Final answer:
Underwater, the muscles used for breathing through a snorkel work to equalize lung and external pressure. A snorkel is ineffective in deep water because the water pressure exceeds what lung expansion can overcome. Thus, snorkeling is only viable in shallow water.
Step-by-step explanation:
Understanding Breathing Underwater with a Snorkel
When a person breathes underwater using a snorkel, the muscles involved in respiration function differently than they do on land. The pressure within the lungs must equalize with the water pressure, which can be much greater than atmospheric pressure. During inhalation, muscles such as the diaphragm and rib cage expand the chest cavity, creating a negative gauge pressure that draws air in through the snorkel. However, the function of these muscles is limited by the snorkel's length and the depth of water.
A snorkel cannot be used in deep water due to the increasing water pressure with depth, which significantly exceeds the pressure humans can compensate for by expanding their lungs. At greater depths, the surrounding water pressure would prevent the muscles from expanding the chest cavity effectively, and without additional equipment like a scuba tank and regulator, it becomes impossible to inhale through a snorkel. The diaphragm and rib cage would face undue pressure, and without the proper pressure differential, air could not be drawn into the lungs.
Despite the body's natural capability to equalize pressures, the limited length of a snorkel makes it impractical and unsafe to use beyond shallow water. Additionally, exhalation underwater through a snorkel is primarily passive due to the elasticity of the lungs, although muscle action can add to this positive pressure to produce forced exhalation if needed.