Introduction & Nitrogenous Wastes

Accumulation of Metabolic Wastes

Animals accumulate ammonia, urea, uric acid, CO₂, water, and ions (Na⁺, K⁺, Cl⁻, phosphate, sulphate, etc.) via metabolic activities or excess ingestion. These must be removed totally or partially.

Major Nitrogenous Wastes

Ammonia, urea, and uric acid are the major nitrogenous wastes produced by animals.

Ammonia

  • Most toxic form
  • Requires large amounts of water for elimination
  • Highly soluble in water

Urea

  • Moderately toxic
  • Requires less water than ammonia
  • Produced in liver via Ornithine cycle

Uric Acid

  • Least toxic
  • Removed with minimum loss of water
  • Excreted as pellet or paste

Types of Nitrogen Excretion

1. Ammonotelism

The process of excreting ammonia.

  • Organisms: Many bony fishes (e.g., Labeo), aquatic amphibians (e.g., tadpole), and aquatic insects
  • Mechanism: Readily soluble; excreted by diffusion across body surfaces or gill surfaces (in fish) as ammonium ions. Kidneys do not play any significant role.

2. Ureotelism

Terrestrial adaptation for water conservation necessitated producing lesser toxic urea or uric acid.

  • Organisms: Mammals, many terrestrial amphibians (e.g., adult frog), and marine fishes
  • Mechanism: Ammonia produced by metabolism is converted into urea in the liver via the Ornithine cycle, which removes CO₂ and ammonia from the blood (NEET 2005). It is released into the blood, filtered, and excreted by the kidneys (NEET 2010).
  • Exception: Some amount of urea may be retained in the kidney matrix to maintain desired osmolarity.

3. Uricotelism

Excretion of nitrogenous waste as uric acid in the form of a pellet or paste with minimum water loss.

  • Organisms: Reptiles (lizards), birds (peacock, pigeon), land snails, and insects (cockroach, housefly) (NEET 2004, 2009, 2011, 2012, 2013)

Survey of the Animal Kingdom (Excretory Structures)

Excretory Structure Animal Examples Primary Function
Protonephridia / Flame cells Platyhelminthes (Flatworms, e.g., Planaria), rotifers, some annelids, and the cephalochordate Amphioxus Primarily concerned with ionic and fluid volume regulation, i.e., osmoregulation
Nephridia Earthworms and other annelids Remove nitrogenous wastes and maintain fluid and ionic balance
Malpighian tubules Most insects, including cockroaches (NEET 2009, 2018) Removal of nitrogenous wastes and osmoregulation
Antennal glands / Green glands Crustaceans like prawns Excretory function and osmoregulation
Kidneys Vertebrates (complex tubular organs) Filtration, excretion, and osmoregulation

Human Excretory System

Anatomical Structure of Kidneys

Components

A pair of kidneys, one pair of ureters, a urinary bladder, and a urethra (NEET 2018)

  • Location: Reddish-brown, bean-shaped structures situated between the levels of the last thoracic and third lumbar vertebra, close to the dorsal inner wall of the abdominal cavity
  • Dimensions & Weight: 10–12 cm long, 5–7 cm wide, 2–3 cm thick; average weight of 120–170 g
  • Hilum: A notch towards the center of the inner concave surface through which the ureter, blood vessels, and nerves enter
  • Renal Pelvis: A broad funnel-shaped space inner to the hilum with projections called calyces
  • Internal Zones:
    • Tough Capsule: Outer protective layer
    • Cortex: Outer zone; extends in between medullary pyramids as renal columns called Columns of Bertini
    • Medulla: Inner zone; divided into a few conical masses called medullary pyramids projecting into the calyces
  • Adrenal Gland: Located at the anterior part of each kidney (NEET 2013)

The Nephron (Structural and Functional Unit)

Quantity: Nearly one million complex tubular structures per kidney (NEET 1997)

Components of a Single Nephron

Consists of a Glomerulus and a Renal Tubule (NEET 1990, 1994)

  1. Glomerulus: A tuft of capillaries formed by the afferent arteriole (fine branch of renal artery). Blood is carried away from it by an efferent arteriole (NEET 2011)
  2. Bowman's Capsule: Double-walled, cup-like structure enclosing the glomerulus
  3. Malpighian Body / Renal Corpuscle: Glomerulus + Bowman's capsule (NEET 2018)
  4. Proximal Convoluted Tubule (PCT): Highly coiled network continuing from Bowman's capsule
  5. Henle's Loop: Hairpin-shaped structure with a descending limb and an ascending limb (NEET 2018)
  6. Distal Convoluted Tubule (DCT): Highly coiled tubular region continuing from the ascending limb
  7. Collecting Duct: A straight tube where DCTs of many nephrons open. Many converge and open into the renal pelvis through medullary pyramids in the calyces.
    Crucial Detail: The collecting duct is NOT part of a single nephron (NEET 1994)

Zonal Distribution

  • Malpighian corpuscle, PCT, and DCT are situated in the cortex
  • The loop of Henle dips into the medulla
  • Convoluted tubules are never part of a renal pyramid (NEET 2011)

Types of Nephrons

Cortical Nephrons (Majority)

  • Loop of Henle is too short
  • Extends only very little into the medulla
  • Vasa recta is absent or highly reduced

Juxtamedullary Nephrons

  • Loop of Henle is very long
  • Runs deep into the medulla
  • Important for urine concentration

Capillary Networks

  • Peritubular Capillaries: A fine capillary network formed around the renal tubule by the emerging efferent arteriole
  • Vasa Recta: A minute vessel of the peritubular network running parallel to Henle's loop, forming a 'U' shape

Urine Formation

Urine formation involves three main steps: Glomerular Filtration, Reabsorption, and Secretion.

