MCAT Biology : Gas Exchange

Study concepts, example questions & explanations for MCAT Biology

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Example Questions

Example Question #1 : Respiratory System

Gas exchange in the lungs is located in the __________.

Possible Answers:

alveoli

bronchioles

larynx

pleural membrane

epiglottis

Correct answer:

alveoli

Explanation:

Bronchioles end is clusters of small air sacs, the alveoli, where gas exchange occurs. Gases diffuse through the walls of the alveoli into the capillaries.

Example Question #1 : Respiratory System

Among the most important pH buffer systems in humans is the bicarbonate buffer, which keeps the blood at a remarkably precise 7.42 pH.  The bicarbonate buffer system uses a series of important compounds and enzymes to make the system function.  Figure 1 depicts the key reactions that take place.

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The activity of this buffer system is mainly controlled by the renal and respiratory systems.  The renal system excretes bicarbonate in the urine, while the respiratory system “blows off” carbon dioxide as needed.   By balancing these two systems as needed, blood pH is maintained in such a narrow range.

When carbon dioxide is removed by the lungs, which of the following describes an accurate change in the system?

Possible Answers:

Bicarbonate builds up in the blood

The pKa of carbonate goes down

The equilibrium shifts toward carbonic acid

The equilibrium shifts away from carbonic acid

The pH of the blood goes down

Correct answer:

The equilibrium shifts away from carbonic acid

Explanation:

As CO2 is blown off by the lungs, the equilibrium is shifted away from carbonic acid via Le Chatelier's principle. The system is attempting to recapture the lost carbon dioxide by producing more, thus depleting the volume of free carbonic anhydrase. Ultimately, the same principle will shift the equilibrium away from bicarbonate.

Example Question #1 : Gas Exchange

Among the most important pH buffer systems in humans is the bicarbonate buffer, which keeps the blood at a remarkably precise 7.42 pH.  The bicarbonate buffer system uses a series of important compounds and enzymes to make the system function.  Figure 1 depicts the key reactions that take place.

Untitled

The activity of this buffer system is mainly controlled by the renal and respiratory systems.  The renal system excretes bicarbonate in the urine, while the respiratory system “blows off” carbon dioxide as needed.   By balancing these two systems as needed, blood pH is maintained in such a narrow range.

A patient undergoes a procedure in a hospital, and begins to reabsorb large quantities of bicarbonate from the kidneys. In the above reaction __________.

Possible Answers:

CO2 production will decrease, while carbonic acid concentration builds up

CO2 quickly builds up

the equilibrium is shifted toward CO2

carbonic anhydrase prevents a shift in equilibrium

the equilibrium is shifted toward bicarbonate

Correct answer:

the equilibrium is shifted toward CO2

Explanation:

Via Le Chatelier's principle, the equilibrium is quickly shifted toward CO2, however, as the passage details, CO2 does not build up. Instead, it is blown off via the respiratory system.

Example Question #4 : Respiratory System

In mammals, what muscles are involved in inhalation?

Possible Answers:

External intercostals and internal intercostals

External intercostals and diaphragm

External intercostals only

Internal intercostals and diaphragm

Diaphragm only

Correct answer:

External intercostals and diaphragm

Explanation:

During inhalation, the external intercostal muscles and diaphragm both contract to help expand the thoracic cavity and draw in air. The internal intercostal muscles are involved in exhalation, and compress the thoracic cavity during contraction.

Example Question #3 : Respiratory System

Which of the following is not one of the four processes that comprise external respiration?

Possible Answers:

Use of oxygen within the mitochondria to generate ATP via oxidative phosphorlyation

Pulmonary ventilation 

Transporation of oxygen and carbon dioxide between the lungs and body tissues via the blood

Exchange of oxygen and carbon dioxide between lung air spaces and the blood via diffusion 

Exchange of oxygen and carbon dioxide between the blood and tissues via diffusion

Correct answer:

Use of oxygen within the mitochondria to generate ATP via oxidative phosphorlyation

Explanation:

The use of oxygen within the mitochondria to generate ATP via oxidative phosphorylation is a part of internal, or cellular, respiration.

The other four processes listed are all parts of external respiration, consisting of the mechanisms used to transport gases between the atmosphere and the body.

Example Question #4 : Respiratory System

There are two types of alvelolar cells that line the alveolar sacs in the lungs. Type I cells participate in gas exchange. What do type II cells do?

Possible Answers:

They also participate in gas exchange

They provide structural support to type I cells

They secrete surfactant, a chemical that helps the alveoli close

They secrete surfactant, a chemical that helps the alveoli stay open

They secrete surfactant and also participate in gas exchange

Correct answer:

They secrete surfactant, a chemical that helps the alveoli stay open

Explanation:

Type II alveolar cells have the ability to differentiate into type I cells if needed. However, they do not participate in gas exchange. They do secrete surfactant, a chemical that helps the alveoli stay open so that gas exchange can occur effectively. 

Example Question #5 : Respiratory System

Which of the following may signal a respiratory abnormality?

          I. Low concentration of oxygen in the alveoli

          II. High concentration of carbon dioxide in the alveoli

          III. Contraction of intercostal muscles upon inhalation

Possible Answers:

I, II, and III

Both I and II

Only III

Only II

Only I

Correct answer:

Both I and II

Explanation:

In a healthy individual, the concentration of oxygen in the alveoli is high and that of carbon dioxide is low. This allows oxygen to diffuse out of the alveoli into the blood stream while carbon dioxide diffuses into the alveoli to be eliminated via exhalation. Intercostal muscles and diaphragm contract, enlarging the thoracic cavity. Therefore, options I and II are both correct choices.

Example Question #5 : Circulatory And Respiratory Systems

The brain is a very delicate structure with little room to move around. Surrounding the brain and the spinal cord are three protective layers in addition to the skull and the vertebral column. Directly surrounding the brain and spinal cord is the pia mater. Following the pia mater is the arachnoid mater. Between the pia mater and the arachnoid mater is the sub-arachnoid space where the cerebrospinal fluid circulates. Finally, the protective layer is the dura mater is loosely attached to the arachnoid mater but is strongly associated with the skull bone.

Depending on the type of injury, a certain type of vein and/or artery are more susceptible to injury. For example, the meningeal artery and vein run through the foramen spinosum and travel between the two layers making up the dura mater. As the artery and the vein are traveling in between the dura mater, there is a vulnerable region at the temple. A strike to the temple region could rupture these vessels and result in a epidural hematoma. 

Traveling from the cerebral cortex to the venous dural sinus (located at certain regions between the two layers of the dura mater) is the cerebral vein. When an injury results in the dura mater shifting away from the arachnoid mater, the cerebral vein could rupture and lead to a subdural hematoma.

When the drainage system is compromised such as during an epidural hematoma, waste product in the blood concentrates. One of the waste products is carbon dioxide. What happens to the blood when the carbon dioxide is not properly removed? 

Possible Answers:

More than one of these

The blood's oxygen level also increases

The blood's pH increases

The hemoglobin's dissociation curve shifts to the left

The blood's pH decreases

Correct answer:

The blood's pH decreases

Explanation:

Without proper venous drainage, carbon dioxide level increases in the blood. As the carbon dioxide concentration increases, it reacts with water to create protons. This reaction will lower the blood's pH. The equilibrium reaction is shown below:

Recall Le Chatelier's principle. When the carbon dioxide on the left side of the equation builds up, it drives the reaction to the right. As such, the hydrogen ion concentration will increase, leading to a more acidic solution (plasma) and a lower pH.

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