Breathing and Exchange of GasesNEET Zoology · Class 11 · NCERT Chapter 14

Introduction

Every cell in your body burns nutrients to produce ATP. This process, called cellular respiration, uses oxygen (O2) and produces carbon dioxide (CO2) continuously. Breathing, or external respiration, is the physical process that delivers O2 from the atmosphere to your blood and removes CO2 from the blood back into the air. Without breathing, cellular respiration would starve of O2 within minutes.

Expect 1 NEET question from this chapter every year. The most reliable areas to score are: respiratory volumes and capacities with their values, partial pressures of O2 and CO2, the sigmoid dissociation curve with the Bohr effect, and how CO2 is mainly carried as bicarbonate.

Respiratory Organs in Animals

Different animals use different structures for gas exchange, depending on where they live and how large they are.

• Body surface (cutaneous respiration): in very small or thin animals (like earthworms and planarians), O2 and CO2 simply diffuse through the moist skin. No special organ is needed.
• Tracheae (tracheal system): insects and many other terrestrial arthropods have a network of air-filled tubes. O2 travels directly to cells through tiny tracheoles. This is very efficient for small, active animals.
• Gills: aquatic animals like most fish and aquatic arthropods use gills. Blood flows through thin gill lamellae in close contact with water. O2 dissolved in water diffuses in and CO2 diffuses out.
• Lungs: terrestrial vertebrates use lungs, which are internal sac-like organs with a large moist surface area. Lungs keep the gas-exchange surface protected from drying out.

Human Respiratory System

The human respiratory system has two main parts: the conducting zone (from the nostrils to the terminal bronchioles) and the respiratory zone (alveolar ducts and alveoli where actual gas exchange happens).

The Conducting Pathway

1. External nostrils: air enters here. The nostrils have hairs (vibrissae) to filter large particles.
2. Nasal chamber (nasal cavity): lined with ciliated, mucus-secreting epithelium. Warms, humidifies and filters the air. The nasal chamber opens into the pharynx.
3. Pharynx: a common pathway for food and air. It leads to the larynx (for air) and the oesophagus (for food).
4. Larynx (voice box): a cartilaginous box at the top of the trachea. Contains the glottis (the opening between the vocal cords) and the epiglottis (a flap that closes over the glottis during swallowing to stop food entering the airway). The vocal cords in the larynx produce sound.
5. Trachea (windpipe): about 11 to 12 cm long. Kept open by C-shaped cartilaginous rings. Lined with ciliated mucus epithelium that sweeps debris upward. The trachea divides at the carina into the two primary bronchi.
6. Primary bronchi: one to each lung. The right primary bronchus is wider, shorter and more vertical. It divides into secondary bronchi (one per lobe: 3 on the right, 2 on the left). Secondary bronchi divide into tertiary (segmental) bronchi serving each bronchopulmonary segment. Bronchi have cartilaginous support.
7. Bronchioles: further branches without cartilage. Walls contain smooth muscle. They end in alveolar ducts and then alveoli.
8. Alveoli: tiny, thin-walled air sacs (about 300 million in two lungs). Their total surface area is about 70 m2 in adults. The wall is simple squamous epithelium (type I pneumocytes) for fast diffusion. Type II pneumocytes secrete surfactant to reduce surface tension and prevent collapse.

The Lungs and Pleura

The right lung has three lobes (superior, middle, inferior); the left lung has two lobes (superior and inferior, with a cardiac notch to accommodate the heart). Each lung is enclosed in two pleural membranes: the visceral pleura (on the lung surface) and the parietal pleura (on the chest wall). The narrow space between them is the pleural cavity, filled with a small amount of pleural fluid. This fluid lubricates the lungs and creates surface tension that keeps the lungs pressed against the chest wall.

The diaphragm is a dome-shaped muscular sheet separating the thoracic cavity from the abdominal cavity. It is the main muscle of breathing.

Mechanism of Breathing

Breathing works by changing the volume of the thoracic cavity. Boyle Law states that pressure and volume are inversely related at constant temperature: when volume increases, pressure falls.

• Inspiration (inhalation): the diaphragm contracts and flattens downward. The external intercostal muscles contract, pulling the ribs upward and outward. Both increase the volume of the thoracic cavity. Lung volume increases; intra-pulmonary pressure falls to about 1 to 3 mm Hg below atmospheric. Air flows in.
• Expiration (exhalation): in quiet breathing, expiration is largely passive. The diaphragm relaxes and domes upward. The ribs fall back. Thoracic volume decreases; intra-pulmonary pressure rises about 1 to 3 mm Hg above atmospheric. Air flows out. In forced expiration, the internal intercostal muscles and abdominal muscles actively contract to push more air out.

A healthy adult breathes about 12 to 20 times per minute at rest, moving about 500 mL (tidal volume) each breath. This gives a minute ventilation of about 6 to 10 litres per minute.

