Secretion vs excretion

Many cells release simple molecules like water, carbon dioxide or sugar. But this isn’t enough for them to be called glands. To be a gland, the substances they release must have a particular role in the body.

In the human body, our glands produce and release things like digestive enzymes, saliva, tears, and hormones. Releasing useful substances like these is called secretion. For example, you could say “the salivary glands secrete saliva”.

Some organs specialise in secretion. Their name often includes the word ‘gland’; for example, sweat glands. Other organs might secrete substances but also have other, more well-known functions.  These organs might not normally be thought of as glands. For example, the small intestine could be called a gland because it produces and releases digestive enzymes, but you will rarely hear anyone call it that.

In contrast to secretion, the release of waste is called excretion. The kidneys excrete urine. The lungs excrete carbon dioxide. These are substances that the body needs to get rid of; they are of no use and could even be damaging to the body if not removed. An organ that excretes waste is not called a gland. 

Inside vs outside

You can divide glands into two groups based on where they send their secretions. Do they send them into the body tissues, or do they send them out of the body?

Glands that send secretions into the body are called endocrine glands, and those that send them outside are called exocrine glands.

For example, hormone-producing glands are endocrine glands. They send their secretions into the bloodstream. And sweat glands are exocrine glands. They secrete sweat onto the skin’s surface.

Surprisingly perhaps, glands that send secretions into the gut are exocrine glands. This might seem strange at first as the gut is inside our bodies. But the space that food moves through as it is digested is counted as being outside. Think of the gut like a tube – it goes through us, but things can pass through from mouth to anus without actually entering our body tissues. The inside surface of this tube is covered with special cells that control how things are absorbed into our bodies – just like our outer bodies are covered in skin.  

Some organs have multiple functions. For example, the pancreas is both an endocrine gland (because it secretes hormones into the blood) and an exocrine gland (because it secretes digestive enzymes into the gut).  

Biology jargon tip: 

Whenever you see a word starting with “endo-“, it tells you that it has something to do with “inside”. The prefix “Endo” comes from an ancient Greek word for “in” or “within”. And whenever you see a word starting with “exo-“, it means something to do with “outside” (again from ancient Greek). For example, the words “endoskeleton” (a skeleton inside the body) and “exoskeleton” (a skeleton on the outside of the body).  

Endocrine glands

Endocrine glands release useful substances into the body tissues. In humans, endocrine glands release hormones directly into the blood.

Hormones are small messenger molecules that travel around the body. The gland secretes huge numbers of hormone molecules into the bloodstream. These are carried by the movement of blood, spreading out through the body to reach the smallest of blood vessels. From there they can reach every cell in the body with their message.

Find out more about how hormones carry their message between the gland and other cells here.

Endocrine glands are really important for keeping the body environment stable in a process called homeostasis . For example, a gland called the pancreas monitors blood sugar and uses hormones to help keep it at healthy levels.

Other endocrine glands control sexual development and reproduction. The most important glands for this are the pituitary gland, ovaries, and testes.

Endocrine glands have many other roles too, including in growth and how our bodies respond to stress. Hormones can even affect our emotions. Together, all the endocrine glands in the body make up one of the body’s main communication networks: the endocrine system.

Doctors and researchers who focus their work on endocrine glands and their hormones are called Endocrinologists. There are many different medical conditions linked to the endocrine system, including common conditions like diabetes mellitus and morning sickness.

Exocrine glands

Exocrine glands secrete useful substances out of the body. Sweat glands and salivary glands are probably the best-known exocrine glands. Less visibly, we have a variety of exocrine glands which secrete substances into the gut to help with digestion, including digestive enzymes.  

Exocrine glands are different from endocrine glands because they secrete through a duct (a tube). The duct is used to make sure their secretions don’t go into the body tissues and instead are released to the surface. 

Fake glands!

You might have heard people talking about the “glands in your neck”, especially when they swell up when you are unwell. These are lymph nodes, sometimes known as lymph glands. However, these organs are not considered true glands by endocrinologists. The lymph nodes’ main job is to filter tissue fluids and act as a base for white blood cells that fight infection. They are similar to exocrine glands because they have a duct, and they release fluid into the blood like an endocrine gland might. But they are just releasing normal body fluids collected from the body tissues.