This question requires you to use your A level knowledge on the structure and function of the lung, as well as water potentials and movement of water by osmosis.
Drowning is defined as death caused by filling of the lungs with fluid. Normally the lungs, specifically the alveolar sacs, are filled with air. This is where exchange of gases across the alveolar and pulmonary capillary membranes into the bloodstream by diffusion occurs. When water is sucked into the alveoli, the diffusion of oxygen (and carbon dioxide) is impeded and leads to death due to lack of oxygen to the brain and other organs (hypoxia due to asphyxiation).
This mechanism is amplified in saltwater drowning: saltwater contains a higher solute concentration than blood plasma, therefore water will move by osmosis out of the pulmonary capillaries and into the alveoli down the water potential gradient. Therefore this further increases the amount of fluid in the lungs and the victim drowns in their own fluids and dies from asphyxia-induced hypoxia.
The difference between drowning in freshwater vs saltwater is the direction of water movement, due to their different water potentials. In freshwater (i.e. pure water), water moves by osmosis from the blood into the alveoli (remember that blood plasma has a lower water potential due to proteins and dissolved solutes). This raises the water potential of the blood plasma, causing an imbalance in ions, which leads to cardiac fibrillation (irregular heart beat) and death.
Your interviewer could then ask you further questions to push your thinking, e.g. about how the ions in the blood are dysregulated and why they are important in maintaining heart beat:
The increase in water in the blood decreases the concentration of all ions (notably sodium)
Water moves into red blood cells (again, by osmosis down its water potential gradient), and this causes them to burst, releasing potassium ions into the blood
Sodium and potassium concentrations are the key ions involved in generating action potentials and their concentrations must be carefully maintained by ion channels pumps. Therefore, changing their ratio will affect action potentials in cardiac cells and could drastically dysregulate heartbeat.
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