Human Physiology II — Excretion and Locomotion — NEET Biology
These two systems add 4–6 questions to the physiology tally every year. Excretion rewards a precise walk through the nephron — where filtration, reabsorption and secretion happen, how the counter-current multiplier concentrates urine, and which hormone tunes which step. Locomotion rewards the sliding-filament mechanism and the exact bones, joints and disorders NCERT names. This chapter builds each system in that ordered, value-rich form the exam quotes almost verbatim.
Part A — Excretory Products and their Elimination
1. Modes of excretion and the human excretory system
Animals excrete nitrogenous waste in three main forms, a trade-off between water and energy cost:
- Ammonotelic (ammonia — most toxic, needs much water): bony fish, aquatic amphibians.
- Ureotelic (urea — less toxic): mammals (including humans), sharks, adult amphibians.
- Uricotelic (uric acid — least toxic, least water, semi-solid): birds, reptiles, insects.
The human excretory system: a pair of kidneys, two ureters, a urinary bladder and a urethra. Each kidney has an outer cortex and inner medulla (with renal pyramids), and contains about one million nephrons — the functional units.
Worked example 1.1. Humans are described as ureotelic. What does this mean, and how does it compare with birds? Ureotelic means the chief nitrogenous waste excreted is urea, which is far less toxic than ammonia and needs moderate water to remove. Birds are uricotelic, excreting uric acid as a semi-solid paste — saving water, useful for flight and for eggs.
2. Structure of the nephron
Each nephron has two parts:
- Malpighian body (renal corpuscle): the glomerulus (a capillary tuft fed by an afferent and drained by an efferent arteriole) enclosed in the cup-shaped Bowman's capsule.
- Renal tubule: proximal convoluted tubule (PCT) → loop of Henle (descending + ascending limbs) → distal convoluted tubule (DCT) → collecting duct.
Cortical nephrons have a short loop; juxtamedullary nephrons have a long loop of Henle dipping deep into the medulla — these drive the concentration of urine.
3. Urine formation — three steps
- Glomerular filtration — blood is filtered under pressure across the glomerulus into Bowman's capsule. The glomerular filtration rate (GFR) is about 125 mL/min (~180 L/day). The filtrate is like plasma minus proteins and cells.
- Tubular reabsorption — ~99% of the filtrate is reabsorbed. The PCT reabsorbs most water, glucose, amino acids and ions (glucose is normally completely reabsorbed).
- Tubular secretion — H⁺, K⁺, NH₃ are secreted into the filtrate to maintain ionic and acid-base balance.
Only about 1–1.5 L of urine is formed per day from ~180 L filtered.
Worked example 3.1. Glucose is present in the glomerular filtrate but absent from normal urine. Explain. Glucose is freely filtered at the glomerulus, but it is completely reabsorbed in the PCT back into the blood. So normal urine has no glucose. Its appearance (glycosuria) signals that blood glucose has exceeded the reabsorptive capacity — as in diabetes mellitus.
4. Concentrating the urine — the counter-current mechanism
The loop of Henle and the vasa recta form a counter-current system that builds an increasing solute (NaCl, urea) gradient from cortex to the inner medulla. Because the medulla is hyperosmotic, water leaves the descending limb and the collecting duct, concentrating the urine. This lets humans produce urine far more concentrated than blood.
- Descending limb: permeable to water (water leaves), impermeable to salt.
- Ascending limb: impermeable to water, actively pumps out NaCl.
5. Regulation of kidney function
Three hormones fine-tune water and salt balance:
- ADH (vasopressin) — from the posterior pituitary; when the body is dehydrated it increases water reabsorption in the DCT/collecting duct → less, more concentrated urine. Its lack causes diabetes insipidus (dilute urine).
- Aldosterone — from the adrenal cortex (part of the renin–angiotensin–aldosterone system, RAAS); promotes Na⁺ (and water) reabsorption, raising blood pressure.
- ANF (atrial natriuretic factor) — from the heart's atria when BP is high; promotes Na⁺ loss and lowers BP (opposes RAAS).
The JGA (juxtaglomerular apparatus) releases renin when blood pressure or GFR falls, triggering the RAAS.
Worked example 5.1. A dehydrated person produces a small volume of concentrated urine. Which hormone is responsible and how? ADH (antidiuretic hormone / vasopressin). Dehydration raises blood osmolarity; the posterior pituitary releases ADH, which increases water reabsorption in the distal tubule and collecting duct — so a small volume of concentrated urine is formed, conserving water.
Other roles / disorders: the kidney also helps make active vitamin D and erythropoietin. Uraemia (urea accumulation) is treated by haemodialysis (an artificial kidney) or transplant; kidney stones and glomerulonephritis are other disorders. Accessory organs of excretion: lungs (CO₂), liver (bile pigments), skin (sweat).
