Excretory Products and their Elimination

Animals accumulate ammonia, urea, uric acid, carbon dioxide, water and ions as a result of metabolism, and these must be removed. The three main modes of nitrogenous excretion are ammonotelism in bony fishes and aquatic amphibians, ureotelism in mammals and terrestrial amphibians, and uricotelism in reptiles, birds and insects. Excretory structures include protonephridia or flame cells in flatworms, nephridia in earthworms, Malpighian tubules in insects and green glands in prawns. You will then study the human excretory system: a pair of kidneys, ureters, the urinary bladder and urethra, with each kidney containing about a million nephrons. You will learn the structure of a nephron, made of the glomerulus, Bowman's capsule, the proximal convoluted tubule, Henle's loop and the distal convoluted tubule, and the difference between cortical and juxtamedullary nephrons. The chapter explains urine formation through glomerular filtration (about 180 litres per day), reabsorption (about 99%) and secretion. It also explains the counter-current mechanism that concentrates urine, regulation by ADH, the renin-angiotensin system and ANF, micturition, the excretory roles of lungs, liver and skin, and disorders like uremia and renal calculi.

Detailed Worksheet: Excretory Products and their Elimination Section A - Definitions (10 marks) 1. Distinguish between ammonotelic and uricotelic animals with one example each. (2 marks) 2. What is glomerular filtration rate? State its normal value. (2 marks) 3. What is the juxtaglomerular apparatus? What is its function? (2 marks) 4. What is uremia? How is it treated? (2 marks) 5. Name the excretory structures of flatworms, earthworms, insects and prawns. (2 marks) Section B - Calculations and Applications (15 marks) 6. The GFR of a healthy person is 125 mL per minute. Calculate the volume of filtrate formed per hour and per day in litres. If 99% of the filtrate is reabsorbed, calculate the volume of urine produced per day. (3 marks) 7. A person excretes 1.5 litres of urine per day containing about 25 g of urea. Calculate the concentration of urea in g per litre. If the person's urine output doubles but urea excretion stays the same, what would be the new concentration? (3 marks) 8. Each kidney contains about 10 lakh nephrons. If the total GFR is 125 mL per minute, calculate the average filtration per nephron per minute in microlitres (1 mL = 1,000 microlitres). (3 marks) 9. The osmolarity of the medullary interstitium rises from 300 mOsmol per litre in the cortex to about 1,200 mOsmol per litre in the inner medulla. Calculate how many times more concentrated the inner medulla is. How does this help concentrate urine? (3 marks) 10. The lungs remove about 200 mL of carbon dioxide per minute. Calculate the volume removed in a day in litres. Name two other organs that help in excretion and the substances they remove. (3 marks) Section C - Diagrams (10 marks) 11. Draw a labelled diagram of the human excretory system. (4 marks) 12. Draw a labelled diagram of a nephron showing its blood vessels and the regions of the tubule. (3 marks) 13. Draw a diagram showing the counter-current mechanism in Henle's loop and the vasa recta. (3 marks) Section D - Analysis and Higher-order Thinking (15 marks) 14. Explain the process of urine formation, describing glomerular filtration, reabsorption and secretion, and the function of each part of the nephron. (5 marks) 15. Explain how the kidney's function is regulated by the hypothalamus, the JGA and the heart. What happens when blood volume or blood pressure falls? (5 marks) 16. 'Animals living in water and on land excrete different nitrogenous wastes.' Explain this statement in terms of toxicity, water availability and energy cost. (5 marks) Instructions: Time allowed 2 hours. Attempt all sections. Draw diagrams neatly with labels. Show calculations with units.