9.1. Metabolic processes produce waste products that can be harmful to the body.

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1 Excretion in Humans Notes The Process of Excretion Metabolic processes produce waste products that can be harmful to the body. 2 These waste products are constantly removed through the process of excretion. 3 Excretion is defined as the constant removal of metabolic waste products from the body. ENcretion 9.2 :[xe,'..&{umares 1 at ion 1 Excretion in Mammals The main excretory products and excretory organs in mammals are given below. What You Will Learn 9.1 The Process of Excretion 9.2 Excretion in Vammals 9.3 Structure of the Kidney 9.4 Function of the Kidney 9.5 Process of Urine Formation 9.6 Kidney Failure Ed uc Excretory product : : r i Carbon dioxide in exhaled air Nitrogenous waste products in urine Bile pigments 2 Metabolic process involved in generating the excretory product Tissue Excretory organ respiration Lungs Deamination of Kidneys excess amino acids Ski n Breakdown of haemoglobin Liver Three major kinds of nitrogenous waste in mammals are ammonia, uric acids and urea.the type that is excreted depends on the amount of water available to aid its removal. Da ny al 3 The main nitrogenous waste in mammals is urea which is excreted through the kidneys Structure of the Kidney The two human kidneys are bean-shaped and located at the back of the abdominal cavity (Figure 9.1). posterior vena cava renal right kidn aorta renal artery left kidney ureter urinary bladder urethra Figure 9.1 The human urinary system 151

2 (t) G LlJ o- o 2 The kidney is enclosed within a protective fibrous capsule.there are three tubes arising from each kidney (Figure 9.2). (a)the renal artery, which carries oxygenated blood to the kidney.the blood is also rich in excretory products such as urea and mineral sa lts. (b) The renal vein, which carries deoxygenated blood away from the kidney.the blood has less urea and mineral salts. (c) The ureter carries urine from the kidney to the urinary bladder. cortex medulla renal artery renal vein pelvis pyramid (zone in medulla) ureter Figure 9.2 Longitudinal section through a kidney 3 The longitudinal section of a kidney shows three distinct regions (Figure 9.2). (a) Cortex (outer region) (b) Medulla (inner region) (c) Pelvis (central cavity to which the ureter directly connects) 4 Each kidney consists of millions of microscopic tubules called nephrons (Figure 9.3). Bowman's le branch of renal artery pyramid of medu lla Figure 9.3 Long the position of pelvis cortex loop of Henle collecting duct itudinal section of a kidney showing nephrons in the kidney 5 Each nephron, which can be up to 14 mm long, consists of the following structures: (a) Bowman's capsule (b) Proximal convoluted tubule (c) Loop of Henle (d) Distal convoluted tubule, which opens into a collecting duct u riniferous tubule (kidney tubule) cortex med wide afierent arteriole of renr artery glomerulus capsule proximal convol tubule branch of renal vein renal venule loop Henle narrow efierent arteriole collecting duct distal convoluted tubule Figure 9.4 Structure of a nephron and its blood supply 5 The Bowman's capsule is a cup-shaped structure and is located in the cortex of the kidney. 7 The Bowman's capsule has a network of capillaries called the glomerulus. 8 Each glomerulus is supplied with blood via an afferent arteriole, which arises from a branch of the renal artery.the blood is carried away from the glomerulus through the efferent arteriole that has a smaller diameter than the afferent arteriole.the efferent arteriole branches into a network of capillaries to surround the tubule before it enters a branch of the renal vein. 9 The walls of the Bowman's capsule and the capillaries of the glomerulus are only one-cell thick and permeable to all small molecules. 10 The uriniferous tubule which continues from the Bowman's capsule consists of three main pa rts. (a) Proximal convoluted tubule (in the cortex) (b)loop of Henle (U-shaped and located in the medulla) (c) Distal convoluted tubule (in the cortex and is joined to the collecting duct) 11 The direction of blood flow in the kidney is as follows: renal artery -+ afferent arteriole -+ glomerulus (network of capillaries) -+ efferent arteriole -+ blood capillaries surrounding the uriniferous tubule + venule -+ renal vein Hence, there are two capillary networks in the kidney; the glomerulus and the capillaries surrounding the uriniferous tubule

