MCAT Biology Review

Chapter 6: The Respiratory System


As we learn about the human body, it may be easy to reduce the complex and varied functions of the lungs to simply focus on breathing and providing a supply of oxygen. The lungs do indeed perform gas exchange, which relies on differences in partial pressures of gases between the alveoli and the blood. Oxygen is taken up by the blood, while carbon dioxide is released for exhalation. Inhalation and exhalation also require pressure differentials created by anatomical structures such as the chest wall, diaphragm, pleurae, and lungs.

However, the lungs are so much more than just bags of air; gas exchange is not the only function of the respiratory system. The respiratory system also serves essential roles in thermoregulation, immunity, and pH regulation. As we go through the individual systems within the human body, take special note of how each system is integrated with other systems. One of the more clear connections is the binding of oxygen to hemoglobin in the lungs and the circulatory system—a concept we will expand upon in the next chapter, along with considering the effects of altitude, pH, and chemicals on this binding.

Concept Summary

Anatomy and Mechanism of Breathing

·        Air is drawn in through the nares, and through the nasal cavity and pharynx, where it is warmed and humidified. It is filtered by nasal hairs (vibrissae) and mucous membranes. It then enters the larynx, followed by the trachea. The trachea divides into two mainstem bronchi, which divide into bronchioles, which divide into continually smaller passages until reaching the alveoli.

·        Alveoli are small sacs that interface with the pulmonary capillaries, allowing gases to diffuse across a one-cell-thick membrane.

·        Surfactant in the alveoli reduces surface tension at the liquid–gas interface, preventing collapse.

·        The pleurae cover the lungs and line the chest wall.

o   The visceral pleura lies adjacent to the lung itself.

o   The parietal pleura lines the chest wall.

o   The intrapleural space lies between these two layers and contains a thin layer of fluid, which lubricates the two pleural surfaces.

·        The diaphragm is a thin skeletal muscle that helps to create the pressure differential required for breathing.

·        Inhalation is an active process.

o   The diaphragm and external intercostal muscles expand the thoracic cavity, increasing the volume of the intrapleural space. This decreases the intrapleural pressure.

o   This pressure differential ultimately expands the lungs, dropping their pressure and drawing in air from the environment. This mechanism is termed negative-pressure breathing.

·        Exhalation may be passive or active.

o   In passive exhalation, relaxation of the muscles of inspiration and elastic recoil of the lungs allow the chest cavity to decrease in volume, reversing the pressure differentials seen in inhalation.

o   In active exhalation, the internal intercostal muscles and abdominal muscles can be used to forcibly decrease the volume of the thoracic cavity, pushing out air.

·        A spirometer can be used to measure lung capacities and volumes.

o   Total lung capacity (TLC) is the maximum volume of air in the lungs when one inhales completely.

o   Residual volume (RV) is the minimum volume of air in the lungs when one exhales completely.

o   Vital capacity (VC) is the difference between the minimum and maximum volume of air in the lungs.

o   Tidal volume (TV) is the volume of air inhaled or exhaled in a normal breath.

o   Expiratory reserve volume (ERV) is the volume of additional air that can be forcibly exhaled after a normal exhalation.

o   Inspiratory reserve volume (IRV) is the volume of additional air that can be forcibly inhaled after a normal inhalation.

·        Ventilation is regulated by the ventilation center, a collection of neurons in the medulla oblongata.

o   Chemoreceptors respond to carbon dioxide concentrations, increasing the respiratory rate when there are high concentrations of carbon dioxide in the blood (hypercarbia or hypercapnia).

o   The ventilation center can also respond to low oxygen concentrations in the blood (hypoxia) by increasing ventilation rate.

o   Ventilation can also be controlled consciously through the cerebrum, although the medulla oblongata will override the cerebrum during extended periods of hypo- or hyperventilation.

Functions of the Respiratory System

·        The lungs perform gas exchange with the blood through simple diffusion across concentration gradients.

o   Deoxygenated blood with a high carbon dioxide concentration is brought to the lungs via the pulmonary arteries.

o   Oxygenated blood with a low carbon dioxide concentration leaves the lungs via the pulmonary veins.

·        The large surface area of interaction between the alveoli and capillaries allows the respiratory system to assist in thermoregulation through vasodilation and vasoconstriction of capillary beds.

·        The respiratory system must be protected from potential pathogens.

o   Multiple mechanisms, including vibrissae, mucous membranes, and the mucociliary escalator, help filter the incoming air and trap particulate matter.

o   Lysozyme in the nasal cavity and saliva attacks peptidoglycan cell walls of gram-positive bacteria.

o   Macrophages can engulf and digest pathogens and signal to the rest of the immune system that there is an invader.

o   Mucosal surfaces are covered with IgA antibodies.

o   Mast cells have antibodies on their surface that, when triggered, can promote the release of inflammatory chemicals. Mast cells are often involved in allergic reactions as well.

·        The respiratory system is involved in pH control through the bicarbonate buffer system.

o   When blood pH decreases, respiration rate increases to compensate by blowing off carbon dioxide. This causes a left shift in the buffer equation, reducing hydrogen ion concentration.

o   When blood pH increases, respiration rate decreases to compensate by trapping carbon dioxide. This causes a right shift in the buffer equation, increasing hydrogen ion concentration.

Answers to Concept Checks

·        6.1

1.    Nares → nasal cavity → pharynx → larynx → trachea → bronchi → bronchioles → alveoli

2.    Inhalation uses the diaphragm and external intercostal muscles; in labored breathing, abdominal muscles and muscles of the neck may also be involved. Passive exhalation uses the recoil of these same muscles; active exhalation also uses the internal intercostal muscles and abdominal muscles.

3.    Surfactant reduces surface tension at the air–liquid interface at the alveoli. This prevents the collapse of alveoli.

4.    Vital capacity is the sum of the inspiratory reserve volume, expiratory reserve volume, and tidal volume: VC = IRV + ERV + TV

5.    When CO2 levels become too low, the brain can decrease the respiratory rate in order to raise CO2 levels.

·        6.2

1.    Immune mechanisms in the respiratory system include vibrissae in the nares, lysozyme in the mucous membranes, the mucociliary escalator, macrophages in the lungs, mucosal IgA antibodies, and mast cells.

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

3.    In respiratory failure, ventilation slows, and less carbon dioxide is blown off. As this occurs, the buffer equation shifts to the right, and more hydrogen ions are generated. This results in a lower pH of the blood.

Shared Concepts

·        Biology Chapter 7

o   The Cardiovascular System

·        Biology Chapter 10

o   Homeostasis

·        General Chemistry Chapter 6

o   Equilibrium

·        General Chemistry Chapter 8

o   The Gas Phase

·        General Chemistry Chapter 10

o   Acids and Bases

·        Physics and Math Chapter 3

o   Thermodynamics