Lesson 6

Essential Question: 

How could drinking too much water lead to death? 

(Hint: Size of cells)

Directions: 

Step #1: Complete the reading

Step #2: Complete the two questions below the article.

Osmosis and Water Intoxication

Cells depend on a careful balance of water moving in and out to survive. This movement of water is called osmosis, which happens when water moves across a semi-permeable membrane. The membrane lets some molecules pass through because of their size, but blocks others. Water moves from areas with more water to areas with less water until the cell reaches equilibrium.

Normally, your body controls how much water is inside and outside your cells. Your kidneys remove extra water and keep your blood at a safe level of concentration. But when a person drinks too much water too quickly, the amount of water in the blood becomes extremely high. This makes the blood more diluted, meaning there is now a higher concentration of water outside the cells than inside.

Because of this difference, water begins rushing into the cells by osmosis. The cells take in more and more water as they try to reach equilibrium. Animal cells do not have a cell wall, so they cannot resist large increases in pressure. As more water fills the inside, the cells begin to swell. Some cells can swell only a little before they start to stretch and weaken.

If the amount of incoming water becomes too extreme, the pressure inside the cells becomes too high. The membrane cannot handle the stress. This can cause the cells to burst. When many cells burst at the same time, important body systems begin to fail. Brain cells are especially sensitive to swelling. If large numbers of brain cells swell and burst, the brain can become dangerously compressed inside the skull.

This condition is known as water intoxication. It can lead to confusion, seizures, coma, or death. It all begins with osmosis and the breakdown of the normal balance of water inside and outside cells. Understanding osmosis helps explain why the amount of water a person drinks must match what the body can safely handle.

Doctors often give a patient with water intoxication a saline solution through an IV. Saline contains salt dissolved in water. Since water follows salt, the added salt in the bloodstream makes the fluid outside the swollen cells more concentrated than the fluid inside them. Because of this difference, water begins to move out of the cells and into the bloodstream through osmosis. This movement removes extra water from the cells so they shrink back to a safe size. By pulling water out, the saline IV prevents the cells from bursting and helps the body return to equilibrium.

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Questions:

1. Explain why water moves into cells during water intoxication. In your answer, describe the difference in concentration inside and outside the cell.

2. A patient with water intoxication is given a saline IV. Explain how the salt in the IV changes the movement of water across the cell membrane and why this helps protect the cells from bursting.

In your medical journals, write down these key concepts. 

Directions: Answer the questions in the below Padlet. Use what you have learned in this lesson and unit to answer the questions.