Osmoregulation

Living things must keep a careful balance of water and salt inside their bodies. This important job is called osmoregulation. It helps maintain homeostasis, which is a steady internal state. Osmoregulation manages the concentration of electrolytes, which are salts dissolved in body fluids. Without this balance, body fluids could become too thin or too thick. This happens because of osmotic pressure. This is a measure of how water wants to move from one place to another. Water naturally moves toward areas with higher osmotic pressure.

There are two main ways animals handle this task. Some are called osmoconformers. These animals match their internal salt levels to the world around them. Most marine invertebrates are osmoconformers. Other animals are called osmoregulators. These animals keep their internal salt and water levels constant even when the environment changes. They must make sure the amount of water and salt going in equals the amount going out. This is a hard job that requires active control.

Fish show us how different environments change the way osmoregulation works. Freshwater fish face a problem because water constantly flows into their bodies. To fix this, they excrete very dilute urine to get rid of the extra water. They also use special cells in their gills to take in salt. Marine fish have the opposite problem. They tend to lose water and gain too much salt from the sea. They use their gills to actively push salt out of their bodies. Some fish, like the flounder, are euryhaline. This means they can live in both fresh and salt water.

Plants also have their own ways to manage water. They do not have organs like kidneys, but they use other methods. Stomata are tiny openings on leaves that help control water loss. A hormone called abscisic acid can make these openings close to save water. Some plants, called xerophytes, live in very dry places like deserts. Cacti are succulents that store water in large tissues. Other plants, called halophytes, live in very salty soil. They can even use salt glands on their leaves to get water from the air.

In humans, the kidneys are the most important part of this system. The kidneys use a process called filtration to clean the blood. This happens in a tiny structure called a nephron. Hormones like antidiuretic hormone, or ADH, tell the kidneys how much water to keep. If your body needs more water, the pituitary gland releases ADH. This makes the kidney tubules more permeable, so more water goes back into your blood. Tiny organisms like the Paramecium use a different way. They use a contractile vacuole to pump excess water out of their cells.