Excretion and Osmoregulation
Short Questions
1. What is excretion?
Answer: Elimination of metabolic waste products from the body.
2. Name the primary nitrogenous waste in humans.
Answer: Urea.
3. What is the functional unit of the kidney?
Answer: Nephron.
4. Which hormone regulates water reabsorption in kidneys?
Answer: Antidiuretic hormone (ADH).
5. What gives urine its yellow color?
Answer: Urochrome.
6. What is the role of the juxtaglomerular apparatus?
Answer: Regulates blood pressure by releasing renin.
7. Which part of the nephron is responsible for selective reabsorption?
Answer: Proximal convoluted tubule (PCT).
8. What is micturition?
Answer: The process of urination.
9. Name an organism that is ammonotelic.
Answer: Bony fish.
10. What is the countercurrent mechanism?
Answer: A process in the loop of Henle to concentrate urine.
11. Which organ excretes volatile substances like CO₂?
Answer: Lungs.
12. What is the composition of sweat?
Answer: Water, NaCl, lactic acid, and urea.
13. What is uremia?
Answer: Excessive urea in blood (>0.05%).
14. Name a guanotelic organism.
Answer: Spider.
15. What is the role of aldosterone in excretion?
Answer: Enhances sodium and water reabsorption.
Long Questions
1. Explain the process of ultrafiltration in the nephron.
Answer: Ultrafiltration occurs in the glomerulus, where high glomerular hydrostatic pressure (55 mm Hg) forces plasma (except proteins and blood cells) through fenestrated capillaries into Bowman’s capsule. The effective filtration pressure (10 mm Hg) is determined by subtracting osmotic pressure (30 mm Hg) and capsular pressure (15 mm Hg) from glomerular pressure. This forms glomerular filtrate at a rate of 125 ml/min.
2. How does the countercurrent mechanism help in urine concentration?
Answer: The countercurrent mechanism in the loop of Henle creates an osmotic gradient in the medulla, with the descending limb allowing water to exit and the ascending limb reabsorbing Na⁺ and Cl⁻. Vasa recta maintain this gradient by exchanging solutes and water. ADH enhances water reabsorption in collecting ducts, concentrating urine up to 1200 mOsm/L.
3. What is the role of the liver in urea production?
Answer: The liver converts toxic ammonia, produced from amino acid deamination, into urea via the ornithine/urea cycle. This process requires 3 ATP molecules per urea molecule and occurs in hepatocytes. Urea, being less toxic, is then excreted by the kidneys.
4. Why are kidneys called retroperitoneal?
Answer: Kidneys are retroperitoneal because they are located behind the peritoneum, the membrane lining the abdominal cavity. They are anchored to the abdominal wall by renal fascia and not fully enclosed by the peritoneum. This positioning protects them and facilitates their connection to blood vessels and ureters.
5. How do skin and lungs contribute to excretion?
Answer: Skin excretes water, NaCl, lactic acid, and urea through sweat glands, primarily for thermoregulation. Lungs eliminate volatile substances like CO₂, water vapor, and compounds from foods like garlic, contributing to respiratory excretion. Both organs supplement kidney function in waste elimination.
6. What is the significance of creatinine as an index of kidney function?
Answer: Creatinine, produced from muscle metabolism, is normally excreted at a steady rate matching its production. Elevated blood creatinine levels (normal: 0.6–1.4 mg/dl) indicate poor renal function, as kidneys fail to filter it effectively. It is a reliable marker for assessing glomerular filtration rate.
7. Why are birds uricotelic in nature?
Answer: Birds are uricotelic to conserve water, as uric acid is least toxic and requires minimal water (5–10 ml/g) for excretion. This adaptation suits their terrestrial and flying lifestyle, reducing water weight. Uric acid is formed via the inosinic acid pathway in the liver.
8. How does RAAS regulate blood volume and pressure?
Answer: The renin-angiotensin-aldosterone system (RAAS) is activated when low blood pressure triggers renin release from the juxtaglomerular apparatus. Renin converts angiotensinogen to angiotensin II, which constricts arterioles and stimulates aldosterone release. Aldosterone enhances Na⁺ and water reabsorption, increasing blood volume and pressure.
9. What are the differences between ammonotelism and ureotelism?
Answer: Ammonotelism involves excreting ammonia, which is highly toxic and requires large amounts of water (300–500 ml/g), typical in aquatic organisms like bony fish. Ureotelism involves excreting urea, which is less toxic and needs less water (~50 ml/g), common in terrestrial mammals. Ureotelism conserves water but requires energy for urea synthesis in the liver.
10. What are the symptoms and causes of kidney stones?
Answer: Kidney stones (renal calculi) cause pain in the back/sides, hazy/reddish urine, frequent urination, and pain during micturition. They form due to high levels of calcium oxalate, uric acid, or cystine, often from low water intake, high-protein diets, or bacterial infections (struvite stones). Diagnosis involves blood tests, urine analysis, X-rays, or sonography.
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