What this chapter is about
This chapter explains how living organisms, particularly humans, remove metabolic waste products from their bodies. During cellular activities such as protein metabolism, harmful substances like ammonia, urea and uric acid are produced. If these accumulate, they become toxic. The process of removing such wastes is called excretion, and the organs involved form the excretory system.
A Class 11 student meets this chapter because understanding excretion connects earlier concepts of digestion, respiration and circulation to the maintenance of internal body balance (homeostasis). The kidneys, which are the primary excretory organs in humans, also regulate water and electrolyte balance, blood pressure and pH. Students learn the detailed structure of the kidney, the nephron (the functional unit), and the mechanism of urine formation.
After studying this chapter, students should be able to describe the types of nitrogenous wastes in different animals, explain the structure of the human urinary system, trace the steps of urine formation (glomerular filtration, tubular reabsorption and secretion), understand regulation by hormones, and recognise common kidney disorders along with medical interventions like dialysis.
Key ideas
- Ammonotelism, ureotelism and uricotelism are three modes of excretion based on the main nitrogenous waste: ammonia (most aquatic animals), urea (mammals, some amphibians) or uric acid (birds, reptiles, insects).
- The human urinary system consists of two kidneys, two ureters, a urinary bladder and a urethra; kidneys filter blood and produce urine.
- Each kidney contains about one million nephrons; each nephron has a glomerulus enclosed by Bowman's capsule, followed by the proximal convoluted tubule (PCT), loop of Henle, distal convoluted tubule (DCT) and collecting duct.
- Urine formation involves three main processes: glomerular filtration (ultrafiltration of blood in the glomerulus), selective reabsorption (recovery of useful substances in tubules) and tubular secretion (active removal of additional wastes into the filtrate).
- The counter-current mechanism in the loop of Henle and vasa recta maintains the medullary osmotic gradient, enabling concentration of urine.
- Hormones regulate kidney function: antidiuretic hormone (ADH) from the posterior pituitary increases water reabsorption; aldosterone from the adrenal cortex increases sodium reabsorption; atrial natriuretic factor (ANF) promotes sodium excretion.
- The juxtaglomerular apparatus (JGA) releases renin, activating the renin-angiotensin mechanism that raises blood pressure and aldosterone secretion.
- Disorders include kidney stones, glomerulonephritis and renal failure; haemodialysis and kidney transplant are common treatments.
Formulas and facts to remember
1. Glomerular filtration rate (GFR): In a healthy adult, approximately 125 mL of filtrate forms per minute in both kidneys, totalling about 180 litres per day.
2. Composition of normal urine: About 95 % water, 2 % urea, and small amounts of creatinine, uric acid, ammonia, sodium chloride and potassium ions.
3. Daily urine output: A healthy adult excretes about 1–1.8 litres of urine per day, depending on fluid intake and climate.
4. Ammonia toxicity: Ammonia is highly soluble and very toxic; aquatic animals excrete it directly because abundant water dilutes it rapidly.
5. Urea synthesis: In mammals, ammonia is converted to less toxic urea in the liver via the ornithine cycle (urea cycle).
6. Osmolarity of medulla: The inner medulla can reach about 1200 mOsmol/L, allowing production of hypertonic urine.
7. ADH deficiency: Leads to diabetes insipidus, characterised by excretion of large volumes of dilute urine and excessive thirst.
8. Haemodialysis principle: Blood is passed through an artificial kidney (dialyser) containing a semipermeable membrane; wastes diffuse out into a dialysing fluid of suitable composition.
Worked examples
### Example 1: Calculating daily filtrate volume
A student reads that the GFR is 125 mL/min. Calculate the total volume of filtrate produced by both kidneys in 24 hours.
Solution
Volume per minute = 125 mL
Minutes in 24 hours = 24 × 60 = 1440 min
Total filtrate = 125 mL/min × 1440 min = 180 000 mL = 180 litres
Thus, kidneys filter about 180 litres of plasma per day; most is reabsorbed, leaving only 1–1.8 litres as urine.
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### Example 2: Identifying excretory mode from habitat
A farmer in coastal Tamil Nadu notices two animals near a pond: a freshwater fish (rohu) and a garden lizard (calotes). Which nitrogenous waste does each excrete, and why?
Solution
Rohu is a freshwater fish. It lives in an aquatic environment with plenty of water to dilute wastes. It excretes ammonia directly through its gills (ammonotelic).
The garden lizard is a terrestrial reptile. Water conservation is essential; ammonia would be too toxic, and urea would need water for excretion. It converts nitrogenous waste to uric acid, a semi-solid paste requiring little water (uricotelic).
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### Example 3: Role of ADH in water balance
During summer, a college student drinks very little water while travelling. Explain how his body prevents excessive water loss in urine.
Solution
Low water intake raises blood osmolarity (blood becomes slightly concentrated). Osmoreceptors in the hypothalamus detect this and stimulate the posterior pituitary to release more antidiuretic hormone (ADH). ADH acts on the collecting ducts and DCT, making them more permeable to water. More water is reabsorbed back into blood, producing a smaller volume of concentrated (hypertonic) urine. This conserves body water until the student drinks again.
Common mistakes
- Thinking ammonia is safe for all animals → ammonia is toxic; only aquatic animals with abundant water can excrete it directly.
- Confusing reabsorption with secretion → reabsorption moves substances from filtrate back to blood; secretion moves substances from blood into the filtrate.
- Believing all 180 litres of filtrate become urine → over 99 % of filtrate is reabsorbed; only about 1.5 litres exits as urine daily.
- Assuming the loop of Henle works the same in cortical and juxtamedullary nephrons → juxtamedullary nephrons have longer loops that extend deep into the medulla, crucial for concentrating urine.
- Mixing up ADH and aldosterone functions → ADH increases water reabsorption; aldosterone increases sodium (and therefore water) reabsorption, but acts via a different mechanism involving Na⁺/K⁺ exchange.
Quick revision
- Ammonotelic: aquatic animals excrete ammonia; ureotelics (mammals) excrete urea; uricotelics (birds, reptiles) excrete uric acid.
- Nephron = Bowman's capsule + PCT + loop of Henle + DCT + collecting duct.
- Urine formation: filtration (glomerulus) → reabsorption (mainly PCT) → secretion (PCT, DCT).
- GFR ≈ 125 mL/min; daily filtrate ≈ 180 L; daily urine ≈ 1.5 L.
- ADH conserves water; aldosterone conserves sodium; ANF promotes sodium excretion.
- Haemodialysis replaces kidney function by filtering blood through an artificial membrane.