What happens to the lungs when drowning occurs?

Drowning isn't the simple process most people imagine. There is a difference between fresh water and salt water drownings. Some drownings are referred to as "dry" due to laryngospasm preventing water fron entering the lungs. In "wet" drowning were large amounts of water enters the lungs several things take place depending on the water, fresh or salt and cleanliness,.In fresh water, a hypotonic solution, it diffuses into the vascular space resulting in hemodilution. It also washes away the sufactant causing the aveoli to collapse.

Salt water, a hypertonic solution, draws water from the bloodstream into the aveoli resulting in pulmonary edema preventing blood moving through the lung tissues from being oxygenated causing hypoxia.

I've left a lot out of this because it would be a 2-3 page response to go into greater detail and describe the entire pathology of drowning. Hope this helps with your question.

There is about five stages to drowing, but there is difference in

fresh water drowning, to salt water drowning in terms of the mechanism for causing suffocation. It only takes a small amount of either kind of water to damage the lungs and interfere with the body's ability to breathe. If fresh water is inhaled, it passes from the lungs to the bloodstream and destroys red blood cells. If salt water is inhaled, the salt causes fluid from the body to enter the lung tissue displacing the air.

polk2525 is correct; the lungs fill with liquid and prevent the exchange of oxygen which eventually leads to suffocation. Not a pleasant way to go.

when drowning occurs the lungs are filled and block air avoiding to enter the heart.

They fill with liquid.