Chapter 7 — Excretion
Key Concepts Quick Recap
| Term | Definition |
|---|
| Excretion | Removal of metabolic waste products from the body |
| Ammonotelic | Excretes ammonia (aquatic animals — fish, aquatic invertebrates) |
| Ureotelic | Excretes urea (mammals, adult amphibians) |
| Uricotelic | Excretes uric acid (reptiles, birds, insects) |
| Nephron | Functional unit of kidney; ~1 million per kidney |
| GFR | Glomerular Filtration Rate — ~125 mL/min; ~180 L/day |
| Juxtaglomerular apparatus (JGA) | Regulates blood pressure and filtration rate; produces renin |
| Renin-Angiotensin system | Blood pressure regulation mechanism by kidney |
| ADH (Vasopressin) | Promotes water reabsorption in collecting duct; reduces urine volume |
| Aldosterone | Promotes Na⁺ (and water) reabsorption; increases blood pressure |
I. Choose the Best Answer (MCQs)
1. The functional unit of the kidney is: (a) Glomerulus (b) Nephron (c) Bowman's capsule (d) Loop of Henle
Answer(b) Nephron Each kidney has ~1 million nephrons. A nephron consists of: Malpighian body (Bowman's capsule + glomerulus) + Proximal Convoluted Tubule (PCT) + Loop of Henle + Distal Convoluted Tubule (DCT) + Collecting duct.
2. Ultrafiltration of blood takes place in: (a) Loop of Henle (b) Collecting duct (c) Bowman's capsule (d) Proximal convoluted tubule
Answer(c) Bowman's capsule Ultrafiltration (glomerular filtration): blood pressure in glomerulus forces plasma (minus large proteins and cells) into Bowman's capsule → glomerular filtrate. The glomerular capillaries are fenestrated (have pores) → high filtration. GFR = 125 mL/min = 180 L/day (but only 1-1.5 L becomes urine after reabsorption).
3. Glucose reabsorption from the glomerular filtrate occurs in: (a) Loop of Henle (b) Collecting duct (c) Proximal convoluted tubule (d) Distal convoluted tubule
Answer(c) Proximal convoluted tubule PCT is the primary site of selective reabsorption. Completely reabsorbs: glucose, amino acids, vitamins. Reabsorbs: 70–80% Na⁺, water, bicarbonate, phosphate. Uses active transport (sodium-glucose cotransport).
4. The hormone that increases water reabsorption in the collecting duct is: (a) Aldosterone (b) Renin (c) ADH (Vasopressin) (d) Angiotensin
Answer(c) ADH (Vasopressin) ADH (antidiuretic hormone) is secreted by the posterior pituitary. High blood osmolarity (dehydration) → ADH secreted → collecting duct becomes more permeable to water → more water reabsorbed → concentrated urine. Alcohol inhibits ADH → increased urine output (diuresis).
5. What is the normal GFR (Glomerular Filtration Rate)? (a) 50 mL/min (b) 125 mL/min (c) 200 mL/min (d) 500 mL/min
Answer(b) 125 mL/min GFR = 125 mL/min = 180 L/day. Of this, 99% is reabsorbed → only 1–1.5 L becomes urine. The JGA monitors GFR; reduced GFR → JGA secretes renin → Renin-Angiotensin system → blood pressure increases → GFR restored.
6. Humans are ureotelic. This means they excrete nitrogenous waste as: (a) Ammonia (b) Uric acid (c) Urea (d) Allantoin
Answer(c) Urea Urea is synthesised in the liver (from ammonia + CO₂ via ornithine cycle / urea cycle). Less toxic than ammonia, needs some water for excretion. Ureotelic animals: mammals (including humans), adult frogs and toads.
7. The countercurrent mechanism in the Loop of Henle is important for: (a) Glucose reabsorption (b) Production of concentrated urine (c) Acid-base balance (d) Blood pressure regulation
Answer(b) Production of concentrated urine The countercurrent mechanism creates an osmotic gradient in the renal medulla:
- Descending loop: permeable to water; hypertonic medulla draws water out → filtrate becomes concentrated.
- Ascending loop: impermeable to water; pumps out NaCl → dilutes filtrate.
- Collecting duct: ADH controls water reabsorption here → concentrated urine produced.
II. Short Answer Questions
Q1. Describe the structure of a nephron.
Answer
A nephron is the structural and functional unit of the kidney. Each kidney has ~1 million nephrons.
Parts:
1. Malpighian body (Renal corpuscle):
Answer
- Bowman's capsule: Double-walled cup; inner wall (visceral layer) has podocytes with filtration slits.
- Glomerulus: Tuft of capillaries inside Bowman's capsule; high blood pressure → ultrafiltration.
2. Proximal Convoluted Tubule (PCT):
Answer
- Lined by cuboidal cells with microvilli (brush border).
- Reabsorbs glucose, amino acids, Na⁺, water (80%), HCO₃⁻.
3. Loop of Henle:
Answer
- Hairpin loop in renal medulla.
- Descending limb: permeable to water only.
- Ascending limb: permeable to salt only (pumps NaCl out).
- Creates osmotic gradient in medulla → concentrated urine.
4. Distal Convoluted Tubule (DCT):
Answer
- Selective reabsorption of Na⁺ (aldosterone-regulated).
- Secretion of H⁺, K⁺, NH₃ → acid-base balance.
5. Collecting duct:
Answer
- Multiple nephrons drain into one collecting duct.
- ADH controls water reabsorption → urine concentration.
Q2. Describe the three steps of urine formation.
Answer
Step 1 — Ultrafiltration (Glomerular filtration): Blood under high pressure (due to afferent arteriole wider than efferent) → plasma filtered through glomerular capillaries + Bowman's capsule → glomerular filtrate. Filtrate contains: water, glucose, amino acids, urea, uric acid, creatinine, ions. NOT filtered: proteins, blood cells (too large). GFR: 125 mL/min.
Step 2 — Selective tubular reabsorption: Useful substances reclaimed from filtrate back into blood:
~99% of filtrate is reabsorbed.
- PCT: 100% glucose, amino acids; 70-80% Na⁺, water, HCO₃⁻.
- Loop of Henle: water (descending), NaCl (ascending).
- DCT: Na⁺ (aldosterone), water.
- Collecting duct: water (ADH).
Step 3 — Tubular secretion: Waste substances secreted from blood/tubular cells into filtrate:
- H⁺, K⁺, NH₃ (from DCT and PCT) — acid-base balance.
- Creatinine, drugs, uric acid.
Final urine: 1–1.5 L/day; concentrated; contains urea, uric acid, creatinine, salts.
Common Mistakes
- Confusing filtration (Bowman's capsule) with secretion (tubular) — two different processes.
- Saying glucose appears in urine normally — glucose is completely reabsorbed in PCT. Glucose in urine (glycosuria) indicates diabetes or renal threshold is exceeded.
- Forgetting that GFR = 125 mL/min but only 1-1.5 L urine produced daily (99% reabsorbed).
- Confusing ADH (water reabsorption) and Aldosterone (Na⁺ reabsorption).
Exam Checklist
- Nephron parts: Bowman's capsule + glomerulus → PCT → Loop of Henle → DCT → collecting duct.
- Urine formation: ultrafiltration → reabsorption → secretion.
- GFR = 125 mL/min; 180 L/day filtered; 1-1.5 L urine.
- PCT: glucose, amino acids completely reabsorbed.
- ADH: water reabsorption; aldosterone: Na⁺ reabsorption.
- Ureotelic (urea): humans, mammals. Ammonotelic (ammonia): fish. Uricotelic (uric acid): birds, reptiles.
