Breathing and Exchange of Gases

In this chapter: respiratory organs in animals; the human respiratory system; mechanism of breathing; respiratory volumes and capacities; exchange of gases; transport of O₂ and CO₂; regulation; disorders.

Respiratory organs in animals

The human respiratory system

External nostrils → nasal chamber → pharynx (common passage for food and air) → larynx (cartilaginous box that produces sound: the voice box) → trachea → primary bronchi → secondary and tertiary bronchi → bronchioles → terminal bronchioles → alveoli.

• During swallowing, the glottis is covered by a thin, elastic cartilaginous flap, the epiglottis, to stop food entering the larynx.
• The trachea runs to the mid-thoracic cavity and divides at the level of the 5th thoracic vertebra into right and left primary bronchi.
• The trachea, primary, secondary and tertiary bronchi and the initial bronchioles are supported by incomplete cartilaginous rings.
• Each terminal bronchiole ends in many thin, irregular-walled, vascularised bags: the alveoli. The branching network of bronchi, bronchioles and alveoli forms the lungs.
• Lungs are covered by a double-layered pleura with pleural fluid between the layers, which reduces friction on the lung surface.
• Conducting part (nostrils to terminal bronchioles): carries air, clears it of foreign particles, humidifies it and brings it to body temperature. Respiratory (exchange) part: alveoli and their ducts, where gases diffuse.
• The lungs lie in an air-tight thoracic chamber: vertebral column behind, sternum in front, ribs on the sides and the dome-shaped diaphragm below.

Mechanism of breathing

Additional abdominal muscles can increase the strength of both. A healthy person breathes 12 to 16 times a minute. Volumes are measured with a spirometer.

Respiratory volumes and capacities

Common trap: residual volume cannot be measured by a simple spirometer, and it is not part of vital capacity. FRC includes RV; IC does not.

Exchange of gases

Gases are exchanged by simple diffusion, in the alveoli and in the tissues, driven by partial pressure gradients (pO₂, pCO₂). The solubility of CO₂ is 20 to 25 times higher than that of O₂, so much more CO₂ can diffuse per unit difference in partial pressure.

The diffusion membrane has three layers: the thin squamous epithelium of the alveoli, the endothelium of the alveolar capillaries, and the basement substance between them. Its total thickness is much less than a millimetre.

Transport of gases

Oxygen

• About 97% is carried by RBCs as oxyhaemoglobin; about 3% is dissolved in plasma.
• Haemoglobin is a red, iron-containing pigment; each molecule can carry at most four O₂ molecules.
• Binding depends mainly on pO₂, and also on pCO₂, H⁺ concentration and temperature. Plotting % saturation against pO₂ gives the sigmoid oxygen dissociation curve.
• In the alveoli (high pO₂, low pCO₂, fewer H⁺, lower temperature), oxyhaemoglobin forms. In the tissues (low pO₂, high pCO₂, more H⁺, higher temperature), it dissociates and releases O₂.
• Every 100 mL of oxygenated blood delivers about 5 mL of O₂ to the tissues under normal conditions.

Carbon dioxide

• About 20 to 25% is carried by haemoglobin as carbamino-haemoglobin; about 70% as bicarbonate; about 7% dissolved in plasma.
• High pCO₂ and low pO₂ in tissues favour CO₂ binding; the reverse in the alveoli releases it.
• RBCs are rich in carbonic anhydrase (small amounts also in plasma), which speeds up: CO₂ + H₂O ⇌ H₂CO₃ ⇌ HCO₃⁻ + H⁺. The reaction runs forwards in tissues and backwards in the alveoli.
• Every 100 mL of deoxygenated blood delivers about 4 mL of CO₂ to the alveoli.

Regulation of respiration

• The respiratory rhythm centre is in the medulla.
• The pneumotaxic centre in the pons can moderate the rhythm centre, reducing the duration of inspiration and altering the respiratory rate.
• A chemosensitive area next to the rhythm centre is highly sensitive to CO₂ and H⁺; an increase activates the centre to remove them. Receptors in the aortic arch and carotid artery also detect changes in CO₂ and H⁺.
• The role of oxygen in regulating the respiratory rhythm is quite insignificant.

Disorders

• Asthma: difficulty in breathing and wheezing due to inflammation of bronchi and bronchioles.
• Emphysema: a chronic disorder in which alveolar walls are damaged, reducing the respiratory surface. Cigarette smoking is a major cause.
• Occupational respiratory disorders: in industries involving grinding or stone-breaking, long exposure to dust can cause inflammation leading to fibrosis (proliferation of fibrous tissue) and serious lung damage. Workers should wear protective masks.

NEET focus

• Respiratory volumes and capacities, including which sums make which capacity.
• The partial pressure table; solubility of CO₂ vs O₂.
• Percentages of O₂ and CO₂ transport forms; 5 mL and 4 mL per 100 mL.
• Factors favouring oxyhaemoglobin formation and dissociation.
• Medulla vs pons; chemoreceptors; disorders.

Practice questions