# Breathing and Exchange of Gases (NEET Biology Class 11): Volumes, Capacities & Gas Transport Source: https://tracknation.in/story/breathing-and-exchange-of-gases-neet-biology-class-11-volumes-capacities-gas-transport (TrackNation — free UPSC notes with PYQs) > NEET Class 11 Breathing and Exchange of Gases, mined from NCERT: respiratory organs across animal groups, the full human respiratory system (path of air, larynx, epiglottis, pleura, conducting vs exchange part), the mechanism of breathing, a complete respiratory volumes and capacities table with exact mL values and formulae, the partial pressure table for O2 and CO2, transport of oxygen and carbon dioxide with exact percentages, the oxygen dissociation curve, regulation of respiration (medulla, ## Respiratory Organs & the Human Respiratory System 🎯 **NEET priority: High-yield.** *3–5 questions every year. The **respiratory volumes and capacities** (with their exact mL values) and the **partial pressure table** are asked almost verbatim — memorise both tables below.* ### Respiratory organs across animals - **Simple diffusion over the body surface** — sponges, coelenterates, flatworms. **Moist cuticle** — earthworms. **Tracheal tubes** — insects. **Gills (branchial respiration)** — most aquatic arthropods, molluscs and fishes. **Lungs (pulmonary respiration)** — amphibians, reptiles, birds, mammals. Amphibians like frogs also use **moist skin (cutaneous respiration)**. ### The human respiratory system — the path of air - **External nostrils → nasal chamber → pharynx → larynx → trachea → primary bronchi → secondary/tertiary bronchi → bronchioles → terminal bronchioles → alveoli.** - The **larynx** is a cartilaginous **sound box**. During swallowing, the **epiglottis** — a thin elastic cartilaginous flap — covers the glottis to stop food entering the larynx. The **trachea divides at the level of the 5th thoracic vertebra**. Trachea, all bronchi and initial bronchioles are supported by **incomplete cartilaginous rings**. - Lungs are covered by a **double-layered pleura** with **pleural fluid** between them to reduce friction. The **outer** pleural membrane contacts the thoracic lining; the **inner** contacts the lung surface. - **Conducting part** (external nostrils → terminal bronchioles): transports air, **clears foreign particles, humidifies, and brings air to body temperature**. **Respiratory/exchange part** (alveoli and their ducts): the actual site of diffusion. - The thoracic chamber is **air-tight**, bounded **dorsally by the vertebral column, ventrally by the sternum, laterally by the ribs, and below by the dome-shaped diaphragm** — so any change in thoracic volume is reflected in the lungs (we cannot alter pulmonary volume directly). ### The five steps of respiration - (1) **Breathing/pulmonary ventilation**; (2) **diffusion** of O₂ and CO₂ across the alveolar membrane; (3) **transport** of gases by blood; (4) **diffusion** between blood and tissues; (5) **utilisation of O₂ by cells** for catabolism, releasing CO₂. ## Mechanism of Breathing, Volumes & Gas Exchange ### Mechanism of breathing - **Inspiration** needs **intra-pulmonary pressure LOWER than atmospheric** (negative pressure). It is initiated by **contraction of the diaphragm** (increasing thoracic volume along the **antero-posterior axis**) plus contraction of the **external inter-costal muscles**, lifting the ribs and sternum (increasing volume along the **dorso-ventral axis**). - **Expiration** occurs when **intra-pulmonary pressure is HIGHER than atmospheric** — from **relaxation** of the diaphragm and inter-costal muscles. - A healthy human breathes **12–16 times per minute**. Volumes are measured with a **spirometer**. ### Respiratory volumes and capacities — the exact numbers | **Volume / Capacity** | **Definition** | **Value / Formula** | | Tidal Volume (TV) | Air inspired or expired during normal respiration | ≈ 500 mL (so 6000–8000 mL per minute) | | Inspiratory Reserve Volume (IRV) | Extra air inspired by forcible inspiration | 2500–3000 mL | | Expiratory Reserve Volume (ERV) | Extra air expired by forcible expiration | 1000–1100 mL | | Residual Volume (RV) | Air