Homeostasis, Excretion, and Drugs PDF

Unit 3: Homeostasis, excretion and drugs Control and co-ordination There are two communicating systems, the nervous system and the endocrine system. The two systems co-ordinate and connect the different parts of the body in order to respond to changes in the internal and external environment. This...

Unit 3: Homeostasis, excretion and drugs Control and co-ordination There are two communicating systems, the nervous system and the endocrine system. The two systems co-ordinate and connect the different parts of the body in order to respond to changes in the internal and external environment. This is important in the regulation of substances within the organism and also helps to change the activity of some part of the organism in response to external stimulus as in reflex action The nervous system is much less developed in plants. However there are differences between the two systems as follows: Both are involved in linking the body parts together in order to respond to stimuli. Both have receptors to receive stimuli and effectors to respond to the stimuli. Both are involved in homeostasis They may involve negative or positive feedback Both involve cell signaling. Both involve signal molecule binding to receptor and both involve chemicals (neurotransmitters and hormones). They may use the same chemical such as adrenaline 27 Hormones Hormones are chemicals secreted by endocrine glands directly into the blood to affect the activity of one or more far target organs (or organ). They are then destructed in the liver and excreted by the kidneys. Adrenaline hormone: Adrenaline is secreted from the medulla of the adrenal gland. It is secreted in response to stressful conditions such as fight, flight and fear. In response to these stressful situations, nerve impulses are sent from the brain to the adrenal medulla which releases adrenaline into the blood. Its actions can be summarized in the following table 28 Homeostasis: It is maintenance of the internal environment constant. The system of homeostasis involves: Receptors (sensor): specialized structures to detect any change in the environment (stimulus) Controller: in nervous communication, it is the central nervous system (CNS) Effectors: it is usually a muscle or a gland. It makes a response to the stimulus Homeostasis depends upon negative feedback: Any change in the internal environment is detected by a specific receptor and an action is done to reverse the change and thus restores the set point. For example, if body temperature rises, a mechanism will occur to lower it back to normal. Again, when the parameter returns, the correcting mechanism stops. The internal environment includes the tissue fluid surrounding the cells and the blood from which tissue fluid is formed. The temperature and pH of this environment should be kept constant for the optimum function of the enzymes catalysing reactions in the cells. The water potential of this fluid should also be kept constant, as excess water would enter or leave the cells by osmosis leading to swelling and rupture of these cells or their dehydration. Blood sugar should also be regulated to provide glucose for cell respiration In reality body systems are constantly active, constantly monitoring and responding to changing conditions Positive feedback is rare in biology. ln positive feedback a signal becomes amplified (not controlled). It is needed in special conditions (not homeostasis) as in case of birth 29 where a hormone is released causing the uterus to contract. Uterine contraction leads to more secretion of the hormone. This leads to stronger contraction of the uterus and so on until the process of birth ends. Another example of positive feed back occurs in blood clotting: When a part of the body is injured, it releases chemicals that activate blood platelets. Platelets are responsible for stopping bleeding by forming clots. An activated platelet in turn activates more platelets, which group together to form a blood clot. (In individuals with hemophilia, the blood lacks enough blood-clotting proteins, causing excessive bleeding after an injury.) There are homeostatic organs which include: The kidneys as they remove poisonous substances which may affect enzymes, they control the levels of salts and water (osmoregulation) and acids in the blood. The liver regulates blood glucose level as it can store any excess glucose as glycogen or turns glycogen back to glucose if the glucose concentration in the blood gets too low. Brain cells are very sensitive to blood glucose level. If it is very low, brain cells stop working and the person becomes unconscious and will die unless injected with glucose. It is also affected badly by very high blood glucose. The lungs keep the concentrations of oxygen and carbon dioxide in best levels in the blood. The skin regulates the temperature of the blood for