Question

question 19
during inspiration, the expansion of the lungs causes:
cohesion of visceral and parietal pleura.
a pressure gradient from alveoli to atmosphere.
a decrease in alveolar pressure.
an increase in intrathoracic pressure from about -6 to -4 mm hg.
question 20
which of the following combinations in arterial blood could cause an increase in the respiratory rate?
increased pco2, decreased arterial pressure, decreased ph, decreased po2
increased pco2, decreased arterial pressure, increased ph, decreased po2
decreased pco2, decreased arterial pressure, increased ph, increased po2
decreased pco2, decreased arterial pressure, decreased ph, decreased po2
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Explanation:

Question 19

• Cohesion of visceral and parietal pleura: This is a constant due to serous fluid, not caused by lung expansion during inspiration.
• Pressure gradient from alveoli to atmosphere: During inspiration, the gradient is from atmosphere to alveoli (air flows in).
• Decrease in alveolar pressure: When lungs expand (volume increases), according to Boyle's law ($P=\frac{k}{V}$, where $k$ is a constant), pressure ($P$) decreases. This creates a pressure gradient for air to flow in (from higher - pressure atmosphere to lower - pressure alveoli).
• Increase in intrathoracic pressure from about - 6 to - 4 mmHg: Intrathoracic pressure becomes more negative (decreases further) during inspiration.

• Chemoreceptors respond to changes in $PCO_2$, $pH$, and $PO_2$.
• Increased $PCO_2$: High $PCO_2$ (hypercapnia) can stimulate chemoreceptors. When $CO_2$ levels are high, it can combine with water in the blood to form carbonic acid ($H_2CO_3$), which dissociates into $H^+$ and $HCO_3^-$, decreasing $pH$ (acidosis).
• Decreased $PO_2$: Low oxygen levels (hypoxia) also stimulate chemoreceptors (especially peripheral chemoreceptors).
• Decreased arterial pressure: Can be a sign of poor perfusion, which may also signal the body to increase respiratory rate (e.g., in shock - like states to get more oxygen and remove more $CO_2$).

• Option 1 (Increased $PCO_2$, decreased arterial pressure, decreased $pH$, decreased $PO_2$):
• Increased $PCO_2$ (hypercapnia), decreased $pH$ (acidosis) from $CO_2$ - related acid - base changes, decreased $PO_2$ (hypoxia), and decreased arterial pressure (which can be a sign of poor circulatory status) all can stimulate the respiratory center (either directly or via chemoreceptors) to increase the respiratory rate.
• Option 2 (Increased $PCO_2$, decreased arterial pressure, increased $pH$, decreased $PO_2$): Increased $pH$ (alkalosis) would not stimulate the respiratory center in the same way as acidosis.
• Option 3 (Decreased $PCO_2$, decreased arterial pressure, increased $pH$, increased $PO_2$): Decreased $PCO_2$ (hypocapnia) and increased $pH$ (alkalosis) are not strong stimulators of increased respiratory rate.
• Option 4 (Decreased $PCO_2$, decreased arterial pressure, decreased $pH$, decreased $PO_2$): Decreased $PCO_2$ (hypocapnia) is not a typical stimulator of increased respiratory rate.

Answer:

a decrease in alveolar pressure.

Question 20