1. Glomerular Filtration (Ultrafiltration)

Volume: On average, 1100–1200 ml of blood is filtered per minute, constituting roughly 1/5th of the blood pumped out by each ventricle of the heart in a minute.

Filtration Barrier (3 Layers):

  1. Endothelium of glomerular blood vessels
  2. Epithelium of Bowman's capsule (composed of podocytes arranged intricately to leave minute spaces called filtration slits or slit pores) (NEET 2011)
  3. Basement membrane between these two layers

Ultrafiltration: Blood is filtered so finely that almost all constituents of plasma except proteins pass into the lumen of Bowman's capsule (NEET 2015)

Glomerular Filtration Rate (GFR)

Amount of filtrate formed by both kidneys per minute. In a healthy individual, it is 125 ml/minute (i.e., 180 liters per day).

Regulation via Juxtaglomerular Apparatus (JGA)

JGA is a sensitive region formed by cellular modifications in the DCT and the afferent arteriole at their contact location.

  • Mechanism: A fall in GFR activates JG cells to release renin (NEET 2012). Renin stimulates glomerular blood flow, bringing GFR back to normal.

2. Reabsorption

The Gradient: Out of 180 liters of filtrate, only 1.5 liters is released as urine daily; 99% of the filtrate must be reabsorbed (NEET 2010)

Mechanisms:

  • Active Transport: Glucose, amino acids, and Na⁺ are actively reabsorbed (NEET 1993, 2015, 2016)
  • Passive Transport: Nitrogenous wastes are reabsorbed passively. Water reabsorption occurs passively in the initial segments.

3. Tubular Secretion

  • Substances Secreted: Tubular cells secrete H⁺, K⁺, and ammonia (NH₃) into the filtrate
  • Significance: Vital for maintaining the ionic, pH, and acid-base balance of body fluids

Function of the Tubules

Segment-wise Transport Mechanisms

Proximal Convoluted Tubule (PCT)

  • Lined by simple cuboidal brush border epithelium which drastically increases surface area (NEET 1990)
  • Reabsorbs nearly all essential nutrients, and 70–80% of electrolytes and water (NEET 2012, 2015)
  • Secretes H⁺, NH₃, and potassium ions into the filtrate; absorbs HCO₃⁻ to maintain pH and ionic balance

Henle's Loop

  • General: Reabsorption is minimum in the ascending limb. Crucial for maintaining high osmolarity of the medullary interstitial fluid (NEET 2015, 2017)
  • Descending Limb: Permeable to water, almost impermeable to electrolytes. Concentrates the filtrate as it moves down (NEET 2010, 2017)
  • Ascending Limb: Impermeable to water, allows transport of electrolytes actively or passively. Dilutes the filtrate as it moves upward (NEET 2017)

Distal Convoluted Tubule (DCT)

  • Conditional reabsorption of Na⁺ and water occurs here under hormonal influence
  • Reabsorbs HCO₃⁻ and selectively secretes H⁺, K⁺, and NH₃ to maintain pH and Na⁺–K⁺ balance in blood

Collecting Duct

  • Extends from the cortex deep into the medulla. Large amounts of water are reabsorbed here to produce concentrated urine
  • Allows passage of small amounts of urea into the medullary interstitium to maintain high osmolarity
  • Maintains pH and ionic balance via selective secretion of H⁺ and K⁺ ions

Mechanism of Concentration of the Filtrate

Counter Current Mechanism

Anatomy: Operated by the Loop of Henle and Vasa Recta (NEET 2019)

  • The flow of filtrate in the two limbs of Henle's loop is in opposite directions
  • The flow of blood in the two limbs of vasa recta is also in a counter-current pattern

The Gradient

Osmolarity increases progressively from the cortex (300 mOsmolL⁻¹) deep into the inner medulla (1200 mOsmolL⁻¹).