optimum function of enzymes. The brain has an overall control of homeostatic processes in the body. It detects changes in the blood flowing. Any detected change, nerve impulses or hormones are sent to the organs concerned to make the necessary adjustments 30 The skin and regulation of body temperature: In warm-blooded animals (homoeothermic), birds and mammals, body temperature is kept constant for the optimum function of the enzymes. In the hypothalamus in the brain there is thermoregulatory centre (acts as a controller). It monitors the blood passing through it and also receives sensory nerve impulses from temperature receptors in the skin. If any change in the temperature is detected, the hypothalamus sends nerve impulses to the skin to correct the temperature. The skin is formed of epidermis, dermis and a fatty tissue. The dermis contains hair follicles attached to erectile muscle, sebaceous glands, sweat glands, blood vessels and different receptors some of which are nerve endings. The fatty tissue acts as insulator. The skin protects the body as it acts as a barrier against bacteria. The melanin pigment prevents damage by the ultraviolet rays of the sunlight. The skin is a sense organ as it contains sense receptors for touch, pressure, heat, cold and pain. The skin is responsible for temperature regulation: If the body temperature rises (stimulus), e.g. during muscular exercise or when the outside temperature is high. it is detected by temperature receptors in the skin (and in the hypothalamus in the brain). A signal is sent to the brain (control center). The brain sends impulses to the skin to respond (effector) as follows: * The arterioles of the skin dilate so that more blood flows to the capillaries. Hot blood loses heat by radiation. * The sweat gland becomes active secreting sweat. Evaporation of the sweat absorbs a lot of heat cooling the body (test the cooling effect of evaporation by wrapping cotton wool around the base of two thermometers leave one piece of cotton wool dry and 31 drop water on the other piece. Observe a fall in the temperature of the second thermometer). N.B. hairs remain flat so that it will not trap air which is an insulator preventing loss of heat. If the body temperature drops as in cold environment, the temperature receptors detect the change. A signal is sent to the brain which in turn sends impulses to the skin so that: The arterioles of the skin constrict decreasing blood flow in the capillaries to decrease heat loss by radiation. The sweat gland becomes inactive so that sweat is not secreted thus no loss by evaporation. The hairs become erect trapping air which is an insulator preventing loss of heat. The layer of fat in the skin acts as insulator. When metabolized, it generates heat. If severe drop in body temperature occurs; involuntary contraction of muscles occurs (shivering) generating a great amount of heat. The liver also produces a lot of heat. We also control our temperature by adding or removing clothing or deliberately taking exercise. 32 Regulation of the blood glucose: Blood glucose should be kept almost constant as very low blood glucose will lead to decrease in respiration of the cells as glucose is the most important substrate for respiration. Less respiration leads to decrease in the release of energy and ability to work. In the brain, decrease glucose concentration may lead to coma since the brain depends on glucose only as a substrate for respiration to release energy. Very high blood glucose may cause dehydration of the cells by osmosis as glucose is soluble and high blood glucose will lower the water potential of the blood. Dehydration of brain cells lead to coma. Regulation of blood glucose occurs as follows: If blood glucose rises (e.g. after a carbohydrate meal); the islets in the pancreas detect and secrete insulin into the blood. Insulin affects most of the cells especially the liver where: It increases the uptake of glucose by the cells It stimulates respiration It stimulates the conversion of excess glucose into glycogen to be stored It promotes the conversion of glucose to fat When blood glucose returns to normal, insulin stops to be secreted. If blood glucose drops (e.g. during starvation) The islets in the pancreas detect and secrete glucagon which is carried by the blood and acts on the liver only where: It stimulates the breakdown of stored glycogen into glucose which is released to the blood returning it to normal. Then the pancreas stops secreting glucagon. The concentration of normal blood glucose ranges from 90 mg100cm -3 (after fasting for 8 hours) to 140 mg100cm-3. If blood glucose reached below 40mg100cm-3, convulsion 33 and coma may occur. Glucose may be excreted in the urine if the blood glucose rises more than 180 mg100cm-3. Excretion: It is the removal from the organism of toxic materials e.g. drugs and spent hormones, waste products