EXCRETORY PRODUCTSANDTHEIR ELIMINATION A survey of animal kingdom presents a variety of excretory structures. In most of the invertebrates, these structures are simple tubular forms whereas vertebrates have complex tubular organs called kidneys. Some of these structures are mentioned here. Protonephridiaor flame cells are the excretory structures in Platyhelminthes (Flatworms, e.g., Planaria), rotifers, some annelids and the cephalochordate – Amphioxus. Protonephridia are primarily concerned with ionic and fluid volume regulation, i.e., osmoregulation. Nephridia are the tubular excretory structures of earthworms and other annelids. Nephridia help to remove nitrogenous wastes and maintain a fluid and ionic balance. Malpighian tubules are the excretory structures of most of the insects including cockroaches. Malpighian tubules help in the removal of nitrogenous wastes and osmoregulation. Antennal glands or green glands perform the excretory function in crustaceans like prawns. 19.1 HUMAN EXCRETORY SYSTEM In humans, the excretory system consists of a pair of kidneys, one pair of ureters, a urinary bladder and a urethra (Figure 19.1). Kidneys are reddish brown, bean shaped structures situated between the levels of last thoracic and third lumbar vertebra close to the dorsal inner wall of the abdominal cavity. Each kidney of an adult human measures 10-12 cm in length, 5-7 cm in width, 2-3 cm in thickness with an average weight of 120170 g. Towards the centre of the inner concave surface of the kidney is a notch called hilum through which ureter, blood vessels and nerves enter. Inner to the hilum is a broad funnel shaped space called the renal pelvis with projections called calyces. The outer layer of kidney is a tough capsule. Inside the kidney, there are two zones, an outer cortex and an inner Figure 19.1 Human Urinary systemmedulla. The medulla is divided into a few conical masses (medullary pyramids) projecting into the calyces (sing.: calyx). The cortex extends in between the 292 BIOLOGY Figure 19.2 Longitudinal section (Diagrammatic) of Kidney medullary pyramids as renal columns called Columns of Bertini (Figure 19.2). Each kidney has nearly one million complex tubular structures called nephrons (Figure 19.3), which are the functional units. Each nephron has two parts – the glomerulus and the renal tubule. Glomerulus is a tuft of capillaries formed by the afferent arteriole – a fine branch of renal artery. Blood from the glomerulus is carried away by an efferent arteriole. The renal tubule begins with a double walled cup-like structure called Bowman’s capsule, which encloses the glomerulus. Glomerulus alongwith Bowman’s capsule, is called the malpighian bodyor renal corpuscle(Figure 19.4). The tubule continues further to form a highly coiled network – proximal convoluted tubule Figure 19.3 A diagrammatic representation of a nephron showing blood vessels, duct and tubule 296 BIOLOGY also in a counter current pattern. The proximity between the Henle’s loop and vasa recta, as well as the counter current in them help in maintaining an increasing osmolarity towards the inner medullary interstitium, i.e., from 300 mOsmolL–1 in the cortex to about 1200 mOsmolL–1 in the inner medulla. This gradient is mainly caused by NaCl and urea. NaCl is transported by the ascending limb of Henle’s loop which is exchanged with the descending limb of vasa recta. NaCl is returned to the interstitium by the ascending portion ofvasa recta. Similarly, small amounts of urea enter the thin segment of the ascending limb of Henle’s loop which is transported back to the interstitium by the collecting tubule. The above described transport of substances facilitated by the special arrangement of Henle’s loop and vasa recta is called the counter current mechanism (Figure. 19.6). This mechanism helps to maintain a concentration gradient Figure 19.6 Diagrammatic representation of a nephron and vasa recta showing counter current mechanisms BIOLOGY minute (GFR). JGA, a specialised portion of the nephrons, plays a significant role in the regulation of GFR. Nearly 99 per cent reabsorption of the filtrate takes place through different parts of the nephrons. PCT is the major site of reabsorption and selective secretion. HL primarily helps to maintain osmolar gradient (300 mOsmolL–1 -1200 mOsmolL–1) within the kidney interstitium. DCT and collecting duct allow extensive reabsorption of water and certain electrolytes, which help in osmoregulation: H+, K+ and NH3 could be secreted into the filtrate by the tubules to maintain the ionic balance and pH of body fluids. A counter current mechanism operates between the two limbs of the loop of Henle and those of vasa recta (capillary parallel to Henle’s loop). The filtrate gets concentrated as it moves down the descending limb but is diluted by the ascending limb. Electrolytes and urea are retained in the interstitium by this arrangement. DCT and collecting duct concentrate the filtrate about four times, i.e., from 300 mOsmolL–1 to 1200 mOsmolL–1, an excellent mechanism of conservation of water. Urine is stored in the urinary bladder till a voluntary signal from CNS carries out its release through urethra, i.e., micturition. Skin, lungs and liver also assist in excretion. EXERCISES 1. Define Glomerular Filtration Rate (GFR) 2. Explain the autoregulatory mechanism of GFR. 3. Indicate whether the following statements are true or false : (a) Micturition is carried out by a reflex. (b) ADH helps in water elimination, making the urine hypotonic. (c) Protein-free fluid is filtered from blood plasma into the Bowman’s capsule. (d) Henle’s loop plays an important role in concentrating the urine. (e) Glucose is actively reabsorbed in the proximal convoluted tubule. 4. Give a brief account of the counter current mechanism. 5. Describe the role of liver, lungs and skin in excretion. 6. Explain micturition. 7. Match the items of column I with those of column II : Column I Column II (a) Ammonotelism (i) Birds (b) Bowman’s capsule (ii) Water reabsorption (c) Micturition (iii) Bony fish (d) Uricotelism (iv) Urinary bladder (d) ADH (v) Renal tubule

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