ICSE Class 10 Biology: Excretory System Notes

Page 1: Introduction and Types of Excretory Products

Excretion is the elimination of harmful and unwanted metabolic waste products, especially nitrogenous wastes, to maintain internal homeostasis including constant volume, composition, pH, and osmotic pressure of blood.

Kidneys are the primary excretory organs in humans. Accessory excretory organs include the lungs, liver, large intestine, and skin.

Types of excretory products:

• Respiratory Wastes: $CO_2$ and water from glucose oxidation.
• Nitrogenous Wastes:
  • Ammonia: Highly toxic product of oxidative deamination of amino acids. Its elimination is termed ammonotelism (seen in Hydra, bony fishes, and tailed amphibians).
  • Urea: Primary excretory product of vertebrates, synthesized in the liver from ammonia and $CO_2$. Its elimination is termed ureotelism.

Page 2: Excretory System Structure and Kidney Anatomy

• Nitrogenous Wastes (cont.):
  • Uric Acid: Chief product in insects, reptiles, birds, and desert animals. Its elimination is termed uricotelism. Joint deposition of crystallized uric acid causes gout; excessive uric acid and calcium oxalate form kidney stones.
• Mineral Salts: Eliminated via sweat, urine, or faeces.
• Water: Makes up $60\%$ of body weight; excess is eliminated via urine, sweat, faeces, and expired air.
• Bile Pigments: Bilirubin from dead RBC hemoglobin breakdown is excreted by liver cells into bile juice.

Human Excretory System Components:

• Pair of kidneys, pair of ureters, urinary bladder, and urethra.

Kidney Structure and Dimensions:

• Reddish-brown, bean-shaped organs located below the stomach on either side of the vertebral column.
• The right kidney is $1\text{--}1.5\,cm$ lower than the left due to the right liver lobe.
• Dimensions: $10\,cm$ long, $5\,cm$ wide, and $3.5\,cm$ thick. Outer margin is convex; inner margin is concave with a notch called the hilus.

Page 3: Nephron Structure and Kidney Histology

• Hilus: Entry and exit point for the renal artery, renal vein, nerve fibers, and ureter; opens into the renal pelvis.
• Capsule: Outer white fibrous tissue layer.
• Internal Regions:
  • Cortex: Darker, outer region with a dotted appearance due to coiled tubule portions.
  • Medulla: Lighter, inner region with a striated appearance containing pyramids.
• Uriniferous Tubule (Nephron): Structural and functional unit of the kidney ($\approx 1.25\text{ million}$ per kidney).
  • Bowman's Capsule: Thin, double-walled cup containing a capillary knot called the glomerulus. Together they form the Malpighian capsule (Renal capsule), where ultrafiltration occurs.
  • Proximal Convoluted Tubule (PCT): Coiled portion located in the cortex.
  • Loop of Henle: U-shaped hairpin loop in the medulla consisting of descending and ascending limbs.

Page 4: Tubules, Blood Supply, Excretory Passages, and Urea Formation

• Distal Convoluted Tubule (DCT): Coiled part in the cortex opening into a collecting duct.
• Collecting Duct: Extends from cortex to medulla, pouring collected urine into the renal pelvis.
• Renal Blood Supply: Renal artery $\rightarrow$ Afferent arteriole $\rightarrow$ Glomerulus $\rightarrow$ Efferent arteriole $\rightarrow$ Secondary capillary network (vasa recta) $\rightarrow$ Renal venule $\rightarrow$ Renal vein $\rightarrow$ Inferior vena cava.
• Ureters: Narrow tubes ($10\text{--}12\text{ inches}$ long, $< 0.5\text{ inch}$ diameter) carrying urine via peristaltic waves.
• Urinary Bladder: Distensible reservoir in the pelvic region holding $0.5\text{--}1\,L$ of urine.
• Urethra: Canal carrying urine out of the body.
  • Female Urethra: $3\text{--}4\,cm$ long; serves as a passageway for urine only.
  • Male Urethra: $18\text{--}20\,cm$ long; serves as a passageway for both urine and semen.
• Urea Formation: Takes place in the liver where oxidative deamination releases ammonia, which combines with $CO_2$ to form urea.

