Decreased PO ₂

• Collapsed alveoli (atelectasis)
1. Airway obstruction
• By the tongue
• By a foreign body
2. Failure to take deep breaths
• Pain (rib fracture, pleurisy)
• Paralysis of respiratory muscles (spinal cord injury, polio)
• Depression of the respiratory center (head injury, drug overdose)
3. Collapse of the whole lung (pneumothorax)
• Fluid in the alveoli
1. Pulmonary edema
2. Pneumonia
3. Near-drowning
4. Chest trauma
• Other gases in the alveoli
1. Smoke inhalation
2. Inhalation of toxic chemicals
3. Carbon monoxide poisoning
• Respiratory arrest

Elevated PCO₂

• Decreased CO₂ elimination (hypoventilation)
1. Decreased tidal volume
• Pain (rib fractures, pleurisy
• Weakness (myasthenia gravis)
• Paralysis (spinal cord injury polio)
2. Decreased respiratory rate
• Head injury
• Depressant drugs
• Stroke
• Increased CO₂ production
1. Fever
2. Muscular exertion
3. Anaerobic metabolism