Excretory Products and Their Elimination Class 11 Notes

Modes of Excretion

Excretion is the removal of metabolic waste products (nitrogenous wastes, CO₂, excess salts and water) from the body. Different organisms use different modes depending on their habitat and nitrogen metabolism.

• Ammonotelism — excretion of ammonia (NH₃), which is highly toxic and highly soluble. Requires large volumes of water for dilution. Typical of aquatic animals: bony fishes, aquatic amphibians, aquatic insects.
• Ureotelism — excretion of urea (CO(NH₂)₂), which is less toxic and less soluble. Requires less water. Typical of mammals, adult amphibians, some marine fishes (elasmobranchs).
• Uricotelism — excretion of uric acid, which is non-toxic, nearly insoluble, and excreted as a paste. Very little water lost. Typical of insects, land snails, reptiles, birds and egg-laying mammals.

Human Excretory System — Kidneys and Nephron

• A pair of bean-shaped kidneys (each ~10–12 cm long, ~50 g) located in the abdominal cavity against the posterior wall, one on either side of the vertebral column.
• A pair of ureters carry urine from each kidney to the urinary bladder (a muscular, distensible sac) for temporary storage.
• A single urethra carries urine out of the body during micturition.

Kidney Structure

• Outer renal cortex — granular, contains the glomeruli and convoluted tubules.
• Inner renal medulla — arranged into cone-shaped renal pyramids; contains the loops of Henle and collecting ducts.
• Renal pelvis — funnel-shaped cavity that collects urine from the pyramids and leads into the ureter.
• Each kidney contains about 1–1.3 million nephrons — the functional units of the kidney.

Nephron — The Functional Unit

• Malpighian body (renal corpuscle) — consists of the glomerulus (a tuft of fenestrated capillaries) enclosed by Bowman's capsule (double-walled cup). Blood enters via the afferent arteriole and exits via the narrower efferent arteriole — the diameter difference creates high pressure for ultrafiltration.
• Proximal convoluted tubule (PCT) — lined with brush-bordered (microvilli) cuboidal cells; major site of reabsorption (glucose, amino acids, Na⁺, K⁺, Cl⁻, HCO₃⁻, water).
• Loop of Henle — U-shaped; thin descending limb (permeable to water, not solutes) and thin ascending limb (permeable to NaCl, not water). Creates the countercurrent gradient in the medulla.
• Distal convoluted tubule (DCT) — site of regulated reabsorption and secretion under hormonal control (aldosterone, ADH).
• Collecting duct — runs through the medulla; final concentration of urine under ADH control.

Urine Formation — Three Processes

1. Glomerular Filtration (Ultrafiltration)

Blood is filtered under high pressure at the glomerulus. The fenestrated endothelium of glomerular capillaries, the basement membrane and the podocytes of Bowman's capsule form a three-layered filtration barrier. Water, ions, glucose, amino acids, urea and uric acid pass through; blood cells and large proteins are retained.

2. Tubular Reabsorption

Of the ~180 L of filtrate formed per day, only about 1–1.8 L is excreted as urine — meaning over 99% of the filtrate is reabsorbed. The PCT reabsorbs the bulk: ~65% of Na⁺, water, glucose, amino acids and HCO₃⁻. The loop of Henle reabsorbs water (descending limb) and NaCl (ascending limb). The DCT and collecting duct carry out fine-tuned, hormonally regulated reabsorption.

3. Tubular Secretion

Certain substances (H⁺, K⁺, NH₃, creatinine, some drugs) are actively transported from the peritubular capillaries into the tubular fluid. This helps maintain blood pH and eliminates substances not filtered at the glomerulus.

Countercurrent Mechanism and Concentration of Urine

• Thin descending limb — permeable to water but not to solutes. Water moves out into the hypertonic medullary interstitium by osmosis, concentrating the filtrate.
• Thin ascending limb — permeable to NaCl (passive diffusion out) but impermeable to water. This dilutes the filtrate and adds solute to the interstitium.
• Thick ascending limb — actively pumps NaCl into the interstitium (Na⁺-K⁺-2Cl⁻ cotransporter); impermeable to water. This is the main driver of the gradient.
• Vasa recta — act as countercurrent exchangers, picking up water and losing solute as they descend into the medulla, and the reverse as they ascend, thereby preserving the medullary gradient.

