EXCRETORY PRODUCTS & THEIR ELIMINATION
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├── EXCRETION MODES
│   ├── Ammonotelism → Ammonia → Aquatic (bony fish, tadpoles, aquatic insects)
│   ├── Ureotelism → Urea → Mammals, adult frogs, marine fish, turtles
│   │   └── Ornithine cycle in LIVER: NH3 → Urea
│   └── Uricotelism → Uric acid (semi-solid) → Birds, reptiles, land snails, insects
│       └── Toxicity: Ammonia > Urea > Uric acid
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├── HUMAN EXCRETORY SYSTEM
│   ├── Organs: Kidneys (×2) → Ureters (×2) → Bladder → Urethra
│   ├── Kidney: 10–12 cm, bean-shaped, retroperitoneal
│   │   ├── Cortex: glomeruli, PCT, DCT
│   │   └── Medulla: renal pyramids, loops of Henle, collecting ducts
│   └── Nephron (~1 million/kidney)
│       ├── Types: Cortical (85%, short loop) | Juxtamedullary (15%, long loop)
│       └── Segments: Bowman's → PCT → Loop of Henle → DCT → Collecting duct
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├── URINE FORMATION (3 processes)
│   ├── 1. Glomerular filtration: GFR = 125 mL/min → 180 L/day filtrate
│   ├── 2. Tubular reabsorption: 99% of filtrate (PCT=65–70%)
│   └── 3. Tubular secretion: H+, K+, NH3 (PCT and DCT)
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├── COUNTER-CURRENT MECHANISM
│   ├── Loop of Henle (multiplier): builds gradient 300–1200 mOsm/L
│   │   ├── Descending: permeable to water (water exits → concentrated)
│   │   └── Ascending: permeable to NaCl (NaCl exits → diluted)
│   └── Vasa recta (exchanger): preserves gradient without washing it out
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├── HORMONAL REGULATION
│   ├── ADH $\frac{hypothalamus}{posterior pituitary}$
│   │   └── Trigger: high osmolarity/dehydration → water reabsorption in DCT + CD
│   ├── Aldosterone (adrenal cortex)
│   │   └── Trigger: angiotensin II → Na+ reabsorption + K+ secretion in DCT
│   ├── ANF (cardiac atria)
│   │   └── Trigger: high blood volume → ↓ Na+ reabsorption → ↓ blood volume
│   └── RAAS cascade
│       └── Low BP → JG cells → Renin → Angiotensinogen → Ang I → ACE → Ang II → Aldosterone
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├── OTHER EXCRETORY ORGANS
│   ├── Lungs: CO2 + H2O vapour
│   ├── Liver: bile pigments + urea synthesis
│   └── Skin: NaCl + small amounts of urea (sweat)
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└── DISORDERS
    ├── Uremia: urea in blood → nausea, coma → dialysis/transplant
    ├── Renal calculi: calcium oxalate stones → renal colic + haematuria
    ├── Glomerulonephritis: inflamed glomeruli → proteinuria + haematuria
    └── Renal failure: GFR drops → oliguria/anuria → dialysis