Excretory Products and their Elimination (NEET Biology Class 11): Nephron, GFR & Counter Current Mechanism
Nitrogenous Wastes, Excretory Structures & Kidney Anatomy
📌 NEET priority: Important. 2–3 questions a year. The nephron segment-by-segment function table, GFR (125 mL/min, 180 L/day) and the renin-angiotensin cascade are the reliable scorers — plus the excretory-structure list, which is pure match-the-pair.
Nitrogenous waste — the three excretory strategies
Type | Waste excreted | Water cost | Animals |
Ammonotelic | Ammonia — highly toxic, very soluble | Needs lots of water; excreted by diffusion across body/gill surface. KIDNEYS play no significant role | Bony fishes, aquatic amphibians, aquatic insects |
Ureotelic | Urea — less toxic; made from ammonia in the LIVER, released into blood, filtered by kidneys | Moderate | Mammals, many terrestrial amphibians, marine fishes |
Uricotelic | Uric acid — as a pellet or paste | MINIMUM water loss | Reptiles, birds, land snails, insects |
Excretory structures across the animal kingdom — match the pair
Structure | Found in | Function |
Protonephridia (flame cells) | Platyhelminthes (e.g. Planaria), rotifers, some annelids, Amphioxus | Primarily OSMOREGULATION — ionic and fluid volume regulation |
Nephridia | Earthworms and other annelids | Remove nitrogenous wastes, maintain fluid and ionic balance |
Malpighian tubules | Most insects, including cockroaches | Removal of nitrogenous wastes and osmoregulation |
Antennal (green) glands | Crustaceans like prawns | Excretion |
The human kidney — structure
A pair of reddish-brown, bean-shaped kidneys sit between the last thoracic and third lumbar vertebra. Each is 10–12 cm long, 5–7 cm wide, 2–3 cm thick, weighing 120–170 g.
The hilum is the notch on the inner concave surface where ureter, blood vessels and nerves enter. Inside it is the funnel-shaped renal pelvis with projections called calyces. Internally there is an outer cortex and an inner medulla with conical medullary pyramids. Cortex extending between the pyramids forms renal columns — the Columns of Bertini.
Each kidney has nearly one million nephrons — the functional units. Each nephron = glomerulus (a tuft of capillaries from the afferent arteriole, drained by the efferent arteriole) + renal tubule.
Tubule path: Bowman's capsule → PCT → Henle's loop (descending + ascending) → DCT → collecting duct. Glomerulus + Bowman's capsule together = the malpighian body / renal corpuscle.
Location matters: the malpighian corpuscle, PCT and DCT are in the CORTEX; the loop of Henle dips into the MEDULLA. Nephrons with a long loop running deep into the medulla are juxta medullary nephrons; most have short loops (cortical nephrons). The vasa recta — a U-shaped vessel parallel to Henle's loop — is absent or highly reduced in cortical nephrons.
Urine Formation & Function of Each Tubule Segment
Urine formation — three processes
1. Glomerular filtration. About 1100–1200 mL of blood is filtered per minute — roughly 1/5th of the blood pumped by each ventricle per minute. Blood passes through three layers: endothelium of glomerular vessels, epithelium of Bowman's capsule, and a basement membrane between them. Bowman's capsule epithelial cells, the podocytes, leave minute filtration slits (slit pores). Everything except proteins passes through — hence it is ultrafiltration.
Glomerular Filtration Rate (GFR) in a healthy individual is ~125 mL/minute = 180 litres per day.
2. Reabsorption. Compare 180 L filtered with only 1.5 L urine released — so nearly 99% of the filtrate is reabsorbed. Glucose, amino acids and Na⁺ are reabsorbed actively; nitrogenous wastes passively; water passively in the early segments.
3. Tubular secretion. Tubular cells secrete H⁺, K⁺ and ammonia into the filtrate, maintaining ionic and acid-base balance.