3 12 All the constituents of blood plasma except plasma proteins and red blood cells are filtered out of the glomerulus into the cavity of the Bowman's capsule.the flow direction of the filtrate in the kidney is as follows: Bowman's capsule -+ proximal convoluted tubule -+ loop of Henle -+ distal convoluted tubule -+ collecting duct --> pelvis -+ ureter -e urinary bladder -+ urethra The filtrate that is drained into the pelvis of the kidney is called urine and will be excreted through the urethra. CONCEPT FOCUS E Function Waste products of metabolism, such as carbon dioxide, water and various forms of nitrogenous wastes, must be constantly removed from the body. Kidneys are the organs responsible for removing nitrogenous wastes from the body, and nephrons are their basic functional units. of the Kidney 1 Kidneys carry out two main functions. (a) Excretion (b) Osmoregulation 2 As excretory organs, the kidneys remove nitrogenous toxic wastes such as urea, uric acid and ammonia. 3 As osmoregulatory organs, the kidneys regulate the balance of water and salts (mainly sodium ions) in the blood by homeostasis, thus regulating the blood osmotic pressure. 6urcK chec{',?.l A Understanding key ideas Tick (/) if the statement is true. 1 One major function of the kidney is to control the concentration of blood by regulating blood osmotic pressure. 2 fhe skin is an organ of excretion. 3 Carbon dioxide is an excretory product in mammals. B Main concepts and facts Each kidney consists of an outer i nner and a central cavity, tt T to which the ureter is directly / connected. Each kidney consists of millions of microscopic tubules called which urine is produced. 9.5 Process of Urine Formation The formation of urine involves three main processes. (a) Ultrafiltration (b)reabsorption (c) Excretion Ultrafiltration Process (Bowman's Capsule and Glomerulus) 1 Blood enters the glomerulus through the afferent arteriole and leaves through the efferent arteriole. 2 The blood pressure in the afferent arteriole is high because it is derived from the renal artery which branches from the aorta.the diameter of the efferent arteries is also smaller than the afferent arteriole. As a result, there is a high resistance in the blood flow.this produces a high hydrostatic pressure in the glomerulus. 3 The high hydrostatic pressure in the blood of the glomerulus causes most of the constituents of the plasma to be filtered out of the glomerulus (through the thin capillary walls with pores) into the cavity of the Bowman's capsule. 4 The process where all the constituents of blood plasma (except red blood cells, platelets and plasma proteins) are filtered under high hydrostatic pressure into the Bowman's capsule is known as ultrafiltration (Figure 9.5). blood from renal artery---+ aflere arteriole wall of Bowman's capsule cavity of capsule to renal vein efferent arteriole glomerulus small molecules are filtered out Figure 9.5The ultrafiltration process of the glomerulus 5 The fluid filtered into the Bowman's capsule is cal led glomeru!ar filtrate. in rg lll o. o 153