remaining in the lungs even after forcible expiration | 1100–1200 mL | | Inspiratory Capacity (IC) | Total air inspired after a normal expiration | TV + IRV | | Expiratory Capacity (EC) | Total air expired after a normal inspiration | TV + ERV | | Functional Residual Capacity (FRC) | Air remaining in lungs after a normal expiration | ERV + RV | | Vital Capacity (VC) | Max air breathed in after forced expiration (or out after forced inspiration) | ERV + TV + IRV | | Total Lung Capacity (TLC) | Total air in lungs at the end of forced inspiration | RV + ERV + TV + IRV (= VC + RV) | ### Exchange of gases — partial pressures (mm Hg) | **Gas** | **Atmospheric air** | **Alveoli** | **Deoxygenated blood** | **Oxygenated blood** | **Tissues** | | O₂ | 159 | 104 | 40 | 95 | 40 | | CO₂ | 0.3 | 40 | 45 | 40 | 45 | - These gradients drive O₂ **alveoli → blood → tissues**, and CO₂ **tissues → blood → alveoli**. **CO₂ solubility is 20–25 times higher** than O₂'s, so far more CO₂ diffuses per unit pressure difference. - The **diffusion membrane has three layers**: the **thin squamous epithelium of alveoli**, the **endothelium of alveolar capillaries**, and the **basement substance** between them. Total thickness is **much less than a millimetre**. ## Transport of Oxygen and Carbon Dioxide ### Transport of gases — the percentages NEET asks | **Gas** | **By RBC / haemoglobin** | **As bicarbonate** | **Dissolved in plasma** | | Oxygen | 97% (as oxyhaemoglobin) | — | 3% | | Carbon dioxide | 20–25% (as carbamino-haemoglobin) | 70% | 7% | ### Transport of oxygen - **Haemoglobin** is a red, **iron-containing** pigment in RBCs. Each haemoglobin molecule carries a maximum of **four O₂ molecules**, reversibly, as **oxyhaemoglobin**. Binding depends chiefly on **pO₂**, and also on pCO₂, H⁺ concentration and temperature. - The **oxygen dissociation curve** is **sigmoid**. **In alveoli** — high pO₂, low pCO₂, low H⁺, low temperature — **favour FORMATION** of oxyhaemoglobin. **In tissues** — low pO₂, high pCO₂, high H⁺, high temperature — **favour DISSOCIATION**. - Every **100 mL of oxygenated blood delivers ~5 mL of O₂** to the tissues. ### Transport of carbon dioxide - RBCs contain a very high concentration of **carbonic anhydrase** (small amounts also in plasma), catalysing **CO₂ + H₂O ⇌ H₂CO₃ ⇌ HCO₃⁻ + H⁺** in **both directions**. - **At the tissues** (high pCO₂): CO₂ diffuses in and is trapped as **bicarbonate**. **At the alveoli** (low pCO₂): the reaction reverses, releasing CO₂. Every **100 mL of deoxygenated blood delivers ~4 mL of CO₂** to the alveoli. ## Regulation, Disorders & Why This Matters for NEET ### Regulation of respiration - The **respiratory rhythm centre**, in the **medulla**, is primarily responsible. The **pneumotaxic centre**, in the **pons**, can moderate it — its signals **reduce the duration of inspiration**, altering respiratory rate. - A **chemosensitive area** adjacent to the rhythm centre is highly sensitive to **CO₂ and hydrogen ions** (not oxygen). Receptors on the **aortic arch and carotid artery** also detect CO₂ and H⁺ changes. **The role of oxygen in regulating respiratory rhythm is quite insignificant** — a favourite NEET trap. ### Disorders of the respiratory system | **Disorder** | **What happens** | **Cause** | | Asthma | Difficulty in breathing, causing wheezing | Inflammation of bronchi and bronchioles | | Emphysema | Alveolar walls are damaged, decreasing respiratory surface | Chronic disorder; a major cause is cigarette smoking | | Occupational respiratory disorders | Inflammation leading to fibrosis (proliferation of fibrous tissue) and serious lung damage | Long exposure to dust in industries involving grinding or stone-breaking; workers should wear protective masks | ### Why this matters for NEET - High-value one-liners: TV = **500 mL**; RV = **1100–1200 mL**; **VC = ERV + TV + IRV**; **FRC = ERV + RV**; alveolar pO₂ = **104**, tissue pO₂ = **40**; O₂ transport = **97% by RBC**; CO₂ = **70% as bicarbonate**; one haemoglobin carries **4 O₂**; trachea divides at the **5th thoracic vertebra**. - **Trap:** respiration is regulated by **CO₂ and H⁺**, NOT by oxygen — NCERT explicitly says oxygen's role is insignificant. Also, **inspiration is active** (muscle contraction) while **normal expiration is passive** (relaxation). And don't confuse **residual volume** (air that can never be expelled) with **expiratory reserve volume** (extra air you CAN forcibly expel). ## Test Yourself: MCQs, PYQs & Active Recall Answer these, then close the article and do an Active Recall. Reveal each answer only after you commit to one. ## Practice Questions **Q1.