Osmoregulation

Osmoregulation is the way living things keep the right balance of water and salts inside their bodies. All organisms, from tiny fish to humans, need to control how much water is in their blood and cells. If there is too much water, cells can swell and burst. If there is too little, cells can shrink and not work well.

This balance is managed by special sensors called osmoreceptors that detect changes in osmotic pressure—the push that makes water move from places with less salt to places with more salt. To stop this movement, organisms use different methods, like the kidneys in humans, which clean the blood and control how much water is kept or removed.

Whether an animal lives in water, like a fish, or on land, like a human, it must keep its internal fluids at just the right concentration. This process, called excretion, also helps get rid of waste products so they don’t build up and make the body sick. Osmoregulation is a key part of keeping any living thing healthy and working properly.

Regulators and conformers

There are two main ways animals manage their internal water and salt levels: osmoconformers and osmoregulators. Osmoconformers adjust their body’s salt levels to match their surroundings. Many sea creatures without a backbone, like invertebrates, are osmoconformers.

Osmoregulators, like most fish, keep their internal salt levels different from their environment. Freshwater fish take in salt from their surroundings and remove extra water by producing very dilute urine. In contrast, saltwater fish lose water and gain salt, so they remove salt through their gills. Some fish, like flounder, can live in both fresh and saltwater by changing their body processes. Sharks keep a high level of a chemical called urea in their blood to save water and also use another chemical to protect their cells.

In plants

Plants do not have special organs for managing water like animals do, but they have other ways to control water loss and balance. The tiny pores on leaves called stomata help regulate how much water a plant loses. When it’s windy, dry, or very hot, plants lose more water, so they use a special hormone called abscisic acid to help close these pores and grow more roots to get extra water.

Some plants, known as xerophytes, live in very dry places like deserts. They have tricks to save water, such as storing it inside their cells or having needle-shaped leaves to reduce water loss. Other plants, called hydrophytes, grow in water and don’t worry much about drying out. There are also halophytes that grow in salty soils and have ways to handle extra salt, and mesophytes that live in temperate zones with plenty of water.

In animals and humans

Kidneys play a big role in keeping our body’s water balance. They decide how much water to keep from the liquid filtered in the kidneys, using special signals from hormones like antidiuretic hormone, aldosterone, and angiotensin II. When the brain senses less water, it tells the kidneys to hold onto more water.

Marine mammals like seals and whales don’t drink much water; they get what they need from their food and body processes. Some, like manatees, often drink fresh water, while sea otters drink saltwater. Fish, especially advanced ray-finned ones, use their gills, kidneys, and digestive systems to keep their body fluids balanced. Some special fish even have extra organs to help manage salt in their bodies.

In protists

Amoeba uses special structures called contractile vacuoles to keep its water balance. These vacuoles collect waste materials, like ammonia, from inside the cell through processes known as diffusion and active transport. When water moves into the cell, the vacuole moves to the cell's surface and pushes the waste out, helping the amoeba stay balanced.

In bacteria

Bacteria respond to changes in water pressure by quickly taking in special chemicals. This helps them stay balanced when their environment gets too salty or too fresh. In a well-studied type of bacteria called E. coli, special systems help control which chemicals get in.

Vertebrate excretory systems

Ammonia is a by-product made when mammals eat protein. Ammonia is very harmful, so animals change it into safer waste. Mammals change it into urea, while birds and reptiles change it into uric acid. This waste leaves the body through a part called the cloacas.

Vertebrates have four main steps to keep water balanced in their bodies. First, blood is filtered in the kidney. Next, most of the filtered fluid goes back into the blood. Then, the leftover fluid becomes urine. Finally, the urine leaves the body. In mammals, it is stored in the urinary bladder and leaves through the urethra. In other animals, the urine mixes with other wastes in the cloaca before leaving.