4 il t JJ a. r C) 5 The glomerular filtrate consists of mainly dissolved small molecules such as inorganic ions, especially sodium ions, glucose. amino acids and urea. 7 The ultrafiltration process occurs at a higher rate in the glomerulus compared with other capillaries because (a) the hydrostatic pressure in the glomerulus is higher than other capillaries because the diameter of the efferent arteriole is smaller than the diameter of the afferent afteriole; (b) the capillaries of the glomerulus are longer than the other capillaries in the body; (c) tfre capillary walls of the glomerulus have pores which cause its permeability to be higher than other capillaries. Reabsorption Process 1 From the Bowman's capsule, the glomerular filtrate flows into the uriniferous tubule. 2 The reabsorption process occurs along the whole uriniferous tubule. Essential solutes and water in the filtrate are reabsorbed into the blood capillaries that surround the tubule. 3 At the proximal convoluted tubule (a) About 75%-80% of water is reabsorbed back into the blood capillaries by osmosis.this occurs because the glomerular filtrate is hypotonic to the blood plasma. (b) All glucose, amino acids and some mineral ions, such as sodium ions (Na*) and chloride ions (Cl-), in the tubule are reabsorbed into the bloodstream by active transport. 4 At the loop of Henle (a) About 15% of water is reabsorbed through osmosis in the descending limb which is permeable to water but not to other solutes. Water, glucose, amino acids, mineral salts (inorganic ions), urea - Reabsorolion (a) All glucose, amino acids and some mineral salts are reabsorbed by active transport (b) Water is reabsorbed by osmosis Reabsorption of water by osmosis <_ <-, <_ <_ <_ (b) Sodium ions and chloride ions are actively transported out of the filtrate in the ascending limb which is impermeable to water. 5 At the distal convoluted tubule and the collecting duct (a) The amount of water and inorganic ions (salts) that will be reabsorbed from the filtrate depends on the body's needs and is controlled by the endocrine system. (b) The rate of reabsorption of water and salts is affected by the quantity of water and salts consumed. lt is controlled by hormones as the walls of the distal convoluted tubule and the collecting duct are more permeable to water if antidiuretic hormone (ADH) is present and more permeable to salts if aldosterone hormone is present. (c) Urea is not reabsorbed throughout the nephron and is excreted in the urine. 6 The remaining filtrate in the tubule which is channelled into the pelvis of the kidney is called urine. lf plenty of water from the filtrate is reabsorbed in the distal convoluted tubule and the collecting duct, then the amount of urine produced is little and concentrated (hypertonic urine). However, if less water is reabsorbed from the filtrate, a larger amount of dilute urine is produced (hypotonic urine). 7 Urine consists of 96% water, 2.5o/o nitrogenous waste products such as urea, uric acid and creatinine, 1.5% inorganic ions and traces of bile pigments. 8 Urine is carried by the ureter from the kidney to the urinary bladder to be stored temporarily and excreted through the urethra. 9 Figure 9.6 shows a summary of the reabsorption process in the kidney tubule > > ' J -> ) Reabsorption of water and salts depends on the body's needs and is controlled by hormones Reabsorption o1 mineral salts (sodium chloride) by active transport Figure 9.6 Summary of the reabsorption process in the kidney tubule 154

