Board exam focus — Breathing and Exchange of Gases (CBSE & HBSE)

Breathing and Exchange of Gases describes how oxygen is supplied to cells and carbon dioxide removed during cellular respiration. The chapter covers respiratory organs across animals and the human respiratory system, the mechanism of breathing with respiratory volumes and capacities, the diffusion-based exchange of gases at alveoli and tissues, the transport of O2 and CO2 (including the oxygen dissociation curve), neural regulation of respiration, and common disorders. Spirometry values and the oxygen dissociation curve are favourite NEET and board topics.

Respiratory Organs and the Human Respiratory System

Why Breathe?

Organisms need O2 for the oxidative breakdown of nutrients (glucose, fatty acids) to derive energy, and they release CO2 that is harmful. The process of exchange of O2 from the atmosphere with CO2 produced by cells is called breathing (respiration).

Respiratory Organs Across Animals

Human Respiratory System

The respiratory passage from outside inward:

1. External nostrils → nasal chamber → nasal pharynx.
2. Pharynx → larynx (sound box) via the glottis; the epiglottis prevents food entry during swallowing.
3. Trachea — a straight tube supported by C-shaped cartilaginous rings (prevents collapse).
4. Trachea divides into right and left primary bronchi, which branch into secondary and tertiary bronchi and finally bronchioles.
5. Terminal bronchioles end in irregular alveoli — the site of gas exchange.

Conducting vs. Respiratory (Exchange) Parts

• Conducting part: nostrils to terminal bronchioles — transports, humidifies, warms and cleans the air; no gas exchange.
• Exchange (respiratory) part: alveoli and their ducts — actual diffusion of O2 and CO2 between blood and atmosphere.

Lungs and Pleura

• We have two lungs covered by a double-layered pleura with pleural fluid between them, reducing friction.
• The outer parietal pleura is in contact with the thoracic lining; the inner visceral pleura is in contact with the lung surface.
• The lungs are situated in the thoracic chamber, anatomically air-tight, formed dorsally by the vertebral column, ventrally by the sternum, laterally by the ribs and on the lower side by the dome-shaped diaphragm.

The arrangement makes the thoracic volume changes directly drive lung volume changes, which is essential for breathing.

Mechanism of Breathing, Volumes and Capacities

Mechanism of Breathing

Breathing involves two stages: inspiration (atmospheric air drawn in) and expiration (alveolar air released). Movement of air is governed by pressure gradients created by changes in thoracic volume.

Inspiration (active process):

• Diaphragm contracts and flattens → increases the volume in the antero-posterior axis.
• External intercostal muscles contract → ribs and sternum lift up → increases volume in the dorso-ventral axis.
• Increased thoracic (and pulmonary) volume → intra-pulmonary pressure falls below atmospheric → air rushes in.

Expiration (normally passive):

• Diaphragm and intercostal muscles relax → thoracic volume decreases.
• Intra-pulmonary pressure rises above atmospheric → air is forced out.

A healthy human breathes 12-16 times per minute. Inspiration occurs when intrapulmonary pressure is less than atmospheric; expiration when it is greater.

Respiratory Volumes and Capacities (Spirometry)

Capacities (sums of volumes):

• Inspiratory Capacity (IC) = TV + IRV
• Expiratory Capacity (EC) = TV + ERV
• Functional Residual Capacity (FRC) = ERV + RV
• Vital Capacity (VC) = TV + IRV + ERV (maximum volume of air a person can breathe in after a forced expiration)
• Total Lung Capacity (TLC) = VC + RV (≈ 5800-6000 mL)

NEET trap: Residual volume cannot be measured by a simple spirometer because that air can never be exhaled. Therefore VC and TLC measurement involving RV need special techniques.

Pulmonary Ventilation

Pulmonary ventilation ≈ TV × breaths per minute. With TV = 500 mL and 12 breaths/min, ventilation ≈ 6000 mL/min, of which about 150 mL × 12 = 1800 mL ventilates the dead space (conducting part).

Gas Exchange, Transport and Regulation; Disorders

Exchange of Gases

Gases are exchanged by simple diffusion based on partial pressure gradients. The rate depends on partial pressure differences, solubility of gases and the thickness of the diffusion membrane.

Since pO2 is high in alveoli (104) and low in deoxygenated blood (40), O2 diffuses from alveoli to blood. Since pCO2 is high in tissues (45) and low in oxygenated blood (40), CO2 diffuses from tissues to blood.

CO2 is about 20-25 times more soluble than O2, so the same partial-pressure gradient moves much more CO2 per unit time.

Transport of Oxygen

• About 97% of O2 is transported by RBCs bound to haemoglobin as oxyhaemoglobin; ~3% is dissolved in plasma.
• Each haemoglobin can carry a maximum of four molecules of O2.
• Oxygen dissociation curve (sigmoid): plot of percentage saturation of Hb vs. pO2.
• A right shift (favouring O2 release) occurs with low pO2, high pCO2, high H+ (low pH), higher temperature — the Bohr effect, which favours unloading of O2 at the actively respiring tissues.

Transport of Carbon Dioxide

• ~70% as bicarbonate (HCO3-) in plasma (formed via carbonic anhydrase in RBCs).
• ~20-25% as carbamino-haemoglobin (bound to Hb).
• ~7% dissolved in plasma.

In tissues (high pCO2): CO2 + H2O → H2CO3 → H+ + HCO3- (catalysed by carbonic anhydrase). In alveoli (low pCO2): the reaction reverses, releasing CO2 to be exhaled.

Regulation of Respiration

• The respiratory rhythm centre in the medulla oblongata is primarily responsible.
• A pneumotaxic centre in the pons can moderate the rhythm centre's functions.
• A chemosensitive area adjacent to the rhythm centre is highly sensitive to CO2 and H+; rise in these activates the centre to increase ventilation. Receptors associated with the aortic arch and carotid artery also recognise such changes. Oxygen has little direct role in normal regulation.

Respiratory Disorders

• Asthma: difficulty in breathing causing wheezing due to inflammation of bronchi and bronchioles.
• Emphysema: chronic disorder in which alveolar walls are damaged, decreasing the respiratory surface; major cause is cigarette smoking.
• Occupational respiratory disorders: long exposure to dust (e.g., grinding/stone-breaking industries) causes inflammation and fibrosis (e.g., silicosis, asbestosis); proper masks and dust-proofing are protective.

Frequently asked questions

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