Animal Physiology: The Body That Runs the Show
Animal physiology is not just another chapter — it’s the chapter that ties together everything from cell biology to ecology. When students ask “why does NEET have so many questions on digestion and circulation?”, the answer is simple: these systems are the functional core of multicellular life, and NEET tests your ability to connect structure with function.
In Class 11, animal physiology spans five major systems: digestion and absorption, breathing and gas exchange, body fluids and circulation, excretion, and locomotion. Together, these systems account for roughly 15–18 marks in NEET every year — that’s not weightage you leave on the table.
We will cover each system with the depth needed for board exams and the precision needed for NEET. No hand-waving about “how complex the body is” — let’s get into the mechanisms.
Key Terms & Definitions
Digestion — The process of breaking down complex food molecules into simpler, absorbable units using mechanical and chemical means.
Peristalsis — Wave-like muscular contractions that move food through the alimentary canal. The key word is involuntary — this is smooth muscle action, not skeletal.
Tidal Volume (TV) — Volume of air breathed in or out in one normal breath. In a healthy adult: ~500 mL. This is the most-tested spirometry value in NEET.
Stroke Volume (SV) — Volume of blood pumped by the left ventricle per beat. Combined with heart rate: Cardiac Output (CO) = SV × HR.
Glomerular Filtration Rate (GFR) — Volume of filtrate formed per minute by both kidneys (~125 mL/min). The entire plasma is filtered 60 times a day — a number NEET loves to ask.
Sarcomere — Functional unit of skeletal muscle, from one Z-line to the next Z-line.
Actin — Thin filament in muscle. Contains the binding sites for myosin heads, which are blocked by tropomyosin at rest.
System 1: Digestion and Absorption
Alimentary Canal — Room by Room
The human gut is 30 feet long but let’s focus on what the NEET paper actually asks.
Mouth — Salivary amylase (ptyalin) begins starch digestion here. pH ~7. Students forget that protein digestion does NOT begin in the mouth.
Stomach — HCl (pH 1.5–2) activates pepsinogen → pepsin. Pepsin cleaves proteins into peptones and proteoses. The gastric mucosa secretes intrinsic factor needed for Vitamin B₁₂ absorption — a classic one-mark NEET question.
Small Intestine — Where 90% of absorption happens. The three segments are duodenum, jejunum, and ileum. Remember the mnemonic D-J-I (Delhi-Jaipur-Indore, if it helps you).
| Enzyme | Source | Substrate | Product |
|---|---|---|---|
| Trypsin | Pancreas | Proteins | Peptides |
| Lipase | Pancreas | Fats | Fatty acids + Glycerol |
| Maltase | Intestinal juice | Maltose | 2 Glucose |
| Sucrase | Intestinal juice | Sucrose | Glucose + Fructose |
| Lactase | Intestinal juice | Lactose | Glucose + Galactose |
Absorption route matters:
- Glucose, amino acids → blood capillaries → portal vein → liver
- Fatty acids, glycerol → lacteals (lymph capillaries) → thoracic duct → blood
Students often write that fats are absorbed directly into blood. They’re not — fats are reassembled into chylomicrons and travel via lacteals first. This distinction costs 1 mark every cycle in NEET.
System 2: Breathing and Gas Exchange
Lung Volumes — The NEET Favourite
You must memorise these values. They appear in almost every NEET paper.
- Tidal Volume (TV) = 500 mL
- Inspiratory Reserve Volume (IRV) = 2500 mL
- Expiratory Reserve Volume (ERV) = 1000 mL
- Residual Volume (RV) = 1100 mL
- Inspiratory Capacity (IC) = TV + IRV = 3000 mL
- Vital Capacity (VC) = TV + IRV + ERV = 4000 mL
- Total Lung Capacity (TLC) = VC + RV = 5100 mL
- Functional Residual Capacity (FRC) = ERV + RV = 2100 mL
NEET 2023 asked for FRC directly. NEET 2021 Shift 2 gave a spirogram and asked students to identify ERV. If you can draw the spirogram from memory, you won’t lose a single lung volume question.
Gas Transport
Oxygen transport: 97% by haemoglobin (as oxyhaemoglobin), 3% dissolved in plasma.
CO₂ transport: 70% as bicarbonate ions (HCO₃⁻) — the most important route. 20–25% bound to haemoglobin (carbaminohaemoglobin), 7% dissolved in plasma.
The Chloride Shift: When CO₂ enters RBCs and converts to HCO₃⁻, the bicarbonate moves out into plasma and Cl⁻ moves in to maintain electrical neutrality. This is also called the Hamburger phenomenon — NEET has asked the mechanism, not just the name.
