Secretion of sweat is a way the body helps to maintain the homeostasis condition of the body. Sweat comprises of majorly water and electrolytes(e.g Sodium, chloride, calcium, potassium etc). A sweat is said to be hypotonic when it has a lower concentration of electrolytes than the cells of the sweat glands. These sweat glands primarily secrete sweat in response to nervous stimuli (e.g exercise) or emotional stimuli. So when large amount of water is drank by a runner who produced hypotonic sweat, his body cells will swell in response to the water intake.
When runner was running he was sweating which results in loss of water from cells and cells become shrink. But at the end of marathon when he drink water his cells will gain water and store it, which results in swallowing of cells.