Respiratory Volumes and Capacities

A spirometer measures respiratory volumes. Here are the key values you need to know for NEET:

• Tidal Volume (TV): about 500 mL. Air moved in or out in a normal breath at rest.
• Inspiratory Reserve Volume (IRV): about 2500 to 3000 mL. Extra air that can be inspired forcefully after a normal inspiration.
• Expiratory Reserve Volume (ERV): about 1000 to 1100 mL. Extra air that can be expired forcefully after a normal expiration.
• Residual Volume (RV): about 1100 to 1200 mL. Air that always remains in the lungs after maximum expiration. Keeps alveoli from collapsing.

Capacities are sums of two or more volumes:

• Inspiratory Capacity (IC): TV + IRV = about 3000 to 3500 mL. Total air that can be inspired after a normal expiration.
• Expiratory Capacity (EC): TV + ERV = about 1500 to 1600 mL. Total air that can be expired after a normal inspiration.
• Functional Residual Capacity (FRC): ERV + RV = about 2100 to 2300 mL. Air remaining in the lungs after a normal (tidal) expiration.
• Vital Capacity (VC): TV + IRV + ERV = about 3500 to 4500 mL. The maximum amount of air you can breathe in and out in one breath. Does NOT include RV. VC is reduced in lung diseases.
• Total Lung Capacity (TLC): VC + RV = about 6000 mL. The total volume the lungs can hold at maximum inspiration.

Exchange of Gases

Gas exchange happens by diffusion: molecules move from an area of high partial pressure to an area of low partial pressure. You do not need to know the actual temperature and pressure calculations, but you do need to know the partial pressure values at each site.

• pO2 in alveolar air: about 104 mm Hg. pO2 in deoxygenated blood arriving at alveoli: about 40 mm Hg. So O2 diffuses from alveoli into blood.
• pCO2 in alveolar air: about 40 mm Hg. pCO2 in deoxygenated blood: about 45 mm Hg. So CO2 diffuses from blood into alveoli.
• At the tissues: pO2 in cells is about 20 to 40 mm Hg (because cells use O2). Oxygenated blood arrives with pO2 about 95 mm Hg. O2 diffuses into cells. pCO2 in cells is about 45 to 50 mm Hg. Blood pCO2 is about 40 mm Hg. CO2 diffuses from cells into blood.

The diffusion membrane at the alveoli has three layers: the alveolar epithelium (squamous), a basement membrane, and the endothelium of the pulmonary capillary. The total thickness is less than 1 micrometre, which allows very rapid diffusion. CO2 diffuses about 20 times faster than O2 through biological membranes because it is far more soluble.

Transport of Gases

Oxygen Transport

About 97% of O2 is transported bound to haemoglobin as oxyhaemoglobin (HbO2). Only about 3% is dissolved in plasma. Each haemoglobin molecule has 4 haem groups; each haem can carry one O2. When all four are occupied, haemoglobin is said to be fully saturated.

The relationship between pO2 and haemoglobin saturation is shown by the oxygen-haemoglobin dissociation curve, which is S-shaped (sigmoid). The S-shape results from cooperative binding: binding of the first O2 makes it easier for the next O2 to bind (and loss of one O2 makes it easier to lose the next). This is a key NEET concept.

The Bohr effect is the shift of this curve caused by CO2, H+ and temperature:

• Right shift (lower affinity, more O2 released): caused by high pCO2, low pH (more H+), or high temperature. This is what happens in active tissues.
• Left shift (higher affinity, more O2 picked up): caused by low pCO2, high pH, or low temperature. This is what happens in the lungs.

Carbon Dioxide Transport

CO2 is transported in three forms:

• Dissolved in plasma: about 7%. CO2 is more soluble than O2 but still most cannot be carried this way alone.
• As carbamino-haemoglobin: about 23%. CO2 binds to the amino groups (not the haem iron) of haemoglobin. This is called carbamino-haemoglobin (HbCO2). It forms in active tissues and releases CO2 in the lungs.
• As bicarbonate ions (HCO3-) in plasma: about 70%. In red blood cells, CO2 reacts with water to form carbonic acid (H2CO3), catalysed by carbonic anhydrase. H2CO3 dissociates to H+ and HCO3-. The HCO3- moves into the plasma in exchange for Cl- (the chloride shift or Hamburger phenomenon). The H+ is buffered by haemoglobin. In the lungs, this entire process runs in reverse: HCO3- re-enters the red blood cell, combines with H+ to form H2CO3, and carbonic anhydrase converts it back to CO2 + H2O. CO2 diffuses into the alveoli.

Regulation of Respiration

Breathing is controlled by the nervous system to match ventilation to metabolic demand.

