Lung volumes and ventilation rates

The conducting zone, from the nose to the terminal bronchioles, only carries, warms and cleans air. Gas exchange happens only in the respiratory zone, where alveoli sit next to capillaries.

• Along the whole airway, from the nose onward:
• Correct: Only in the respiratory zone: respiratory bronchioles, alveolar ducts and alveoli:
• Only in the trachea and the main bronchi:

Contraction of the diaphragm and external intercostals enlarges the chest. By Boyle's law, the larger volume lowers alveolar pressure below atmospheric pressure, and air flows in down that pressure gradient.

• Correct: The diaphragm contracts, the chest enlarges, and alveolar pressure falls below atmospheric pressure:
• Air is pushed in because atmospheric pressure rises during each breath:
• The lungs actively expand themselves by contracting muscle in the alveolar walls:

Resistance varies with the fourth power of the radius, so halving the radius raises resistance 2 × 2 × 2 × 2 = 16 times.

• It doubles:
• Correct: It rises about 16-fold:
• It halves:

Vital capacity = TV + IRV + ERV = 500 + 3,000 + 1,100 = 4,600 mL. Residual volume is not part of it, because it can never be breathed out.

• 3,500 mL: 3,500 mL is TV + IRV, the inspiratory capacity.
• 5,800 mL: 5,800 mL adds the residual volume too. That sum is total lung capacity.
• Correct: 4,600 mL: Correct. 500 + 3,000 + 1,100 = 4,600 mL.
• 2,300 mL: 2,300 mL is ERV + RV, the functional residual capacity.

A quiet breath out ends at about 2,500 mL, the bottom of the tidal band. The deepest breath in reaches 6,000 mL. The difference, 6,000 − 2,500 = 3,500 mL, is the inspiratory capacity (TV + IRV).

• About 500 mL: About 500 mL is the tidal volume alone, a quiet breath, not the deepest possible one.
• About 6,000 mL: 6,000 mL is the total lung capacity, which counts air already in the lungs at the start.
• About 4,800 mL: About 4,800 mL is the vital capacity, measured from the fullest breath out, not from a quiet one.
• Correct: About 3,500 mL: Correct. From about 2,500 up to 6,000 mL is about 3,500 mL.

Alveolar ventilation = (tidal volume − dead space) × rate = (500 − 150) × 12 = 350 × 12 = 4,200 mL/min = 4.2 L/min.

• 6.0 L/min: 6.0 L/min is the minute ventilation, 500 × 12, which still includes the dead space.
• Correct: 4.2 L/min: Correct. 350 mL of fresh air reaches the alveoli on each of 12 breaths.
• 1.8 L/min: 1.8 L/min is the dead space ventilation, 150 × 12: the air wasted each minute.
• 7.8 L/min: 7.8 L/min adds the dead space instead of subtracting it: (500 + 150) × 12.

Doubling the rate and halving the depth keeps minute ventilation the same, but the dead space is a fixed cost on every breath. With smaller breaths, a larger share of each one is wasted, and alveolar ventilation falls by almost half.

• Minute ventilation: no change. 12 × 500 and 24 × 250 both equal 6,000 mL/min, so the total air moved each minute is unchanged.
• Anatomical dead space: no change. Anatomical dead space is the volume of the conducting airways, set by their size, not by how she breathes.
• Share of each breath that is dead space: up. 150 mL is now 60% of a 250 mL breath, instead of 30% of a 500 mL breath.
• Alveolar ventilation: down. (250 − 150) × 24 = 2,400 mL/min, down from (500 − 150) × 12 = 4,200 mL/min, because the fixed dead space is paid on twice as many smaller breaths.

FEV1/FVC = 1.6 ÷ 3.9 = 0.41, far below about 0.70. She can blow out a nearly normal total volume, but slowly, because narrowed airways raise resistance. That is obstruction, typical of asthma.

• Correct: Obstructive: narrowed airways slow the breath out: Correct. A low FEV1/FVC ratio is the mark of obstruction.
• Restrictive: her lungs and chest wall cannot expand fully: Restriction shrinks the FVC but leaves the FEV1/FVC ratio normal or high. Her ratio is very low.
• Normal: her FVC is close to the typical value: A near-normal FVC does not make the test normal; the speed of emptying, FEV1/FVC, is badly reduced.
• Neither pattern can be judged without her residual volume: Residual volume is needed to confirm restriction, but a low FEV1/FVC ratio shows obstruction from spirometry alone.

The snorkel adds its volume to the dead space: 150 + 150 = 300 mL. Alveolar ventilation falls from (500 − 150) × 12 = 4.2 L/min to (500 − 300) × 12 = 2.4 L/min. Deeper breaths help most, because each extra milliliter of tidal volume goes straight to the alveoli once the dead space is filled.

• Correct: It falls from 4.2 to 2.4 L/min; breathe more deeply: Correct. Deeper breathing restores alveolar ventilation most efficiently.
• It is unchanged, because the snorkel is outside his body: Any tube the breath must pass through adds dead space, wherever it is. The stale air left in the snorkel is breathed back in first.
• It falls from 4.2 to 2.4 L/min; breathe faster with the same small breaths: The fall is right, but faster breathing with the same small breaths pays the larger dead space on every extra breath, so it restores alveolar ventilation poorly.
• It rises, because the snorkel adds extra air to each breath: The snorkel holds air that is breathed back and forth unused; it adds dead space, not fresh air.

Air reaching alveoli with no blood flow cannot exchange gas, so those alveoli become alveolar dead space. Total dead space, anatomical plus alveolar, rises, and she must breathe more to deliver the same useful ventilation.

• Anatomical dead space rises, because the airways widen: Anatomical dead space is the volume of the conducting airways, which a clot in an artery does not change.
• Correct: Alveolar dead space rises, so total dead space rises: Correct. Ventilated alveoli with no blood flow are alveolar dead space.
• Dead space is unchanged, because her airways are normal: Normal airways keep anatomical dead space normal, but dead space also includes alveoli that get air and no blood.
• Total dead space falls, because less blood reaches the lungs: Losing blood flow to ventilated alveoli wastes their air, which raises dead space rather than lowering it.