AP Biology : Understanding Pressure Differentials
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Example Questions
Example Question #1 : Understanding Respiratory Functions
Which of the given circulatory system elements has the highest partial pressure of oxygen?
Left atrium
Inferior vena cava
Superior vena cava
Right atrium
Pulmonary arteries
Left atrium
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 #1 : Understanding Respiratory Functions
Which statement describes the pressure in the lungs during exhalation?
Lung pressure must be equal to atmospheric pressure
Lung pressure should not change during the process
Lung pressure must be lower than atmospheric pressure
Lung pressure must be higher than atmospheric pressure
Lung pressure must be higher than atmospheric pressure
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 #1 : Understanding Pressure Differentials
What causes air to enter human lungs?
The decreased concentration of 
Smooth muscle in the trachea guides 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.
Air diffuses into the lungs due to the decreased 
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.
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 #4 : Understanding Pressure Differentials
To initiate inhalation, the diaphragm contracts, making the pressure inside the lungs __________.
zero
None of these
higher than the pressure outside the body
lower than the pressure outside the body
the same as the pressure outside the body
lower than the pressure outside the body
Inhalation happens by making the pressure in the lungs lower relative to the pressure outside the body. When the diaphragm contracts, it increases the volume of the thoracic cavity. By Boyle's law, there is an inverse relationship between pressure and volume of a gas. Thus the pressure of the thoracic cavity decreases and since air will flow from high to low pressure, this pulls air into the lungs during inhalation.
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