Human Physiology — NEET Class 11 revision
Everything in the NEET Human Physiology unit in one place: crisp concept notes, side-by-side differentiate tables, the doubts students get wrong most often, and quick-recall facts — plus interactive heart and neuron models you can tap through.
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Tap through the interactive models for this unit — each part explained in plain English with an exam-accurate NEET note.
Key concepts
Unit 5 covers breathing and gas transport; blood, lymph, heart and circulation; excretion and osmoregulation; movement and skeletal muscle; neural control; and the endocrine system, including major disorders listed in the NEET syllabus.
Respiratory organs & human pathway
Animals use body surface, gills, tracheae or lungs depending on habitat and organisation. In humans, air passes through nasal passages, pharynx, larynx, trachea, bronchi and bronchioles to alveoli.
- Alveoli = main gas-exchange surface
Mechanism of breathing
Inspiration is mainly active: diaphragm contracts and external intercostals lift ribs, increasing thoracic volume and lowering intrapulmonary pressure. Quiet expiration is mainly passive.
- Inspiration: volume ↑, pressure ↓
- Expiration: volume ↓, pressure ↑
Respiratory volumes & capacities
Tidal volume is about 500 mL. IRV, ERV and residual volume combine to form capacities such as vital capacity and total lung capacity.
- TV ≈ 500 mL
- Vital capacity = IRV + TV + ERV
Gas exchange & transport
O₂ and CO₂ diffuse down partial-pressure gradients. Most O₂ is carried by haemoglobin; most CO₂ is transported as bicarbonate.
- O₂: mainly oxyhaemoglobin
- CO₂: mainly HCO₃⁻
Regulation & respiratory disorders
Respiratory rhythm is generated in the medulla and modified by pons and chemoreceptor input, especially CO₂/H⁺. Asthma, emphysema and occupational respiratory disorders are key syllabus examples.
- CO₂/H⁺ strongly influence ventilation
- Asthma · emphysema · occupational disorders
Blood composition
Blood consists of plasma plus formed elements: RBCs, WBCs and platelets.
- RBC: haemoglobin/O₂ transport
- WBC: defence · platelets: clotting
Blood groups & coagulation
ABO groups depend on A/B antigens and antibodies; Rh compatibility matters in transfusion and pregnancy. Coagulation converts fibrinogen to fibrin through a cascade.
- ABO + Rh systems
- Fibrin mesh stabilises clot
Lymph
Lymph is tissue fluid returned to blood through lymphatic vessels. It transports absorbed fats from intestinal lacteals and participates in immunity.
- Returns interstitial fluid
- Lacteals transport absorbed lipids
Heart & double circulation
The four-chambered heart maintains pulmonary and systemic circuits. Valves ensure one-way flow.
- Right heart → lungs
- Left heart → body
Cardiac cycle, output & ECG
At about 75 beats/min, a cardiac cycle lasts about 0.8 s. Cardiac output = heart rate × stroke volume. ECG records electrical events.
- Cardiac output ≈ 5 L/min at rest
- ECG: P wave · QRS complex · T wave
Cardiac regulation & disorders
The SA node is the normal pacemaker; autonomic nerves and hormones modify heart rate. Important disorders include hypertension, coronary artery disease, angina and heart failure.
- SA node = pacemaker
- Hypertension · CAD · angina · heart failure
Modes of nitrogen excretion
Animals may excrete ammonia, urea or uric acid depending on water availability and adaptation.
- Ammonotelic · ureotelic · uricotelic
- Humans are ureotelic
Nephron & glomerular filtration
The nephron is the functional unit of the kidney. High glomerular pressure filters water and small solutes into Bowman's capsule while cells and most proteins remain in blood.
- GFR ≈ 125 mL/min
- Filtration is pressure-driven
Tubular reabsorption & secretion
Useful substances are selectively reabsorbed and wastes/ions can be secreted into the tubule. The PCT reabsorbs most filtered water and solutes.
- PCT: major reabsorption site
- Glucose is normally completely reabsorbed
Counter-current mechanism & osmoregulation
The loop of Henle and vasa recta maintain a medullary osmotic gradient. ADH raises collecting-duct water permeability, concentrating urine.
- ADH ↑ → water reabsorption ↑
- Counter-current system preserves medullary gradient
Kidney regulation & disorders
Renin-angiotensin-aldosterone, ANF and ADH regulate volume and pressure. Diabetes insipidus involves deficient ADH action. Uraemia, renal failure, calculi and nephritis are key disorders.
- RAAS retains Na⁺/water · ANF opposes volume expansion
- Dialysis substitutes for major kidney filtering functions
Other excretory organs
Lungs remove CO₂ and water; skin removes sweat; liver eliminates bile pigments, cholesterol and detoxified products.
- Kidneys are primary, but excretion is multi-organ
Types of movement & skeleton
Ciliary, flagellar and muscular movements occur in the body. The skeleton supports, protects and provides levers; joints allow different ranges of movement.
- Fibrous · cartilaginous · synovial joints
Sliding-filament mechanism
Ca²⁺ exposes actin binding sites. Myosin cross-bridges cycle using ATP, sliding actin over myosin. Filaments do not shorten.
- A band constant
- I band + H zone shorten
Musculoskeletal disorders
Myasthenia gravis, tetany, muscular dystrophy, arthritis, osteoporosis and gout are the listed disorders.
- Myasthenia: neuromuscular junction
- Gout: urate crystal deposition
Neuron & nervous-system divisions
A neuron has dendrites, cell body and axon. The human nervous system includes CNS and PNS; the autonomic/visceral system includes sympathetic and parasympathetic divisions.