Part B — Locomotion and Movement
6. Types of movement and muscle
Movements are amoeboid (pseudopodia — WBCs), ciliary (trachea, oviduct) and muscular (limbs, tongue, organs). Muscle is contractile tissue of three kinds:
- Skeletal (striated, voluntary): attached to bones; moves the body.
- Smooth (unstriated, involuntary): walls of internal organs.
- Cardiac (striated, involuntary): heart; branched, with intercalated discs.
A skeletal muscle is bundles of muscle fibres; each fibre is packed with myofibrils made of repeating sarcomeres — the contractile unit between two Z-lines.
7. Structure of the sarcomere and the sliding-filament theory
The sarcomere contains two filaments:
- Thin filament — actin (with troponin and tropomyosin).
- Thick filament — myosin (whose heads form cross-bridges).
Banding: the A-band (dark, thick filaments) stays constant; the I-band (light, thin filaments only) and the H-zone shorten during contraction.
Sliding-filament theory: a nerve impulse releases Ca²⁺ from the sarcoplasmic reticulum → Ca²⁺ binds troponin, exposing the actin binding sites → myosin heads bind actin (cross-bridges) and, powered by ATP, pull the thin filaments toward the sarcomere centre. The filaments slide (they don't shorten); the sarcomere shortens. Relaxation follows when Ca²⁺ is pumped back.
Worked example 7.1. During muscle contraction which bands change and which stay the same? The A-band remains constant in length (the thick filaments don't shorten), while the I-band and H-zone shorten as the thin filaments slide inward. The Z-lines are pulled closer, shortening each sarcomere — the essence of the sliding-filament model.
8. The skeletal system
The human skeleton has 206 bones, in two divisions:
- Axial skeleton (80): skull (cranium + facial), vertebral column (26 vertebrae), sternum, ribs (12 pairs — 7 true, 3 false, 2 floating).
- Appendicular skeleton (126): limb bones + girdles (pectoral = clavicle + scapula; pelvic = hip bones).
Joints allow movement:
- Fibrous (immovable — skull sutures), cartilaginous (slightly movable — between vertebrae), synovial (freely movable).
- Synovial types: ball-and-socket (shoulder, hip), hinge (knee, elbow), pivot (atlas–axis), gliding, saddle (thumb).
Worked example 8.1. Classify the shoulder joint and the knee joint, and state their movement. The shoulder is a ball-and-socket synovial joint allowing movement in all directions (rotation). The knee is a hinge synovial joint allowing movement in one plane (flexion/extension), like a door hinge.
Disorders: arthritis (joint inflammation), osteoporosis (decreased bone mass, low oestrogen/Ca²⁺), gout (uric-acid crystals in joints), myasthenia gravis (autoimmune, muscle weakness), muscular dystrophy, tetany (low Ca²⁺).
9. Common traps NEET sets here
- Ammonotelic/ureotelic/uricotelic — fish/mammals/birds respectively; toxicity and water cost fall in that order.
- GFR ≈ 125 mL/min (~180 L/day) but only ~1.5 L urine — reabsorption is ~99%.
- Glucose fully reabsorbed in PCT — none in normal urine.
- ADH → water reabsorption (concentrated urine); its lack → diabetes insipidus. Don't confuse with diabetes mellitus (insulin/glucose).
- Ascending limb pumps salt, is water-impermeable; descending limb loses water — counter-current basis.
- In contraction the A-band is constant; I-band and H-zone shorten; filaments slide, they don't shorten.
- Ca²⁺ triggers contraction (binds troponin); ATP powers the myosin cross-bridge.
- 206 bones; axial 80, appendicular 126; 12 rib pairs (7 true, 3 false, 2 floating).
- Ball-and-socket (shoulder/hip) vs hinge (knee/elbow).
10. Memory aids
- "Fish-Ammonia, Mammal-Urea, Bird-Uric" — the three excretory modes.
- "180 filtered, 1.5 out" — GFR versus urine volume (99% reabsorbed).
- "ADH Adds water back" — its antidiuretic effect.
- "A-band Always the same" — the band that stays constant in contraction.
- "Ca²⁺ starts, ATP powers" — the two ions/molecules of contraction.
- "7 True, 3 False, 2 Float" — the 12 pairs of ribs.
- "Ball rotates, Hinge swings" — the two main synovial joints.
11. Exam protocol
- Excretory modes: ammonotelic (fish) / ureotelic (mammals) / uricotelic (birds).
- Nephron parts and the three steps (filtration → reabsorption → secretion); GFR ~125 mL/min; ~1.5 L urine.
- Counter-current: descending loses water, ascending pumps salt → concentrated urine.
- Hormones: ADH (water), aldosterone (Na⁺/BP up), ANF (BP down); JGA/renin → RAAS.
- Muscle: sarcomere (actin/myosin); sliding-filament (Ca²⁺ triggers, ATP powers; A-band constant, I/H shorten).
- Skeleton: 206 bones (axial 80 / appendicular 126); joints (ball-and-socket, hinge, pivot); disorders (arthritis, osteoporosis, gout).