Function of each tubule segment — the table NEET asks from
Segment | Lining / structure | What it does |
PCT (proximal convoluted tubule) | Simple cuboidal BRUSH BORDER epithelium — increases surface area | Reabsorbs nearly ALL essential nutrients and 70–80% of electrolytes and water. Maintains pH/ionic balance by secreting H⁺ and ammonia and absorbing HCO₃⁻ |
Descending limb of Henle's loop | Permeable to WATER, almost impermeable to electrolytes | Filtrate becomes CONCENTRATED as it moves down |
Ascending limb of Henle's loop | IMPERMEABLE to water, allows electrolyte transport | Reabsorption is MINIMUM here. Filtrate gets DILUTED going up; maintains high osmolarity of the medullary interstitium |
DCT (distal convoluted tubule) | — | CONDITIONAL reabsorption of Na⁺ and water; reabsorbs HCO₃⁻; selectively secretes H⁺, K⁺ and NH₃ to maintain pH and Na-K balance |
Collecting duct | Extends from cortex deep into medulla | Large amounts of water reabsorbed here to produce CONCENTRATED urine. Allows small amounts of UREA into the medullary interstitium to keep up osmolarity. Also secretes H⁺ and K⁺ |
Counter Current Mechanism & Regulation of Kidney Function
Counter current mechanism — how concentrated urine is made
Filtrate flows in opposite directions in the two limbs of Henle's loop, and blood likewise in the two limbs of the vasa recta — a counter current. Their proximity maintains an increasing osmolarity towards the inner medulla: from 300 mOsmol/L in the cortex to about 1200 mOsmol/L in the inner medulla.
This gradient is caused mainly by NaCl and urea. NaCl is transported by the ascending limb of Henle's loop, exchanged with the descending vasa recta, and returned by the ascending vasa recta. Urea enters the thin ascending limb and is returned by the collecting tubule.
Result: human kidneys can produce urine nearly four times more concentrated than the initial filtrate.
Regulation of kidney function — three hormonal mechanisms
Mechanism | Trigger | What happens | Net effect |
ADH (vasopressin) | Excessive fluid loss activates OSMORECEPTORS, stimulating the hypothalamus; ADH released from the NEUROHYPOPHYSIS | Facilitates water reabsorption from the latter parts of the tubule, preventing diuresis. Also constricts blood vessels | Conserves water; raises blood pressure and hence GFR. Rising fluid volume switches osmoreceptors off (feedback) |
Renin-Angiotensin | Fall in glomerular blood flow/pressure/GFR activates the JG cells of the JGA | Renin converts angiotensinogen → angiotensin I → angiotensin II. Angiotensin II is a powerful VASOCONSTRICTOR and also triggers the ADRENAL CORTEX to release ALDOSTERONE, which reabsorbs Na⁺ and water from the distal tubule | Increases glomerular blood pressure, GFR and blood pressure |
Atrial Natriuretic Factor (ANF) | Increased blood flow to the atria of the heart | ANF causes VASODILATION | DECREASES blood pressure — acts as a CHECK on the renin-angiotensin mechanism |
The juxta glomerular apparatus (JGA) is a sensitive region formed where the DCT contacts the afferent arteriole.
Micturition, Other Organs, Disorders & Why This Matters for NEET
Micturition & urine characteristics
Stretching of the filling bladder triggers stretch receptors → signals to the CNS → motor messages cause contraction of bladder smooth muscle and simultaneous relaxation of the urethral sphincter. This release is micturition; the neural mechanism is the micturition reflex.
An adult excretes 1–1.5 litres of urine per day — light yellow, slightly acidic (pH 6.0), with about 25–30 g of urea per day.
Glucose in urine = glycosuria; ketone bodies in urine = ketonuria — both indicative of diabetes mellitus.
Other organs that assist excretion
Lungs remove about 200 mL of CO₂ per minute plus significant water. Liver (largest gland) secretes bile containing bilirubin, biliverdin, cholesterol, degraded steroid hormones, vitamins and drugs. Sweat glands excrete a watery fluid with NaCl, small amounts of urea and lactic acid (primary function is cooling). Sebaceous glands eliminate sterols, hydrocarbons and waxes via sebum, which oils the skin.
Disorders
Uremia — accumulation of urea in blood from kidney malfunction; may lead to kidney failure. Treated by haemodialysis, in which blood drained from an artery is pumped through an artificial kidney dialysing unit.
Why this matters for NEET
High-value one-liners: GFR = 125 mL/min = 180 L/day; 99% of filtrate reabsorbed; PCT reabsorbs 70–80% of electrolytes and water; osmolarity 300 → 1200 mOsmol/L; urine pH 6.0, 1–1.5 L/day; each kidney has one million nephrons.
Trap: the DESCENDING limb of Henle's loop is permeable to water but not electrolytes; the ASCENDING limb is the exact opposite — swapping these is the single most common error here. Also, ANF opposes the renin-angiotensin system (vasodilation vs vasoconstriction). And ammonotelic animals' kidneys play no significant role in ammonia removal — it diffuses out across body/gill surfaces.
Test Yourself: MCQs, PYQs & Active Recall
Answer these, then close the article and do an Active Recall. Reveal each answer only after you commit to one.
