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State Board · Class 11 · Biology

Lesson 15: Excretion and Osmoregulation

7 concepts · 56 practice questions with answers · Free to read

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Excretion and nitrogenous wastes

Link to Std 9–10

Std 8–10 introduced the kidney and excretion; here we study the nephron, urine formation, ADH and RAAS in detail.

From daily life

Household waste must go out daily; body waste goes out through excretion.

Simple definition

Excretion = removing metabolic wastes formed in cells. Animals are ammonotelic, ureotelic or uricotelic.

Nitrogenous wastesDeamination and nitrogenous wastesAmino aciddeaminationAmmonia (toxic)AmmonotelicAmmoniafish, protozoaUreotelicUrea; ~50 ml/gmammals, sharks, frogsUricotelicUric acid; 5–10 ml/gbirds, insects, reptilesExcretion = removing metabolic wastes (not undigested matter)Wastes: CO₂, water, ammonia, urea, uric acid, creatinine, saltsBilirubin → faeces; urochrome → urineGuanotelism: spiders, scorpions, penguins (guanine)Water availability decides the mode of excretionSharks retain urea to stay isotonic with sea water

In the figure : Deamination and the three modes of excretion.

Rule 1 : Wastes and deamination — Metabolic wastes form inside cells; undigested matter never enters cells. Wastes: CO₂, water, ammonia, urea, uric acid, creatinine, salts; bilirubin (via faeces), urochrome (via urine). Excess carbohydrate and fat can be stored, amino acids cannot → deamination: amino group is removed to make ammonia.

  • Easy : Deamination = removing the amino group.
  • Medium : Ammonia is the most toxic.
  • Exam-type (CET) : Bilirubin = faeces; urochrome = urine.

Rule 2 : Three types — Ammonotelism: ammonia; needs lots of water (300–500 ml/g); aquatic animals, bony fish, protozoa. Ureotelism: urea (ornithine cycle in liver; 3 ATP per urea); ~50 ml water/g; mammals, sharks, adult frogs. Uricotelism: uric acid (5–10 ml water/g; least toxic; costs more energy); birds, insects, reptiles, land snails. Spiders, scorpions, penguins: guanotelism.

  • Easy : More water → ammonia; less → uric acid.
  • Medium : Urea cycle in the liver.
  • Exam-type (CET) : Sharks retain urea.
Remember

A-U-U: Ammonia (wet), Urea (medium), Uric (dry).

Common mistake and CET trap

‘Uric acid is most toxic’ is wrong (least). Trap: ‘birds are ureotelic’ is wrong.

Shortcut

Memory clue : "Fish flush ammonia; birds excrete dry uric acid".

Spot the difference

TypeWater / gExample
Ammonotelic300–500 mlFish
Ureotelic~50 mlHuman
Uricotelic5–10 mlBirds

Practice questions

Easy
  1. 1What is excretion?
  2. 2What is deamination?
  3. 3Which type is human?
Medium
  1. 4Why uric acid in birds?
  2. 5Where is urea made?
  3. 6Guanotelic animals?
Challenging
  1. 7How is bilirubin excreted?
  2. 8Why urea in sharks?
Show answers (try them yourself first)
  1. 1Removing metabolic wastes.
  2. 2Removing the amino group.
  3. 3Ureotelic.
  4. 4To save water.
  5. 5In the liver.
  6. 6Spiders, scorpions, penguins.
  7. 7Through faeces.
  8. 8To stay isotonic with sea water.

Osmoregulation and excretory organs of animals

From daily life

A sea bird drinks salty water and removes the salt through glands near its nostrils.

Simple definition

Osmoregulation = control of salt and water balance. Animals are osmoconformers or osmoregulators. Excretory organs: contractile vacuole, nephridia, Malpighian tubules, green glands, kidney.

Excretory organsOsmoregulation and excretory organsAnimalOrganProtozoaContractile vacuoleSpongediffusion → osculumFlatwormProtonephridia (flame cells)EarthwormMetanephridia (nephrostome)InsectsMalpighian tubulesCrustaceansGreen glandsEchinodermsno special organMammalsKidney (nephrons)Osmoconformer: body fluid = surroundings (most marine)Osmoregulator: independent control (freshwater, terrestrial)Stenohaline: narrow salinity; euryhaline: wide (barnacle, clam)Albatross: salt glands near nostrils (active transport)Homeostasis = steady internal environment

In the figure : Animals and their excretory organs.