System 3: Body Fluids and Circulation
Heart Structure — What NEET Tests
The human heart has four chambers. The right side handles deoxygenated blood, the left side handles oxygenated blood. Know the valves:
- Tricuspid — between right atrium and right ventricle (3 cusps)
- Bicuspid (Mitral) — between left atrium and left ventricle (2 cusps)
- Semilunar valves — at the base of pulmonary artery and aorta
Cardiac Cycle Duration: ~0.8 seconds at a heart rate of 75 bpm.
- Atrial systole: 0.1s
- Ventricular systole: 0.3s
- Joint diastole: 0.4s
Conduction System
SA Node (Pacemaker) → AV Node → Bundle of His → Purkinje fibres. This is the sequence of electrical impulse conduction. The SA node sets the intrinsic rate at 70–75 bpm.
A good way to remember the conduction sequence: SA → AV → BH → PF — think “Some Animals Breathe Perfectly Fast”.
System 4: Excretory Products and Their Elimination
Nephron — The Core Unit
Each kidney contains ~1 million nephrons. The nephron has:
- Bowman’s Capsule + Glomerulus — filtration
- Proximal Convoluted Tubule (PCT) — maximum reabsorption (glucose, amino acids, Na⁺, 70% water)
- Loop of Henle — concentration of urine (creates medullary gradient)
- Distal Convoluted Tubule (DCT) — selective reabsorption and secretion, regulated by ADH and aldosterone
- Collecting Duct — final water reabsorption, response to ADH
- GFR = 125 mL/min = 180 L/day
- Urine output = ~1.5 L/day
- That means 178.5 L is reabsorbed — 99% of filtrate
Hormonal Control:
- ADH (Vasopressin) — increases water reabsorption from DCT and collecting duct. Released when blood osmolarity rises.
- Aldosterone — increases Na⁺ (and water) reabsorption from DCT. Released by adrenal cortex.
- ANF (Atrial Natriuretic Factor) — causes vasodilation and reduces blood pressure; antagonises aldosterone.
Students confuse ADH and aldosterone. ADH targets water; aldosterone targets sodium. In NEET MCQs, options are designed to make you swap the two. Read them carefully.
System 5: Locomotion and Movement
Sliding Filament Theory — The Mechanism NEET Loves
Here’s the sequence during muscle contraction:
- Motor neuron releases ACh at neuromuscular junction
- ACh binds to receptors → action potential along sarcolemma → travels into T-tubules
- Ca²⁺ released from sarcoplasmic reticulum
- Ca²⁺ binds to troponin → tropomyosin shifts → exposes active sites on actin
- Myosin heads bind to actin → power stroke (myosin head bends, pulling actin toward M-line)
- ATP binds to myosin → head detaches
- ATP hydrolysis → myosin head re-cocks
- Cycle repeats as long as Ca²⁺ and ATP are available
NEET 2022 asked: “Which of the following ions is required for binding of myosin to actin?” Answer: Ca²⁺ (not Mg²⁺, not Na⁺). The ATP hydrolysis needs Mg²⁺ as a cofactor, but the binding step requires Ca²⁺. Don’t confuse the two roles.
Sarcomere changes during contraction:
- I-band shortens
- H-zone shortens
- A-band remains the same length (thick filaments don’t change)
- Sarcomere shortens overall
Solved Examples
Example 1 — Easy (CBSE Level)
Q: A person breathes in 500 mL of air with each breath and takes 16 breaths per minute. What is the minute ventilation? How much of this actually reaches the alveoli?
Solution:
- Minute ventilation = 500 × 16 = 8000 mL/min
- Dead space volume = ~150 mL per breath (anatomical dead space in trachea + bronchi)
- Alveolar ventilation = (500 − 150) × 16 = 350 × 16 = 5600 mL/min
The remaining 2400 mL/min never participates in gas exchange — it stays in the conducting zone.
Example 2 — Medium (NEET Level)
Q: The glomerular filtration rate is 125 mL/min. A substance X is filtered but neither reabsorbed nor secreted. Its plasma concentration is 1 mg/mL. What is the amount of X excreted per day?
Solution:
- GFR = 125 mL/min = 125 × 60 × 24 mL/day = 180,000 mL/day = 180 L/day
- Amount of X filtered = 180,000 mL × 1 mg/mL = 180,000 mg = 180 g/day
- Since X is neither reabsorbed nor secreted, all filtered X is excreted
- Amount excreted = 180 g/day
This is why inulin is used clinically to measure GFR — it satisfies exactly these conditions.
Example 3 — Hard (JEE Main / NEET High Difficulty)
Q: During strenuous exercise, CO₂ production increases fivefold. Explain what happens to: (a) breathing rate, (b) blood pH, and (c) haemoglobin’s affinity for O₂.
