cooler objects  Humans lose almost half of their heat energy through radiation ▫ Conduction  Transfer of heat between objects of differing temperatures that are in direct contact  Heat moves from warmer to cooler object  45 Mechanisms of Heat Transfer ▫ Convection  Transfer of heat energy by air (or water) currents  e.g. Heat loss on windy day  Warm air moves away from skin surface  Clothing  Helps trap air & prevents convective air current  Retains heat close to body ▫ Evaporation  Heat loss from body through water evaporation from skin surface or respiratory tract  46 Body Temperature is Homeostatically Regulated Thermoneutral zone ▫ Environmental temperature range (25–30°C) in which heat generated by normal metabolism is sufficient to maintain body temperature ▫ Env. temp. above thermoneutral zone  Heat gain > Heat loss ▫ Env. temp. below thermoneutral zone  Heat loss > Heat gain ▫ In both cases, the body must use homeostatic compensation to maintain a constant internal temp.  Thermoregulatory reflexes  47 Thermoregulatory Pathways Thermoreceptors ▫ Central thermoreceptors  Located in hypothalamus, CNS & abdominal organs  Monitor core temperature ▫ Peripheral thermoreceptors  Located in skin  Monitor skin temperature throughout body Integrating centre ▫ In hypothalamus for temperature regulation Effectors ▫ Skeletal muscles, smooth muscle in arterioles in skin, sweat glands, adrenal medulla, thyroid gland  48 Thermoregulatory Pathways Stimuli Receptors Integrating centre Effectors + Responses Responses © Sherwood L. Human Physiology: From Cells to Systems. Brooks/Cole.  49 Thermoregulatory Pathways (Minor) (Feedforward) (−ve feedback) (Major) (smooth muscles) Adrenaline  50 Thermoregulatory Pathways Stimulus Stimulus Response  51 Thermoregulatory Responses 1. Alteration in cutaneous blood flow ▫ To conserve heat:  Vasoconstriction  Blood flow through cutaneous blood vessels is close to zero ▫ To release heat:  Vasodilation  Nearly 1/3 of cardiac output flow through cutaneous blood vessels  52 Thermoregulatory Responses ▫ Neural regulation  Most arterioles in the body are under sympathetic control 1. Core body temperature falls 2. Hypothalamus selectively activates sympathetic neurones innervating cutaneous arterioles 3. Vascular smooth muscles contract 4. Cutaneous arterioles constrict 5. Resistance increases 6. Blood divert to lower-resistance blood vessels (interior of the body)  Keeps warmer core blood away from cooler skin surface & reduces heat loss  Opposite happens when core body temp. increases ▫ Local control  Vasodilator released by vascular endothelium  53 Thermoregulatory Responses 2. Heat loss by sweating ▫ Evaporation of sweat  Enhances surface heat loss ▫ 2–3 millions sweat glands ▫ Cooling by evaporation, rate depending on humidity  54 Thermoregulatory Responses 3. Heat generation by movement & metabolism ▫ Unregulated heat production  Voluntary muscle contraction  Normal metabolic pathways ▫ Regulated heat production  Maintain temperature homeostasis in low env. temp.  e.g. Shivering  Signals from hypothalamic thermoregulatory centre initiate skeletal muscle tremors  Produces 5–6 times as much heat as resting muscle  55 Thermogenesis Low core temperature High core temperature Decreases heat loss Increases heat loss ▫ Vasoconstriction of skin ▫ Vasodilation of skin vessels vessels ▫ Sweating ▫ Behavioural response (warm ▫ Behavioural response (cooler clothing, curling up body, etc.) clothing, seeking shade, etc.) Increases heat production Decreases heat production ▫ Increasing muscle tone ▫ Decreasing muscle tone & ▫ Shivering voluntary activity ▫ Increasing adrenaline ▫ Decreasing adrenaline secretion secretion ▫ Increasing food appetite ▫ Decreasing food appetite