Predict what would happen to the tidal volume and inspiratory reserve volume if the phrenic nerves were severed. Which muscles would contract to try to compensate for this?

Verified step by step guidance

1. Understand that the phrenic nerves are responsible for innervating the diaphragm, which is the primary muscle involved in breathing.
2. Recognize that if the phrenic nerves are severed, the diaphragm would be paralyzed, leading to a significant reduction in tidal volume, as the diaphragm cannot contract to facilitate normal inhalation.
3. Consider that the inspiratory reserve volume would also decrease because the diaphragm's inability to contract would limit the capacity to take a deep breath beyond the normal tidal volume.
4. Identify that accessory muscles of respiration, such as the intercostal muscles, sternocleidomastoid, and scalene muscles, would attempt to compensate by contracting to elevate the rib cage and increase thoracic volume.
5. Acknowledge that while these accessory muscles can aid in breathing, they are not as effective as the diaphragm, and breathing would be more labored and less efficient.

Key Concepts

Tidal Volume

Tidal volume refers to the amount of air inhaled or exhaled during normal breathing. It is a critical measure of lung function and respiratory health. If the phrenic nerves, which innervate the diaphragm, are severed, the tidal volume would likely decrease due to the diaphragm's inability to contract effectively, leading to shallower breaths.

Inspiratory Reserve Volume

Inspiratory reserve volume (IRV) is the additional amount of air that can be inhaled after a normal inhalation. Severing the phrenic nerves would impair the diaphragm's function, potentially reducing the IRV as the body struggles to take deeper breaths. This could lead to a reliance on accessory muscles for deeper inhalation.

Accessory Muscles of Breathing

Accessory muscles of breathing, such as the sternocleidomastoid and scalene muscles, assist in inhalation, especially when the diaphragm is compromised. If the phrenic nerves are severed, these muscles would contract more actively to compensate for the reduced tidal volume and inspiratory reserve volume, allowing for some degree of ventilation despite the loss of diaphragm function.