- Homeostasis is the regulation of the chemical and physical factors to maintain a constant internal environment.- The physical factors in the internal environment are body temperature and blood pressure while the chemical factors are partial pressure of oxygen and carbon dioxide, osmotic pressure and sugar levels. - Homeostasis involves monitoring changes in the external and internal environments by means of receptors and adjusting the change through a negative feedback mechanism.

Changes in Blood Osmotic Pressure to Urine Output- The water content of the blood determines the blood osmotic pressure. - When the body loses water through sweating, the osmotic pressure of blood increases. - The content of water in the body is regulated through homeostasis. More water is reabsorbed into the blood by the kidneys, hence decreasing the amount of urine eliminated from the body.

Similarly when the osmotic pressure of blood is low owing to the high water content in the blood, less water is reabsorbed into the bloodstream. - Excess water from the kidneys is eliminated as urine, increasing the volume of urine output.

- Urine is produced by the kidneys and excreted out of the body.- The kidneys are situated in the dorsal wall of the abdomen (Figure 3.17). - Each kidney has an outer cortex, an inner medulla and a pelvis which opens into the ureter (Figure 3.18).

- Each kidney consists of numerous tubular units called nephrons. - Each nephron consists of a Bowmans capsule which begins in the cortex. - The tubule continues on from the Bowmans capsule to form the proximal convoluted tubule which leads to the medulla to form a U-shaped loop called the loop of Henle.

- The tubule leads on to form the distal convoluted tubule before it opens into a collecting duct. - In the Bowmans capsule is a little mass of capillaries called the glomerulus which arise from a branch of the renal artery. - A network of capillaries leave the glomerulus to surround the tubule before it enters a branch of the renal vein (Figure 3.19).

Kidneys- Regulate blood pH.- Remove waste products from the circulating blood.- Regulate salt balance and water content of the blood by excreting more or less salt, and increasing the intake or loss of water.- Regulate the osmotic pressure and ionic levels in the blood.- Regulate the chemical composition of the blood.Waste products excreted by the kidneys:(a) from the metabolic reactions in the bodyExample: breakdown of amino acids urea and creatinine breakdown of nucleic acids uric acid

(b) from the foreign substances in the dietExample: Drugs or toxins

- The endothelium and the podocytes of the glomerulus form a filtration membrane.- The membrane allows the passage of water and solutes from the blood into the capsular space.- High hydrostatic pressure forces small molecules into Bowmans capsule.- The pressure is maintained and enhanced as the afferent arteriole has a larger diameter than the efferent arteriole.- As blood enters the glomerulus, ultrafiltration takes place when the high pressure forces fluid through the filtration membrane and into the capsular space. - The filtrate contains no protein and blood cells as they are too large.- The fluid which enters the capsular space is known as glomerular filtrate.

- The diameter of the efferent arteriole which carries blood out of the glomerulus is smaller than the afferent arteriole which carries blood into the glomerulus. - This causes a high resistance and high pressure in the flow of blood in the glomerulus (Figure 3.20). - As a result, there is a high hydrostatic pressure in the glomerulus. Ultrafiltration occurs at the glomerulus.

- The high hydrostatic pressure in the glomerulus causes many constituents of the blood to be filtered out from the glomerulus into the Bowmans capsule. - The glomerular filtrate has the same composition as the plasma except that it does not contain any of the larger components such as red blood cells and plasma proteins.

- Selective reabsorption takes place when substances move across the walls of the renal tubule into the capillary network. - It occurs at the proximal convoluted tubule. - The watery filtrate which is low in salt but high in wastes such as urea will arrive at the distal convoluted tubule and more water, sodium and chloride ions are reabsorbed. - The main components in the glomerular filtrate are water, urea, glucose, amino acids and salts such as sodium ions. As the filtrate passes along the length of the nephron, reabsorption occurs (Figure 3.21).

- At the proximal convoluted tubule, about 65% of the water in the filtrate is reabsorbed back into the surrounding blood capillaries by osmosis. - All the glucose, amino acids, vitamins and some salts are reabsorbed by active transport. - Urea is not reabsorbed. As the filtrate (water, salts, urea) passes along the loop of Henle, about 20% of the water and some salts are reabsorbed into the blood capillaries.- At the distal convoluted tubule and collecting duct, the amount of water and salts that are reabsorbed into the blood capillaries depends on the content of water and salt in the blood. - The reabsorption of water and salts is regulated by the endocrine system.

- Along the tubule, waste products such as urea, uric acid and ammonia are pumped out of the blood capillaries into the distal convoluted tubule by active transport. - This process is called secretion. - Some drugs and other toxic substances are also secreted out of the blood capillaries by simple diffusion (Figure 3.21). - The final glomerular filtrate which remains in the collecting duct is called urine. - The major constituents of urine are approximately 96% water, 2.5% nitrogenous products such as urea, uric acid, ammonia and creatinine, 1.5% salts and other trace elements such as bile pigments.- From the collecting ducts, the urine is channelled into the pelvis and carried out of the kidney by the ureter to the urinary bladder before it is excreted through the urethra.

Osmoregulation- The kidneys also function as an organ of osmoregulation by regulating the salts and water balance in the body to maintain the blood osmotic pressure. - This process is called osmoregulation. - The water and salt content in the blood determines the osmotic pressure in the blood. - The osmotic pressure is high when there is an excess of salts and less water in the blood. - Conversely, the osmotic pressure is low when the blood has more water and less amount of salts.