Key Solutes Responsible

NaCl and Urea

  • NaCl is transported out by the ascending limb of Henle's loop and exchanged into the descending limb of vasa recta
  • NaCl is returned to the medullary interstitium by the ascending portion of vasa recta
  • Small amounts of urea enter the thin segment of the ascending limb of Henle's loop and are transported back to the interstitium by the collecting duct

Significance

The medullary interstitial gradient enables easy osmotic passage of water out of the collecting tubule, concentrating the urine. Human kidneys can concentrate urine up to 4 times compared to the initial filtrate (from 300 mOsmolL⁻¹ to 1200 mOsmolL⁻¹). If Henle's loop were absent, urine would be highly dilute (NEET 2003)

Regulation of Kidney Function

Hormonal Feedback Control Loops

Hypothalamus (via ADH / Vasopressin)

  • Activated by: osmoreceptors detecting excessive fluid loss (changes in blood volume, fluid volume, or ionic concentration)
  • Action: Stimulates neurohypophysis to release Antidiuretic Hormone (ADH) / Vasopressin
  • Function: Facilitates water reabsorption from later parts of the tubule (DCT/Collecting duct), preventing diuresis. When water intake is high, ADH release is suppressed (NEET 2011)
  • Vascular Effect: ADH acts as a vasoconstrictor, increasing blood pressure, which enhances glomerular blood flow and GFR

Juxtaglomerular Apparatus (Renin-Angiotensin-Aldosterone System - RAAS)

  • A fall in glomerular blood flow, pressure, or GFR triggers JG cells to release Renin (NEET 2012)
  • Cascade:
    Angiotensinogen (from Liver) →[Renin] Angiotensin I → Angiotensin II (NEET 2006)
  • Angiotensin II Actions:
    • Powerful vasoconstrictor → directly increases glomerular blood pressure and GFR
    • Signals Adrenal Cortex to release Aldosterone
  • Aldosterone Action: Causes active reabsorption of Na⁺ and water from distal parts of the tubule, increasing blood pressure and GFR (NEET 2014, 2017)

The Heart (ANF Check Mechanism)

  • An increase in blood flow/volume to the atria of the heart triggers the release of Atrial Natriuretic Factor (ANF) (NEET 2017, 2020)
  • Action: Causes vasodilation and decreases blood pressure. It acts as an explicit counter-check on the RAAS mechanism

Micturition

Mechanism

Urine is stored in the bladder until a voluntary signal is provided by the CNS.

Micturition Reflex

  • Stretching of the bladder wall as it fills activates stretch receptors
  • Signals are sent to the CNS, which transmits motor commands initiating contraction of smooth muscles of the bladder and simultaneous relaxation of the urethral sphincter
  • Important: Complete removal of stretch receptors results in no micturition (NEET 2009)

Urine Characteristics

  • Adult human excretes 1–1.5 liters of urine per day
  • Light yellow, watery fluid, slightly acidic (pH ~ 6.0)
  • Excretes 25–30 gm of urea per day

Clinical Diagnosis

  • Glycosuria: Presence of glucose in urine (NEET 2018)
  • Ketonuria: Presence of ketone bodies in urine. Prolonged fasting also yields ketones in urine (NEET 2005)
  • Both conditions are key diagnostic clinical indicators for Diabetes mellitus
  • Hematuria: Presence of RBCs in urine

Role of Other Organs in Excretion

Lungs

Eliminate large amounts of CO₂ (approx. 200 mL/minute) and significant quantities of water daily.

Liver

Largest gland. Secretes bile containing bilirubin, biliverdin, cholesterol, degraded steroid hormones, vitamins, and drugs; these pass out with digestive wastes.

Skin

  • Sweat Glands: Produce sweat (watery fluid containing NaCl, small amounts of urea, and lactic acid). Primary role is thermoregulation (cooling effect), but assists in waste removal
  • Sebaceous Glands: Eliminate sterols, hydrocarbons, and waxes through sebum, providing a protective oily covering

Note: Small amounts of nitrogenous wastes can also be eliminated through saliva.

Disorders of the Excretory System

Uremia

Malfunctioning of kidneys leading to high accumulation of urea in the blood; highly toxic, can cause kidney failure.

Hemodialysis

Process to remove excess urea from uremic patients.

  • Procedure: Blood is drained from a convenient artery, mixed with an anticoagulant (heparin), and pumped into an artificial kidney
  • Dialyzing Unit: Contains a coiled cellophane tube surrounded by a dialyzing fluid that has the same composition as plasma except it lacks nitrogenous wastes
  • Mechanism: Wastes diffuse down the concentration gradient out of the blood
  • Return: Cleared blood is pumped back through a vein after adding anti-heparin
  • Side-effects of long term use: Can cause reduced RBC production due to decreased erythropoietin secretion, and reduced calcium absorption from the GIT (NEET 2019)

Kidney Transplantation

Ultimate method for correcting acute renal failure. Uses a functioning kidney from a living donor, preferably a close relative, to minimize rejection by the host's immune system.

Renal Calculi

Stones or insoluble masses of crystallized salts (like oxalates) formed within the kidney (NEET 2018)

Glomerulonephritis

Inflammation of the glomeruli of the kidney (NEET 2018)

🔑 Key Points to Remember

  • Ammonia is most toxic, uric acid is least toxic
  • Each kidney has ~1 million nephrons
  • Normal GFR = 125 ml/min (180 L/day)
  • 99% of filtrate is reabsorbed
  • Counter-current mechanism creates osmotic gradient (300-1200 mOsmolL⁻¹)
  • RAAS increases BP and GFR; ANF decreases BP
  • Collecting duct is NOT part of nephron