Topic 3 Homeostasis PDF

Topic 3: Homeostasis 1. Homeostasis is the ability of an organism to maintain a constant environment in its body within small tolerance limits necessary for life. 2. Environment in the body is referring to internal / cellular environment. 3. Some important internal environmental factors that the...

Topic 3: Homeostasis 1. Homeostasis is the ability of an organism to maintain a constant environment in its body within small tolerance limits necessary for life. 2. Environment in the body is referring to internal / cellular environment. 3. Some important internal environmental factors that the body needs to regulate are: a) body temperature b) blood pH c) blood pressure d) concentrations of dissolved substances in body fluids e) concentration of blood glucose f) concentration of O2 & CO2 g) concentration of metabolic wastes 4. Tolerance limits are defined as the upper & lower limits of a normal range of a factor. 5. Within these limits the body functions normally but when outside this range the body dysfunctions will take place. 6. Homeostasis provides the body with a degree of independence of the environment. 7. Independence of the environment is used as a criterion of the success of an organism. 8. On this basis mammals are seen as successful group because they are able to maintain constant levels of activity despite fluctuations in external / internal environmental conditions. 9. This steady state is maintained by feedback systems, usually operated through negative feedback. Feedback systems 1. A feedback system is a circular situation in which the body responds to a stimulus, with the response altering the original stimulus. 2. There are 2 types of feedback system: a) Negative feedback Involved in maintaining the conditions within a narrow range. The responses will counteract the original stimulus & restore the system to its original state. ie. the response reverses the direction of the stimulus. For eg. controlling the blood glucose level 1  b) Positive feedback In this situation, a stimulus will cause responses which increase the stimulus even further in the same direction. ie. positive feedback acts as an amplifier of response. Therefore, it does not contribute to homeostasis. It can be damaging to the body. For eg. high fever / heat stroke (an increased body temperature causes an increased metabolic rate that produces more heat, which raises the temperature even further). But in some conditions, it becomes useful. For eg. uterine contractions during childbirth through the release of oxytocin. Negative feedback system (stimulus-response model) 1. The system consists of: a) Receptor (detector) specialised cells either in brains / organs such as pancreas. detect the stimulus. A stimulus never stays exactly constant but fluctuates around the set value (set point). b) Modulator (processing centre) responsible for processing information from receptor. sending information to effector. c) Effector can be organs (eg. skin, liver & kidneys), muscles / glands. carries out a response that reverses the stimulus. 2. The communication in the feedback system may be by hormones / nerve impulses. 2 The stimulus-response model 3 Thermoregulation 1. It is important because: a) at high body temperature (hyperthermia) enzymes are denatured, causing metabolic reactions to fail. b) at low body temperature (hypothermia) enzymes are inactivated, causing metabolic activities to slow down. 2. Thermoregulation involves controlling the amount of heat lost & heat gained across the body surface. 3. Conduction, convection & radiation may move heat into & out of the body. 4. However, evaporation can only remove heat. 5. Conduction = transfer of heat from the hotter to the colder of two surfaces in contact. 6. Convection = transfer of heat by the movement of air / liquid past a surface. 7. Radiation = transfer of heat from the hot object to a colder object not in contact by infra-red waves. 8. Evaporation = the change of liquid to vapour, accompanied by cooling. 9. Heat also can be gained through metabolism. 10. During cell respiration, food is oxidised to release energy. 11. Some energy (ATP) is used to carry out cellular works such as active transport & cell division. 12. However, most of the energy is released as heat energy. 13. The basis of thermoregulation is a balance between heat gain & heat loss. 14. A constant body temperature is achieved only if there is a heat balance. 15. Thermoregulation is controlled by a region of the brain called the hypothalamus. 16. The temperature it monitors is the body core temperature. 17. The major homeostatic organ involved in thermoregulation is the skin because it has a large surface area. 4 Components of thermoregulatory system 1. Receptors: a) Peripheral thermoreceptors in the skin (warm & cold receptors) detect changes in the external temperature. b) Central thermoreceptors in the hypothalamus detect changes in the blood temperature. 2. Modulator: Thermoregulatory centre in the hypothalamus. 3. Effectors: a) Skin-based effectors i. Sweat glands ii. Hair erector muscles iii. Skin arterioles / capillaries b) Skeletal muscles c) Endocrine glands i. Adrenal gland ii. Thyroid gland 5 Responding to decreased temperature (increase heat gain & reduce heat loss) 1. Effectors: a) Skin-based effectors i. Sweat glands not stimulated. No sweat production. Therefore, no evaporation of sweat. ii. Hair erector muscles contract. Raises the hairs & trap a layer of air. The air becomes an insulating layer which helps to reduce heat loss. iii. Skin arterioles / capillaries Vasoconstriction. Less blood flows to the surface of the skin. So, there is less heat loss by radiation. b) Skeletal muscles shiver (involuntary contraction) produce heat. c) Endocrine glands i. Adrenal gland stimulated to release adrenaline. Both hormones increase the body’s metabolic ii. Thyroid gland rate in the liver to generate heat. stimulated to release thyroxine. 