Question

Describe how Boyle’s Law contributes to one type of ventilation mechanism (fish 2-stroke, amphibian buccal, crocodilian, avian, mammalian). Explain what happens in each step of the process with respect to volume and pressure changes caused by muscle contraction/relaxation of the body cavities involved

Answer 1

We can breathe air in and out of our lungs because of Boyle's law. According to Boyle's law, if a given amount of gas has a constant temperature, increasing its volume decreases its pressure, and vice-versa. When you inhale, muscles increase the size of your thoracic (chest) cavity and expand your lungs.

Air flows into the lungs largely due to a difference in pressure; atmospheric pressure is greater than intra-alveolar pressure, and intra-alveolar pressure is greater than intrapleural pressure. Air flows out of the lungs during expiration based on the same principle; pressure within the lungs becomes greater than the atmospheric pressure.A respiratory cycle is one sequence of inspiration and expiration. In general, two muscle groups are used during normal inspiration: the diaphragm and the external intercostal muscles. Additional muscles can be used if a bigger breath is required. When the diaphragm contracts, it moves inferiorly toward the abdominal cavity, creating a larger thoracic cavity and more space for the lungs. Contraction of the external intercostal muscles moves the ribs upward and outward, causing the rib cage to expand, which increases the volume of the thoracic cavity. Due to the adhesive force of the pleural fluid, the expansion of the thoracic cavity forces the lungs to stretch and expand as well. This increase in volume leads to a decrease in intra-alveolar pressure, creating a pressure lower than  atmospheric pressure. As a result, a pressure gradient is created that drives air into the lungs. The process of normal expiration is passive, meaning that energy is not required to push air out of the lungs. Instead, the elasticity of the lung tissue causes the lung to recoil, as the diaphragm and intercostal muscles relax following inspiration. In turn, the thoracic cavity and lungs decrease in volume, causing an increase in interpulmonary pressure. The interpulmonary pressure rises above atmospheric pressure, creating a pressure gradient that causes air to leave the lungs.

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