What is the endocrine system

The endocrine system is made up of the body’s hormone-releasing glands, the hormones they release, and the organs that these hormones affect. Hormones act as chemical messengers, carrying signals that can reach every cell in the body.

Communication between cells

Imagine you are just one of the trillions of cells in a human body. You have a cell membrane, lots of useful organelles, and a rich mix of interesting molecules. But where are you? You have no idea. You don’t know you are part of a body. You have no brain, clock nor task list. You do have your DNA and you are a particular type of cell – maybe a liver cell, or a skin cell, or a neuron. So you are set up to perform certain functions. But you have little or no direct information about the body’s overall state or needs. This is the experience of every cell. But somehow, trillions of cells like this – none with any conscious awareness of the existence of the rest of the body – work together and create a single organism that moves and feels and thinks and reacts to its environment. How does any one cell ever know what it should be doing? Each cell is able to react to changes in its local environment. What is the pH? Is there enough sugar? And some cells can release chemicals to send signals to the cells next door. But this isn’t enough – in a human, cells need to react to things happening in completely different parts of the body. If the brain detects that you are dehydrated, it can’t solve this problem by itself – it needs to get a signal to the kidney. If the pancreas (an organ near the stomach) detects that blood sugar is too high, it needs to signal to the liver and other organs. And the uterus needs a signal to tell it an egg is on its way. 

Endocrine system vs nervous system

The endocrine system and nervous system have different strengths and weaknesses as communication networks. Working together, they can do things that neither system could achieve alone. The nervous system sends lightning-quick messages with pin-point accuracy. Nerves allow us to react quickly: to find our balance, catch a ball, or pull our hand away from a fire. They carry messages between organs, including sensory information. And of course neurons also form our brain. It takes billions of complex and rapid signals between neurons for us to make sense of the world, to learn, and have creative thought. The endocrine system is very different. Its network can reach every cell in the body with slower, longer-lasting messages. It can also change the strength of its signals, increasing or decreasing their effect over long periods of time. But these two systems aren’t separate. Signals from nerves can cause the release of hormones, and hormones can affect how nerves respond. The two systems work together to make sure every cell in our body is doing the right thing at the right time. Plants don’t have nerves, so they rely on their endocrine system to send signals between different tissues and organs.

Why do we need an endocrine system?

Imagine you just ate a huge piece of sweet, sticky (and delicious) cake. Your gut is now absorbing the sugar from the cake into your bloodstream. Sugar is useful to the body, but there is a lot of it coming in, and it’s unhealthy to have too much sugar in your blood at one time. To reduce your blood sugar, you need your muscle cells and liver cells to let sugar in, to move it away from the blood. But they are unable to sense the high blood sugar level themselves. Your pancreas spots the problem, but how can it send a signal to so many different cells, in so many different parts of your body, all at once? The answer is hormones. Hormones are chemical messengers. By releasing billions of hormone molecules into the bloodstream, the pancreas is able to signal to every cell in the entire body. Hormones are used to signal the status of the body and to tell organs what they should be doing (and when). Hormones can be released in sudden bursts, or their levels can be slowly raised or lowered through time. They are hugely important in the development and control of nearly all organs and processes. The endocrine system has three interacting parts – the endocrine glands, the hormones, and the target cells. 

Endocrine glands

Organs that release hormones into the bloodstream are called endocrine glands. They contain special cells that produce and release the hormones directly into the blood. There are many endocrine glands in the body, including the pancreas, ovaries, testes, thyroid, pituitary gland and adrenal glands.   Endocrine glands are triggered to release their hormones by a variety of different signals. Such signals might come from the nervous system, from other parts of the endocrine system, or from direct monitoring of the environment in the body. You can find out more about endocrine glands in this endocrine gland article.

Hormones

Hormones are small molecules. They are released in huge numbers, and are carried along in the blood like rubber ducks in a stream. The blood carries them to every part of the body, so that they can reach every cell. Hormones can be made from different types of molecule, including proteins and cholesterol. The thing that makes them a hormone is that they travel away from where they were made, and they change the activity of another cell. The job of a hormone is to carry a message. It doesn’t directly take action when it arrives at the target cell. It just gives a signal to the machinery of the cell that it should change what it’s doing. After their job is done, hormones are broken down. This step is often carried out at the target cells, or in the liver. 

Target cells

“Target cells” are the cells which are able to respond to a particular hormone’s signal. As hormones are unable to navigate or steer, they can’t aim for their target cells. They have to reach them just by chance. This is why large numbers of hormone molecules are released at the same time.   The hormone’s target cells will have special protein receptors that are able to grab hold of (bind) the hormone. The receptor fits around the hormone like a glove fitting around a hand. The protein and hormone’s shapes are flexible and move to fit perfectly around each other. (This is also called lock and key binding.) When the target cell’s receptor binds the hormone, this causes a change in the shape of other parts of the receptor. This then causes further changes of activity in the cell. Different target cells can respond differently to the same hormone. This is one of the great powers of hormones and lets them have a widespread effect on the body. Find out more about hormones and how they affect their target cells in our article What is a hormone? 

Endocrinology

Endocrinology is the study of the endocrine system. There are many diseases and medical conditions related to the endocrine system. Endocrinologists are doctors who treat these conditions. They need an understanding of every part of the system, including the glands, hormones and the cells that are affected by the hormones. As endocrinologists learn more about the complexities of the endocrine system, new treatments are constantly being developed to help people with endocrine diseases, including diabetes mellitus (view article here).