# AP Biology : Understanding Pressure Differentials

## Example Questions

### Example Question #1 : Respiratory Physiology

Which of the given circulatory system elements has the highest partial pressure of oxygen?

Pulmonary arteries

Right atrium

Left atrium

Inferior vena cava

Superior vena cava

Left atrium

Explanation:

The left atrium receives blood from the pulmonary veins, which carry blood that was freshly oxygenated from the lungs to the heart. The partial pressure of oxygen is always highest soon after oxygenation, thus blood returning from the lungs would have a high partial pressure.

The superior and inferior vena cavae return deoxygenated blood from the body to the heart, and would have very low oxygen partial pressures. The right atrium receives this deoxygenated blood from the vena cavae and transfers it to the right ventricle. From the ventricle, the deoxygenated blood is transported to the lungs via the pulmonary arteries. It only becomes oxygenated again after reaching the lungs, and then returns to the heart through the pulmonary veins.

### Example Question #2 : Respiratory Physiology

Which statement describes the pressure in the lungs during exhalation?

Lung pressure must be higher than atmospheric pressure

Lung pressure must be lower than atmospheric pressure

Lung pressure should not change during the process

Lung pressure must be equal to atmospheric pressure

Lung pressure must be higher than atmospheric pressure

Explanation:

This question requires a basic understanding of general chemistry and/or general physics. Remember that gas will only move from an area of high pressure to low pressure; thus, if air is moving out of the lungs, the pressure inside of the lungs must be greater than the pressure outside of the lungs. The point at which air does not move in or out of the lungs is a signal that the pressure of the gas inside of the lungs is equal to that of atmospheric pressure.

In a biological sense, remember that the diaphragm contracts to cause inhalation, which results from negative or decreased pressure in the lungs. When the diaphragm relaxes, the pressure in the lungs must increase again. The increase in pressure forces the air out of the lungs and back into the atmospheric environment.

### Example Question #3 : Respiratory Physiology

What causes air to enter human lungs?

Cillia in the trachea guide the flow of air into the lungs.

The contraction of the diaphragm increases the volume of the lungs, decreasing the pressure relative to the air outside the body.

Smooth muscle in the trachea guides the flow of air into the lungs.

Air diffuses into the lungs due to the decreased  content relative to the air outside our body.

The decreased concentration of  in the lungs decreases the density of that air, and denser air outside the body "sinks" into the lungs.

The contraction of the diaphragm increases the volume of the lungs, decreasing the pressure relative to the air outside the body.

Explanation:

Increasing the volume of a container (in this case, the lungs) while keeping the contents (air molecules) the same will decrease the pressure. If no barrier is present (as when holding your breath), pressure will tend to equalize between areas of differing pressure. In order to equalize the pressure, air molecules from outside the body rush into the expanded lungs. The concentration of oxygen does not impact inhalation, and cilia are not used to inhale.

### Example Question #15 : Respiratory System

To initiate inhalation, the diaphragm contracts, making the pressure inside the lungs __________.

the same as the pressure outside the body

zero

None of these

higher than the pressure outside the body

lower than the pressure outside the body