# Excretory Products and their Elimination (NEET Biology Class 11): Nephron, GFR & Counter Current Mechanism Source: https://tracknation.in/story/excretory-products-and-their-elimination-neet-biology-class-11-nephron-gfr-counter-current-mechanism (TrackNation β€” free UPSC notes with PYQs) > NEET Class 11 Excretory Products and their Elimination, mined from NCERT: the three nitrogenous-waste strategies in a table, excretory structures across the animal kingdom (protonephridia, nephridia, Malpighian tubules, antennal glands), full human kidney and nephron anatomy, urine formation with exact GFR values, a segment-by-segment tubule function table (PCT, Henle's loop limbs, DCT, collecting duct), the counter current mechanism and osmolarity gradient, a table of the three hormonal regulat ## Nitrogenous Wastes, Excretory Structures & Kidney Anatomy πŸ“Œ **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 β€” plus the excretory-structure list, which is pure match-the-pair.* ### Nitrogenous waste β€” the three excretory strategies | **Type** | **Waste excreted** | **Water cost** | **Animals** | | Ammonotelic | Ammonia β€” 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 β€” less toxic; made from ammonia in the LIVER, released into blood, filtered by kidneys | Moderate | Mammals, many terrestrial amphibians, marine fishes | | Uricotelic | Uric acid β€” as a pellet or paste | MINIMUM water loss | Reptiles, birds, land snails, insects | ### Excretory structures across the animal kingdom β€” match the pair | **Structure** | **Found in** | **Function** | | Protonephridia (flame cells) | Platyhelminthes (e.g. Planaria), rotifers, some annelids, Amphioxus | Primarily OSMOREGULATION β€” 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 β€” 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 β€” the **Columns of Bertini**. - Each kidney has nearly **one million nephrons** β€” 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 β†’ PCT β†’ Henle's loop (descending + ascending) β†’ DCT β†’ 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** β€” a U-shaped vessel parallel to Henle's loop β€” is **absent or highly reduced in cortical nephrons**. ## Urine Formation & Function of Each Tubule Segment ### Urine formation β€” three processes - **1. Glomerular filtration.** About **1100–1200 mL of blood is filtered per minute** β€” 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 β€” 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** β€” 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 β€” the table NEET asks from | **Segment** | **Lining / structure** | **What it does** | | PCT (proximal convoluted tubule) | Simple cuboidal BRUSH BORDER epithelium β€” 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) | β€” | 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 β€” 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** β€” 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 β€” 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 β†’ angiotensin I β†’ 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 β€” 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** β†’ signals to the CNS β†’ 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** β€” 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** β€” both indicative of **diabetes mellitus**. ### Other organs that assist excretion - **Lungs** remove about **200 mL of COβ‚‚ 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** β€” 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 β†’ 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 β€” 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 β€” 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 **Answer: (b)** β€” 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 **Answer: (b)** β€” Uricotelic animals β€” reptiles, birds, land snails and insects β€” 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 **Answer: (b)** β€” 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 **Answer: (a)** β€” 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 **Answer: (a)** β€” 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 **Answer: (d)** β€” 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 **Answer: (c)** β€” 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 **Answer: (c)** β€” 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 **Answer: (d)** β€” 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 **Answer: (a)** β€” 180 litres are filtered daily but only about 1.5 litres of urine is released β€” 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 **Answer: (a)** β€” 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 **Answer: (b)** β€” 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 **Answer: (c)** β€” 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 **Answer: (d)** β€” 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 **Answer: (a)** β€” 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 **Answer: (b)** β€” 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 **Answer: (c)** β€” ANF causes vasodilation and thereby decreases blood pressure β€” 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 **Answer: (c)** β€” Glucose in urine is glycosuria and ketone bodies in urine is ketonuria β€” both indicative of diabetes mellitus. **Q19.** Nearly all the glucose and amino acids, and about 70-80% of the electrolytes and water, are reabsorbed in the part marked - (a) B - (b) E - (c) F - (d) C **Answer: (a)** β€” B is the proximal convoluted tubule (PCT). Its brush-border epithelium greatly increases surface area; nearly all essential nutrients are reabsorbed here, and it also secretes hydrogen ions, ammonia and potassium. **Q20.** Which labelled part is permeable to water but almost impermeable to electrolytes? - (a) C - (b) B - (c) D - (d) E **Answer: (a)** β€” C is the descending limb of the loop of Henle: permeable to water, almost impermeable to electrolytes, so the filtrate becomes concentrated as it descends. The ascending limb [D] is the reverse: impermeable to water, allowing transport of electrolytes. **Q21.** Conditional reabsorption of sodium and water and selective secretion of hydrogen and potassium ions occurs in the part marked - (a) A - (b) E - (c) C - (d) B **Answer: (b)** β€” E is the distal convoluted tubule (DCT), where reabsorption of Na+ and water is conditional (under hormonal control) and H+, K+ and NH3 are secreted selectively to maintain pH and ion balance. **Q22.** The structure marked A, formed of the glomerulus and the double-walled cup around it, is called the - (a) loop of Henle - (b) juxtaglomerular apparatus - (c) vasa recta - (d) renal (Malpighian) corpuscle **Answer: (d)** β€” The glomerulus together with Bowman's capsule forms the renal or Malpighian corpuscle, where ultrafiltration takes place. ### NEET Previous Year Questions (PYQs) *Real NEET previous-year questions on this chapter, with explanations in our own words.* **Q23.** 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 **Answer: (c)** β€” The opposite flow directions in the two limbs of Henle's loop and of the vasa recta form the counter current mechanism. **Q24.** 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 **Answer: (a)** β€” Human kidneys can produce urine nearly four times more concentrated than the initial filtrate. **Q25.** Vasa recta is absent or highly reduced in: (NEET PYQ) - (a) The collecting duct - (b) Juxta medullary nephrons - (c) Cortical nephrons - (d) All nephrons **Answer: (c)** β€” Vasa recta is absent or highly reduced in cortical nephrons, which have short loops of Henle. **Q26.** The average pH of human urine is: (NEET PYQ) - (a) 7.4 - (b) 8.5 - (c) 4.0 - (d) 6.0 **Answer: (d)** β€” Urine is a light yellow watery fluid which is slightly acidic, with a pH of about 6.0. **Q27.** Uremia, the accumulation of urea in blood, is treated by: (NEET PYQ) - (a) Vasodilation - (b) Haemodialysis - (c) Increasing ADH - (d) Micturition **Answer: (b)** β€” Uremia is treated by haemodialysis, in which blood from an artery is pumped through an artificial kidney. **Q28.** 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 **Answer: (c)** β€” On average, 25 to 30 grams of urea is excreted out per day. **Q29.** Our lungs remove approximately how much CO2 per minute? (NEET PYQ) - (a) 200 mL - (b) 5000 mL - (c) 2000 mL - (d) 20 mL **Answer: (a)** β€” The lungs remove large amounts of CO2, approximately 200 mL per minute, along with significant quantities of water. **Q30.** 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 **Answer: (d)** β€” 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.