Chapter 5.1: Breathing and Exchange of Gases

1. Respiratory Organs in Animals (Recall)

• Sponges, Coelenterates, Flatworms: Exchange occurs through the entire body surface by simple diffusion.
• Earthworms: Use their moist cuticle (skin) for cutaneous respiration.
• Insects: Have a network of tubes (tracheal system) that transport atmospheric air directly to the cells.
• Aquatic Arthropods and Molluscs: Use special vascularized structures called gills (branchial respiration).
• Terrestrial Vertebrates: Use vascularized bags called lungs (pulmonary respiration).
• Amphibians (e.g., Frogs): Can respire through their moist skin and lungs.

2. Respiratory System in Humans

• The Pathway of Air: Nostrils → Nasal Passage → Nasal Chamber → Pharynx → Larynx (Sound Box) → Trachea → Primary Bronchi → Secondary Bronchi → Tertiary Bronchi → Bronchioles → Terminal Bronchioles → Alveoli.
• Organs Involved:
  • Nose: Air is filtered by hairs, warmed, and moistened.
  • Pharynx: A common passage for food and air.
  • Larynx: A cartilaginous box that helps in sound production. The epiglottis, a cartilaginous flap, prevents the entry of food into the larynx during swallowing.
  • Trachea (Windpipe): A straight tube extending up to the mid-thoracic cavity, supported by C-shaped cartilaginous rings to prevent collapse.
  • Bronchi, Bronchioles, and Alveoli: The trachea divides into right and left primary bronchi. Each bronchus undergoes repeated divisions to form secondary and tertiary bronchi and bronchioles, ending up in very thin terminal bronchioles. The terminal bronchioles give rise to a number of vascularized, balloon-like structures called alveoli.
  • Lungs: A pair of spongy, air-filled organs located on either side of the chest (thorax). They are covered by a double-layered pleura, with pleural fluid between them to reduce friction.
  • Diaphragm: A large, dome-shaped muscle at the base of the lungs.

3. Mechanism of Breathing

Breathing involves two stages: inspiration (inhalation) and expiration (exhalation).

• Inspiration (Inhalation):

  • It is an active process.

  • The diaphragm contracts and flattens.

  • The external intercostal muscles contract, lifting the ribs and sternum upwards and outwards.

  • This increases the volume of the thoracic cavity, which in turn increases the volume of the lungs.

  • The pressure inside the lungs (intra-pulmonary pressure) decreases to less than the atmospheric pressure.

  • Air rushes from outside into the lungs.

  • Expiration (Exhalation):

  Suction, not Pushing Humans use a negative pressure breathing system. We don't "push" air in; we expand our chest cavity to create a vacuum (lower pressure), and the outside air is "sucked" in to fill the space.

  • It is a passive process.
  • The diaphragm relaxes and returns to its dome shape.
  • The external intercostal muscles relax, and the ribs and sternum return to their original position.
  • This decreases the volume of the thoracic cavity and the lungs.
  • The pressure inside the lungs increases to more than the atmospheric pressure.
  • Air is expelled from the lungs.

4. Exchange of Gases

Gas exchange occurs in the alveoli and the tissues. It is a passive process of diffusion based on pressure/concentration gradients.

• Alveolar Gas Exchange:

  • The partial pressure of oxygen (pO2) in the alveoli is high (104 mmHg), while in the deoxygenated blood in the pulmonary capillaries, it is low (40 mmHg).
  • Therefore, oxygen diffuses from the alveoli into the blood.
  • The partial pressure of carbon dioxide (pCO2) in the deoxygenated blood is high (45 mmHg), while in the alveoli, it is low (40 mmHg).
  • Therefore, carbon dioxide diffuses from the blood into the alveoli.

• Tissue Gas Exchange:

  • In the tissues, the pO2 is low (40 mmHg) because cells are continuously using oxygen for metabolism. The pO2 in the oxygenated blood is high (95 mmHg).
  • Therefore, oxygen diffuses from the blood into the tissues.
  • In the tissues, the pCO2 is high (45 mmHg) due to metabolic activity. The pCO2 in the oxygenated blood is low (40 mmHg).
  • Therefore, carbon dioxide diffuses from the tissues into the blood.

5. Transport of Gases

Transport of Oxygen

• Dissolved in Plasma: About 3% of O2 is transported in a dissolved state through blood plasma.

• As Oxyhaemoglobin: About 97% of O2 is transported by red blood cells (RBCs). Oxygen binds with haemoglobin (a red-coloured iron-containing protein) in a reversible manner to form oxyhaemoglobin.