Practice Questions
Q1. Animals that excrete ammonia are called:
(a) Osmotelic
(b) Ammonotelic
(c) Uricotelic
(d) Ureotelic
Show answer
Answer: (b) — Bony fishes, aquatic amphibians and aquatic insects are ammonotelic; ammonia diffuses out across body or gill surfaces.
Q2. Uric acid is excreted as a pellet or paste by:
(a) Mammals
(b) Reptiles, birds, land snails and insects
(c) Aquatic amphibians
(d) Bony fishes
Show answer
Answer: (b) — Uricotelic animals — reptiles, birds, land snails and insects — excrete uric acid with minimum water loss.
Q3. Protonephridia or flame cells are the excretory structures of:
(a) Prawns
(b) Platyhelminthes such as Planaria
(c) Earthworms
(d) Cockroaches
Show answer
Answer: (b) — Protonephridia occur in Platyhelminthes, rotifers, some annelids and Amphioxus, and are primarily for osmoregulation.
Q4. Malpighian tubules are the excretory structures of:
(a) Most insects including cockroaches
(b) Prawns
(c) Planaria
(d) Earthworms
Show answer
Answer: (a) — Malpighian tubules serve most insects; antennal or green glands serve crustaceans like prawns.
Q5. Each human kidney contains approximately how many nephrons?
(a) One million
(b) One lakh
(c) One thousand
(d) One billion
Show answer
Answer: (a) — Each kidney has nearly one million nephrons, which are its functional units.
Q6. The cortex extending between the medullary pyramids forms renal columns called:
(a) Calyces
(b) Vasa recta
(c) Bowman's capsule
(d) Columns of Bertini
Show answer
Answer: (d) — Cortical tissue extending between the medullary pyramids forms the renal columns known as Columns of Bertini.
Q7. Glomerulus along with Bowman's capsule is called the:
(a) Renal pelvis
(b) Vasa recta
(c) Malpighian body or renal corpuscle
(d) Juxta glomerular apparatus
Show answer
Answer: (c) — Glomerulus plus Bowman's capsule together form the malpighian body, also called the renal corpuscle.
Q8. The epithelial cells of Bowman's capsule that leave filtration slits are called:
(a) Brush border cells
(b) JG cells
(c) Podocytes
(d) Osmoreceptors
Show answer
Answer: (c) — Podocytes are arranged so as to leave minute filtration slits or slit pores, enabling ultrafiltration.
Q9. Glomerular Filtration Rate in a healthy individual is approximately:
(a) 500 mL/minute
(b) 1200 mL/minute
(c) 25 mL/minute
(d) 125 mL/minute
Show answer
Answer: (d) — GFR is about 125 mL per minute, which works out to 180 litres per day.
Q10. What percentage of the glomerular filtrate is reabsorbed by the renal tubules?
(a) 99 per cent
(b) 50 per cent
(c) 75 per cent
(d) 90 per cent
Show answer
Answer: (a) — 180 litres are filtered daily but only about 1.5 litres of urine is released — nearly 99 per cent is reabsorbed.
Q11. The PCT reabsorbs approximately what proportion of electrolytes and water?
(a) 70-80 per cent
(b) 20-30 per cent
(c) 50 per cent
(d) 99 per cent
Show answer
Answer: (a) — The PCT, lined by brush border epithelium, reabsorbs nearly all essential nutrients and 70-80 per cent of electrolytes and water.
Q12. The descending limb of Henle's loop is:
(a) Permeable to both equally
(b) Permeable to water but almost impermeable to electrolytes
(c) Impermeable to both
(d) Impermeable to water but permeable to electrolytes
Show answer
Answer: (b) — The descending limb is permeable to water and almost impermeable to electrolytes, concentrating the filtrate as it descends.
Q13. The osmolarity gradient in the kidney runs from about:
(a) 300 throughout
(b) 1200 mOsmol/L in cortex to 300 in medulla
(c) 300 mOsmol/L in cortex to 1200 in inner medulla
(d) 1200 throughout
Show answer
Answer: (c) — Osmolarity increases from about 300 mOsmol/L in the cortex to about 1200 mOsmol/L in the inner medulla, caused mainly by NaCl and urea.
Q14. Antidiuretic hormone (ADH) is released from the:
(a) JG cells
(b) Adrenal cortex
(c) Atria of the heart
(d) Neurohypophysis
Show answer
Answer: (d) — Osmoreceptors stimulate the hypothalamus to release ADH or vasopressin from the neurohypophysis.