5 BOLOGY BULLETN The following table shows the substances present n certain parts of the excretory system of a healthy Sodiumions Chloride ions ++ Urea ++ Glucose Protein Water Erythrocytes Key: + present.++ - absent CONCEPT FOCUS +- Selective absorption of needed materials takes place along the tubule of the nephron by active transport. Since the glomerular filtrate is hypotonic to the blood plasma, water will move out of the tubules into the surrounding tissue fluid by osmosis. Both ultrafiltration and selective reabsorption help to maintain the salt-water balance within the body. 0smoregulation 1 Osmoregulation is the process of regulating the blood osmotic pressure by regulating the water content and salt concentration in the body. 2 Osmoregulation is an example of homeostasis which is brought about by the negative feedback system. 3 The negative feedback system is a corrective mechanism to restore the deviated osmotic pressure in the blood to its normal level. 4 The kidneys carry out osmoregulation by coordinating the rate of reabsorption of water and salts (especially sodium and chloride ions) during the formation of urine. 5 The amount of water and salts in the blood will determine the osmotic pressure of the blood. 6 Reabsorption of water is controlled by the antidiuretic hormone (ADH) which is released by the pituitary gland. 7 Reabsorption of salts is controlled by the aldosterone hormone which is produced by the adrenal gland. The mechanism of osmoregulation (A) When the blood osmotic pressure is high 1 The blood osmotic pressure is raised when water is lost excessively through sweating or after a salty meal where a large amount of salt is consumed ]T + E!uFo- + 2 The high osmotic pressure is detected by the osmoreceptors in the hypothalamus. 3 The pituitary gland is stimulated to release the antidiuretic hormone (ADH). 4 The adrenal gland is less stimulated and thus less aldosterone hormone is released. 5 Antidiuretic hormone increases the permeability of the walls of the distal convoluted tubule and the collecting duct towards water. 6 Hence, more water and less salt are reabsorbed from the tubules into the blood capillaries. 7 This lowers the blood osmotic pressure to its optimum level. 8 As a result, a small amount of concentrated urine is produced. (B) When the blood osmotic pressure is low 1 The blood osmotic pressure is lowered when an excessive amount of water is consumed. 2 The low osmotic pressure in the blood is detected by the osmoreceptors in the hypothalamus. 3 The adrena! gland is stimulated to release the aldosterone hormone. 4 The pituitary gland is less stimulated and the release of ADH is greatly reduced. 5 The aldosterone hormone causes the walls of the distal convoluted tubule and the collecting duct to become more permeable to salts and less permeable to water. 6 Hence, more salt and less water are reabsorbed from the tubules into the blood capillaries. 7 This increases the blood osmotic pressure to its optimum level. 8 As a result, a large amount of dilute urine is produced. (J 155

6 9 Figure 9.7 shows a schematic diagram of the negative feedback mechanisms in osmoregulation. tre U c o Hypothalamus in brain detected by increases decreases delected by CONCEPT FOCUS 1 Pituitary gland (a) is stimulated to release more ADH (b) the walls o{ distal convoluted tubule and collecting duct become more permeable to water (c) more water is reabsorbed into the blood capillary 2 Adrenal gland (a) less stimulated and thus less aldosterone hormone is released (b) reabsorption of salt inlo the blood capillary is reduced 3 Urine formed is small in volume but concenlrated 1 Pituitary gland (a) less stimulated and thus less ADH is released (b) reabsorption ot water into the blood capillary is reduced 2 Adrenal gland (a) is stimulated to release more aldosterone hormone (b) the walls ol the distal convoluted tubule and the collecting duct become more permeable to salt (c) more salt is reabsorbed into the blood capillary 3 Urine formed is large in volume and is dilute Figure 9.7 Osmoregulation by kidney through negative feedback mechanisms The osmotic regulation in the body is maintained with the help of a negative feedback mechanism involving the antidiuretic hormone (ADH) and aldosterone hormone. 9.6 Oq{noreceptor$ iri hypothalamus High blood osmotic pressure Low blood osmolic pressure Osmoreceptors in hypothalamus Hypothalamus in brain Kidney Failure 1 For patients with impaired kidney function, the kidney cannot remove the excess water, mineral salts or urea. Hence, these substances remain in the blood. 2 Kidneys that are damaged by disease or injury fail to carry out ultrafiltration at the glomerulus, and are thus unable to regulate the blood osmotic pressure, filter the blood and remove :,1;;;;,1,, Cstieclive meehaili$ms by efiqdor,,,,,,,, Normal blood osmolic pressure Blood osmotic pressure is decreased negative leedback Blood osmotic mechanism pressure is increased negatire feedbaci mechanism the unwanted waste products.these problems can be overcome through haemodialysis. 3 Haemodialysis is the treatment for patients with impaired kidney function to filter the blood by dialysis using an artificial kidney machine (Figure 9.8). (a) Dialysis involves separating the small molecules from the larger ones by allowing them to diffuse through a partially permeable membrane. (b)during haemodialysis, blood is pumped from the radial artery in the wrist, passed through the dialysis machine where it is filtered and returned to the cephalic vein in the wrist. Heparin is added to prevent the blood from clotting.the blood is circulated many times through the machine during a treatment session which may take up to four hours or more. 156

