How do most freshwater fish maintain water balance?

Freshwater fish face a constant challenge: their bodies are more concentrated with dissolved salts than the surrounding water. This means water tends to flow *into* their bodies, causing them to swell. To overcome this, they have developed several mechanisms to maintain water balance:

1. Drinking Very Little Water: Unlike saltwater fish, freshwater fish drink very little water. They actually rely on the water that enters their bodies through osmosis.

2. Producing Large Amounts of Dilute Urine: To get rid of excess water, freshwater fish produce large amounts of very dilute urine. This helps flush out excess water and maintain their salt concentration.

3. Actively Absorbing Salts: Freshwater fish actively absorb salts from their environment through their gills. This helps them replenish the salts lost through urine and maintain their internal salt levels.

4. Specialized Cells in Gills: Their gills have specialized cells called chloride cells that actively pump salts from the water into the bloodstream, further enhancing salt absorption.

5. Limited Surface Area: Freshwater fish have a smaller surface area compared to saltwater fish, reducing the amount of water that can enter their bodies through osmosis.

These mechanisms work together to ensure freshwater fish maintain a stable internal environment, despite the constant influx of water from their surroundings.