  • Hb + 4O2 ↔ Hb(O2)4

• Oxyhaemoglobin Dissociation Curve:

  • A sigmoid (S-shaped) curve obtained by plotting the percentage saturation of haemoglobin with oxygen against the partial pressure of oxygen.
  • Factors Affecting the Curve:
    • Shift to the Right: Indicates a lower affinity of haemoglobin for oxygen, meaning oxygen is more easily released to the tissues. This occurs with:
      • High pCO2
      • High H+ concentration (low pH) - Bohr Effect
      • High temperature
    • Shift to the Left: Indicates a higher affinity of haemoglobin for oxygen. This occurs with:
      • Low pCO2
      • Low H+ concentration (high pH)
      • Low temperature

Transport of Carbon Dioxide

• Dissolved in Plasma: About 7% of CO2 is transported dissolved in blood plasma.
• As Carbamino-haemoglobin: About 20-25% of CO2 is transported by haemoglobin. CO2 binds to the amino groups of globin chains to form carbamino-haemoglobin.
  • Haldane Effect: While the Bohr effect explains O2 release in tissues, the Haldane effect explains CO2 loading. Deoxygenation of the blood increases its ability to carry CO2. Conversely, oxygenation of blood in the lungs displaces CO2 from haemoglobin.

• As Bicarbonate Ions: About 70% of $\text{CO}_2$ is transported as bicarbonate ions ($\text{HCO}_3^-$).

  • $\text{CO}_2$ from the tissues enters the RBCs and reacts with water in the presence of the enzyme carbonic anhydrase to form carbonic acid ($\text{H}_2\text{CO}_3$): $\text{CO}_2 + \text{H}_2\text{O} \overset{\text{Carbonic Anhydrase}}{\rightleftharpoons} \text{H}_2\text{CO}_3$
  • Carbonic acid is unstable and rapidly dissociates into hydrogen ions ($\text{H}^+$) and bicarbonate ions ($\text{HCO}_3^-$): $\text{H}_2\text{CO}_3 \rightleftharpoons \text{H}^+ + \text{HCO}_3^-$

• Chloride Shift (Hamburger's Phenomenon):

  • As bicarbonate ions ($\text{HCO}_3^-$) accumulate inside the RBCs, they diffuse out into the plasma down their concentration gradient.
  • To maintain electrochemical neutrality across the erythrocyte membrane, chloride ions ($\text{Cl}^-$) from the plasma diffuse into the RBCs in exchange. This reciprocal exchange of anions is called the Chloride Shift (mediated by the Band 3 anion exchange protein).
  • In the Lungs (Reverse Chloride Shift): The process is reversed. Chloride ions ($\text{Cl}^-$) diffuse out of the RBC back into the plasma, while bicarbonate ($\text{HCO}_3^-$) enters the RBC, combining with $\text{H}^+$ to form carbonic acid, which is converted to $\text{CO}_2$ and water by carbonic anhydrase and exhaled.

6. Regulation of Breathing

• Respiratory Rhythm Centre: Located in the medulla oblongata of the brainstem. It is primarily responsible for generating and maintaining the respiratory rhythm.
• Pneumotaxic Centre: Located in the pons region of the brainstem. It can moderate the function of the respiratory rhythm centre. A neural signal from this centre can reduce the duration of inspiration and thereby alter the respiratory rate.
• Hering-Breuer Reflex: Stretch receptors in the walls of the bronchi and bronchioles send inhibitory signals to the respiratory rhythm centre when the lungs are over-stretched. This prevents over-inflation of the lungs during forceful inspiration.
• Chemosensitive Area: Situated adjacent to the rhythm centre, it is highly sensitive to CO2 and hydrogen ions. An increase in these substances activates this centre, which in turn signals the rhythm centre to make necessary adjustments in the respiratory process to eliminate them.
• Peripheral Chemoreceptors: Located in the aortic arch and carotid artery, they also recognize changes in CO2 and H+ concentration and send necessary signals to the rhythm centre for remedial actions.

7. Respiratory Volumes and Lung Capacities

• Tidal Volume (TV): Volume of air inspired or expired during a normal respiration (approx. 500 mL).
• Inspiratory Reserve Volume (IRV): Additional volume of air a person can inspire by a forcible inspiration (approx. 2500-3000 mL).
• Expiratory Reserve Volume (ERV): Additional volume of air a person can expire by a forcible expiration (approx. 1000-1100 mL).
• Residual Volume (RV): Volume of air remaining in the lungs even after a forcible expiration (approx. 1100-1200 mL).
• Inspiratory Capacity (IC): Total volume of air a person can inspire after a normal expiration (TV + IRV).
• Expiratory Capacity (EC): Total volume of air a person can expire after a normal inspiration (TV + ERV).
• Functional Residual Capacity (FRC): Volume of air that will remain in the lungs after a normal expiration (ERV + RV).
• Vital Capacity (VC): The maximum volume of air a person can breathe in after a forced expiration (ERV + TV + IRV).
• Total Lung Capacity (TLC): Total volume of air accommodated in the lungs at the end of a forced inspiration (RV + ERV + TV + IRV or Vital Capacity + Residual Volume).

8. Disorders of the Respiratory System

• Asthma: Difficulty in breathing causing wheezing due to inflammation of bronchi and bronchioles. It is often triggered by allergens.
• Emphysema: A chronic disorder in which the alveolar walls are damaged, leading to a decrease in the respiratory surface area. A major cause is cigarette smoking.
• Occupational Respiratory Disorders: Certain industries, especially those involving grinding or stone-breaking, produce a lot of dust. Long-term exposure can give rise to inflammation leading to fibrosis (proliferation of fibrous tissues) and causing serious lung damage. Examples include Silicosis (from silica dust) and Asbestosis (from asbestos dust).