Excretory Products and their Elimination

Types of Nitrogenous Excretion

• The form of nitrogenous waste depends on the habitat and availability of water:
• Ammonotelic: excrete ammonia (NH3) — very toxic; needs large amounts of water to dilute. Most aquatic animals (bony fishes, aquatic invertebrates)
• Ureotelic: excrete urea — less toxic; requires less water. Mammals (including humans), adult amphibians, cartilaginous fishes (sharks)
• Uricotelic: excrete uric acid — least toxic; insoluble; excreted as paste/crystals with minimal water. Reptiles, birds, insects, land snails

Human Excretory System

• Organs:
• Kidneys (pair): primary organs; filter blood and form urine
• Ureters: carry urine from kidneys to urinary bladder
• Urinary bladder: stores urine
• Urethra: expels urine; longer in males

Structure of the Kidney

• Each kidney has ~1 million nephrons (functional units). Kidney regions:
• Cortex (outer): contains Bowman's capsules, proximal/distal convoluted tubules, glomeruli
• Medulla (inner): contains loops of Henle and collecting ducts; forms renal pyramids
• Pelvis: funnel-shaped; collects urine from pyramids → ureter

Structure of the Nephron

1. 1.Malpighian Body (Renal Corpuscle): Bowman's capsule + Glomerulus (knot of capillaries); site of ultrafiltration
2. 2.Proximal Convoluted Tubule (PCT): in cortex; simple cuboidal cells with microvilli (brush border); most reabsorption occurs here
3. 3.Loop of Henle: hairpin-shaped; descends into medulla; creates osmotic gradient; descending limb permeable to water; ascending limb permeable to ions (impermeable to water)
4. 4.Distal Convoluted Tubule (DCT): in cortex; hormone-regulated reabsorption of Na+ and water; selective secretion of K+ and H+
5. 5.Collecting Duct: collects urine from several nephrons; passes through medulla; regulated by ADH; opens into renal pelvis

Urine Formation — Three Processes

1. Ultrafiltration (Glomerular Filtration):
Occurs in glomerulus. Blood pressure forces water, ions, glucose, amino acids, urea, uric acid into Bowman's capsule = glomerular filtrate (~180 L/day in humans). Large proteins and blood cells are NOT filtered. Filtration depends on glomerular filtration rate (GFR) ≈ 125 mL/min (180 L/day).

• 2. Selective Reabsorption (Tubular Reabsorption):
• Most filtrate is reabsorbed back into blood from the tubules:
• PCT: reabsorbs ~70-80% of water (osmosis), almost all glucose and amino acids (active transport), most Na+, K+, Cl-, HCO3-; secretes H+, NH3, drugs
• Loop of Henle: Descending loop: water exits (into hypertonic medullary interstitium); Ascending loop: Na+, K+, Cl- reabsorbed actively in thick segment (water cannot follow)
• DCT: Na+ reabsorption (aldosterone-stimulated); water reabsorption (ADH-regulated); secretion of K+ and H+
• Collecting duct: water reabsorption (regulated by ADH — antidiuretic hormone); concentration of urine

3. Tubular Secretion:
Active secretion of substances from blood INTO the tubule to be excreted: H+ (maintain blood pH), K+ (DCT), NH4+, creatinine, uric acid, drugs (e.g., penicillin)

Counter-Current Mechanism

The counter-current multiplier (Loop of Henle) and counter-current exchanger (Vasa recta — blood vessels alongside loop) maintain the osmotic gradient in the medulla (300 mOsm in cortex → 1200 mOsm in inner medulla).
This gradient is essential for concentrating urine. ADH increases the permeability of DCT and collecting duct to water, allowing water to move out into the hypertonic medullary interstitium → concentrated urine.

Hormonal Regulation of Urine Formation

• ADH (Antidiuretic hormone / Vasopressin): released by posterior pituitary when blood osmolarity rises or blood volume falls; increases water reabsorption in DCT and collecting duct → concentrated, low-volume urine. Deficiency → Diabetes insipidus (large volume of dilute urine)
• Aldosterone: released by adrenal cortex (RAAS pathway); promotes Na+ reabsorption in DCT → water follows → blood volume and pressure increase
• ANF (Atrial Natriuretic Factor): released by heart atria when blood pressure is HIGH; inhibits ADH and aldosterone; promotes Na+ and water excretion → lowers blood pressure

Micturition

Urine stored in bladder causes stretch → micturition reflex → bladder contracts (detrusor muscle) and sphincters relax → urination. Adults have voluntary control over the external urethral sphincter.

Other Organs of Excretion

• Lungs: excrete CO2 and water vapour
• Skin: excretes water, NaCl, small amounts of urea via sweat
• Liver: detoxifies ammonia → urea; excretes bile pigments (bilirubin from Hb breakdown) into bile → faeces

Disorders of the Excretory System

• Uremia: accumulation of urea in blood due to kidney failure; treated by dialysis or transplant
• Renal calculi (kidney stones): deposits of calcium oxalate, uric acid; cause severe pain (renal colic)
• Glomerulonephritis: inflammation of glomeruli; reduces filtration
• Diabetes mellitus: glucose appears in urine (glycosuria) when blood glucose exceeds renal threshold (~180 mg/dL)