Rule 1 : Osmoconformers and osmoregulators — Osmoconformers: body fluid isoosmotic with surroundings (most marine). Osmoregulators: independent of surroundings; freshwater and terrestrial. Stenohaline: tolerate narrow salinity; euryhaline: wide (barnacle, clam). Albatross, sea turtles, marine iguanas: salt glands remove salt by active transport. Homeostasis = constant internal environment.

  • Easy : Conform = go with the flow; regulate = control.
  • Medium : Steno = narrow; eury = wide.
  • Exam-type (CET) : Salt glands are near the nostrils.

Rule 2 : Excretory organs across animals — Protozoa: contractile vacuole. Sponge: diffusion → osculum. Platyhelminthes: protonephridia (flame cells; blind-ended tubes). Earthworm: metanephridia (nephrostome + nephridiopore). Insects: Malpighian tubules (dry uric acid). Crustaceans: green glands. Echinoderms: no special organ; tube feet/diffusion. Mammals: kidney (nephron).

  • Easy : Flame cells = flatworm.
  • Medium : Metanephridia = earthworm.
  • Exam-type (CET) : Green gland = crustacean.
Remember

Flat → flame; earthworm → meta; insect → Malpighi; crab → green.

Common mistake and CET trap

‘All marine animals regulate’ is wrong (most conform). Trap: ‘echinoderms have kidneys’ is wrong.

Shortcut

Memory clue : "Flame–flat, meta–earthworm, Malpighi–moth, green–crab".

Spot the difference

AnimalOrgan
FlatwormProtonephridia
EarthwormMetanephridia
InsectMalpighian tubules
PrawnGreen glands

Practice questions

Easy
  1. 1What is an osmoconformer?
  2. 2What is stenohaline?
  3. 3Example of euryhaline?
Medium
  1. 4How does albatross remove salt?
  2. 5Where are flame cells?
  3. 6What is a nephrostome?
Challenging
  1. 7Insect organ?
  2. 8What is homeostasis?
Show answers (try them yourself first)
  1. 1Body fluid matches the surroundings.
  2. 2Tolerates narrow salinity.
  3. 3Barnacle, clam.
  4. 4Salt glands.
  5. 5In Platyhelminthes.
  6. 6Funnel of the metanephridium.
  7. 7Malpighian tubules.
  8. 8Constant internal environment.

Human excretory system

From daily life

The kidney is a daily blood filter; ureter, bladder and urethra are pipe and tank.

Simple definition

Human excretory system: a pair of kidneys, two ureters, one urinary bladder, one urethra.

Urinary systemUrinary systemKidneyKidneyUreterBladderUrethraKidney L.S.Cortex (Malpighian bodies, tubules)Medulla : pyramidsLoop of Henle + collecting ductsColumns of Bertini; papillaCalyx → Pelvis → Ureter3 coverings: fascia, fat, capsuleBlood: ~¼ of cardiac outputPosition: T12–L3; retroperitoneal~1 million nephrons eachRoles: wastes, water, pH; renin, erythropoietin, calcitriolUreter 25–30 cm; opens obliquely into bladder (no backflow)Bladder: detrusor muscle; trigone; capacity ~700 mlMicturition: internal (involuntary) + external (voluntary) sphincter

In the figure : Urinary system and a kidney section.

Rule 1 : Kidney — Bean-shaped; from T12 to L3; behind the peritoneum = retroperitoneal; convex outer, concave inner side (hilus: renal artery in, renal vein and ureter out). Three coverings: renal fascia, adipose capsule (shock absorber), renal capsule. L.S.: cortex (outer; Malpighian bodies, tubules), medulla (inner; Henle loops, collecting ducts) → renal pyramids; columns of Bertini; papilla → minor calyx → major calyx → renal pelvis → ureter. About a million nephrons per kidney.

  • Easy : Cortex outside; medulla inside.
  • Medium : Papilla → minor → major calyx → pelvis.
  • Exam-type (CET) : Retroperitoneal = behind the peritoneum.

