Quick Revision Notes

Class 11 Biology: Excretory Products and Their Elimination

MP Board Education

Excretion is the process of removal of nitrogenous wastes as well as excess of water, salt, and metabolic wastes from the animal body.

Ammonotelism (Ammonotelic animals): Elimination of nitrogenous waste in the form of Ammonia (NH_3).
- Examples: Bony fishes, aquatic amphibians, and aquatic insects.
- Key point: Requires large amounts of water for elimination.
Ureotelism (Ureotelic animals): Elimination of nitrogenous waste in the form of Urea.
- Examples: Mammals, many terrestrial amphibians, and marine fishes.
- Key point: Less toxic than ammonia, requires moderate water for disposal.
Uricotelism (Uricotelic animals): Elimination of nitrogenous waste in the form of Uric acid (pellets or paste).
- Examples: Reptiles, birds, land snails, and insects.
- Key point: Least toxic, conserved water to the maximum (minimum water loss).

The human excretory system consists of a pair of kidneys, one pair of ureters, a urinary bladder, and a urethra.

Kidney Structure:
- Bean-shaped organs situated between the levels of last thoracic and 3rd lumbar vertebra close to the dorsal inner wall of the abdominal cavity.
- Hilum: A notch on the inner concave surface of the kidney through which ureter, blood vessels, and nerves enter.
- Renal Cortex: Outer zone of the kidney.
- Renal Medulla: Inner zone divided into conical masses called renal pyramids.
- Columns of Bertini (Renal columns): Extension of the cortex into the medullary pyramids.

Each kidney has nearly 1 million complex tubular structures called Nephrons.

Each nephron has two main parts:

Glomerulus: A tuft of capillaries formed by the afferent arteriole (fine branch of renal artery). Blood is drained from the glomerulus by the efferent arteriole.
Renal Tubule: Begins with a double-walled cup-like structure called Bowman's capsule.
- Bowman's capsule + Glomerulus = Malpighian body / Renal corpuscle.
- Proximal Convoluted Tubule (PCT): Highly coiled network.
- Henle’s Loop: Has a descending limb and an ascending limb.
- Distal Convoluted Tubule (DCT): Further coiled region.
- Collecting Duct: Straight tube where multiple DCTs open.

Urine formation involves three main processes:

Glomerular Filtration (Ultrafiltration):
- Blood is filtered through 3 layers: endothelium of glomerular blood vessels, epithelium of Bowman's capsule, and a basement membrane between them.
- Glomerular Filtration Rate (GFR): The amount of filtrate formed by the kidneys per minute.
- Normal value: 125 mL/minute (approx. 180 litres a day).
Reabsorption:
- 99% of the filtrate is reabsorbed by the renal tubules.
- Active reabsorption: Glucose, amino acids, Na^+, etc.
- Passive reabsorption: Nitrogenous wastes, water.
Secretion:
- Tubular cells secrete substances like H^+, K^+, and ammonia into the filtrate to maintain ionic balance and acid-base balance of body fluids.

PCT: Reabsorbs 70-80% of electrolytes and water. Maintains pH and ionic balance by selective secretion of H^+, ammonia, and absorption of HCO_3^-.
Henle’s Loop:
- Descending limb: Permeable to water, impermeable to electrolytes (concentrates filtrate).
- Ascending limb: Impermeable to water, permeable to electrolytes (dilutes filtrate).
DCT: Conditional reabsorption of Na^+ and water. Reabsorption of HCO_3^- and selective secretion of H^+ and K^+.
Collecting Duct: Allows passage of small amounts of urea into medullary interstitium to keep osmolarity. Reabsorption of water to produce concentrated urine.

The flow of filtrate in the two limbs of Henle’s loop is in opposite directions and thus forms a counter-current.
The flow of blood through the vasa recta also is in a counter-current pattern.
Role: Helps in maintaining an increasing osmolarity towards the inner medullary interstitium (300 mOsmolL^-1 in cortex to 1200 mOsmolL^-1 in inner medulla).
Result: Enables the production of concentrated urine, thus conserving water.

ADH (Anti-Diuretic Hormone / Vasopressin):
- Released by the posterior pituitary when osmoreceptors are triggered by body fluid volume/osmolarity changes.
- Prevents diuresis by facilitating water reabsorption from distal parts of tubules.
JGA (Juxta-Glomerular Apparatus):
- A sensitive region formed by cellular modification in the distal convoluted tubule and the afferent arteriole at the location of their contact.
- A fall in glomerular blood flow/pressure activates JGA to release Renin.
- Renin converts angiotensinogen in blood to Angiotensin I and then Angiotensin II.
- Angiotensin II acts as a powerful vasoconstrictor (increases BP) and also activates the adrenal cortex to release Aldosterone (increases Na^+ and water reabsorption).
ANF (Atrial Natriuretic Factor):
- An increase in blood flow to the atria of the heart causes the release of ANF.
- ANF causes vasodilation and checks the renin-angiotensin mechanism (acts as a check on ADH/Renin).

Definition: The process of release of urine from the urinary bladder.
Mechanism: Urine filled in the urinary bladder stretches its wall, generating signals to the CNS via the CNS sensory nerves. The motor messages cause the smooth muscles of the bladder to contract and the urethral sphincter to relax, releasing urine.
Neural mechanism: Micturition reflex.
Normal human output: 1 to 1.5 litres of urine per day containing 25-30 gm of urea.

Lungs: Remove CO_2 (~18 L/day) and significant quantities of water.
Liver: Secretes bilirubin, biliverdin, cholesterol, degraded steroid hormones, vitamins, and drugs via bile.
Skin: Sweat and sebaceous glands eliminate small amounts of NaCl, urea, lactic acid, and sterols/hydrocarbons through sebum.

Uremia: Accumulation of urea in blood due to kidney malfunction. Can lead to kidney failure.
Renal Failure: Kidneys stop working; corrected by Hemodialysis (artificial kidney).
Calculi (Kidney Stones): Stone or insoluble mass of crystallized salts (oxalates, etc.) formed within the kidney.
Glomerulonephritis: Inflammation of glomeruli of kidneys.