Regulation of Kidney Function

Renin–Angiotensin–Aldosterone System (RAAS)

• When blood pressure or blood volume drops, juxtaglomerular cells of the kidney release renin.
• Renin converts angiotensinogen (from liver) to angiotensin I, which is converted to angiotensin II by ACE (angiotensin-converting enzyme, mainly in lungs).
• Angiotensin II is a potent vasoconstrictor (raises BP) and stimulates aldosterone release from the adrenal cortex.
• Aldosterone acts on the DCT and collecting duct, increasing Na⁺ and water reabsorption — raising blood volume and pressure.

Antidiuretic Hormone (ADH / Vasopressin)

• Produced by the hypothalamus, stored and released by the posterior pituitary.
• Acts on the collecting ducts and DCT, inserting aquaporin-2 channels, increasing water permeability.
• High ADH → more water reabsorbed → concentrated, low-volume urine. Low ADH → dilute, high-volume urine.
• Alcohol inhibits ADH release, causing diuresis (frequent, dilute urine).

Micturition and Accessory Excretory Organs

Micturition is the reflex act of urination. When the bladder fills to ~300–500 mL, stretch receptors send impulses via sensory nerves to the sacral spinal cord, triggering a parasympathetic reflex that contracts the detrusor muscle and relaxes the internal urethral sphincter. The external urethral sphincter (voluntary, skeletal muscle) can be consciously controlled.

Other Organs of Excretion

• Lungs — excrete CO₂ and water vapour (volatile waste).
• Skin — excretes sweat containing water, NaCl, urea and small amounts of other waste via sweat glands.
• Liver — deaminates amino acids producing urea (urea cycle); detoxifies drugs and poisons; breaks down haemoglobin into bile pigments (bilirubin) excreted in faeces.

Disorders of the Excretory System

• Uraemia — build-up of urea and other nitrogenous wastes in the blood due to kidney failure. Causes nausea, vomiting, fatigue and can be fatal if untreated.
• Renal failure (acute or chronic) — the kidneys lose the ability to filter blood adequately. Chronic kidney disease (CKD) progresses through stages; end-stage requires dialysis or transplant.
• Renal calculi (kidney stones) — crystallisation of calcium oxalate, calcium phosphate or uric acid in the renal pelvis or ureter. Causes severe pain (renal colic).
• Glomerulonephritis — inflammation of the glomeruli, often post-streptococcal, causing proteinuria and haematuria.
• Haemodialysis — an artificial kidney machine filters blood outside the body using a semipermeable membrane; the patient's blood flows on one side and dialysing fluid on the other. Typically needed 2–3 times per week for several hours.

Solved Examples

Example: A person produces about 1.5 L of urine per day. If the GFR is 125 mL/min, what percentage of the glomerular filtrate is actually reabsorbed?

Solution: Total filtrate per day = 125 mL/min × 60 min × 24 h = 180,000 mL = 180 L. Urine produced = 1.5 L. Reabsorbed = 180 − 1.5 = 178.5 L. Percentage reabsorbed = (178.5 / 180) × 100 ≈ 99.2%. This confirms that over 99% of the filtrate is reabsorbed, mainly by the PCT.

Example: Explain why a person who drinks a large quantity of water produces dilute urine, while a dehydrated person produces concentrated urine.

Solution: When water intake is high, blood osmolarity drops, the hypothalamus reduces ADH secretion from the posterior pituitary, and the collecting ducts become less permeable to water. More water remains in the filtrate, producing large volumes of dilute urine. Conversely, during dehydration, blood osmolarity rises, ADH release increases, aquaporin channels are inserted in the collecting duct walls, more water is reabsorbed, and a small volume of concentrated urine is produced.