** The thin elastic cartilaginous flap that prevents food entering the larynx during swallowing is the: - (a) Pleura - (b) Diaphragm - (c) Glottis - (d) Epiglottis **Answer: (d)** — The epiglottis covers the glottis during swallowing to prevent food entering the larynx. **Q2.** The trachea divides into right and left primary bronchi at the level of the: - (a) 1st lumbar vertebra - (b) 5th thoracic vertebra - (c) 10th thoracic vertebra - (d) 2nd thoracic vertebra **Answer: (b)** — The trachea extends to the mid-thoracic cavity and divides at the level of the 5th thoracic vertebra. **Q3.** The part of the respiratory system from external nostrils to terminal bronchioles is called the: - (a) Respiratory part - (b) Pleural cavity - (c) Conducting part - (d) Exchange part **Answer: (c)** — The conducting part transports air, clears foreign particles, humidifies it and brings it to body temperature. **Q4.** Inspiration is initiated by the contraction of: - (a) The pleura - (b) The diaphragm and external inter-costal muscles - (c) Internal inter-costal muscles only - (d) Abdominal muscles only **Answer: (b)** — Diaphragm contraction increases thoracic volume antero-posteriorly; external inter-costals lift the ribs and sternum dorso-ventrally. **Q5.** A healthy human being breathes approximately how many times per minute? - (a) 30-40 - (b) 12-16 - (c) 6-8 - (d) 20-25 **Answer: (b)** — On average, a healthy human breathes 12-16 times per minute. **Q6.** Tidal volume in a healthy human is approximately: - (a) 2500 mL - (b) 500 mL - (c) 4500 mL - (d) 1200 mL **Answer: (b)** — Tidal volume — air inspired or expired during normal respiration — is about 500 mL. **Q7.** Vital Capacity is the sum of: - (a) ERV + TV + IRV - (b) RV + TV - (c) TV + IRV - (d) ERV + RV **Answer: (a)** — Vital Capacity = ERV + TV + IRV. Adding residual volume to it gives Total Lung Capacity. **Q8.** Functional Residual Capacity (FRC) is: - (a) TV + ERV - (b) TV + IRV - (c) ERV + RV - (d) RV only **Answer: (c)** — FRC, the air remaining in the lungs after a normal expiration, equals ERV + RV. **Q9.** The partial pressure of oxygen in the alveoli is approximately: - (a) 40 mm Hg - (b) 104 mm Hg - (c) 159 mm Hg - (d) 95 mm Hg **Answer: (b)** — Alveolar pO2 is 104 mm Hg, compared with 159 in atmospheric air and 40 in deoxygenated blood and tissues. **Q10.** The solubility of CO2 compared to O2 is approximately: - (a) 20-25 times higher - (b) Equal - (c) 2 times higher - (d) 100 times higher **Answer: (a)** — CO2 solubility is 20-25 times higher than that of O2, so far more CO2 diffuses per unit pressure difference. **Q11.** The diffusion membrane in the alveoli is made up of how many major layers? - (a) Three - (b) Two - (c) One - (d) Five **Answer: (a)** — Three layers: the thin squamous epithelium of alveoli, the endothelium of alveolar capillaries, and the basement substance between them. **Q12.** What percentage of oxygen is transported by RBCs in the blood? - (a) 100 per cent - (b) 97 per cent - (c) 80 per cent - (d) 70 per cent **Answer: (b)** — About 97 per cent of O2 is transported by RBCs; the remaining 3 per cent is dissolved in plasma. **Q13.** The majority of carbon dioxide in blood is transported as: - (a) Bicarbonate - (b) Carbamino-haemoglobin - (c) Carbonic anhydrase - (d) Dissolved CO2 in plasma **Answer: (a)** — About 70 per cent of CO2 is carried as bicarbonate; 20-25 per cent as carbamino-haemoglobin and 7 per cent dissolved in plasma. **Q14.** Each haemoglobin molecule can carry a maximum of how many oxygen molecules? - (a) Eight - (b) Four - (c) One - (d) Two **Answer: (b)** — Each haemoglobin molecule binds a maximum of four O2 molecules, reversibly, forming oxyhaemoglobin. **Q15.