Gas exchange

Diffusion is the net movement of molecules down their concentration gradient, driven by their own random motion. It needs no ATP.

• From lower to higher concentration, using ATP:
• Correct: From higher to lower concentration, without ATP:
• In whichever direction a carrier protein moves them:

The respiratory membrane is the thin barrier between alveolar air and blood: type I alveolar cells, the capillary endothelium and their fused basement membranes, about half a micrometer thick.

• Correct: The alveolar wall, the capillary wall and their fused basement membranes:
• The visceral and parietal pleura:
• The wall of a terminal bronchiole and its cartilage:

Alveolar ventilation = (tidal volume − dead space) × rate = (500 − 150) × 12 = 4,200 mL/min. The 6.0 L/min minute ventilation includes the dead space.

• 6.0 L/min:
• 1.8 L/min:
• Correct: 4.2 L/min:

Water vapor takes 47 mm Hg of the total, leaving 760 − 47 = 713 mm Hg for the dry gases. Oxygen's share is 713 × 0.209 = 149, about 150 mm Hg.

• About 159 mm Hg: 159 mm Hg is the PO2 of dry air. It ignores the water vapor, which takes part of the total pressure.
• Correct: About 150 mm Hg: Correct. (760 − 47) × 0.209 ≈ 150 mm Hg.
• About 713 mm Hg: 713 mm Hg is the pressure left for all the dry gases together, not oxygen's share of it.
• About 169 mm Hg: 169 mm Hg adds the water vapor pressure instead of subtracting it.

Dry gases share 523 − 47 = 476 mm Hg. Oxygen's share is 476 × 0.209 = 99.5, about 100 mm Hg: two-thirds of the sea-level value, although the air is still 20.9% oxygen.

• About 109 mm Hg: 109 mm Hg (523 × 0.209) forgets to subtract the water vapor first.
• About 150 mm Hg: 150 mm Hg is the sea-level value. The oxygen fraction is the same here, but the total pressure is lower.
• About 476 mm Hg: 476 mm Hg is the pressure of all the dry gases together.
• Correct: About 100 mm Hg: Correct. (523 − 47) × 0.209 ≈ 100 mm Hg.

Diffusion rate depends on solubility as well as gradient. Carbon dioxide dissolves about 20 times more readily, so 6 mm Hg × 20 moves it about as readily as 120 mm Hg would move oxygen: more than enough to clear the carbon dioxide you make.

• Carbon dioxide is pumped across by active transport: Both gases cross by simple diffusion. No ATP is spent moving them.
• Carbon dioxide molecules are smaller than oxygen molecules: A carbon dioxide molecule is actually heavier than an oxygen molecule. Its advantage is solubility, not size.
• The respiratory membrane has channels only for carbon dioxide: Both gases dissolve through the membrane's lipid and water; neither needs a special channel.
• Correct: Carbon dioxide is about 20 times more soluble than oxygen: Correct. High solubility makes up for the small gradient.

Diffusion continues until the blood's partial pressures match the alveolar air. At rest, this happens within the first third of the capillary, so blood leaves at about 100 and 40 mm Hg.

• PO2 about 140, PCO2 about 6 mm Hg: Diffusion can only bring the blood up to the alveolar value, never past it, because the gradient disappears once they match.
• PO2 about 70, PCO2 about 43 mm Hg: Halfway values would mean diffusion stopped early. In healthy lungs at rest, there is plenty of time to finish.
• Correct: PO2 about 100, PCO2 about 40 mm Hg: Correct. Blood leaves in balance with alveolar air.
• PO2 about 40, PCO2 about 46 mm Hg: These are the values of blood arriving from the tissues. They would stay the same only if no exchange happened, as in an unventilated alveolus.

Internal respiration runs down the gradient from blood to tissue. With the tissue at 20 instead of 40 mm Hg, the gradient grows from about 55 to about 75 mm Hg, so more oxygen diffuses out of each pass of blood, with no signal needed.

• It falls, because there is less oxygen in the tissue: The tissue's low PO2 is what steepens the gradient; it pulls more oxygen out of the blood.
• It rises, because arterial blood carries more dissolved oxygen: Arterial PO2 is unchanged, so the dissolved oxygen in arterial blood is unchanged too.
• It stays the same, because arterial PO2 has not changed: Oxygen movement depends on the difference between blood and tissue, and the tissue value has changed.
• Correct: It rises, because the blood-to-tissue gradient is steeper: Correct. A steeper gradient means faster diffusion into the tissue.

Poorly ventilated alveoli have a low PO2. Smooth muscle in the nearby pulmonary arterioles responds to the low PO2 by contracting: hypoxic pulmonary vasoconstriction. Blood is sent to better-ventilated alveoli, which keeps the ventilation–perfusion ratio closer to normal.

• Nearby pulmonary arterioles dilate and draw blood away from the blocked area: The response is constriction of the arterioles supplying the poorly ventilated alveoli, not dilation.
• The mucus plug presses on the pulmonary capillaries: The plug sits inside the airway; it does not squeeze the blood vessels.
• Correct: The alveolar PO2 falls and the nearby pulmonary arterioles constrict: Correct. Low alveolar PO2 triggers local vasoconstriction.
• The heart pumps less blood to the lungs: Cardiac output to the lungs as a whole is unchanged; blood is redistributed within the lungs.

In a systemic tissue such as muscle, low PO2 dilates arterioles, bringing more blood where oxygen is being used up. In the lungs, low alveolar PO2 constricts pulmonary arterioles, sending blood away from alveoli that are getting too little air.

• Lung arterioles dilate; muscle arterioles constrict: This reverses both responses.
• Both constrict: Constriction is the lung response only. Muscle arterioles dilate when oxygen runs low.
• Correct: Lung arterioles constrict; muscle arterioles dilate: Correct. The two circulations respond in opposite directions.
• Both dilate: Dilation is the systemic response only. Lung arterioles constrict.