Rule 2 : Ureter, bladder, urethra, micturition — Ureter: 25–30 cm muscular tube; opens obliquely into the bladder (no backflow). Bladder: in the pelvis; detrusor muscle; trigone (openings of ureters + urethra); capacity ~700 ml. Micturition: when half full, stretch receptors → spinal cord → bladder contracts + internal sphincter relaxes (involuntary); external sphincter (voluntary) relaxes → urine leaves. No control up to 2 years. The kidney makes renin, erythropoietin, calcitriol.

  • Easy : Internal involuntary; external voluntary.
  • Medium : Trigone = triangle.
  • Exam-type (CET) : Erythropoietin = RBC production.
Remember

K-U-B-U: Kidney, Ureter, Bladder, Urethra.

Common mistake and CET trap

‘Ureter has a valve’ is wrong (oblique entry). Trap: ‘cortex is inner’ is wrong.

Shortcut

Memory clue : "Cortex = cover (outer); medulla = middle (inner)".

Spot the difference

RegionContains
CortexMalpighian bodies, tubules
MedullaHenle loops, collecting ducts
PelvisUrine to the ureter

Practice questions

Easy
  1. 1What is retroperitoneal?
  2. 2What passes through the hilus?
  3. 3Where are pyramids?
Medium
  1. 4Length of ureter?
  2. 5What is the trigone?
  3. 6Bladder capacity?
Challenging
  1. 7External sphincter?
  2. 8Role of erythropoietin?
Show answers (try them yourself first)
  1. 1Behind the peritoneum.
  2. 2Renal artery, vein, ureter.
  3. 3In the medulla.
  4. 425–30 cm.
  5. 5Triangular area at bladder base.
  6. 6~700 ml.
  7. 7Voluntary (striated).
  8. 8Stimulates RBC production.

Structure of the nephron

From daily life

A nephron is a tiny blood-cleaning factory; each kidney has about a million.

Simple definition

Nephron = structural and functional unit of the kidney: renal corpuscle (Bowman's capsule + glomerulus) + renal tubule (PCT, loop of Henle, DCT).

NephronNephronBowman's capsule+ GlomerulusPCTLoop of HenledescendingascendingDCTCollecting ductBowman: visceral = podocytes (filtration slits); parietal = squamousAfferent wider than efferent → high pressure in glomerulusPCT: cuboidal + brush border; selective reabsorptionLoop: descending permeable to water; ascending notDCT: tubular secretion, pH control; vasa recta around loopCortical (most) and juxtamedullary (long loop) nephronsJGA = JG cells (afferent) + macula densa (DCT)

In the figure : Parts of a nephron.

Live figure

Structure of the nephron

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Rule 1 : Renal corpuscle and PCT — Bowman's capsule: cup-like; inner (visceral) wall has podocytes (filtration slits); outer (parietal) wall thin squamous. Glomerulus: fenestrated capillaries; afferent arteriole (wider) → efferent arteriole (narrower) → high pressure → ultrafiltration. PCT: highly coiled; cuboidal cells + brush border; peritubular capillaries; selective reabsorption.

  • Easy : Afferent wide, efferent narrow.
  • Medium : Podocyte = foot-like cell.
  • Exam-type (CET) : Brush border = more surface.

Rule 2 : Henle loop, DCT, JGA, types — Loop of Henle ‘U’: descending (thin, water-permeable, squamous), ascending (thick, water-impermeable, cuboidal); vasa recta nearby. DCT: cuboidal; tubular secretion, pH, water reabsorption. Collecting tubule/duct: reabsorbs water. JGA: JG cells (afferent arteriole) + macula densa (DCT) → BP regulation. Cortical nephrons (most, short loop) and juxtamedullary (long loop, vasa recta).

  • Easy : Descending = water out.
  • Medium : JGA = BP guardian.
  • Exam-type (CET) : Juxtamedullary = long loop.
Remember

B-P-L-D-C: Bowman, PCT, Loop, DCT, Collecting.

Common mistake and CET trap

‘Efferent is wider’ is wrong. Trap: ‘ascending limb permeable to water’ is wrong.

Shortcut

Memory clue : "Afferent = arrives (wide); efferent = exits (narrow)".