Solution:
(a) Breathing Rate: Rising CO₂ → rising H⁺ (CO₂ + H₂O → H₂CO₃ → H⁺ + HCO₃⁻) → detected by central and peripheral chemoreceptors → increased respiratory rate and depth. This is the primary driver of hyperpnoea during exercise (not O₂ drop, as students commonly assume).
(b) Blood pH: Drops. More CO₂ → more carbonic acid → more H⁺ → pH falls (respiratory acidosis). The kidneys compensate over hours by excreting more H⁺ and retaining HCO₃⁻.
(c) Hb-O₂ Affinity: Decreases. This is the Bohr Effect — lower pH (higher H⁺) and higher CO₂ shift the O₂-Haemoglobin dissociation curve to the right. Hb releases O₂ more readily to working muscles. Exactly what we need during exercise — the body self-regulates oxygen delivery.
Exam-Specific Tips
For NEET
Animal physiology contributes 12–15 questions in NEET annually. The highest-yield chapters are: Digestion (3–4 Q), Circulation (3–4 Q), Excretion (3–4 Q), Breathing (2–3 Q), Locomotion (1–2 Q). Master these five in this priority order.
- Diagram-based questions (heart chambers, nephron, sarcomere) appear every year. Draw these 10 times each — muscle memory matters.
- For enzyme questions, know both the substrate and product — options often swap one or the other.
- PYQ analysis: NEET 2024 asked about the role of carbonic anhydrase in CO₂ transport. NEET 2023 asked about the portal system. These aren’t surprises if you’ve done PYQs from 2018–2024.
For CBSE Board Exams
- 3-mark and 5-mark questions on digestion and excretion are standard in Section B and C.
- For 3-markers on circulation, always write: structure → function → control mechanism.
- Diagrams carry 1 mark each when asked — label all parts, even if you draw a rough sketch.
Common Mistakes to Avoid
Mistake 1 — Mixing up Trypsin and Pepsin: Pepsin works in the stomach (acidic, pH 1.5–2). Trypsin works in the small intestine (alkaline, pH 8). If the question says “gastric juice,” it’s pepsin.
Mistake 2 — A-band stays constant: During muscle contraction, students often say “A-band shortens.” It does not — A-band = length of thick filaments (myosin), and myosin does not shorten. Only I-band and H-zone shorten.
Mistake 3 — Renal threshold for glucose: Glucose normally does not appear in urine because all of it is reabsorbed in PCT. When blood glucose exceeds ~180 mg/dL (renal threshold), it appears in urine. Don’t write “kidneys don’t filter glucose” — they do filter it, they just reabsorb it completely.
Mistake 4 — Double circulation vs closed circulation: All vertebrates have closed circulation. Double circulation (blood passes through heart twice per circuit) is specific to birds and mammals. Amphibians have incomplete double circulation. Don’t use the terms interchangeably.
Mistake 5 — SA node location: The SA node is in the right atrium (not right ventricle). The AV node is between right atrium and right ventricle. Students swap these in diagrams and lose easy marks.
Practice Questions
Q1. Which of the following enzymes is correctly matched with its substrate? (A) Pepsin — Maltose (B) Trypsin — Proteins (C) Lipase — Nucleic acids (D) Amylase — Fats
Answer: (B)
Trypsin is a protease secreted by the pancreas that cleaves proteins into smaller peptides. Pepsin acts on proteins (not maltose), lipase acts on fats (not nucleic acids), and amylase acts on starch (not fats). This question tests whether you know the substrate-enzyme pair, not just the enzyme name.
Q2. The vital capacity of lungs does NOT include: (A) Tidal Volume (B) IRV (C) ERV (D) Residual Volume
Answer: (D)
Vital Capacity = TV + IRV + ERV = 4000 mL. Residual Volume cannot be expelled even with maximum forced expiration, so it is never part of VC. It is part of Total Lung Capacity and FRC. This is a classic NEET distractor question.
Q3. The Bohr effect refers to: (A) Increased O₂ affinity of Hb at high CO₂ (B) Decreased O₂ affinity of Hb at high CO₂ (C) Increased Hb synthesis at high altitude (D) Decreased CO₂ transport at high temperature
Answer: (B)
The Bohr effect states that increased CO₂ (and H⁺) reduces haemoglobin’s affinity for O₂, causing rightward shift of the dissociation curve. This facilitates O₂ release to metabolically active tissues. Remember: exercise produces CO₂ → Hb releases more O₂ → working muscles get what they need.
