NEET Physics: Waves – NCERT Masterclass & 100 MCQ Quiz

EXCRETORY PRODUCTS AND THEIR ELIMINATION

NCERT Masterclass & NEET Biology Study Module

1. Modes of Excretion

Animals accumulate ammonia, urea, uric acid, carbon dioxide, water and ions like $Na^+$, $K^+$, $Cl^-$, phosphate, sulphate, etc., either by metabolic activities or by excess ingestion. Nitrogenous wastes are the major focus here.

Toxicity Order: Ammonia (Most toxic, requires massive water) > Urea > Uric Acid (Least toxic, requires minimum water).

• Ammonotelism: Excreting ammonia. Done by bony fishes, aquatic amphibians, aquatic insects. It easily diffuses across body surfaces or gill surfaces as ammonium ions ($NH_4^+$). Kidneys do not play any significant role here.
• Ureotelism: Excreting urea. Mammals, many terrestrial amphibians, and marine fishes. Ammonia is converted to urea in the Liver and released into the blood.
• Uricotelism: Excreting uric acid. Reptiles, birds, land snails, and insects. Excreted as a pellet or paste with minimum water loss.

2. Human Excretory System & Nephron Anatomy

Consists of a pair of kidneys, one pair of ureters, a urinary bladder and a urethra. [Image of Human Excretory System]

• Kidneys: Reddish brown, bean-shaped, situated between the levels of the last thoracic and third lumbar vertebra. The inner concave surface has a notch called the hilum. Inner to the hilum is a broad funnel-shaped space called the renal pelvis with projections called calyces.
• Kidney Zones: Outer cortex and inner medulla. Medulla is divided into conical masses (medullary pyramids). Cortex extends in between the medullary pyramids as renal columns called Columns of Bertini.
• Nephron: Functional unit of the kidney. Has two parts: Glomerulus and Renal Tubule. [Image of Structure of a Nephron]

The Glomerulus along with Bowman's capsule is called the Malpighian body or Renal corpuscle.
Types of Nephrons: Cortical nephrons (85%, loop of Henle is short and extends only very little into medulla, vasa recta absent or reduced) and Juxtamedullary nephrons (15%, loop of Henle is very long and runs deep into medulla, highly developed vasa recta).

3. Urine Formation

• Glomerular Filtration: Non-selective, passive process. On average, 1100-1200 ml of blood is filtered by the kidneys per minute (roughly 1/5th of the cardiac output). Glomerular Filtration Rate (GFR) is approx 125 ml/minute (180 Litres per day). Podocytes (epithelial cells of Bowman's capsule) are arranged intricately, leaving minute spaces called filtration slits.
• Tubular Reabsorption: Of the 180 Litres of filtrate formed, 99% is reabsorbed, leading to only 1.5 Litres of urine. Glucose, amino acids, $Na^+$ are reabsorbed actively. Nitrogenous wastes are absorbed passively. Water reabsorption in initial segments is passive.
• Tubular Secretion: Cells secrete substances like $H^+$, $K^+$, and ammonia into the filtrate. Crucial for maintaining ionic and acid-base balance of body fluids.

4. Functions of the Tubules

PCT (Proximal Convoluted Tubule): Lined by simple cuboidal brush border epithelium (increases surface area). 70-80% of electrolytes and water are reabsorbed here.
Henle's Loop:
- Descending limb: Permeable to water, impermeable to electrolytes. Filtrate gets concentrated.
- Ascending limb: Impermeable to water, permeable to electrolytes. Filtrate gets diluted.
DCT (Distal Convoluted Tubule): Conditional reabsorption of $Na^+$ and water. Secretion of $H^+$, $K^+$, and $NH_3$.
Collecting Duct: Large amounts of water could be reabsorbed here to produce concentrated urine. Allows passage of small amounts of urea into medullary interstitium to keep osmolarity.

5. Mechanism of Concentration of the Filtrate (Counter Current Mechanism)

Mammals have the ability to produce concentrated urine. The Henle's loop and vasa recta play a significant role. The flow of filtrate in the two limbs of Henle's loop is in opposite directions and forms a counter current. The flow of blood through the two limbs of vasa recta is also counter current.

This mechanism maintains a concentration gradient in the medullary interstitium (from 300 $mOsmL^{-1}$ in the cortex to about 1200 $mOsmL^{-1}$ in the inner medulla). This gradient is mainly caused by NaCl and Urea.

• NaCl is transported by the ascending limb of Henle's loop, exchanged with the descending limb of vasa recta, and returned to the interstitium by the ascending limb of vasa recta.
• Urea enters the thin segment of the ascending limb of Henle's loop and is transported back to the interstitium by the collecting duct.

6. Regulation of Kidney Function (RAAS)

Function is regulated by hormonal feedback mechanisms involving the hypothalamus, JGA, and heart.

• ADH (Vasopressin): Released by posterior pituitary when body fluid volume falls. Facilitates water reabsorption from latter parts of the tubule, preventing diuresis.
• ANF (Atrial Natriuretic Factor): Released by the atria of the heart when blood pressure increases. Causes vasodilation and decreases blood pressure. Acts as a check on the RAAS mechanism.

7. Disorders of Excretory System

• Uremia: Malfunctioning of kidneys leads to accumulation of urea in blood, highly toxic. Treated with Hemodialysis. Cellophane tube (dialyzing unit) is suspended in a dialyzing fluid having the same composition as plasma EXCEPT nitrogenous wastes. Anti-heparin is added to blood drawn from an artery.
• Renal calculi: Stone or insoluble mass of crystallised salts (oxalates) formed within the kidney.
• Glomerulonephritis: Inflammation of glomeruli of kidney.

← Back to NEET Resource Hub

🚀 NEET EXCRETORY SYSTEM MEGA QUIZ (100 MCQ)

Solve the 5 parts below to master Nephron function, Urine Formation, RAAS, and Counter Current.

← Back to NEET Resource Hub