Q15. Renin, released by the JG cells, converts:
(a) Angiotensinogen to angiotensin I
(b) Angiotensin I to angiotensin II
(c) Aldosterone to renin
(d) ANF to ADH
Show answer
Answer: (a) — Renin converts angiotensinogen in blood to angiotensin I, which is then converted to angiotensin II.
Q16. Aldosterone, released from the adrenal cortex, causes:
(a) Secretion of potassium only
(b) Reabsorption of Na+ and water from the distal tubule
(c) Increased urine output
(d) Vasodilation and lowered blood pressure
Show answer
Answer: (b) — Angiotensin II activates the adrenal cortex to release aldosterone, which reabsorbs Na+ and water from distal tubule parts.
Q17. Atrial Natriuretic Factor (ANF) acts by:
(a) Vasoconstriction, raising blood pressure
(b) Increasing renin release
(c) Vasodilation, decreasing blood pressure
(d) Stimulating aldosterone
Show answer
Answer: (c) — ANF causes vasodilation and thereby decreases blood pressure — acting as a check on the renin-angiotensin mechanism.
Q18. The presence of glucose in urine is called:
(a) Diuresis
(b) Uremia
(c) Glycosuria
(d) Ketonuria
Show answer
Answer: (c) — Glucose in urine is glycosuria and ketone bodies in urine is ketonuria — both indicative of diabetes mellitus.
NEET Previous Year Questions (PYQs)
Real NEET previous-year questions on this chapter, with explanations in our own words.
Q19. The counter current mechanism in the kidney is facilitated by: (NEET PYQ)
(a) The collecting duct alone
(b) Bowman's capsule and glomerulus
(c) Henle's loop and vasa recta
(d) The PCT and DCT
Show answer
Answer: (c) — The opposite flow directions in the two limbs of Henle's loop and of the vasa recta form the counter current mechanism.
Q20. Human kidneys can produce urine how many times more concentrated than the initial filtrate? (NEET PYQ)
(a) Four times
(b) Two times
(c) Equal concentration
(d) Ten times
Show answer
Answer: (a) — Human kidneys can produce urine nearly four times more concentrated than the initial filtrate.
Q21. Vasa recta is absent or highly reduced in: (NEET PYQ)
(a) The collecting duct
(b) Juxta medullary nephrons
(c) Cortical nephrons
(d) All nephrons
Show answer
Answer: (c) — Vasa recta is absent or highly reduced in cortical nephrons, which have short loops of Henle.
Q22. The average pH of human urine is: (NEET PYQ)
(a) 7.4
(b) 8.5
(c) 4.0
(d) 6.0
Show answer
Answer: (d) — Urine is a light yellow watery fluid which is slightly acidic, with a pH of about 6.0.
Q23. Uremia, the accumulation of urea in blood, is treated by: (NEET PYQ)
(a) Vasodilation
(b) Haemodialysis
(c) Increasing ADH
(d) Micturition
Show answer
Answer: (b) — Uremia is treated by haemodialysis, in which blood from an artery is pumped through an artificial kidney.
Q24. Approximately how much urea is excreted by an adult human per day? (NEET PYQ)
(a) 180 g
(b) 100-120 g
(c) 25-30 g
(d) 2-3 g
Show answer
Answer: (c) — On average, 25 to 30 grams of urea is excreted out per day.
Q25. Our lungs remove approximately how much CO2 per minute? (NEET PYQ)
(a) 200 mL
(b) 5000 mL
(c) 2000 mL
(d) 20 mL
Show answer
Answer: (a) — The lungs remove large amounts of CO2, approximately 200 mL per minute, along with significant quantities of water.
Q26. In ammonotelic animals, the removal of ammonia is achieved mainly by: (NEET PYQ)
(a) The kidneys
(b) Malpighian tubules
(c) The liver
(d) Diffusion across body or gill surfaces
Show answer
Answer: (d) — Ammonia is readily soluble and is excreted by diffusion across body surfaces or gill surfaces; kidneys play no significant role.
Active Recall Prompt
Write everything you can recall about Excretory Products and their Elimination, naming each part first: the three nitrogenous waste strategies (ammonotelic, ureotelic, uricotelic) with their animals; the excretory structures across the animal kingdom; the human kidney (dimensions, hilum, cortex/medulla, nephron parts, cortical vs juxta medullary nephrons, vasa recta); urine formation (filtration with GFR values, reabsorption, secretion); the function of each tubule segment; the counter current mechanism and the osmolarity gradient; regulation of kidney function (ADH, renin-angiotensin, ANF); micturition and urine characteristics; the role of other organs; and disorders. Begin each fact with its topic and end it with a full stop.