- Osmoregulation is controlled by two hormones that are the antidiuretic hormone and aldosterone. - Both hormones work through a negative feedback mechanism to regulate the water and salt content in the blood by the process of reabsorption. - The antidiuretic hormone controls the reabsorption of water while the aldosterone controls the reabsorption of salts. - The reabsorption of both water and salts determine the final concentration and volume of urine.Mechanism of Osmoregulation

Mechanism of Osmoregulation

Consequences of Impaired Kidney Function- Kidneys of some people may be damaged by disease, drugs or injury.- Actually, a person can survive with one kidney. - However, if both kidneys stop functioning, the blood osmotic pressure and blood volume will become unstable.- The build-up of toxic wastes in the body can threaten the life. - Hence, people who suffer from kidney failure have to undergo haemodialysis.- Haemodialysis is a process of filtering blood through an artificial way that takes over the functions of a failed kidney.- Alternative way to treat kidney failure.- The failed kidney of the patient is removed and then replaced with the donors kidney.- However, there is a risk of rejection of transplanted organ.

Regulation of Body Temperature- Thermoreceptors (cold receptors and warm receptors) in the skin detect changes in the environmental temperature (Figure 3.25) while those in the hypothalamus detect changes in the temperature of the blood flowing past it. - The hypothalamus acts as the temperature regulatory centre. - It causes the effectors to respond by negative feedback through corrective mechanism to restore the temperature back to its normal level. - The effectors respond by physical means (involving the skin) and by the process of metabolism (which involves the endocrine glands).

Regulation of body temperature by physical meansThe main methods of regulating heat gain and heat loss through the skin is summarised.

In a cold environment(a) Action of muscles- Voluntary muscular activity is increased such as rubbing the hands to keep warm - Involuntary muscles contract and relax frequently leading to shivering to produce heat(b) Action of adrenal glands- Adrenal glands stimulate the secretion of adrenaline which causes an increase in the metabolic rate- More heat is produced(c) Action of thyroid gland- The thyroid gland is stimulated to secrete thyroxine- Metabolic rate is increased. - More heat is produced In a warm environment(a) Action of muscles - Voluntary muscular activity is reduced. Less heat is lost- Involuntary muscles activity is reduced. Less heat is lost(b) Action of adrenal glands- Adrenal glands not stimulated- Less or no adrenaline released- Metabolic rate is reduced- Less or no heat is produced(c) Action of thyroid gland- Thyroid gland not stimulated- Too little thyroxine is secreted- Metabolic rate is normal- Less heat is producedRegulation of body temperature by metabolism process

Concept of Homeostasis- The process by which the condition of the internal environment is maintained in spite of the changing external environment is called homeostasis. - To maintain homeostasis, both the nervous system and the endocrine system are involved in internal communication. - If communication is by hormones, then the effectors which carry out the response are the target organs. - If communication is by nerve impulses, then the effectors that bring about the corrective responses are the muscles and glands.

- In plants, response is controlled by chemicals called plant hormones or plant growth regulators.- A plant hormone is a chemical substance which is produced by the plant and influences the growth and development of the plant. - It brings about its effect near the site of its formation or it may be transported to other parts of the plant where it acts. - Examples of plant hormones are auxins, and ethylene (ethene).Organic compounds which are produced in a small amount.

Effects of Auxins on Growth Responds

Effects of Auxins on Growth Responds

Figure: Effects of auxins on the growth response in shoots

- Hormones are produced to regulate growth and development.- Auxins control the growth movements of shoots and roots in plants. - Auxins are secreted in the apical meristem at the tips of the shoots.- This type of response where the direction of growth movement is determined by the direction of the stimulus is called tropism.

- High concentration of auxins:(i) speeds up the growth of shoots(ii) slows down the growth of roots(iii) increases the rate of cell divisionRole of Auxins in Tropism

Use of Hormones in AgricultureHormone Auxins1. Use of hormone : In hormone rooting powdersExplanationPromote the growth of adventitious roots from the cut ends of stems. It helps cuttings to produce roots more quickly.2. Use of hormone : In selective weed killersExplanationKill the dicot weeds while leaving cereal crops or grasses (monocot) of a lawn largely unharmed.3. Use of hormone : In fruit settingExplanationPrevent young developing fruits from falling off prematurely.Certain flowers that are sprayed with auxins can form fruits without being fertilised. Such fruits are seedless, for example, seedless grapes.The process is called parthenocarpy.

4. Use of hormone : In producing bushy plantsExplanationGardeners cut off the apical bud to reduce level of auxins so that the lateral buds can grow into side shoots and produce bushier plants.5. Use of hormone : In promoting growth of plants and fruitsExplanationPromote growth of plant cells. Gardeners use auxins to stimulate growth of fruits, to delay fruit ripening and prevent fruit from falling off the plant before it is ripe (abscission).Use of Hormones in AgricultureHormone Auxins

Hormone Ethylene4. Use of hormone : In promoting the ripening of fruitsExplanationFruits are usually imported in an unripe state to prevent damage. However, the fruits can then be ripened quickly for sale by exposing them to ethylene gas.Use of Hormones in Agriculture