Respiratory system

Respiratory system

Oxygen Hemoglobin Dissociation Curve

The oxygen-hemoglobin dissociation curve is a curve of oxygen-saturated hemoglobin against oxygen tension in the Blood. Oxygen is primarily transported bound to a hemoglobin tetramer, or in the dissolved form, with hemoglobin accounting for 98% of all oxygen transported. In an unbound state, the hemoglobin predominates in the tense (T) form, while in the bound state, hemoglobin exists in the rested (R) form. T form requires a higher PO2 to allow oxygen binding. In the R form, oxygen binding occurs with lower PO2. The T state also has a lower affinity for oxygen than the R state (Kaufman et al., 2022). The oxyhemoglobin dissociation curve plots oxygen tension on the x-axis against oxygen saturation on the y-axis. The curve is sigmoid-shaped. It plateaus at high oxygen tension and is steeper at lower oxygen tension.

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Oxygen loading is favored when the curve shifts leftwards while unloading is selected when the curve changes to the right. The curve moves to the right in the tissues where carbon dioxide concentration is high and the pH is low, causing oxygen to be unloaded into the tissues. Oxygen is delivered to metabolically active tissues that use oxygen and glucose and releases carbon dioxide and organic acids via this mechanism. As CO2 concentration increases, the pH decreases, and the oxygen-rich hemoglobin offloads the oxygen to the tissues. The released CO2 forms carbonic acid, which dissociates in the lungs into CO2 and water.

Lung Involvement in Maintaining Normal pH of the Blood

The lungs are involved in the respiratory regulation of acid-base balance. The CO2 produced in the body via metabolic action readily diffuses into the nearby capillary bed. A small proportion of CO2 dissolves in Blood and is transported to the lungs directly unchanged. Most of the CO2 released in the tissues diffuses into the red blood cells, where it readily reacts with water to form carbonic acid. The levels of CO2 and carbonic acid are in equilibrium in the body. As the level of CO2 increases due to increased metabolic activity or as seen in breath holding, more carbonic acid is formed, thus lowering the blood pH. Carbonic acid dissociates in to H+ and bicarbonate (HCO3). The bicarbonate formed is transported to the lungs, where a reverse reaction occurs: the bicarbonate is converted to carbonic acid and further to CO2 and water. The CO2 released diffuses into the alveoli and is breathed out (Higgins, 2022). The high CO2 concentration in the Blood triggers respiratory chemoreceptors that elicit hyperventilation as a reflex response to increase the excretion of CO2.

Intrapulmonary Pressure, Atmospheric Pressure, and Air Flow

Intrapulmonary pressure refers to the force exerted by respiratory gases within the alveoli. Atmospheric pressure is the force exerted on a surface by the air above that surface as a result of gravitational pull. Boyle’s law asserts that at a constant temperature, the volume of a gas is inversely proportional to the pressure exerted on it (Kenny & Ponichtera, 2022). As the volume increases, the pressure exerted on the gas decreases, and as volume decreases, the pressure exerted on the gas increases.

Air flows from a region of high pressure to an area of low pressure. Air flows in and out of the lungs due to pressure differences between the lung cavity and the atmosphere. During everyday inspiration, the diaphragm contracts and assumes a flatter shape, and the volume of the chest cavity increases. This lowers the intra-alveolar pressure, causing air to rush into the lungs (Powers & Dhamoon, 2022). During normal expiration, the diaphragm relaxes and assumes a domicile shape. The thoracic cavity’s volume decreases. This causes a pressure build-up in the interalveolar space. When this pressure exceeds the atmospheric pressure, exhalation occurs.

Compliance, Alveolar Surface Tension, and Airway Resistance

Compliance is defined as the expansibility of the lungs and the chest wall. Dynamic compliance is the change in the expansibility of the lungs in the presence of airflow. Static compliance, on the other hand, is the lungs’ expansibility in the absence of airflow. High lung compliance shows that little pressure differential is required to change the volume of the lungs, while lower lung compliance means that a high-pressure differential is necessary to change the volume of the lungs. Lower lung compliance is seen in restrictive lung pathologies such as fibrosis and interstitial lung disease (Edwards & Annamaraju, 2022). Increases in lung compliance can also be seen in obstructive pathologies such as asthma and chronic obstructive pulmonary disease.

