Question

A young woman is snorkeling and (as an observant physiology student) you understand that she must increase her tidal volume and/or her breathing frequency to maintain her alveolar ventilation rate. Why?

Answer 1

snorkel is a device used for breathing air from above the surface when the wearer's head is face downwards in the water with the mouth and the nose submerged. It may be either separate or integrated into a swimming or diving mask

Snorkels constitute respiratory dead space. When the user takes in a fresh breath, some of the previously exhaled air which remains in the snorkel is inhaled again, reducing the amount of fresh air in the inhaled volume, and increasing the risk of a buildup of carbon dioxide in the blood, which can result in hypercapnia. The greater the volume of the tube, and the smaller the tidal volume of breathing, the more this problem is exacerbated. A smaller diameter tube reduces the dead volume, but also increases resistance to airflow and so increases the work of breathing. Including the internal volume of the mask in the breathing circuit greatly expands the dead space. Occasional exhalation through the nose while snorkeling with a separate snorkel will slightly reduce the buildup of carbon dioxide, and may help in keeping the mask clear of water, but in cold water it will increase fogging. To some extent the effect of dead space can be counteracted by breathing more deeply and slowly, as this reduces the dead space ratio and work of breathing.

Alveolar ventilation rate = respiratory rate *(tidal volume - dead space )

so, to maintain the alveolar ventilation rate, the respiratory rate and tidal volume should be increased. so that the excess dead space can be overcomed.