- CNS: brain + spinal cord
- PNS connects CNS to body
Generation & conduction of nerve impulse
Resting membrane potential reflects ion gradients. Depolarisation mainly involves Na⁺ entry and repolarisation K⁺ exit. Myelin enables saltatory conduction.
- Depolarisation: Na⁺ in
- Repolarisation: K⁺ out
Chemical synapse
An arriving action potential opens Ca²⁺ channels, causing neurotransmitter release. Chemical synaptic transmission is normally one-way.
- Ca²⁺ triggers vesicle fusion
Endocrine system
Major endocrine structures include hypothalamus, pituitary, pineal, thyroid, parathyroids, adrenals, pancreas and gonads.
- Hormones act as chemical messengers
Hormone action & feedback
Peptide hormones generally act through membrane receptors and second messengers; steroid/thyroid hormones act mainly through intracellular receptors. Negative feedback stabilises hormone levels.
- Receptor specificity + feedback control
Major endocrine hormones
Pituitary controls several glands; thyroid hormones regulate metabolism; PTH raises blood Ca²⁺; insulin lowers blood glucose; glucagon raises it; adrenal hormones mediate stress; gonadal hormones regulate reproduction.
- Insulin ↓ glucose · glucagon ↑ glucose
- PTH ↑ blood Ca²⁺
Endocrine disorders
Key syllabus examples include pituitary dwarfism and acromegaly, congenital hypothyroidism (historically termed cretinism in older syllabus language), goitre, exophthalmic goitre, diabetes mellitus and Addison's disease.
- GH excess in adults → acromegaly
- Insulin deficiency/action defect → diabetes mellitus
- Adrenal cortex hypofunction → Addison's disease
Differentiate the major human-physiology controls
Direction, location and regulator are the fastest clues.
ECG
| P wave | Atrial depolarisation |
| QRS complex | Ventricular depolarisation |
| T wave | Ventricular repolarisation |
Nephron segments
| PCT | Bulk reabsorption of water, Na⁺, nutrients |
| Descending limb | Water-permeable; filtrate concentrates |
| Ascending limb | Water-impermeable; salts leave |
| Collecting duct | ADH-regulated water reabsorption |
High-yield distinctions
These pairs appear repeatedly in NEET physiology.
Pulmonary artery vs pulmonary vein ★
Pulmonary artery
- Right ventricle → lungs
- Deoxygenated blood
- Artery = away from heart
Pulmonary vein
- Lungs → left atrium
- Oxygenated blood
- Vein = toward heart
Inspiration vs expiration ★
Inspiration
- Mainly active
- Thoracic volume rises
- Pressure falls
Expiration
- Quiet phase mainly passive
- Thoracic volume falls
- Pressure rises
ADH vs ANF
ADH
- Promotes water reabsorption
- Concentrates urine
- Raises effective volume
ANF
- Promotes Na⁺/water loss
- Opposes RAAS
- Lowers volume/pressure
Depolarisation vs repolarisation ★
Depolarisation
- Voltage-gated Na⁺ channels open
- Na⁺ enters
- Membrane becomes less negative
Repolarisation
- K⁺ channels open
- K⁺ leaves
- Membrane returns negative
Common NEET doubts
Does an artery always carry oxygenated blood?
No. Artery means away from the heart; the pulmonary artery carries deoxygenated blood.
Is most CO₂ transported dissolved in plasma?
No. Most CO₂ is transported as bicarbonate ions.
Is the QRS complex ventricular contraction itself?
It represents ventricular depolarisation, the electrical event that precedes contraction.
Does ADH add water to urine?
No. It increases water reabsorption from the collecting ducts, so urine volume falls and concentration rises.
Is glucose normally present in final urine?
No. Filtered glucose is normally reabsorbed; significant glucose in urine suggests abnormal blood glucose handling.
Do actin and myosin filaments shorten during contraction?
No. They slide past one another; the sarcomere shortens.
Does a stronger stimulus make a taller action potential?
No. Action potentials are all-or-none; stronger stimuli are usually encoded by firing frequency.
Are all hormones proteins?
No. Some are peptides/proteins, some are steroids, and thyroid hormones are amino-acid derivatives.
Does insulin raise blood glucose?
No. Insulin lowers it; glucagon raises it.
Is diabetes insipidus the same as diabetes mellitus?
No. Diabetes insipidus is mainly a water-balance disorder involving ADH; diabetes mellitus is a glucose-regulation disorder involving insulin.
Quick recall
Q. Normal tidal volume?
About 500 mL.
Q. How is most O₂ carried in blood?
Bound to haemoglobin as oxyhaemoglobin.
Q. How is most CO₂ carried?
As bicarbonate (HCO₃⁻).
Q. What is cardiac output?
Heart rate × stroke volume; about 5 L/min at rest in a typical adult.
Q. What does the P wave represent?
Atrial depolarisation.
Q. Normal GFR?
About 125 mL/min.
Q. What does ADH do to the collecting duct?
Increases water permeability and water reabsorption.
Q. What changes during muscle contraction?
Sarcomere, I band and H zone shorten; A band remains constant.
Q. Which ions dominate depolarisation and repolarisation?
Na⁺ entry and K⁺ exit, respectively.
Q. What is the body's normal cardiac pacemaker?
SA node.
Q. Which hormone lowers blood glucose?
Insulin.
Q. Which gland/hormone disorder causes acromegaly?
Excess growth hormone in adults, usually from the anterior pituitary.
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