Spot the difference

PartFeature
PCTBrush border; reabsorption
DescendingWater-permeable
AscendingImpermeable; absorbs NaCl
DCTSecretion, pH

Practice questions

Easy
  1. 1Parts of nephron.
  2. 2Where are podocytes?
  3. 3Why is afferent wider?
Medium
  1. 4Why brush border in PCT?
  2. 5Where is vasa recta?
  3. 6Where is macula densa?
Challenging
  1. 7Where are JG cells?
  2. 8Two types of nephrons.
Show answers (try them yourself first)
  1. 1Corpuscle + PCT + loop + DCT.
  2. 2In Bowman's visceral wall.
  3. 3Builds high pressure for filtration.
  4. 4Increases absorptive area.
  5. 5Around the loop of Henle.
  6. 6Start of the DCT.
  7. 7In afferent arteriole wall.
  8. 8Cortical; juxtamedullary.

Urine formation

From daily life

Like straining tea, the kidney first filters everything, then takes the useful part back.

Simple definition

Urine formation has three steps: ultrafiltration, selective reabsorption, tubular secretion. Filtration rate 125 ml/min (180 L/day); 99% is reabsorbed.

Urine formationUrine formation: three steps1 Ultrafiltration2 Reabsorption3 SecretionGlomerulus → capsulePCT (+ DCT) → bloodblood → DCT / CTEFP = GHP − (BCOP + CHP)10 = 55 − (30 + 15) mm HgGFR = 125 ml/min = 180 L/day~99% of filtrate is reabsorbedFiltrate: protein-free plasma (alkaline); no RBC/proteinsHigh threshold (glucose, amino acids, Na⁺, Ca⁺⁺, vit C) = activeLow threshold (water, sulphate, nitrate) = passiveSecretion: creatinine, K⁺, H⁺ → urine turns acidic~600 ml blood per minute through each kidneyNormal urine: pale yellow (urochrome); acidicGlucosuria: when blood glucose exceeds the threshold

In the figure : The three steps and the pressure arithmetic.

Rule 1 : Ultrafiltration — High pressure in the glomerulus: GHP ≈ 55 mm Hg. Opposed by blood osmotic pressure ≈ 30 + capsular pressure ≈ 15. EFP = 55 − (30 + 15) = 10 mm Hg. Plasma (except proteins and cells) filters into the capsule = glomerular filtrate (alkaline; urea, glucose, amino acids, ions). ~600 ml blood per minute per kidney; GFR = 125 ml/min = 180 L/day.

  • Easy : EFP = 55 − 45 = 10.
  • Medium : Cells and proteins are not filtered.
  • Exam-type (CET) : GFR = 125 ml/min.

Rule 2 : Reabsorption and secretion — PCT does selective reabsorption: high-threshold substances (glucose, amino acids, vit C, Na⁺, K⁺, Ca⁺⁺, Cl⁻) actively; low-threshold (water, sulphates, nitrates) passively; PCT + DCT together take back ~99%. Tubular secretion (DCT, collecting tubule): creatinine, K⁺, H⁺ pass from blood into the tubule; urine turns acidic; pH regulation. The only route of excretion in marine bony fish and desert amphibians. Normal urine: pale yellow (urochrome).

  • Easy : High threshold = active uptake.
  • Medium : Secretion = blood → tubule.
  • Exam-type (CET) : Above glucose threshold → glucosuria.
Remember

F-R-S: Filter, Reabsorb, Secrete; EFP = 55 − 45 = 10.

Common mistake and CET trap

‘Glucose is always in urine’ is wrong (PCT reabsorbs). Trap: ‘EFP = 55’ is wrong.

Shortcut

Memory clue : "FRS: Filter → Return → Send out".

Spot the difference

StepSiteDirection
UltrafiltrationGlomerulusBlood → capsule
ReabsorptionPCT, DCTTubule → blood
SecretionDCT, CTBlood → tubule

Practice questions

Easy
  1. 1Compute EFP: 55, 30, 15.
  2. 2What is GFR?
  3. 3Proteins in filtrate?
Medium
  1. 4% reabsorbed?
  2. 5Example of high threshold?
  3. 6What is secreted?
Challenging
  1. 7Why acidic urine?
  2. 8When does glucosuria occur?
Show answers (try them yourself first)
  1. 155 − (30 + 15) = 10 mm Hg.
  2. 2125 ml/min (180 L/day).
  3. 3Absent.
  4. 4~99%.
  5. 5Glucose, amino acids.
  6. 6Creatinine, K⁺, H⁺.
  7. 7Because of H⁺ secretion.
  8. 8When blood glucose exceeds threshold.