** Conditions in the tissues that favour dissociation of oxygen from oxyhaemoglobin are: - (a) High pO2, low pCO2, low H+ - (b) Low pCO2 and low temperature - (c) Low pO2, high pCO2, high H+, higher temperature - (d) High pO2 and low temperature **Answer: (c)** — In tissues, low pO2, high pCO2, high H+ concentration and higher temperature all favour dissociation of oxygen. **Q16.** The respiratory rhythm centre is located in the: - (a) Cerebellum - (b) Hypothalamus - (c) Medulla - (d) Pons **Answer: (c)** — The respiratory rhythm centre is in the medulla; the pneumotaxic centre in the pons moderates it. **Q17.** Emphysema, in which alveolar walls are damaged, is majorly caused by: - (a) Bacterial infection - (b) Cigarette smoking - (c) Dust from stone-breaking - (d) Allergic inflammation **Answer: (b)** — Emphysema is a chronic disorder decreasing respiratory surface; one of its major causes is cigarette smoking. ### NEET Previous Year Questions (PYQs) *Real NEET previous-year questions on this chapter, with explanations in our own words.* **Q18.** Which of the following is the correct formula for Total Lung Capacity? (NEET PYQ) - (a) Vital capacity + Residual volume - (b) TV + ERV - (c) ERV + RV - (d) TV + IRV **Answer: (a)** — TLC = RV + ERV + TV + IRV, which equals Vital Capacity plus Residual Volume. **Q19.** The pneumotaxic centre that moderates the respiratory rhythm centre is located in the: (NEET PYQ) - (a) Medulla - (b) Cerebrum - (c) Pons - (d) Spinal cord **Answer: (c)** — The pneumotaxic centre in the pons region can reduce the duration of inspiration, altering respiratory rate. **Q20.** The enzyme present in high concentration in RBCs that facilitates CO2 transport is: (NEET PYQ) - (a) Amylase - (b) Catalase - (c) Carbonic anhydrase - (d) Carboxypeptidase **Answer: (c)** — Carbonic anhydrase catalyses CO2 + H2O to H2CO3 to HCO3- + H+ in both directions. **Q21.** Every 100 mL of oxygenated blood delivers approximately how much O2 to the tissues? (NEET PYQ) - (a) 20 mL - (b) 1 mL - (c) 50 mL - (d) 5 mL **Answer: (d)** — Under normal physiological conditions, every 100 mL of oxygenated blood delivers around 5 mL of O2 to the tissues. **Q22.** Which factor plays an insignificant role in the regulation of respiratory rhythm? (NEET PYQ) - (a) Hydrogen ion concentration - (b) Signals from the aortic arch - (c) Carbon dioxide concentration - (d) Oxygen **Answer: (d)** — NCERT explicitly states that the role of oxygen in the regulation of respiratory rhythm is quite insignificant. **Q23.** Residual volume in a healthy human is approximately: (NEET PYQ) - (a) 4500 mL - (b) 2500-3000 mL - (c) 500 mL - (d) 1100-1200 mL **Answer: (d)** — Residual volume — the air remaining even after a forcible expiration — averages 1100 to 1200 mL. **Q24.** Asthma is characterised by difficulty in breathing due to: (NEET PYQ) - (a) Damaged alveolar walls - (b) Loss of haemoglobin - (c) Fibrosis of lung tissue - (d) Inflammation of bronchi and bronchioles **Answer: (d)** — Asthma causes wheezing due to inflammation of the bronchi and bronchioles; damaged alveolar walls describe emphysema. **Q25.** Insects transport atmospheric air within the body using: (NEET PYQ) - (a) Gills - (b) A network of tracheal tubes - (c) Moist cuticle - (d) Lungs **Answer: (b)** — Insects have a network of tracheal tubes; earthworms use moist cuticle and aquatic arthropods use gills. ### Active Recall Prompt Write everything you can recall about **Breathing and Exchange of Gases**, naming each part first: **respiratory organs** across animal groups; the **human respiratory system** (the full path of air, larynx, epiglottis, pleura, conducting vs exchange part, the thoracic chamber); the **mechanism of breathing** (pressure gradients, which muscles do what); **respiratory volumes and capacities** with their values and formulae; **exchange of gases** (partial pressures, the diffusion membrane); **transport of oxygen and carbon dioxide** (percentages, oxyhaemoglobin, the dissociation curve, carbonic anhydrase); **regulation of respiration**; and the **disorders**. Begin each fact with its topic and end it with a full stop.