Biology: Class 11 — Chapter 14: Breathing and Exchange of Gases

Alfonso Corti (1822–1888) was an Italian anatomist who described the organ of Corti located on the basilar membrane of the cochlea, containing hair cells that convert sound vibrations into nerve impulses.

Mechanisms of breathing vary depending on habitats and levels of organisation:

• Simple Diffusion: Lower invertebrates (sponges, coelenterates, flatworms) over their entire body surface.
• Cutaneous Respiration: Earthworms (moist cuticle), amphibians (frogs through moist skin).
• Tracheal System: Insects (network of tracheal tubes).
• Branchial Respiration: Gills in aquatic arthropods, molluscs, and fishes.
• Pulmonary Respiration: Lungs in terrestrial forms (reptiles, birds, mammals, amphibians).

The human respiratory tract consists of:

1. External Nostrils: Open above the upper lips.
2. Nasal Chamber: Reached via the nasal passage.
3. Pharynx: Common passage for food and air.
4. Larynx: Cartilaginous sound box. During swallowing, the glottis is covered by the epiglottis (elastic cartilaginous flap) to prevent food entry.
5. Trachea: Straight tube extending to mid-thoracic cavity, dividing at the 5th thoracic vertebra into right and left primary bronchi.
6. Bronchioles & Alveoli: Bronchi branch into secondary, tertiary bronchi, and bronchioles, ending in thin, vascularised, bag-like alveoli.

Note: Trachea, primary/secondary/tertiary bronchi, and initial bronchioles are supported by incomplete cartilaginous rings.

The lungs are situated in the anatomically air-tight thoracic chamber:

• Boundaries: Dorsally by the vertebral column, ventrally by the sternum, laterally by the ribs, and lower side by the dome-shaped diaphragm.
• Pleura: Double-layered pleural membrane with pleural fluid reduces friction on the lung surface.
• Conducting Part: External nostrils to terminal bronchioles (transports, clears, humidifies, and warms air).
• Exchange Part: Alveoli and their ducts (site of actual diffusion of $O_2$ and $CO_2$).

1. Breathing or pulmonary ventilation.
2. Diffusion of gases ($O_2$ and $CO_2$) across the alveolar membrane.
3. Transport of gases by blood.
4. Diffusion of gases between blood and tissues.
5. Utilisation of $O_2$ by cells for catabolic reactions (cellular respiration).

Movement of air is driven by pressure gradients between the lungs and atmosphere:

• Inspiration: Active process. Contraction of the diaphragm (increases antero-posterior volume) and external intercostal muscles (lifts ribs and sternum, increasing dorso-ventral volume). This decreases intra-pulmonary pressure below atmospheric pressure, drawing air in.
• Expiration: Passive process. Relaxation of diaphragm and intercostal muscles reduces thoracic volume, increasing intra-pulmonary pressure above atmospheric pressure, expelling air.
• Breathing Rate: Healthy human breathes 12–16 times/minute. Volume is measured clinically using a spirometer.

• Tidal Volume (TV): Volume inspired/expired during normal respiration (~500 mL; 6000–8000 mL/min).
• Inspiratory Reserve Volume (IRV): Additional volume inspired forcibly (2500–3000 mL).
• Expiratory Reserve Volume (ERV): Additional volume expired forcibly (1000–1100 mL).
• Residual Volume (RV): Volume remaining in lungs after forcible expiration (1100–1200 mL).
• Inspiratory Capacity (IC): $IC = TV + IRV$
• Expiratory Capacity (EC): $EC = TV + ERV$
• Functional Residual Capacity (FRC): $FRC = ERV + RV$
• Vital Capacity (VC): Maximum air a person can breathe in after forced expiration ($VC = ERV + TV + IRV$).
• Total Lung Capacity (TLC): Total air accommodated at the end of forced inspiration ($TLC = RV + ERV + TV + IRV$).

Gas exchange occurs at alveoli and tissues by simple diffusion governed by:

• Pressure/Concentration Gradient: Measured as partial pressure ($pO_2$ and $pCO_2$).
• Solubility of Gases: $CO_2$ solubility is 20–25 times higher than $O_2$.
• Thickness of Diffusion Membrane: Three-layered membrane (alveolar squamous epithelium, capillary endothelium, and basement substance) is less than 1 mm thick.

• Alveoli: $pO_2 = 104$, $pCO_2 = 40$
• Deoxygenated Blood: $pO_2 = 40$, $pCO_2 = 45$
• Oxygenated Blood: $pO_2 = 95$, $pCO_2 = 40$
• Tissues: $pO_2 = 40$, $pCO_2 = 45$

97% of $O_2$ is transported by RBCs as oxyhaemoglobin; 3% is dissolved in plasma.

• Oxygen Dissociation Curve: A sigmoid curve plotting percentage saturation of haemoglobin against $pO_2$.
• Association Factors (Alveoli): High $pO_2$, low $pCO_2$, low $H^+$ concentration, and lower temperature.
• Dissociation Factors (Tissues): Low $pO_2$, high $pCO_2$, high $H^+$ concentration, and higher temperature.
• Delivery: Every 100 mL of oxygenated blood delivers ~5 mL of $O_2$ to tissues under normal conditions.

Transported in three forms: 70% as bicarbonate, 20–25% as carbamino-haemoglobin, and 7% dissolved in plasma.

• Bicarbonate Formation: Facilitated by the enzyme carbonic anhydrase in RBCs and plasma:

• Delivery: Every 100 mL of deoxygenated blood delivers ~4 mL of $CO_2$ to the alveoli.

• Respiratory Rhythm Centre: Located in the medulla; primarily responsible for regulation.
• Pneumotaxic Centre: Located in the pons; moderates rhythm centre activity and limits inspiration duration.
• Chemosensitive Area: Located adjacent to the rhythm centre; highly sensitive to $CO_2$ and hydrogen ions ($H^+$).
• Receptors: Aortic arch and carotid artery receptors detect changes in $CO_2$ and $H^+$ to signal the rhythm centre.
• Note: The role of oxygen in respiratory rhythm regulation is quite insignificant.

• Asthma: Difficulty in breathing causing wheezing due to inflammation of bronchi and bronchioles.
• Emphysema: Chronic disorder where alveolar walls are damaged, decreasing respiratory surface; majorly caused by cigarette smoking.
• Occupational Respiratory Disorders: Fibrosis (proliferation of fibrous tissues) and lung damage caused by long exposure to dust in grinding or stone-breaking industries.