of metabolism e.g. CO2 and nitrogenous waste products such as urea and uric acid and substances in excess of requirement e.g. excess water and salts. Excretory organs are the liver, lungs, kidneys and the skin. The liver is considered as an excretory organ since it excretes bile pigments as bilirubin formed from breakdown of haemoglobin. It is excreted in bile which is carried to the intestine till it egested with faeces. The liver receives amino acids absorbed from the intestine and carried to the liver by the hepatic portal vein. Some amino acids are transported by the blood to every cell to make its own proteins. Some amino acids are used by the liver to synthesise plasma 34 proteins such as fibrinogen which is used in blood clotting. The excess amino acids are deaminated where the nitrogen containing part of the amino acid is removed to form ammonia and the rest is converted to glycogen.. Ammonia is very alkaline and toxic; the liver combines it with CO2 to form urea which is much less poisonous. Urea is transported in the blood plasma to reach the kidney to be excreted. Although it is less harmful than ammonia, the body cannot tolerate high urea concentration in the blood. If kidneys did not remove urea (and other wastes), it would build up in the blood and cause damage to tissues and reduce the ability of the organs to function. The liver breaks down alcohol and other drugs. Also it breaks spent hormones. The products of breaking are transported by the blood to be excreted by the kidney. The lungs get rid of carbon dioxide produced from every aerobically respiring cell. Carbon dioxide dissolves in fluids such as tissue fluid and blood forming carbonic acid. This acidity can affect the enzymes and may be fatal. The lungs also lose a great deal of water vapour but this is unavoidable and is not to control water content in the body. If carbon dioxide increases in the blood, the brain detects and sends impulses to breathing muscles to increase the rate and depth of breathing thus getting rid of the excess carbon dioxide. 35 Salts are lost in the sweat and the kidneys excrete excess salts. Regulation of salt concentration is important as they affect water potential of the blood. The kidneys remove urea and other nitrogenous waste products from the blood. They also expel excess water, salts, hormones and drugs. The skin expels the sweat which consists of water, salts and urea dissolved in it. In this sense it is an excretory organ. However, sweating is a response of a rise in body temperature and not to a change in the blood composition. Therefore, some consider the skin not an excretory organ The excretory (urinary) system is formed of: *Two kidneys: each receives a renal artery, a branch from the aorta, carrying oxygenated blood but with excretory substances. Renal vein comes out from each kidney, and opens in the vena cava,. It carries deoxygenated blood but with much less excretory substances. The kidneys have two functions: 1-excretory function through formation of the urine which is mainly water with dissolved wastes. 2-osmoregulation: keeping water potential of the body constant. *Two ureters: each comes out from each kidney. The ureter carries the urine, transports it by peristalsis to the urinary bladder. *Urinary bladder: for temporary store of urine until its release. 36 *Urethra: carries the urine from the urinary bladder to the outside. A section in the kidney will show 3 zones: an outer cortex, an inner medulla (pyramid in shape) then the start of the ureter (pelvis of the kidney) The unit of the kidney is the nephron: it is formed of Bowman’s capsule (A) which leads to tubules. From the capsule to the proximal convoluted tubule ( B) in the cortex. This is followed by loop of Henle (C) which runs in the medulla. This followed by the distal convoluted tubule (D) which finally opens in the collecting duct (E) running in the medulla parallel to loop of Henle. The collecting duct opens into the pyramids to the pelvis which carries the urine to the ureter. The renal artery branches inside the kidney and sends a branch to every nephron. This is called afferent arteriole. This arteriole branches to form a network of capillaries inside the capsule. This network of 37 capillaries is called glomerulous. The capillaries collect and come out of the capsule as efferent arteriole which branches into capillaries around the tubules. The branches finally collect to form a branch which joins the renal vein. Excretion depends upon two processes; these are ultrafiltration and selective active reabsorption. Ultrafiltration occurs from the blood in the glomerulus into the capsule. Afferent arteriole is wider than the efferent. This makes the blood in the glomerulus to be under high pressure. The filtrate is formed of water with dissolved nutrients e.g. glucose, amino acids, salts… and wastes e.g. urea, uric acid…Large molecules