Anatomy & Physiology of Respiration – Overview 
 
1. Functional Overview of Respiration 

Respiration is the set of processes that supply oxygen (O₂) to the body’s cells and remove carbon dioxide (CO₂), a waste product of metabolism. Its ultimate purpose is to support cellular energy production (ATP).

1. Pulmonary Ventilation (Breathing)
Function: Move air in and out of the lungs
Inspiration: Air enters the lungs due to diaphragm contraction and chest expansion
Expiration: Air leaves the lungs as muscles relax
Outcome: Fresh air reaches the alveoli

2. External Respiration (Gas Exchange in Lungs)
Function: Exchange gases between air and blood
Where: Alveoli and pulmonary capillaries
Process:
O₂ diffuses from alveoli → blood
CO₂ diffuses from blood → alveoli
Outcome: Blood becomes oxygenated

3. Transport of Respiratory Gases
Function: Deliver gases between lungs and tissues
Oxygen:
Mostly carried by hemoglobin in red blood cells
Carbon dioxide:
Transported mainly as bicarbonate ions in plasma
Outcome: O₂ reaches tissues; CO₂ returns to lungs

4. Internal Respiration (Gas Exchange in Tissues)
Function: Exchange gases between blood and body cells
Process:
O₂ diffuses from blood → cells
CO₂ diffuses from cells → blood
Outcome: Cells receive oxygen for metabolism

5. Cellular Respiration
Function: Use oxygen to produce energy (ATP)
Where: Mitochondria
Process:
Glucose + O₂ → ATP + CO₂ + H₂O
Outcome: Energy for cellular activities

Key Points:

Respiration maintains:

Oxygen supply for ATP production
Removal of carbon dioxide
Acid–base balance (pH regulation)

 
2. Anatomy of the Respiratory System 
 
 
 
A. Upper Respiratory Tract 

Nose & nasal cavity: air filtration, humidification, warming 
Paranasal sinuses: lighten skull, resonance for speech 
Pharynx: nasopharynx (respiratory), oropharynx & laryngopharynx (shared with digestive system) 
 
B. Lower Respiratory Tract 
 
1. Conducting Zone (no gas exchange) 

Larynx, trachea, bronchi → bronchioles → terminal bronchioles 

Features: 
Ciliated pseudostratified columnar epithelium (proximal) 
Goblet cells (mucus production) 
Smooth muscle increases distally 
 
2. Respiratory Zone (gas exchange) 
 
Respiratory bronchioles → alveolar ducts → alveolar sacs 
 
C. Alveoli 
~300 million alveoli, surface area ≈ 70 m² 
Type I pneumocytes – thin squamous cells for gas diffusion 
Type II pneumocytes – produce surfactant, regenerate Type I cells 
Alveolar macrophages – immune defence 
Blood–air barrier: alveolar epithelium + fused basement membrane + capillary endothelium 
 
3. Physiology of Respiration 
 
A. Pulmonary Ventilation 
Inspiration (active): diaphragm contracts, external intercostals elevate ribs → thoracic volume ↑, pressure ↓ 
Expiration: passive at rest (elastic recoil), forced expiration uses internal intercostals & abdominal muscles.

B. Lung Volumes & Capacities 
 

C. Gas Exchange (External Respiration) 
 
Driven by partial pressure gradients 
O₂: alveoli → blood 
CO₂: blood → alveoli 
Fick’s Law: Rate ∝ (Surface Area × ΔP) / Thickness 
 
D. Transport of Respiratory Gases 
Oxygen 
98% bound to haemoglobin, 2% dissolved in plasma 
Oxyhaemoglobin dissociation curve: right shift → ↓ O₂ affinity (↑ CO₂, ↑ H⁺, ↑ temperature, ↑ 2,3-BPG) (Bohr Effect) 
 
Carbon dioxide 
70% as bicarbonate, 23% carbaminohaemoglobin, 7% dissolved 
Chloride shift: HCO₃⁻ exchanged with Cl⁻ across RBC membrane 
 
E. Internal Respiration 
O₂ diffuses from blood → tissues 
CO₂ diffuses from tissues → blood 
 
4. Regulation of Respiration 
 
A. Neural Control 

Medulla: dorsal respiratory group (inspiration), ventral respiratory group (forced breathing) 
Pons: apneustic center (prolongs inspiration), pneumotaxic center (limits inspiration) 
 
B. Chemoreceptors 

Central (medulla): respond to ↑ CO₂ via ↓ CSF pH 
Peripheral (carotid & aortic bodies): respond to ↓ PaO₂ (<60 mmHg), ↑ CO₂, ↓ pH 
 
5. Clinical Correlations 

Surfactant deficiency → neonatal respiratory distress syndrome 

COPD: 
Emphysema → ↓ alveolar surface area 
Chronic bronchitis → ↑ airway resistance 

Restrictive lung disease → ↓ lung compliance 
V/Q mismatch → most common cause of hypoxemia