SN6 SHORT NOTE 5M ★ High-yield

Define dead space. Describe its types and explain its significance in alveolar ventilation.

Physiology · 1st Professional — Pre-Clinical

Dead space = that part of each tidal volume which does not take part in gas exchange.

Type	Definition	Value
Anatomical	Volume of conducting airways (nose → terminal bronchioles); no alveolar epithelium, so no exchange possible	150 mL (2 mL/kg)
Alveolar	Alveoli ventilated but not perfused	Negligible in health
Physiological	Anatomical + alveolar	≈ 150 mL in health

Alveolar ventilation = (TV − Dead space) × Respiratory rate

Gas exchange depends on alveolar ventilation, not minute ventilation
Deep slow breathing is more efficient than rapid shallow breathing at the same minute volume

Pattern	TV	RR	Minute volume	Alveolar ventilation
Normal	500 mL	12/min	6000 mL/min	(500−150)×12 = 4200 mL/min
Rapid shallow	250 mL	24/min	6000 mL/min	(250−150)×24 = 2400 mL/min
Deep slow	1000 mL	6/min	6000 mL/min	(1000−150)×6 = 5100 mL/min

VD/VT = (PACO2 − PECO2) / PACO2

Normal VD/VT = 0.2–0.35
Rises markedly in pulmonary embolism — a useful index of alveolar dead space

↑ dead space → pulmonary embolism, ventilator tubing, over-long tracheostomy tube
Tracheostomy ↓ anatomical dead space by about half (bypasses upper airway) → helps in respiratory muscle weakness

All questions in Respiratory System