Concentration of urine and its regulation

From daily life

With little water in summer, urine turns dark and scanty: the body conserves water.

Simple definition

The countercurrent mechanism makes urine up to 4× more concentrated (1200 vs 300 mOsm/L). Regulated by ADH and RAAS.

Concentration and regulationCountercurrent mechanism300 mOsm/L600 mOsm/L900 mOsm/L1200 mOsm/LLoopH₂O ↓NaCl ↑Vasa rectaMax: urine 1200 vs blood 300 mOsm/LDescending: water leaves → fluid concentratesAscending: Na⁺, Cl⁻ reabsorbed → diluteUrea recycling → gradient in deep medullaVasa recta = exchanger; keeps the gradientADH: makes collecting duct permeable to waterADH (posterior pituitary): via osmoreceptors; negative feedbackNo ADH → diabetes insipidus (lots of dilute urine)RAAS: low BP → JG cells renin → angiotensin I → IIAngiotensin II: vasoconstriction, Na⁺ uptake, aldosteroneAldosterone: Na⁺ + water uptake in DCT/CDANP: blocks Na⁺ reabsorption (atrial wall) → natriuresisCamel: long loop → concentrated urine

In the figure : Countercurrent mechanism and ADH/RAAS.

Rule 1 : Countercurrent mechanism — Loops of Henle of juxtamedullary nephrons and the vasa recta. Descending limb: water leaves → fluid concentrates. Ascending limb: Na⁺, Cl⁻ reabsorbed → fluid dilutes; salt builds up in the medulla → osmotic gradient (300 → 1200 mOsm/L). ADH makes the collecting duct permeable to water → water returns → concentrated urine. The deep collecting duct is permeable to urea: urea recycling. The vasa recta (countercurrent exchange) keeps the gradient. Camels have long loops.

  • Easy : Descending concentrates; ascending dilutes.
  • Medium : Multiplier = loop; exchanger = vasa recta.
  • Exam-type (CET) : Camel: long loop.

Rule 2 : ADH and RAAS — ADH: osmoreceptors in the hypothalamus detect raised blood osmolarity (sweat, salty food) → posterior pituitary releases ADH → water reabsorption in last DCT + collecting duct → less urine; osmolarity normalises → less ADH (negative feedback). No ADH → diabetes insipidus. RAAS: low BP/blood volume → JG cells release renin → angiotensinogen → angiotensin I → II (ACE). Ang II: constricts arterioles, boosts Na⁺ uptake in PCT, triggers aldosterone (adrenal cortex) → Na⁺ + water in DCT/CD. ANP (atrial wall) acts oppositely: natriuresis + diuresis.

  • Easy : ADH = saves water.
  • Medium : Renin → Ang I → II → aldosterone.
  • Exam-type (CET) : ANP opposes ADH/RAAS.
Remember

RAAS order: Renin → Angiotensin → Aldosterone (R-A-A).

Common mistake and CET trap

‘ADH increases urine’ is wrong (reduces). Trap: ‘ANP conserves Na⁺’ is wrong.

Shortcut

Memory clue : "ADH = Add water Here".

Spot the difference

PointADHRAAS
Triggerosmolarity ↑BP/volume ↓
KeyWater uptakeNa⁺ + water
SourcePosterior pituitaryJG cells → adrenal

Practice questions

Easy
  1. 1Max urine concentration?
  2. 2What happens in the descending limb?
  3. 3Role of vasa recta?
Medium
  1. 4Source of ADH?
  2. 5Where are osmoreceptors?
  3. 6Cause of diabetes insipidus?
Challenging
  1. 7Who releases renin?
  2. 8Source of aldosterone?
Show answers (try them yourself first)
  1. 11200 mOsm/L.
  2. 2Water leaves; fluid concentrates.
  3. 3Maintains the gradient.
  4. 4Posterior pituitary.
  5. 5Hypothalamus.
  6. 6Lack of ADH.
  7. 7JG cells.
  8. 8Adrenal cortex.

Disorders, dialysis and transplant

From daily life

If a filter clogs, the house fills with waste; dialysis is an artificial filter for failed kidneys.