such as plasma proteins and blood cells cannot be filtered. It is similar to tissue fluid i.e. plasma minus plasma proteins. Selective active reabsorption: As filtrate passes along the proximal convoluted tubules, all glucose, amino acids and most of the water and salts are selectively and actively reabsorbed and return back to the blood leading to concentration of wastes like urea. The remaining solution of excess water and salts, nitrogenous waste products is now called the urine. Urine is carried along the collecting duct which transports the urine and passes it to the pelvis of the kidney. Loop of Henle is responsible for creating concentrated medulla. The longer the loop, the more the concentration of the medulla. This concentrated medulla allows water to be reabsorbed from the collecting duct Water balance and osmoregulation: The body gains water from food and drinks in addition to water formed from aerobic respiration in cells. On the other hand water is lost from the body in the urine, by 38 evaporation from the skin particularly during sweating. Air from the lungs is saturated with water vapour. Water is also lost in faeces. Despite these gains and losses, the concentration of body fluids is kept almost constant by the kidney which adjusts the concentration of blood flowing through them. : If water potential of blood becomes low as for example due to excessive sweating in hot weather and during exercise, osmoreceptors in the hypothalamus in the brain will detect this stimulus and sends signals to the pituitary gland to secrete hormone ADH (antidiuretic hormone). The hormone is carried by the blood to the collecting ducts and the distal convoluted tubule of the nephrons. ADH will increase the permeability of the duct to water. Thus allowing more water to be reabsorbed to the blood. This corrects the water potential of the blood (negative feedback). The urine becomes small in volume and concentrated. The opposite occurs when water potential of the blood rises for example after drinking lots of water: ADH will not be secreted and so less water is reabsorbed from the collecting duct. The urine is large in volume and is diluted. This corrects the water potential of the blood again. N.B Loop of Henle is responsible for creating concentrated medulla. The longer the loop, the more concentration of the medulla. This concentrated medulla allows water to be reabsorbed from the collecting duct if it is permeable to water under the effect of ADH. 39 Desert animals have very long loop of Henle so that more water is reabsorbed from the collecting duct; thus conserving water in the body not losing it in the urine. This is opposite to mammals living in fresh water that have short loop of Henle. Fish in freshwater have an environment with too little salt. They have developed adaptations to get rid of extra water and hold onto salt. Their kidneys are specially designed to produce lots of dilute urine. The kidney absorbs lots of salt to maintain homeostasis. In addition, their gills have proteins that actively take salt from the water and bring it into the body. Fish in saltwater have the opposite problem. A high salt concentration in the environment drives water out of the animal. Saltwater fish have kidneys that produce very little urine and keep most of their water inside their body. They also do the opposite process in their gills; instead of bringing in extra salt, the gills in saltwater fish pump the salt out. 40 How the body breaks down alcohol Drugs are chemicals which affect body work. Nicotine, carbon monoxide and tar are chemicals entering the body from tobacco smoking. Tar affects the gas exchange system badly and may. Cause cancer. Nicotine and carbon monoxide affects the circulatory system and may cause coronary heart disease. Alcohol is a drug that is harmful to the body. It is depressant to the nervous system. It has immediate and long-term bad effects. Immediately it is especially dangerous to drivers as it prolongs reaction time, the person becomes less cautious with bad judgment. Long term effects include damage to many organs especially the liver. The liver breaks down and filters out harmful substances in the blood. After swallowing an alcoholic drink, about 25 %of the alcohol is absorbed straight from stomach into the bloodstream. The rest is mostly absorbed from small intestine. Once alcohol has entered bloodstream it remains in body until it is processed. About 90-98 %of alcohol is broken down by enzymes in liver. The other 2-10 % of alcohol is removed in urine, breathed out through lungs or excreted in sweat. The products of breaking the alcohol in the liver are toxic to the liver and leads to fatty liver and cirrhosis (fibrosis) of the liver. Generally the kidneys filter out toxins and the breakdown products (of toxins) from blood and pass them out in urine. They are not selectively reabsorbed. 41 42