Q4. Which hormone promotes Na⁺ reabsorption in the DCT? (A) ADH (B) Aldosterone (C) ANF (D) Renin
Answer: (B)
Aldosterone (secreted by adrenal cortex) acts on DCT and collecting duct to increase Na⁺ reabsorption. Water follows osmotically. ADH promotes water reabsorption specifically. ANF (from heart atria) opposes aldosterone — it causes natriuresis (Na⁺ excretion). Renin is an enzyme that triggers the RAAS cascade, not a direct effector.
Q5. During a single cardiac cycle at 75 bpm, the duration of joint diastole is: (A) 0.1 s (B) 0.3 s (C) 0.4 s (D) 0.8 s
Answer: (C)
Total cardiac cycle = 0.8 s. Atrial systole = 0.1 s. Ventricular systole = 0.3 s. Joint diastole = 0.8 − 0.1 − 0.3 = 0.4 s. During joint diastole, all chambers are relaxed and blood flows passively into the atria and then into ventricles. This is the resting phase of the heart.
Q6. Which part of the nephron is impermeable to water but permeable to electrolytes? (A) PCT (B) Descending limb of Loop of Henle (C) Ascending limb of Loop of Henle (D) Collecting duct
Answer: (C)
The ascending limb of the loop of Henle is impermeable to water but actively transports Na⁺ and Cl⁻ out into the medullary interstitium. This creates the hyperosmotic medullary gradient essential for concentrating urine. The descending limb is the opposite — permeable to water, not to electrolytes.
Q7. The intrinsic factor secreted by gastric mucosa is essential for absorption of: (A) Vitamin D (B) Vitamin B₁₂ (C) Iron (D) Calcium
Answer: (B)
Intrinsic factor (a glycoprotein) secreted by parietal cells of gastric mucosa is required for absorption of Vitamin B₁₂ (cobalamin) in the ileum. Deficiency of intrinsic factor → Vitamin B₁₂ deficiency → pernicious anemia. This is a 1-marker that NEET repeats roughly every 2–3 years.
Q8. In the sliding filament model, which band does NOT change length during contraction? (A) I-band (B) H-zone (C) A-band (D) Sarcomere
Answer: (C)
A-band = zone of thick filaments (myosin). Since myosin filaments do not shorten during contraction, the A-band length remains constant. The I-band (thin filaments only) and H-zone (myosin only, no overlap) both shorten as actin slides over myosin. The sarcomere as a whole shortens.
FAQs
What is the difference between digestion and absorption?
Digestion is the breakdown of complex food molecules into simpler units (mechanical + chemical). Absorption is the uptake of these simpler units into the blood or lymph. Both processes are needed — unabsorbed nutrients serve no purpose to the body.
Why does urine become concentrated when we’re dehydrated?
Low blood volume → hypothalamus detects high osmolarity → ADH released from posterior pituitary → collecting duct becomes more permeable to water → more water reabsorbed → urine volume decreases and concentration increases. The kidneys are trying to conserve water for the body.
What is double circulation and why do birds and mammals have it?
Double circulation means blood passes through the heart twice in one complete circuit: once through pulmonary circulation (heart → lungs → heart) and once through systemic circulation (heart → body → heart). Complete separation of oxygenated and deoxygenated blood allows high metabolic rates — essential for homeothermy (warm-bloodedness) in birds and mammals.
Why do we breathe faster during exercise?
The primary stimulus is rising CO₂ levels, not falling O₂. Muscles produce more CO₂ → blood CO₂ rises → chemoreceptors (carotid and aortic bodies, medullary centres) detect this → breathing rate and depth increase. O₂ levels rarely drop significantly during normal exercise in healthy individuals.
What happens if GFR falls?
Falling GFR means less filtrate formed → waste products (urea, creatinine) accumulate in blood → condition called uremia. The body can compensate short-term by increasing tubular secretion, but chronic GFR reduction leads to kidney failure requiring dialysis.
How is the heart rate regulated?
Two divisions of the autonomic nervous system act on the SA node. Sympathetic nerves (via noradrenaline) increase heart rate. Parasympathetic nerves (vagus nerve, via ACh) decrease heart rate. At rest, vagal tone dominates — that’s why cutting the vagus nerve causes tachycardia, not bradycardia.
What is the role of bile in digestion?
Bile does not contain enzymes — it’s an emulsifier. Bile salts break fat globules into smaller droplets (emulsification), increasing surface area for lipase action. Bile also contains bilirubin (from RBC breakdown) and bicarbonate (to neutralise stomach acid in duodenum).
Is rigor mortis related to ATP?
Yes, directly. After death, ATP production stops. Without ATP, myosin heads cannot detach from actin — they remain locked in the bound state. This causes the characteristic stiffness of rigor mortis. When proteins eventually decompose, rigor mortis resolves.