6 Responding to increased temperature (reduce heat gain & increase heat loss) 1. Effectors: a) Skin-based effectors i. Sweat glands stimulated to produce & release sweat. Evaporation of sweat removes heat from skin surface. ii. Hair erector muscles relax. Lowers the hairs & therefore, minimised trapped air layer. So, allows heat loss. iii. Skin arterioles / capillaries Vasodilation. More blood flows / closer to the surface of the skin. So, increases heat loss by radiation. b) Skeletal muscles not stimulated & hence, no shivering. No heat production. c) Endocrine glands i. Adrenal gland not stimulated & hence, no adrenaline secretion. No heat production. ii. Thyroid gland not stimulated & hence, no thyroxine secretion. No heat production. 7 Physiological & behavioural mechanisms for thermoregulation 8 Stimulus-response model for thermoregulation 9 Regulation of blood sugar levels 1. It is important because: a) high blood glucose level (hyperglycaemia). increases blood osmotic pressure as H2O moves out of cells. As a result, chemical reactions may stop. b) low blood glucose level (hypoglycaemia). Cells do not have enough glucose for respiration. So, metabolic reactions may not be able to take place. Cells cannot function normally. 2. There are several organs involved in the regulation of blood sugar levels: a) Liver b) Pancreas c) Adrenal glands Role of liver 1. It converts glucose into glycogen for storage or glycogen to glucose for release into the blood. 10 Role of pancreas 1. There are clusters of hormone-secreting cells called the islets of Langerhans. 2. The cells in the clusters are consisting of α cells & β cells. 3. α cells secrete glucagon whereas β cells secrete insulin. 4. The effects of insulin: 11 5. The effects of glucagon: ❖ Gluconeogenesis = conversion of amino acids & fats to glucose in the liver. Regulation of blood glucose by insulin & glucagon (negative feedback) 12 Role of adrenal glands 1. Adrenal glands: consists of 2 parts: a) adrenal medulla (inner) o secretes adrenaline & noradrenaline b) adrenal cortex (outer) o secretes glucocorticoids (cortisol) A summary of blood glucose homeostasis 13 Treatment of diabetes mellitus using gene therapy 1. It involves taking a copy of the insulin gene & finding an effective way of getting it into the targeted cells so that they can produce the insulin. 2. A vector (eg. virus) is used to carry the insulin gene & deliver it into the cell. 3. Once inside the cell, the gene is expressed to produce the therapeutic insulin. 14 Regulation of body fluid concentrations 1. Osmoregulation is very important because: a) too much H2O cells undergo cytolysis as H2O enters the cells by osmosis. b) too little H2O chemical reactions stop, blood pressure drops & toxic wastes accumulated. 2. Like thermoregulation, the basis of osmoregulation is a balance between fluid gain & fluid loss. 3. A constant composition of body fluids is achieved when fluid gain is equal to fluid loss. 15 The kidneys 1. They are important excretory & osmoregulatory homeostatic organs. 16 2. The functional units of the kidneys are called nephrons. 17 Antidiuretic hormone (ADH) 1. aka vasopressin. 2. It is released from posterior pituitary gland. 3. It causes the distal convoluted tubule & collecting duct to become more permeable to H2O. Negative feedback in osmoregulation Aldosterone 1. In addition to ADH, aldosterone also plays an important role in osmoregulation. 2. It is released from adrenal cortex. 3. It increases the permeability of kidney tubules so that more sodium (salt) is reabsorbed into the bloodstream. 4. As a result, more water is reabsorbed. 18 Regulating water intake by thirst mechanism 19 Regulation of gas concentrations 1. The levels of the respiratory gases must be regulated properly so that a continuous supply of O2 is provided for all cells to carry out cell respiration. 2. At the same time, CO2 is also produced during cell respiration & is required to be removed from body cells as it is a toxic waste product. 3. Too much will cause pH to drop which can damage the cells & disrupt the metabolism. 4. CO2 is carried to the lungs by blood & later excreted in expired air while O2 from inspired air is delivered to the cells by blood. 5. Therefore, both respiratory & circulatory systems are involved in the regulation of gas concentrations. Control of breathing 1. Breathing rate is regulated by respiratory centre located in medulla oblongata. 2. The respiratory centre contains 2 regions: a) Inspiratory centre b) Expiratory centre 20 4. CO2 & H+ (pH) are the 2 main chemical factors that affect breathing rate. 5. The increases in both chemical concentrations are detected by chemoreceptors: a) Central chemoreceptors located in medulla oblongata. detect the changes in CO2 concentration in blood. b) Peripheral chemoreceptors located in aorta (aortic bodies) & carotid arteries (carotid bodies). detect the changes in H+ concentration in blood. Negative feedback control of breathing rate through changes in the concentration of CO2 & the blood pH 21 Voluntary control of breathing 1. The voluntary control comes from the connections between cerebral cortex & spinal cord. 2. It bypasses the respiratory centre in medulla oblongata. 3. It provides protection so that harmful/irritating gases & water are prevented from entering the lungs. 22