Learning Objectives
- Understand the human respiratory system and its components
- Learn the mechanism of breathing (inspiration and expiration)
- Understand gas exchange and transport of respiratory gases
- Study respiratory volumes, capacities, and disorders
Key Concepts
Human Respiratory System
Consists of: External nostrils → Nasal cavity → Pharynx → Larynx → Trachea → Primary bronchi → Secondary bronchi → Tertiary bronchi → Bronchioles → Alveoli. Nostrils have mucus and hair for filtering. Larynx (voice box) contains vocal cords. Trachea has C-shaped cartilage rings. Lungs: Right lung has 3 lobes, left has 2 lobes. Each lung is covered by double-layered pleural membrane with pleural fluid reducing friction. Alveoli are the functional units — thin-walled, highly vascularized for gas exchange.
Mechanism of Breathing
Inspiration (Inhalation): Diaphragm contracts (flattens), external intercostal muscles contract → rib cage moves upward and outward → thoracic volume increases → intrapulmonary pressure decreases (below atmospheric) → air rushes in. Expiration (Exhalation): Diaphragm relaxes (dome-shaped), internal intercostal muscles contract → rib cage moves downward and inward → thoracic volume decreases → intrapulmonary pressure increases (above atmospheric) → air pushed out. Normal breathing is passive expiration; forced breathing involves active expiration.
Respiratory Volumes and Capacities
Tidal Volume (TV): ~500 mL, air breathed in/out during normal breathing. Inspiratory Reserve Volume (IRV): ~2500-3000 mL, extra air that can be forcibly inhaled. Expiratory Reserve Volume (ERV): ~1000-1100 mL, extra air that can be forcibly exhaled. Residual Volume (RV): ~1100-1200 mL, air remaining after forced expiration. Inspiratory Capacity (IC): TV + IRV. Functional Residual Capacity (FRC): ERV + RV. Vital Capacity (VC): TV + IRV + ERV (~3400-4800 mL). Total Lung Capacity (TLC): VC + RV (~5000-6000 mL).
Exchange of Gases
Occurs by simple diffusion based on partial pressure gradients. At alveoli: pO2 is high (~104 mmHg), pCO2 is low (~40 mmHg). In deoxygenated blood: pO2 is low (~40 mmHg), pCO2 is high (~45 mmHg). O2 diffuses from alveoli → blood; CO2 diffuses from blood → alveoli. Factors affecting diffusion: partial pressure gradient, solubility of gases, membrane thickness.
Transport of Gases
O2 Transport: ~97% bound to haemoglobin as oxyhaemoglobin (HbO2), ~3% dissolved in plasma. Each Hb molecule binds 4 O2. Oxygen dissociation curve: Sigmoid (S-shaped). Factors shifting curve to right (decreased O2 affinity): increased pCO2, increased H+ (low pH), increased temperature, increased 2,3-BPG. Bohr effect: CO2 and H+ decrease O2 binding affinity of Hb.
CO2 Transport: ~70% as bicarbonate ions (HCO3-) in plasma, ~23% as carbaminohaemoglobin (HbCO2), ~7% dissolved in plasma. In tissues: CO2 + H2O → H2CO3 → H+ + HCO3- (catalyzed by carbonic anhydrase in RBCs). Chloride shift: HCO3- moves out of RBC, Cl- moves in to maintain electrical neutrality.
Regulation of Respiration
Respiratory centre in medulla oblongata and pons (pneumotaxic centre). Chemoreceptors detect changes in CO2, O2, and H+ levels. Increased CO2 is the most powerful stimulus for breathing.
Respiratory Disorders
Asthma: Spasm of bronchial muscles, inflammation, difficulty breathing. Emphysema: Alveolar walls damaged, reduced surface area (common in smokers). Occupational diseases: Silicosis (silica dust), asbestosis (asbestos fibres).
Summary
The respiratory system facilitates gas exchange between blood and atmosphere. Breathing is driven by pressure changes due to diaphragm and intercostal muscle movements. O2 is mainly transported by haemoglobin; CO2 mainly as bicarbonate ions. Partial pressure gradients drive diffusion of gases at alveoli and tissues.
Important Terms
- Alveoli: Thin-walled air sacs in lungs where gas exchange occurs
- Bohr effect: Decreased O2 affinity of Hb in presence of high CO2/H+
- Vital capacity: Maximum volume of air exhaled after maximum inhalation (TV+IRV+ERV)
- Chloride shift: Exchange of Cl- and HCO3- across RBC membrane
- Carbonic anhydrase: Enzyme in RBCs catalyzing CO2 + H2O reaction
- Pneumotaxic centre: Centre in pons that limits inspiration duration
- Pleura: Double membrane covering lungs
- Partial pressure: Pressure exerted by an individual gas in a mixture
Quick Revision
- Right lung: 3 lobes; Left lung: 2 lobes
- Inspiration: diaphragm contracts, volume increases, pressure decreases
- O2 transport: 97% as HbO2, 3% dissolved
- CO2 transport: 70% as HCO3-, 23% as HbCO2, 7% dissolved
- Oxygen dissociation curve: sigmoid (S-shaped)
- Bohr effect: high CO2 reduces Hb-O2 affinity
- Vital capacity = TV + IRV + ERV (~4800 mL)
- Respiratory centre: medulla oblongata; pneumotaxic centre: pons