7 bubble trap direction of blood dialysis solution t vein artery pump (c) ln the machine, the blood flows through the coiled tubes.the tubes are coiled to increase the surface area for diffusion. The dialysis solution in the tubes flows in the opposite direction to the blood flow.this maximises the exchange of materials between the blood and the dialysis solution by diffusion. (d)the dialysis solution contains glucose, amino acids and inorganic ions which are dissolved in water. The concentration of the salts in the dialysis solution is maintained at the same level (isotonic) as the blood plasma.this ensures that only dissolved substances with a higher concentration in the blood are able to diffuse from the blood into the dialysis solution (ultrafiltration). (e)the dialysis solution is maintained at body temperature (37 "C) and is constantly flowing.this ensures that the substances that have diffused into the solution from the blood are carried away immediately. (f) The walls of the tubes are semi-permeable to allow small molecules to diffuse across the membrane into the fluid. Large molecules such as plasma proteins and erythrocytes cannot diffuse through the membrane. When a balance is reached, the blood has the same concentration of dissolved substances as in the dialysis solution and the blood osmotic pressure is maintained at its equilibrium level. (g)the dialysis solution is free of nitrogenous waste such as urea. Hence, these wastes may diffuse from the blood into the dialysis solution and are removed or carried away in the fluid. Thus, haemodialysis treatment helps to remove the nitrogenous wastes coiled tube which is permeable to small molecules only t dialysis solution Figure 9.8 A summary of how a dialysis machine works Dialysis solution which contains glucose, mineral ions, amino acids and water. The concentration of salt is isotonic to the blood plasma to prevent the loss of mineral salts required by the cells Jrom the blood during dialysis. from the body and also regulates the blood osmotic pressure to its optimum level. (h)a patient with both kidneys damaged needs to undergo haemodialysis treatment two to three times a week for at least four hours per session. 4 Another method to treat impaired kidney function is to have a kidney transplant. (Figure 9.9). (a)a kidney transplant is a procedure that replaces the impaired kidney of a patient with a healthy kidney from a donor. (b)however, the patient has to wait for a suitable kidney.the donor's kidney must be compatible to the recipient's so that the kidney will not be rejected by the recipient's body. Diseased kidneys Vena cava Aorta Bladder Transplanted kid Transplanted ureter Figure 9.9 Kidney transplant (c) A healthy person can donate a kidney and survive with the other kidney because osmoregulation can still be carried out even with only one kidney functioning. EE U r () 157

8 ctr E UJ o- () (d) ln a kidney transplant, a new kidney is placed inside the lower abdomen.the aftery and the vein of the new kidney are connected to the aorta and vena cava.the damaged kidneys are left in place unless they are causing infection or high blood pressure. CK CHECK?.2 A Understanding key ideas Tick (/) if the statement is true. 1 The process of maintaining the concentration of water, salts and other mineral ions is called osmoregulation. 2 When the osmotic pressure of the blood is high, more urine is produced. 3 Ultrafiltration occurs along the kidney tubule. 4 Antidiuretic hormone causes the distal convoluted tubules and collecting ducts to become more permeable to salt. 5 Urine is formed in the nephron. 6 The mechanism of osmoregulation occurs by negative feedback mechanism. 7 Natural kidneys regulate body fluid composition by filtration and reabsorption, while artificial kidney machines can only filter. B Main concepts and facts The formation of urine involves two main processes, which are high and ln the glomerulus, the causes part of the blood plasma to diffuse out of the blood capillaries and enter into the cavity of the The glomerular filtrate then flows along the and mineral salts occurs. where reabsorption of water 158

9 EXCRETON N HUMANS Process of excretion Skin Excretion Lu ngs Excretory organs Liver Kidney failure Exc Kidneys Function of ki d ney Osmoregulation Structu re of kidneys with the help of o Antidiuretic hormone (ADH) o Aldosterone G, UJ r () 159