• Respiratory rhythm centre (medulla oblongata): generates the basic rhythm of breathing. Even without any input, this centre fires regularly to produce inspiration followed by passive expiration.
• Pneumotaxic centre (pons): moderates the rhythm centre. It sends inhibitory signals to switch off inspiration before the lungs over-inflate. This smooths the respiratory cycle.
• Chemical regulation: the most important stimulus is CO2. A small rise in blood pCO2 (or fall in pH) stimulates the medullary centre to increase the rate and depth of breathing. This rapidly blows off excess CO2. Low O2 (hypoxia) also stimulates breathing via peripheral chemoreceptors in the carotid and aortic bodies, but CO2 is the dominant driver. Interestingly, very high O2 does not slow breathing significantly.

Disorders of the Respiratory System

• Asthma: a condition of inflamed, narrowed airways (mainly bronchioles). Triggered by allergens, cold air, or exercise. Airways become hypersensitive; smooth muscle contracts; mucus production increases. Results in wheezing, chest tightness and shortness of breath. Managed with bronchodilators and anti-inflammatory inhalers.
• Emphysema: progressive destruction of alveolar walls, usually caused by long-term smoking. Alveoli merge into larger, abnormal air spaces. The surface area for gas exchange is greatly reduced. Patients have chronic breathlessness and a barrel chest. The damage is irreversible.
• Occupational respiratory disorders: caused by long-term inhalation of industrial dust particles. Examples: silicosis (silica dust), asbestosis (asbestos fibres), and coal workers pneumoconiosis (black lung, from coal dust). Fibrosis of lung tissue reduces elasticity and gas exchange. Symptoms take years to develop.

Worked NEET Problems

Summary Cheat Sheet

• Breathing vs cellular respiration: breathing is physical (moving air); cellular respiration is chemical (breaking down glucose for ATP).
• Respiratory organs: body surface (earthworm), tracheae (insects), gills (fish, crustaceans), lungs (terrestrial vertebrates).
• Conducting path: nostrils, nasal chamber, pharynx, larynx (glottis + epiglottis), trachea, primary bronchi, secondary bronchi, tertiary bronchi, bronchioles, alveolar ducts, alveoli.
• Epiglottis closes over the glottis during swallowing to prevent food entering the airway.
• Alveoli: about 300 million; surface area about 70 m2; type I pneumocytes for diffusion; type II pneumocytes secrete surfactant.
• Right lung: 3 lobes. Left lung: 2 lobes (cardiac notch).
• Inspiration: diaphragm contracts (flattens), external intercostals contract (ribs up and out), thoracic volume increases, pressure falls, air enters.
• Expiration at rest: passive; diaphragm relaxes (domes up), ribs fall, volume decreases, pressure rises, air exits.
• Tidal volume (TV): 500 mL. IRV: 2500 to 3000 mL. ERV: 1000 to 1100 mL. RV: 1100 to 1200 mL.
• Vital capacity: TV + IRV + ERV (about 3500 to 4500 mL). Total lung capacity: VC + RV (about 6000 mL).
• pO2 alveoli: 104 mm Hg. pO2 deoxygenated blood: 40 mm Hg. O2 diffuses into blood.
• pCO2 deoxygenated blood: 45 mm Hg. pCO2 alveoli: 40 mm Hg. CO2 diffuses out of blood.
• CO2 diffuses 20 times faster than O2 through the diffusion membrane.
• O2 transport: 97% as oxyhaemoglobin (Hb + O2 at haem iron); 3% dissolved.
• Dissociation curve: S-shaped (sigmoid) due to cooperative binding.
• Bohr effect: high pCO2, low pH, high temperature = right shift = more O2 released to tissues.
• CO2 transport: 7% dissolved; 23% as carbamino-haemoglobin (at amino groups); 70% as HCO3- (bicarbonate) in plasma via carbonic anhydrase + chloride shift.
• Carbonic anhydrase: enzyme in RBCs that converts CO2 + H2O to H2CO3 (and back in lungs).
• Chloride shift (Hamburger phenomenon): HCO3- leaves RBC into plasma; Cl- enters RBC to balance charge.
• Respiratory rhythm centre: medulla oblongata (generates rhythm). Pneumotaxic centre: pons (moderates rhythm, stops over-inflation).
• Main chemical stimulus for breathing: rise in blood pCO2 (fall in pH). Not O2.
• Asthma: inflamed, narrowed bronchioles. Emphysema: destroyed alveolar walls, reduced surface area. Occupational disorders: dust particles cause fibrosis.

Next: use the interactive learning widgets to explore the respiratory system diagram, compare inspiration and expiration, and try the Bohr effect on the oxygen-haemoglobin dissociation curve, or work through the 14+ NEET PYQs with full solutions. To time yourself, take the free 10-question mock test. You can also explore related chapters: Body Fluids and Circulation for how blood transports gases, Excretory Products and Their Elimination for another major homeostasis chapter, or Neural Control and Coordination for how the brain controls breathing.