Breathing and Exchange of Gases Class 11 Notes

Human Respiratory System — Structure

The human respiratory system consists of a pair of lungs and a system of air passages that conduct air from the external environment to the alveoli where gas exchange occurs.

The Air Passage (Conducting Zone)

• External nostrils → nasal cavity (warms, moistens and filters air) → pharynx (common passage for air and food) → larynx (voice box; contains vocal cords) → trachea (C-shaped cartilaginous rings prevent collapse).
• Trachea divides into two primary bronchi (one per lung) → secondary (lobar) bronchi → tertiary (segmental) bronchi → bronchioles → terminal bronchioles → respiratory bronchioles → alveolar ducts → alveolar sacs → alveoli.
• Bronchial tree branching: about 23 generations of branching from trachea to alveoli; the last few generations (respiratory zone) are the site of gas exchange.

Alveoli — The Functional Units

• About 300 million alveoli in two lungs provide a total surface area of about 70–100 m² for gas exchange.
• Type I pneumocytes (simple squamous epithelium) form the thin gas-exchange surface; Type II pneumocytes secrete pulmonary surfactant (dipalmitoylphosphatidylcholine) that reduces surface tension and prevents alveolar collapse.
• Each alveolus is surrounded by a dense network of pulmonary capillaries. The air–blood barrier is only about 0.2 µm thick.

Mechanism of Breathing — Inspiration and Expiration

Breathing is a mechanical ventilation process driven by pressure changes between the atmosphere and the intrapulmonary (intra-alveolar) space. Air flows from higher to lower pressure.

Inspiration (Active Process)

• Diaphragm contracts and flattens (increases vertical thoracic diameter).
• External intercostal muscles contract, pulling the ribs upward and outward (increases anteroposterior and lateral diameters).
• Thoracic volume increases → intrapleural pressure drops → lungs expand → intrapulmonary pressure falls below atmospheric pressure → air rushes in.

Expiration (Normally Passive)

• Diaphragm relaxes and returns to dome shape; external intercostals relax; ribs move downward and inward due to elastic recoil of the lungs and thoracic wall.
• Thoracic volume decreases → intrapulmonary pressure rises above atmospheric → air is pushed out.
• Forced expiration additionally uses internal intercostals and abdominal muscles (active process).

Respiratory Volumes and Capacities

Spirometry measures the volumes of air moved during breathing. These volumes and their combinations (capacities) are clinically important for diagnosing respiratory disease.

Volumes

• Tidal volume (TV) — volume of air inspired or expired in a normal breath ≈ 500 mL.
• Inspiratory reserve volume (IRV) — additional volume that can be forcefully inspired after a normal inspiration ≈ 2500–3000 mL.
• Expiratory reserve volume (ERV) — additional volume that can be forcefully expired after a normal expiration ≈ 1000–1100 mL.
• Residual volume (RV) — volume of air remaining in the lungs even after the most forceful expiration ≈ 1100–1200 mL.

Capacities

• Inspiratory capacity (IC) = TV + IRV ≈ 3000–3500 mL.
• Expiratory capacity (EC) = TV + ERV ≈ 1500–1600 mL.
• Functional residual capacity (FRC) = ERV + RV ≈ 2100–2300 mL.
• Vital capacity (VC) = TV + IRV + ERV ≈ 4600 mL (maximum volume that can be breathed out after a maximum inspiration).
• Total lung capacity (TLC) = TV + IRV + ERV + RV ≈ 5800 mL (total volume the lungs can hold).

Exchange of Gases — Partial Pressures

Gas exchange across the respiratory membrane and at tissue level occurs by simple diffusion along partial pressure gradients. Henry's law governs the amount of gas dissolved in a liquid.

• At the alveoli: PO₂ (alveolar air) = 104 mm Hg > PO₂ (pulmonary capillary blood) = 40 mm Hg → O₂ diffuses into blood. PCO₂ (blood) = 45 mm Hg > PCO₂ (alveolar air) = 40 mm Hg → CO₂ diffuses into alveoli.
• At the tissues: PO₂ (systemic capillary blood) = 95 mm Hg > PO₂ (tissue cells) = 40 mm Hg → O₂ delivered to cells. PCO₂ (tissue cells) = 45 mm Hg > PCO₂ (blood) = 40 mm Hg → CO₂ enters blood.
• CO₂ is about 20–25 times more soluble than O₂ in the respiratory membrane, so CO₂ diffuses faster despite a smaller pressure gradient.

Transport of Oxygen and Carbon Dioxide

Oxygen Transport

• ~97% of O₂ is transported bound to haemoglobin (Hb) as oxyhaemoglobin (HbO₂). Each Hb molecule has four haem groups, each binding one O₂ molecule — so one Hb carries up to 4 O₂.
• ~3% is dissolved in plasma (Henry's law).
• At the tissues, the lower PO₂, higher PCO₂, lower pH and higher temperature (Bohr effect) reduce Hb's affinity for O₂, promoting O₂ unloading.

Carbon Dioxide Transport

• ~70% as bicarbonate ions (HCO₃⁻) in plasma — CO₂ enters RBCs, carbonic anhydrase catalyses CO₂ + H₂O → H₂CO₃ → H⁺ + HCO₃⁻. HCO₃⁻ is exchanged for Cl⁻ (chloride shift / Hamburger phenomenon).
• ~20–25% as carbaminohaemoglobin (CO₂ bound to the amino groups of Hb, not to the haem group) — forms at tissues, released at lungs.
• ~7% dissolved in plasma.

Regulation of Breathing and Disorders

• Pneumotaxic centre in the pons — modulates the output of the medullary centre; reduces the duration of inspiration and thereby increases the breathing rate.
• Carbon dioxide-sensitive receptors (central and peripheral chemoreceptors) — rising blood CO₂ (and therefore H⁺) stimulates breathing to blow off CO₂. This is the primary drive for breathing under normal conditions.
• Low blood O₂ stimulates peripheral chemoreceptors in the carotid and aortic bodies only when PO₂ falls below about 60 mm Hg.

Common Respiratory Disorders

• Asthma — reversible bronchospasm with wheezing, caused by allergens, cold air or stress. Treatment: bronchodilators (salbutamol).
• Emphysema — chronic destruction of alveolar walls, reducing surface area and elastic recoil. Strongly associated with cigarette smoking. Irreversible.
• Occupational respiratory disorders — prolonged inhalation of industrial dusts causes fibrosis: byssinosis (cotton dust), silicosis (silica), asbestosis (asbestos).

Solved Examples

Example: A spirometry test shows: TV = 500 mL, IRV = 3000 mL, ERV = 1100 mL, RV = 1200 mL. Calculate the vital capacity and total lung capacity.

Solution: Vital capacity (VC) = TV + IRV + ERV = 500 + 3000 + 1100 = 4600 mL. Total lung capacity (TLC) = VC + RV = 4600 + 1200 = 5800 mL. These are normal values for a healthy adult male.

Example: Why does a person feel breathless at high altitude (e.g. 4500 m) even though they are breathing normally?

Solution: At high altitude, atmospheric pressure is lower, so the partial pressure of O₂ is reduced. The PO₂ in inspired air falls to about 80 mm Hg (from 159 mm Hg at sea level), which reduces alveolar PO₂ and therefore the O₂ saturation of haemoglobin. Peripheral chemoreceptors detect the low PO₂ and increase the breathing rate, but the reduced ambient PO₂ limits the amount of O₂ that can be loaded — causing breathlessness and fatigue (mountain sickness).