Alveolar surface tension refers to the cohesive state between the alveoli’s liquid-liquid and liquid gas interfaces. Increased alveolar surface tension increases cohesion between liquid gas interfaces within the alveoli, pulling the alveoli sac closed. This increases the energy required to expand the lungs for ventilation to occur, thus increasing the work on breathing. Such is the case with respiratory distress syndrome. The alveoli release a surfactant fluid to lower alveoli tension, thus enhancing breathing efficiency.

Airway resistance refers to the resistance to airflow through the respiratory tract resulting from changes in transpulmonary pressure. Airway resistance decreases as the radius of the airway increases. This aids in the air passage and is pivotal during inspiration. Conversely, during expiration, the high intrathoracic pressure causes the narrowing of the airway and, therefore, increases airway resistance.

Albuterol in Asthma

In asthma, bronchoconstriction occurs, resulting in airway blockade. This causes the symptomatic wheezing sound and shortness of breath that highlights asthmatic attacks. Albuterol, a selective beta 2 agonist, acts by binding to beta-2 adrenergic receptors in the bronchial smooth muscle to cause relaxation of the bronchial smooth muscles. This relieves airway blockade seen in asthma disease and helps improve the symptoms of wheezing sounds and shortness of breath seen in an asthmatic attack (Johnson et al., 2022). Albuterol also inhibits the release of immediate inflammatory mediators such as mast cells, thus dampening the inflammatory response that characterizes asthmatic attacks.

References

Edwards, Z., & Annamaraju, P. (2022). Physiology, Lung Compliance. Ncbi.nlm.nih.gov. Retrieved 30 August 2022, from https://www.ncbi.nlm.nih.gov/books/NBK554517/.

Higgins, C. (2022). An introduction to acid-base balance in health and disease. Acutecaretesting.org. Retrieved 30 August 2022, from https://acutecaretesting.org/en/articles/an-introduction-to-acidbase-balance-in-health-and-disease.

Johnson, D., Merrell, B., & Bounds, C. (2022). Albuterol. Ncbi.nlm.nih.gov. Retrieved 30 August 2022, from https://www.ncbi.nlm.nih.gov/books/NBK482272/.

Kaufman, D., Kandle, P., Murray, I., & Dhamoon, A. (2022). Physiology, Oxyhemoglobin Dissociation Curve. Ncbi.nlm.nih.gov. Retrieved 30 August 2022, from https://www.ncbi.nlm.nih.gov/books/NBK499818/.

Kenny, B., & Ponichtera, K. (2022). Physiology, Boyle’s Law. Ncbi.nlm.nih.gov. Retrieved 30 August 2022, from https://www.ncbi.nlm.nih.gov/books/NBK538183/.

Powers, K., & Dhamoon, A. (2022). Physiology, Pulmonary Ventilation, and Perfusion. Ncbi.nlm.nih.gov. Retrieved 30 August 2022, from https://www.ncbi.nlm.nih.gov/books/NBK539907/.

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Describe the oxygen-hemoglobin dissociation curve. Explain how it allows for body tissues to receive oxygen and eliminate carbon dioxide wastes.

Respiratory system

How are the lungs involved in maintaining the normal pH of the Blood?

Describe the relationship between intrapulmonary pressure, atmospheric pressure, and airflow during everyday inspiration and expiration, referring to Boyle’s law.

Several physical factors that influence the efficiency of pulmonary ventilation are compliance, alveolar surface tension, and airway resistance. Briefly describe each factor.

Albuterol is a selective beta-2 adrenergic agonist, which means it explicitly activates beta-2 adrenergic receptors on the smooth muscle in the airways. How does this improve asthma symptoms?

*Some questions adapted from Pearson’s Mastering Instructor Resources for Amerman, E. C. (2019) Human anatomy & physiology (2nd ed.).

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