Pulmonary ventilation

A fluid, including air, flows down a pressure gradient: from higher pressure to lower pressure. With no pressure difference there is no flow.

• Correct: Pressure is higher at A than at B:
• Pressure is higher at B than at A:
• The two pressures are equal:

The external intercostals run down and forward between neighboring ribs. Contracting, they lift the ribs up and out, which enlarges the chest. The internal intercostals pull the ribs down.

• Pull the ribs down and in:
• Correct: Pull the ribs up and out:
• Flatten the diaphragm:

The pleural cavity is a very narrow gap between visceral and parietal pleura, holding only a thin film of serous pleural fluid.

• Air:
• Correct: A thin film of pleural fluid:
• Loose connective tissue:

Boyle's law: P₂ = P₁ × V₁ ÷ V₂ = 760 × 500 ÷ 400 = 950 mm Hg. A smaller volume gives a higher pressure.

• 608 mm Hg: 608 comes from 760 × 400 ÷ 500, the ratio upside down. That would be the answer for an expansion, not a squeeze.
• Correct: 950 mm Hg: Correct. 760 × 500 ÷ 400 = 950 mm Hg.
• 860 mm Hg: 860 comes from adding the 100 mL change to 760. Pressure and volume are related by a ratio, not by adding the difference.
• 660 mm Hg: 660 comes from subtracting the 100 mL change from 760. Pressure changes by a ratio, and a squeeze raises it.

Intrapleural pressure falls during inspiration, from about −4 to about −6 mm Hg by 2 seconds, the end of breathing in.

• About 0 mm Hg: 0 is where alveolar pressure sits at the end of breathing in, not intrapleural pressure.
• About −1 mm Hg: −1 is the lowest alveolar pressure, reached at mid-inspiration.
• Correct: About −6 mm Hg: Correct. At the end of inspiration intrapleural pressure is about −6 mm Hg.
• About −4 mm Hg: −4 is the intrapleural pressure at rest, at the start of breathing in and the end of breathing out.

Nerve signal, muscle contraction, larger thorax, lower intrapleural pressure, lung expansion, lower alveolar pressure by Boyle's law, and finally airflow down the gradient.

• Correct order: 1. Signals travel down the phrenic nerves 2. The diaphragm contracts and flattens 3. The thoracic cavity enlarges 4. Intrapleural pressure falls 5. The lungs expand 6. Alveolar pressure falls below atmospheric and air flows in

Air in the pleural cavity raises intrapleural pressure toward atmospheric. Transpulmonary pressure disappears, the lung's own recoil collapses it, and the chest wall's movements no longer expand it.

• Intrapleural pressure: up. Air enters until the pressure in the cavity approaches atmospheric pressure, so it rises from about −4 toward 0.
• Transpulmonary pressure: down. Transpulmonary pressure is alveolar minus intrapleural pressure; with intrapleural pressure near 0, it falls toward 0.
• Volume of the left lung: down. Without transpulmonary pressure to hold it open, the lung recoils toward its hilum and collapses.
• Airflow into the left lung with each breath: down. The collapsed lung is no longer held to the chest wall, so it barely expands when the chest does, and little air flows into it.

Type II alveolar cells usually make enough surfactant only by about 34 to 36 weeks. Without it, the water lining her alveoli has high surface tension, compliance is low, and many alveoli collapse at the end of each breath and must be reopened with great effort.

• Her pleural cavities contain air: Air in a pleural cavity would collapse a lung, but nothing here suggests a pneumothorax; her whole lungs are stiff.
• Her bronchioles are constricted: Narrow bronchioles raise resistance, which slows airflow; they would not make the lung tissue itself stiff.
• Too few elastic fibers make her lungs recoil weakly: Fewer elastic fibers would make the lungs easier to inflate, not stiffer: compliance would rise, not fall.
• Correct: Too little surfactant in her alveoli: Correct. Surfactant deficiency means high surface tension and low compliance.

Resistance is proportional to 1 ÷ r⁴. If r falls to one third, r⁴ falls to (1/3)⁴ = 1/81, so resistance rises 81-fold.

• 3 times: 3 would apply if resistance depended on radius alone. It depends on the fourth power of the radius.
• 9 times: 9 uses the square of the radius change. Resistance depends on the fourth power.
• 27 times: 27 uses the cube of the radius change. Resistance depends on the fourth power.
• Correct: 81 times: Correct. 3⁴ = 81.

The lungs' elastic fibers and surface tension pull them inward; the chest wall tends to spring outward. The pleural fluid film holds the two together, so the film is pulled from both sides and its pressure falls below atmospheric.

• Correct: Lungs recoil inward while the chest wall springs outward: Correct. Opposing recoil of lungs and chest wall lowers the pressure in the film between them.
• The pleural cavity contains a small pocket of low-pressure air: A healthy pleural cavity contains no air at all, only a thin film of fluid. Air there would be a pneumothorax.
• The diaphragm is contracted between breaths, pulling on the pleura: Between breaths the diaphragm is relaxed; the negative pressure is present even with all breathing muscles at rest.
• Alveolar pressure is below atmospheric at rest: Between breaths alveolar pressure equals atmospheric pressure, because no air is flowing.

Expanding lung tissue pulls the airways open during inspiration. During expiration the lungs shrink, that pull weakens, and in forced expiration the rising pressure around the airways squeezes them, so already-narrowed bronchioles narrow further or close, trapping air.

• Her expiratory muscles are weaker than her diaphragm: Muscle strength is not the problem; she can recruit her abdominal muscles. Narrowed airways limit the flow.
• Surfactant is used up during each breath out: Surfactant is not consumed breath by breath, and it acts in the alveoli, not on airway radius.
• Her vocal folds close during each breath out: The vocal folds open for breathing; the wheeze comes from air forced through narrowed bronchioles.
• Correct: Her airways narrow further as her lungs shrink: Correct. Airway radius falls during expiration, and resistance rises with the fourth power of that fall.