What is homeostasis?

Homeostasis is the process by which organisms keep a healthy environment for their cells to live in. This includes controlling (regulating) things like temperature and the levels of water, salt, and sugar in the body. Homeostatic processes involve different organs, tissues, and cells. To coordinate their efforts, signals need to be sent across the body. The body’s main communication networks are the nervous system and the endocrine system. This article focuses on the endocrine system, which uses hormones as chemical messengers. The trick to how homeostasis works is negative feedback loops, which are explained below.

What does homeostasis do?

Imagine how you would keep a healthy environment for a pet. If the room gets too cold, you turn on the heating. If their water dish is empty, you refill it. If the air is bad, you open a window. You give them food, and remove their waste.  Cells also need to be kept in healthy conditions. They need to be warm (but not too hot), at the right pH (not too acid or alkaline), and to be bathed in fluid containing just the right amount of salt. They need oxygen and sugar (but not too much), and to have their waste cleared away.  The difference between pets and the cells in our bodies is that our cells have to provide these conditions themselves. They look after each other. They need to work as a team: different organs, tissues, and cells have different roles to play. Together they regulate the body’s internal environment to keep all the cells comfortable. And this isn’t something you need to control by thought. Ok if you get too cold, you might decide to put on a coat. But that’s not homeostasis - homeostatic processes respond automatically when the environment gets out of balance. You don’t need to think, or even be awake, for homeostasis to rebalance your internal environment. Which is good because it’s happening all the time.

What is a healthy environment for a cell?

Cells are made of chemistry. They are built from, and stuffed full of, a huge mix of different molecules. These molecules are constantly interacting, with a dizzying number of chemical reactions taking place at any moment in time.  A lot of what goes on inside a cell involves proteins. Proteins are made from long strings of amino acids. By folding up into unique shapes, different proteins can carry out a large number of different functions. Proteins are used to read DNA, break apart sugar for its energy, and control what goes into, and out of, the cell. Other proteins act as enzymes, speeding up particular chemical reactions that are needed for the cell’s activity. But chemical reactions need certain conditions to work as expected. Some reactions don’t work if the environment is too acid, or too alkaline. And proteins may fall out of shape if it gets too salty, too hot, or if the pH is wrong. All these conditions need to be controlled, just as they would be in a chemistry laboratory.

Why blood matters

Cells can’t go out shopping. They can only use molecules from their immediate environment. Like somebody unable to leave their home, cells need to have their food and necessities delivered to their door, and their waste collected and removed. In the human body, the main delivery service is the blood. Food, oxygen and other useful molecules arrive from the bloodstream. Waste is cleared in a similar way. This means that keeping the blood in a healthy state is key to providing cells with a healthy environment.  In humans, hormones are used to control homeostatic processes including regulation of the amount of sugar and calcium in the blood, and regulation of blood pressure.  Blood is also used to carry hormones across the body. By releasing hormones into the blood, an endocrine gland can send a signal that reaches cells in many different parts of the body.

Constant change

Cells are busy places; they are constantly active. They consume useful molecules and produce waste. Individual cells don’t sleep, although they may have different activity levels at different times. On a larger scale, the whole-body environment is frequently unbalanced by normal, everyday events. Every time we eat a meal, a flood of new molecules enter our blood. When we hold our breath, our blood oxygen levels fall. When we stand up, our blood pressure drops. Cells use up sugar and oxygen in even their quietest times. But when we exercise, muscle cells use up much more sugar and oxygen, and produce much more carbon dioxide. Even thinking hard uses up extra sugar - as you read this article, your busy brain is consuming more sugar than usual. All of these events change our body’s internal environment. This means the body environment is never actually balanced - rather it is always balancing. Homeostatic processes are constantly working to keep a healthy environment for our cells.

Negative Feedback Loops

At the start of this article, it says that the trick to how homeostasis works is negative feedback loops. So what are they? To explain this, I’d like you to imagine again that you are looking after a pet. One of the conditions you will need to keep stable is temperature. Maybe you have a heater to help with this.  If the heater were on all the time, the room would get too hot. So it has a thermostat. The thermostat measures the room’s temperature and turns the heater off when the temperature gets too high. This means that the heater’s action (warming up the room) directly leads to it stopping that action (when the room gets too warm). But when this happens, the room cools, which leads to the heater turning itself back on again. This is an example of a negative feedback loop:

• It’s a feedback loop because the result of the heater’s action (the room warming up) affects its behaviour (whether or not it stays on).
• It’s a negative feedback loop because the result of the heater’s action (the room warming up) leads to it stopping that action.

Negative feedback loops are really common in biology. They are often used to slow or stop a process when it’s had enough of an effect.  If you want a more detailed explanation, see our article about Feedback Loops here.  

Balancing to and fro

Your heater might be able to warm the room, but what if it’s a sunny day and it gets too hot? You would also need a way to cool it. The heater alone can’t keep the temperature stable.  In the body, homeostasis always involves separate processes that can change the environment in both directions.  For example: if blood sugar is too high, the body needs to be able to lower it. This is done using a hormone called insulin, which acts to lower blood sugar. Insulin’s release is controlled by a negative feedback loop: when blood sugar is high, insulin is released, which lowers blood sugar, which stops the release of insulin.  But if blood sugar falls too low, just stopping insulin isn’t enough. There needs to be a different process to raise it. A different hormone – glucagon – is used for this purpose. By switching between releases of insulin and glucagon, the body can raise or lower blood sugar as required to keep it at a healthy balance. If you want to know more about this example of homeostasis, see our article on sugar control.  Year of last review 2024