Learn how the body eliminates metabolic toxic wastes. By the end of this module, you will understand:
Estimated Reading Time: 25 minutes | Difficulty Level: Hard | Prerequisites: Fluids circulation.
Excretory physiology is key to managing renal failure, treating hypertension (via diuretics blocking RAAS), and performing hemodialysis.
Waste pathway: \[\text{Systemic Blood} \xrightarrow{\text{Glomerulus Filtration}} \text{Filtrate} \xrightarrow{\text{Loop of Henle Counter-Current}} \text{Concentrated Urine}\]
The nephron is the functional unit of the kidney (about a million per kidney). Filtrate passes through: Bowman's capsule (with the glomerulus) → PCT → Loop of Henle → DCT → collecting duct. Urine forms in three steps:
Water reabsorption is fine-tuned by ADH (vasopressin) in the collecting duct; ANF opposes it. The countercurrent mechanism of the Loop of Henle and vasa recta maintains the medullary gradient that concentrates urine.
The kidney builds a rising osmotic gradient from 300 mOsm/L in the cortex to ~1200 mOsm/L deep in the medulla, which then draws water out of the collecting duct to concentrate urine:
| Condition | Feature |
|---|---|
| Uraemia | Toxic urea builds up in blood from kidney failure; treated by dialysis |
| Renal calculi | Kidney "stones" — crystallised salts (e.g. oxalate) in the renal tubules |
| Nephritis / glomerulonephritis | Inflammation of the glomeruli |
An artificial kidney (haemodialysis) removes wastes across a semipermeable membrane using a dialysing fluid without the nitrogenous wastes.
❌ Misconception: The descending limb of Loop of Henle is permeable to electrolytes.
✔ Correction: The descending limb is highly permeable to water but virtually impermeable to electrolytes, concentrating the filtrate as it moves down.
1. (NEET PYQ) Which hormone acts as an antagonist to the RAAS pathway by causing vasodilation when blood pressure increases?
Correct Answer: B. ANF (Atrial Natriuretic Factor)
Explanation: ANF is released by heart wall cells in response to high blood pressure. It causes vasodilation, thereby lowering blood pressure and checking the RAAS pathway.
The sarcomere (between two Z-lines) is the contractile unit of a myofibril:
| Band / line | Contains | On contraction |
|---|---|---|
| A-band (dark, anisotropic) | Whole thick (myosin) filaments + overlap | Length unchanged |
| I-band (light, isotropic) | Thin (actin) only, bisected by Z-line | Shortens |
| H-zone | Thick filaments only (centre of A-band) | Shortens / disappears |
| Z-line / M-line | Anchor thin / thick filaments | Z-lines move closer |
Key point: neither actin nor myosin changes length — they only slide past each other, so the A-band stays constant while the sarcomere, I-band and H-zone shorten.
The human skeleton has 206 bones in two divisions: the axial skeleton (skull, vertebral column, ribs, sternum — 80 bones) and the appendicular skeleton (limbs and their girdles — 126 bones). Bones meet at joints:
| Disorder | Feature |
|---|---|
| Myasthenia gravis | Auto-immune attack on the neuromuscular junction → muscle fatigue/weakness |
| Tetany | Rapid muscle spasms from low blood Ca²⁺ |
| Muscular dystrophy | Progressive degeneration of skeletal muscle (genetic) |
| Arthritis / Osteoporosis / Gout | Joint inflammation / decreased bone mass (low oestrogen) / uric-acid crystal deposition in joints |
| Feature | Red Fibers | White Fibers |
|---|---|---|
| Myoglobin | High | Low |
| Mitochondria | Abundant | Few |
| Respiration | Aerobic | Anaerobic (Lactic acid) |
1. (NEET PYQ) During muscle contraction, which of the following bands remains constant in length?
Correct Answer: C. A-Band
Explanation: Under the sliding filament model, the myosin (A-band) length remains constant, while the actin (I-band) and H-zone shorten during contraction.
The brain has three parts: forebrain (cerebrum — thinking; thalamus; hypothalamus — homeostasis), midbrain, and hindbrain (cerebellum — balance; pons; medulla — controls heartbeat, breathing).
A reflex arc gives a rapid, involuntary response: receptor → sensory (afferent) neuron → spinal cord → motor (efferent) neuron → effector, bypassing conscious brain processing. Synapses are electrical (fast, direct current flow) or chemical (a neurotransmitter such as acetylcholine crosses the cleft — one-way and slightly slower).
1. (NEET PYQ) The active transport sodium-potassium pump moves:
Correct Answer: B. 3 Na+ out and 2 K+ in
Explanation: The electrogenic Na+/K+ pump consumes 1 ATP to actively pump 3 sodium ions out of axoplasm and 2 potassium ions inside.
| Gland | Key hormone(s) | Disorder (hypo / hyper) |
|---|---|---|
| Hypothalamus | Releasing/inhibiting hormones controlling the pituitary | — |
| Pituitary (anterior) | GH, TSH, ACTH, FSH, LH, Prolactin | GH deficit → dwarfism; excess → gigantism / acromegaly |
| Pituitary (posterior) | ADH, Oxytocin | ADH deficit → diabetes insipidus |
| Thyroid | Thyroxine (needs iodine), calcitonin | Hypo → goitre, cretinism; hyper → Graves' disease |
| Parathyroid | Parathormone (raises blood Ca²⁺) | Hypo → tetany |
| Adrenal | Cortisol, aldosterone (cortex); adrenaline (medulla) | Deficit → Addison's disease |
| Pancreas (Islets) | Insulin (β), glucagon (α) | Insulin deficit → diabetes mellitus |
| Gonads | Testosterone / oestrogen & progesterone | Regulate reproduction & secondary sexual characters |
| Pineal | Melatonin | Regulates the circadian (sleep–wake) rhythm |
| Adrenal medulla | Adrenaline & noradrenaline | "Fight-or-flight" — raises heart rate & blood glucose |
| Chemical Group | Examples | Receptor & Pathway |
|---|---|---|
| Peptides | Insulin, Glucagon | Cell surface, cAMP / IP3 second messenger |
| Steroids | Cortisol, Aldosterone, Estrogen | Intracellular nuclear, regulates genes |
1. (NEET PYQ) Which of the following lipid-soluble hormones binds with intracellular receptors inside the cell nucleus?
Correct Answer: D. Cortisol
Explanation: Cortisol is a steroid hormone. Being lipid-soluble, it readily crosses cell membranes to bind with intracellular nuclear receptors.