Simple definition

Kidney disorders: kidney stones, uremia, nephritis, ARF, CKD. Treatment: haemodialysis (and kidney transplant). Skin and lungs also excrete.

Disorders and dialysisDisorders and dialysisKidney stonesCalcium · struvite · uric acid · cystineback pain, pink urineUremiablood urea > 0.05%can lead to kidney failureNephritisglomeruli leaky; protein in urineoedemaARFsudden; after bleeding; <400 ml/dayserum creatinine risesCKDgradual, irreversible fall in GFRsmall kidneys, anaemiaHaemodialysisWhen kidney function falls below 5–7%Blood outside the body through cellophane tube; isosmotic dialysateHeparin prevents clotting; cleaned blood returnsPeritoneal dialysis: uses the abdominal membraneUrine test: albumin = filter injury; ketones = diabetes/starvationLeucocytes in urine: sign of infection

In the figure : Disorders and haemodialysis.

Rule 1 : Disorders — Kidney stones (renal calculi): calcium (oxalate/phosphate), struvite (bacterial infection), uric acid (low water + high protein), cystine (genetic); back pain, pink urine. Uremia: blood urea above 0.05% (normal 0.01–0.03%). Nephritis: glomeruli become leaky → proteinuria → oedema. ARF: sudden; after bleeding; <400 ml/day; creatinine rises. CKD: GFR falls slowly and irreversibly; anaemia. Urine albumin = filter injury; ketones = diabetes/starvation; leucocytes = infection.

  • Easy : Uremia: urea > 0.05%.
  • Medium : Nephritis = protein in urine + oedema.
  • Exam-type (CET) : Creatinine = index of kidney function.

Rule 2 : Haemodialysis and other organs — When kidney function drops below 5–7%: blood (from the radial artery) is taken outside through a cellophane tube; the tube sits in dialysate (isosmotic with plasma); excess salts/wastes diffuse out; heparin prevents clotting; cleaned blood returns to a vein. Skin: sweat (water, NaCl, lactic acid, urea; mainly thermoregulation), sebum. Lungs: CO₂, water vapour, volatile spice compounds. Liver: bilirubin, urea formation.

  • Easy : Dialysate = isosmotic.
  • Medium : Heparin = anticoagulant.
  • Exam-type (CET) : Sweat = cooling + a little excretion.
Remember

Dialysis = dialysate + drain waste + heparin.

Common mistake and CET trap

‘Uremia = uric acid in blood’ is wrong (urea). Trap: ‘ARF is gradual’ is wrong (sudden).

Shortcut

Memory clue : "ARF = abrupt; CKD = creeping".

Spot the difference

DisorderKey sign
StonesBack pain
UremiaUrea > 0.05%
NephritisProteinuria + oedema
ARF / CKDSudden / gradual

Practice questions

Easy
  1. 1What is uremia?
  2. 2Four types of stones.
  3. 3Why oedema in nephritis?
Medium
  1. 4What is ARF?
  2. 5What is CKD?
  3. 6When is dialysis needed?
Challenging
  1. 7Role of heparin?
  2. 8Why creatinine?
Show answers (try them yourself first)
  1. 1Blood urea above 0.05%.
  2. 2Calcium, struvite, uric acid, cystine.
  3. 3Protein loss lowers blood osmotic pressure.
  4. 4Sudden fall in kidney function.
  5. 5Gradual irreversible GFR decline.
  6. 6Function below 5–7%.
  7. 7Prevents clotting.
  8. 8Index of kidney function.

Quick revision of the whole lesson

ConceptKey fact
WastesAmmonia / urea / uric acid
OrgansFlame cell, nephridia, Malpighi, kidney
KidneyCortex + medulla; ~1 million nephrons
NephronCorpuscle, PCT, loop, DCT
UrineEFP = 10; 99% reabsorbed
RegulationADH; RAAS; 1200 mOsm/L
DisordersStones, uremia, nephritis, ARF/CKD

🎯 Most important points for the exam

  • Three modes of excretion.
  • Excretory organs of animals.
  • L.S. of kidney.
  • Nephron and JGA.
  • EFP calculation; three steps.
  • Countercurrent, ADH, RAAS.
  • Disorders and dialysis.
Study mantra

Understand the concept → draw the figure → memorise the key points → solve questions yourself.