MCAT Biology Review

Chapter 6: The Respiratory System

6.2 Functions of the Respiratory System

No organ system functions alone. The lungs function in gas exchange, but this is only part of the respiratory story. The lungs are lined with a tremendous number of capillaries that can also be used in thermoregulation. The lungs also represent a pathway into the body and serve an immune function to prevent invaders from gaining access to the bloodstream. Finally, as we touched on already, lungs also allow for control of pH within the blood by controlling carbon dioxide concentrations. Therefore, the lungs are integrated with many other body systems, including the cardiovascular, immune, renal, and nervous systems.


Gas exchange is, of course, the primary function of the lungs. Each alveolus is surrounded by a network of capillaries. The capillaries bring deoxygenated blood from the pulmonary arteries, which originate from the right ventricle of the heart. The walls of the alveoli are only one cell thick, which facilitates diffusion of carbon dioxide from the blood into the lungs and oxygen into the blood. The oxygenated blood returns to the left atrium of the heart via the pulmonary veins.


Diffusion of gases occurs across a very thin membrane between the alveoli and the capillary. However, certain diseases may cause fibrosis, or scarring, of this membrane, resulting in less effective diffusion. Other diseases may cause a limitation of ventilation (gas flow) or perfusion (blood flow) to the lung. All of these mechanisms can cause hypoxia—low blood oxygen levels—although they accomplish that same end result through different means.

The driving force for gas exchange is the pressure differential of the gases. When it initially arrives at the alveoli, blood has a relatively low partial pressure of oxygen and a relatively high partial pressure of carbon dioxide, facilitating transfer of each down its respective concentration gradient, as shown in Figure 6.5. Because the gradient between the blood and air in the lungs is already present as the blood enters the lungs, no energy is required for gas transfer.

Figure 6.5. Gas Exchange in the Alveolus


O2 in the alveoli flows down its partial pressure gradient from the alveoli into the pulmonary capillaries, where it can bind to hemoglobin for transport. Meanwhile, CO2 flows down its partial pressure gradient from the capillaries into the alveoli for expiration.

How would our respiratory systems adjust if we moved to higher altitudes where less oxygen is available? First, we would breathe more rapidly to try to avoid hypoxia; second, the binding dynamics of hemoglobin to oxygen would be altered to facilitate the unloading of oxygen at the tissues. As we will discuss in Chapter 7 of MCAT Biology Review, the natural response of hemoglobin to the decreased carbon dioxide concentration in the environment would actually be to decrease the unloading of oxygen to tissues, so other mechanisms can counteract and override this phenomenon to allow adequate delivery of oxygen. We could make more red cells to carry the oxygen. In the long term, we could develop more blood vessels (vascularization), which would facilitate the distribution of oxygen to tissues.


In order to maximize gas exchange, there is a tremendous surface area over which the alveoli and capillaries interact. Because the entire respiratory tract is highly vascular, it can also be used for thermoregulation, or the regulation of body temperature. Heat—the transfer of thermal energy—is regulated through the body surfaces by vasodilation and vasoconstriction. As capillaries expand, more blood can pass through these vessels, and a larger amount of thermal energy can be dissipated. As capillaries contract, less blood can pass through them, conserving thermal energy. The capillaries within the nasal and tracheal capillary beds are most frequently used for these purposes within the respiratory system. While these capillary beds provide a mechanism for thermoregulation, humans predominantly regulate temperature using capillaries and sweat glands in the skin, or rapid muscle contraction (shivering).


As mentioned above, the lungs provide a large interface for the body to interact with the outside world. While this is important for gas exchange and thermoregulation, it also comes with potential risks—pathogens such as bacteria, viruses, and fungi can cause infections within the lung, or can attempt to gain access to the body through the rich vascularity of the alveolar membranes. By necessity, the lungs must be able to fight off potential invaders. The first line of defense occurs within the nasal cavity, with small hairs (vibrissae) that help to trap particulate matter and potentially infectious particles. The nasal cavity also contains an enzyme called lysozyme. Also found in tears and saliva, lysozyme is able to attack the peptidoglycan walls of gram-positive bacteria. The internal airways are lined with mucus, which traps particulate matter and larger invaders. Underlying cilia then propel the mucus up the respiratory tract to the oral cavity, where it can be expelled or swallowed; this mechanism is called the mucociliary escalator.