Page 5: Physiology of Urine Formation

Urine formation occurs through three processes:

• Ultrafiltration: High hydrostatic pressure in glomerular capillaries (afferent arteriole diameter > efferent arteriole diameter) forces blood liquid into Bowman's capsule cavity to form glomerular/nephric filtrate.
• Selective Re-absorption: Absorption of essential substances (glucose, $Na^+$, $K^+$, $Cl^-$ ions, water) from nephric filtrate back into secondary capillaries at the PCT, Loop of Henle, DCT, and collecting ducts.
• Tubular Secretion: Secretion of harmful substances (creatinine, $K^+$ ions, ammonia, drugs) from blood into filtrate across DCT walls.

Quantitative Renal Statistics:

• Daily cardiac blood output: $8000\,L$; Kidneys receive $1900\,L$ ($25\%$ of heart blood output per beat).
• Daily nephric filtrate produced: $180\,L$; Daily urine output: $1\text{--}1.5\,L$.
• Entire blood volume is filtered approximately $60\text{ times/day}$.

Page 6: Properties of Urine, Micturition, and Osmoregulation

Physical Properties and Chemical Composition of Normal Urine:

• Daily Volume: $1\text{--}1.5\,L\,\text{day}^{-1}$.
• Colour: Clear yellow/amber due to urochrome pigment.
• pH: Range $5\text{--}8$ (typically $6.0$, acidic).
• Odour: Faint aromatic due to urinod; becomes ammoniacal on standing.
• Specific Gravity: 1.003\text{--}1.004$.\n- Organic Constituents: Urea (2.3\%$), Uric acid ($0.7\%$), Creatinine ($1.5\%).\n- Inorganic Constituents: NaCl$($9.0\%$),$KCl$($2.5\%$),$H_2SO_4$($1.8\%$),$NH_3$($0.6\%$), Calcium ($0.2\%$), Phosphate ($1.7\%$$).

Micturition and Regulation:

• Micturition: Process of expelling urine from the urinary bladder through the urethra.
• Anti-diuretic Hormone (ADH): Secreted by posterior pituitary lobe; regulates water re-absorption in nephrons.
• Diuretics: Substances like tea, coffee, and alcohol that increase urine formation.
• Osmoregulation: Maintenance of constant water and salt levels in body fluids.

Page 7: Kidney Functions, Disorders, and Artificial Kidney

Functions of Kidneys:

• Excretion of nitrogenous waste products.
• Regulation of water balance, electrolyte balance, and acid-base balance.
• Elimination of pigments and drugs.
• Regulation of blood pressure by secreting the hormone renin.

Disorders of Kidneys:

• Haematuria: Presence of blood in urine.
• Uraemia: Accumulation of excess urea in blood due to kidney failure.
• Glycosuria: Presence of glucose in urine.
• Kidney Stones: Precipitation of uric acid and calcium oxalate salts in the renal system.
• Albuminuria: Presence of albumin protein in urine due to damaged Bowman's capsule or hypertension.

Artificial Kidney (Hemodialysis):

• Medical device used during renal failure to clear surplus water, excess salts, and toxic wastes via dialysis.

Page 8: Dialysis Mechanism

Dialysis Mechanism:

• Blood from a patient's artery is pumped through selectively permeable cellophane tubing immersed in a dialysing solution.
• Dialysing fluid has the same composition as normal blood plasma except it contains no urea.
• Nitrogenous wastes and excess salts diffuse down concentration gradients through cellophane pores into the bath fluid.
• Purified blood is returned to a vein of the patient.
• Plasma proteins remain in the blood because cellophane pores are too small for large protein molecules.