Breathing (pulmonary ventilation) is the mechanical process of taking air in (inspiration/inhalation) and expelling it (expiration/exhalation). Respiration is the cellular process of using O2 to generate ATP. The two are related but distinct.
Human Respiratory System
- The respiratory tract in humans includes:
- Nostrils → Nasal cavity: Air is filtered (hairs, mucus), warmed, and humidified
- Pharynx: Common passage for air and food
- Larynx (voice box): Contains vocal cords; the epiglottis covers it during swallowing
- Trachea: Reinforced by C-shaped cartilaginous rings; lined with cilia and goblet cells
- Bronchi (2): Enter the two lungs; right bronchus is wider, shorter, and more vertical
- Bronchioles: Finer branches; terminal bronchioles → respiratory bronchioles → alveolar ducts
- Alveoli: Tiny air sacs; site of gas exchange; ~300 million in humans; total surface area ~70 m2
Each lung is enclosed in a double-layered pleural membrane (pleura). The space between layers contains pleural fluid that reduces friction.
Mechanism of Breathing
Breathing involves changes in lung volume driven by pressure changes.
- Inspiration (active process):
- Diaphragm contracts (flattens)
- External intercostal muscles contract → ribs move outward and upward
- Thoracic volume increases → lung volume increases → pressure in lungs falls below atmospheric pressure
- Air rushes in (from high to low pressure)
- Expiration (passive during quiet breathing):
- Diaphragm and external intercostal muscles relax
- Thoracic volume decreases → lung pressure rises above atmospheric pressure
- Air is pushed out
During forced expiration: internal intercostal muscles and abdominal muscles contract actively.
Respiratory Volumes and Capacities
- Tidal Volume (TV): Air moved in/out in one normal breath ≈ 500 mL
- Inspiratory Reserve Volume (IRV): Additional air that can be inhaled forcefully ≈ 2500-3000 mL
- Expiratory Reserve Volume (ERV): Additional air that can be exhaled forcefully ≈ 1000-1100 mL
- Residual Volume (RV): Air that cannot be expelled even after maximum exhalation ≈ 1100-1200 mL; prevents lung collapse
- Capacities (sums of volumes):
- Inspiratory Capacity (IC) = TV + IRV ≈ 3500 mL
- Expiratory Capacity (EC) = TV + ERV ≈ 1500 mL
- Functional Residual Capacity (FRC) = ERV + RV ≈ 2200-2300 mL
- Vital Capacity (VC) = TV + IRV + ERV ≈ 4000-4600 mL (maximum air that can be moved in/out)
- Total Lung Capacity (TLC) = VC + RV ≈ 5800 mL
Exchange of Gases
Gas exchange at the alveoli and tissues occurs by diffusion (Fick's Law: rate proportional to surface area x partial pressure gradient / thickness).
- Partial pressure (in mmHg):
- Atmospheric air: O2 = 159 mmHg; CO2 = 0.3 mmHg
- Alveolar air: pO2 ≈ 104 mmHg; pCO2 ≈ 40 mmHg
- Deoxygenated blood (arriving at lungs): pO2 ≈ 40 mmHg; pCO2 ≈ 45 mmHg
- Oxygenated blood (leaving lungs): pO2 ≈ 95 mmHg; pCO2 ≈ 40 mmHg
- Tissues: pO2 ≈ 40 mmHg; pCO2 ≈ 45 mmHg
O2 diffuses from alveoli into blood (high to low pO2) and CO2 diffuses from blood into alveoli (high to low pCO2).
- Efficiency factors for alveolar exchange:
- Large surface area of alveoli (~70 m2)
- Very thin alveolar membrane (~1 micron)
- Rich capillary network
- CO2 is 20-25 times more soluble than O2 in body fluids
Transport of O2 and CO2
Oxygen transport: ~97% bound to haemoglobin (Hb) as oxyhaemoglobin (HbO2); ~3% dissolved in plasma.
- Oxygen-Haemoglobin Dissociation Curve: S-shaped (sigmoidal); shows % saturation of Hb vs pO2.
- Curve shifts right (decreased O2 affinity) with: increased CO2, decreased pH, increased temperature, increased 2,3-DPG → facilitates O2 unloading at tissues (Bohr effect)
- Curve shifts left (increased O2 affinity) with opposite conditions → Hb holds O2 tighter (facilitates loading)
- 1.CO2 transport (in blood):
- 2.~70% as bicarbonate ions (HCO3-) in plasma — most important: CO2 + H2O ⇌ H2CO3 ⇌ H+ + HCO3- (catalysed by carbonic anhydrase in RBCs); HCO3- moves out of RBC in exchange for Cl- (chloride shift)
- 3.~20-25% as carbaminohaemoglobin (CO2 bound to Hb protein, not haem)
- 4.~7% dissolved in plasma
Respiratory Disorders
- Asthma: Narrowing of bronchioles (spasm + inflammation); difficulty in breathing, wheezing
- Emphysema: Damage and loss of alveolar walls; reduced gas exchange surface; mainly caused by smoking
- Pneumonia: Infection of alveoli (bacterial/viral); alveoli fill with fluid
- Occupational respiratory diseases: Coal miner's lung (anthracosis), silicosis, asbestosis
Common mistakes
- Breathing is mechanical; cellular respiration is biochemical — do not confuse them.
- Residual volume (RV) is NOT exhaled even after maximum effort — it prevents lung collapse.
- CO2 is mainly transported as bicarbonate (70%), NOT as carbaminohaemoglobin.
- The Bohr effect: increased CO2/decreased pH decreases Hb-O2 affinity (curve shifts right), facilitating O2 release at active tissues.
Summary
Breathing involves pressure-volume changes in the thorax. Gas exchange at alveoli and tissues occurs by diffusion along partial pressure gradients. O2 is mainly transported by haemoglobin; CO2 mainly as bicarbonate. Several volumes and capacities describe lung function, with vital capacity and residual volume being clinically significant.