Learning Exercise

Physiology: Modeling asthma - can that cause death?

This assignment helps students sort out their understanding of diffusion and bulk flow as two separate processes, both of which influence gas exchange in the human respiratory system. They construct a clearer understanding of the structures and functions in the respiratory system as they explore various models showing how asthma causes pathological function.
Course: Human Physiology for Non-Science Majors

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Sample student responses

High quality:

Asthma is a disease characterized by intermittent "attacks" in which the smooth muscles of the airway contract repeatedly, increasing airway resistance enough to impair ventilation. In the designed experiment, both models will experience difficulty with respiration. In the scenario involving breathing through a straw, the subject will experience a higher resistance. Although this method is effective at oxygen exchange for a limited time, the subject will eventually reach a point of exhaustion. In time, breathing through the small diameter will be simlilar to what patient's experience with broncho constriction in where the inflammed bronchi prevent adequate oxygen exchange thereby preventing diffusion between the alveoli and the blood. Constant airway resistance will inevitably lead to a state of hypoxia in where there is a deficiency of oxygen at the tissue level.

In the second model, the diameter is much larger, however so is the length of the tube. In this case, the amount of dead space, or volume of inspired air that cannot be exchanged with the blood, will lead to hypoxia. This scenario may also lead to the condition of hypercapnia in which there is a higher partial pressure of cabon dioxide in the arteries.

In both scenarios, the decrease partial pressure gradient of carbon dioxide will lead to a decrease in the bulk flow, thus leading to inefficient gas exchange. Both scenarios will eventually lead to ineffective diffusion of both gases beween the alvioli and the blood which will prevent both model participants from practicing each of the models for the entire day.

Mid quality

When comparing both of the models with a asthma patient, all would suffer physiological consequences of hypoxia, not enough O2 in the cells, and hypercapnia, two much CO2 in the blood, which go hand in hand. When this happens it causes the partial pressures of CO2 to increase and O2 decrease to dangerous levels. This is because all increase the airway resistance due to either a narrower passage, as in the straw; increase length, as in the hose, and the asthma, due to brachoconstiction. It also decreases the rate and bulk flow of breathing. Also, with not as much air coming in or going out, it increases the dead space and limits the amount of exchange of O2 and CO2 . The students will not be able to practice this because they would not get enough O2 in their body and would pass out and drop the hose and straw and begin to breath normally.

Low quality

In the respiratory system, the changes in the volume of the chest during ventilation cause pressure gradients that create air flow. The movement of air during ventilation is called bulk flow. Partial pressure gradient is important in respiratory physiology. It is the pressure of a single gas species such as oxygen. Asthma is an obstructive lung disease which is associated with bronchoconstriction and airway edema. Both cause an increase of airway resistance and decrease of air flow. If the diameter of the bronchioles decreases, then there is a decrease in the amount of fresh air that reaches the lungs. Diffusion is the movement of molecules from an area of high concentration to low concentration. Hypoxia is when the diffusion of oxygen from the alveolus to the blood is low and there is little oxygen in the cells as a result. Hypercapnia is high concentrations of carbon dioxide in the blood. There are portions of the respiratory system that do not exchange gases with the blood that are known as dead space. With this information that I have found, I believe that the student breathing through the McDonald's straw will not be able to practice asthma for an entire day because of the small diameter. That student will not get enough oxygen to his/her lungs. However, the student with the vaccum hose will probably last the whole day because of the large diameter and enough oxygen can pass through.

Technical Notes

Respiratory system overview http://sln.fi.edu/biosci/systems/respiration.html

Lung anatomy and lung disease resources - Canadian Lung Association resources http://www.lung.ca/copd/anatomy/normal.html

Ventilation - Definitions and background information

Ventilation and endurance performance - Good explanations describing ventilation within a context for athletic training at http://home.hia.no/~stephens/ventphys.htm


Basic intro to normal lung function and asthma.


bulk flow, dead space, partial pressure gradient, bronchoconstriction, airway resistance, hypercapnia, hypoxia

Learning Objectives

To distinguish diffusion from bulk flow as two separate processes, both of which play important roles in gas exchange functions of the human respiratory system. The student should describe the structures and functions of the respiratory system represented by various models of asthma as a cause of pathological function.