Pneumonia is an infection of the lung most often caused by bacteria or viruses. Atypical pneumonia, commonly called walking pneumonia (because the infection does not require hospitalization and does not leave the patient bedridden), is often caused by very small bacterium calledMycoplasma pneumoniae. This bacterium causes a prolonged cough because it damages epithelial cells lining the lung and paralyzes the cilia lining the respiratory tract. The lack of cilia makes it much more difficult to clear mucus from the lungs. The cough lasts until the respiratory epithelial cells have recovered and the cilia are once again functional.

The lungs, especially the alveoli, also contain numerous immune cells, including macrophages. Macrophages can engulf and digest pathogens and signal to the rest of the immune system that there is an invader. Mucosal surfaces also contain IgA antibodies that help to protect against pathogens that contact the mucous membranes. Finally, mast cells also populate the lungs. These cells have preformed antibodies on their surfaces. When the right substance attaches to the antibody, the mast cell releases inflammatory chemicals into the surrounding area to promote an immune response. Unfortunately, these antibodies are often reactive to substances such as pollen and molds, so mast cells also provide the inflammatory chemicals that mediate allergic reactions.


The respiratory system plays a role in pH balance through the bicarbonate buffer system in the blood:

CO2 (g) + H2O (l) ⇌ H2CO3 (aq) ⇌ H+ (aq) + HCO3 (aq)


The division within the sciences is largely artificial; the MCAT often contains questions that integrate multiple science disciplines. A question located in the Biological and Biochemical Foundations of Living Systems section may require knowledge of general chemistry. In fact, 10% of this section is chemistry: 4% general chemistry and 6% organic chemistry.

Questions regarding the bicarbonate buffer system are MCAT favorites, and you are very likely to see it in some form on Test Day. This equation represents an opportunity for the MCAT to test understanding of basic chemistry concepts, such as Le Châtelier’s principle, as well as how disturbances in pH may affect respiration.

The body attempts to maintain a pH between 7.35 and 7.45. When the pH is lower, and hydrogen ion concentration is higher (acidemia), acid-sensing chemoreceptors just outside the blood–brain barrier send signals to the brain to increase the respiratory rate. Further, an increasing hydrogen ion concentration will cause a shift in the bicarbonate buffer system, generating additional carbon dioxide. As described earlier, the respiratory centers in the brain are sensitive to this increasing partial pressure of carbon dioxide and will also promote an increase in respiratory rate.


Metabolic acidosis—a production of excess acid by any mechanism besides hypoventilation—is a common occurrence in medicine. Anaerobic respiration can generate lactic acid; individuals with type 1 diabetes mellitus can produce ketoacids when they are hypoinsulinemic; certain poisons, like methanol and formaldehyde, can produce organic acids. In each of these cases, one of the primary methods of compensation is increasing respiration rate.

As the respiratory rate increases, more carbon dioxide is blown off. This will also push the buffer equation to the left, but notice the difference: the shift to the left in the previous paragraph was caused by an increase in hydrogen ion concentration, which elevated the concentration of carbon dioxide. Here, the removal of carbon dioxide causes a shift to the left that allows the hydrogen ion concentration to drop back to normal.

If the blood is too basic (alkalemia), then the body will seek to increase acidity. How can the lungs contribute to this? If the respiratory rate is slowed, then more carbon dioxide will be retained, shifting the buffer equation to right and producing more hydrogen ions and bicarbonate ions. This results in a lower pH.


If H+ is an acid and HCO3 is a base, then why doesn’t increasing both of them maintain a constant pH? The reason is because H+ is a strong acid, while HCO3 is a weak base. Just like a titration, discussed in Chapter 10 of MCAT General Chemistry Review, this combination will shift the pH of the solution toward the acidic range.

Overall, the lungs play a role in the immediate adjustment of carbon dioxide levels and, by extension, hydrogen ion levels. However, the lungs do not work alone to maintain proper pH. The kidneys also play a role by modulating secretion and reabsorption of acid and base within the nephron. This is a much slower response, however, and represents long-term compensation. For more information on kidney function and homeostasis, see Chapter 10 of MCAT Biology Review.


This equation is essential to Test Day success:

CO2 (g) + H2O (l) ⇌ H2CO3 (aq) ⇌ H+ (aq) + HCO3 (aq)

It is likely to be tested in both the Biological and Biochemical Foundations of Living Systems and the Chemical and Physical Foundations of Biological Systems sections.

MCAT Concept Check 6.2:

Before you move on, assess your understanding of the material with these questions.

1.    What are some of the mechanisms used in the respiratory system to prevent infection?

2.    What is the chemical equation for the bicarbonate buffer system?

3.    Respiratory failure refers to inadequate ventilation to provide oxygen to the tissues. How would the pH change in respiratory failure?