In this explainer, we will learn how to recall the types of endocrine glands and the characteristics and functions of hormones in the human body.
The functions the human body carries out are generally controlled by the nervous and endocrine systems. The endocrine system, which we will focus on here, involves hormones.
Hormones are released in definite doses by different organs, called glands, that belong to the endocrine system.
Key Term: Endocrine System
The endocrine system is a series of glands that produce and secrete hormones that the body uses for a wide range of functions.
You can see the main glands of the human endocrine system in Figure 1 below.
As you can see in Figure 1, both males and females usually have pineal, pituitary, thyroid, and thymus glands as well as a hypothalamus. The pancreas also acts as a gland in addition to its other digestive functions. Most humans have two adrenal glands, one above each kidney. Males tend to have two testes, while females tend to have two ovaries.
Hormones are chemical messengers that travel throughout the body to cause specific effects. Hormones are secreted when glands are activated by a change in the body, such as a rise in blood glucose concentration, or when a gland is stimulated by another hormone or a nervous impulse.
Hormones are chemical messengers that travel throughout an organism’s body, usually in the blood or another transport medium.
Once released from a gland, hormones travel in a liquid transport medium, usually blood plasma, to act upon target organs and cells. Once hormones reach their target cells, they bind to receptors inside the cell or to the cell’s cell surface membrane. The word hormone comes from the Greek word meaning to arouse activity as hormones “activate” an effect elsewhere in the body from where they are secreted by binding to these target cell receptors.
You can see a diagram showing an outline of how hormones travel throughout the body in Figure 2 below.
Example 1: Defining the Term Hormone
With reference to the human endocrine system, which of the following best defines the term hormone?
- A hormone is a form of neurotransmitter that is used to communicate between different nerve cells.
- A hormone is an electrical impulse that is transmitted from an endocrine gland to specific cells in the body via the nervous system.
- A hormone is a chemical messenger that is transported from endocrine glands to target organs via the bloodstream.
- A hormone is a biological catalyst that is used by the body to speed up the rate of chemical reactions.
The human endocrine system is a series of glands that produce and secrete hormones, sometimes described as chemical messengers, that the body uses for a wide range of functions. These glands secrete hormones directly into the bloodstream, where they travel throughout the body in blood plasma to act upon target cells and cause an effect.
While they do interact, the endocrine system and the nervous system differ in numerous ways. In the nervous system, electrical impulses are transmitted along specialized cells called nerve cells, otherwise known as neurons. Between each nerve cell is a junction called a synapse across which chemicals called neurotransmitters diffuse to convey a signal from one nerve cell to the next one.
We can deduce that the option indicating that a hormone is a type of neurotransmitter used to communicate between nerve cells is, therefore, incorrect as this describes the nervous system and not the endocrine system as the question requests.
We can also tell that the reference to a hormone as an electrical impulse is incorrect. Hormones are chemical messengers involved in the endocrine system and not electrical signals involved in the nervous system.
Hormones are chemical messengers that are released from endocrine glands, and they act on target cells in target organs, so this option appears correct. It also accurately describes how the majority of hormones are transported: via the bloodstream.
The option referring to hormones as biological catalysts is incorrect. This, instead, describes an enzyme, a different sort of molecule that speeds up the rate of chemical reactions.
Therefore, the best definition of the term hormone is that a hormone is a chemical messenger that is transported from endocrine glands to target organs via the bloodstream.
Let’s look at an overview of the human endocrine system and the types of the glands it contains.
There are three types of glands: endocrine glands, exocrine glands, and mixed glands. All glands release substances, but the nature of the substances they release and the mechanism by which they do this differ depending on the gland type.
An endocrine gland is one that produces hormones and secretes them directly into the bloodstream.
Key Term: Endocrine Gland
An endocrine gland is a group of specialized cells that secrete hormones into the blood.
An exocrine gland produces substances, such as enzymes, and secretes them via a duct onto the surface of the body. These surfaces can be internal, such as the mouth, into which exocrine salivary glands secrete saliva via ducts. They may, alternatively, be external surfaces, such as the skin, onto which exocrine sweat glands secrete sweat, also via ducts.
Key Term: Exocrine Gland
An exocrine gland is a group of specialized cells that secrete substances via a duct.
Finally, mixed glands are those that can have both endocrine and exocrine functions.
Key Term: Mixed Gland
A mixed gland is a group of specialized cells capable of both endocrine functions, releasing hormones into the blood, and exocrine functions, releasing other substances, such as enzymes, via a duct.
Now that we know a gland can either be endocrine, exocrine, or mixed, let’s look at specific examples of endocrine and mixed glands within the endocrine system.
Let’s look at some examples of endocrine glands first, starting with the adrenal glands that you can see in Figure 3 below.
Humans typically have two adrenal glands, one above each of our two kidneys, as you could see in Figure 1. Each adrenal gland contains an outer section called the adrenal cortex and an inner section called the adrenal medulla, which are responsible for releasing different hormones with very different functions. You can see an outline of the hormones produced by each section in Figure 3 above.
For example, cortisol is a hormone released by the adrenal cortex, responsible for regulating the metabolism of carbohydrates and the use of fats and proteins as a source of energy. Adrenaline is a different hormone released from the adrenal medulla and is responsible for the fight-or-flight response in the human body, allowing us to react effectively to stressful situations.
You may have noticed that many of the endocrine glands in Figure 1 are common to both males and females. However, males have glands called testes, the singular form of which is testis, while females have glands called ovaries, whose singular form is ovary. These glands form part of the male and female reproductive systems, respectively, and are collectively called the gonads.
The male gonads—the testes—release hormones called androgens. Androgens can be testosterone, androstenedione, and other hormones that work together to control the growth of male sex organs and develop secondary sexual characteristics at puberty such as facial hair or the deepening of voice.
The female gonads—the ovaries—release two main hormones: estrogen and progesterone. Estrogen causes the development of secondary sexual characteristics when a woman reaches puberty such as the growth of her breast tissue. Estrogen and progesterone, along with two hormones released from the pituitary gland called FSH and LH, play an important role in regulating the menstrual cycle and preparing the female body for a potential pregnancy.
Relaxin is another hormone released from the ovaries, and also from the placenta, during pregnancy. Relaxin, as the name suggests, is primarily responsible for relaxing parts of the female reproductive system, toward the end of pregnancy, to aid the delivery of a child.
Example 2: Identifying the Functions of Hormones in the Human Body
Which of the following is not a function of hormones in the human body?
- Transmitting electrical signals between neurons
- Helping maintain a constant internal environment
- Stimulating the development of secondary sexual characteristics
- Regulating metabolic functions
Hormones are chemical messengers. They are secreted from endocrine glands to be transported throughout the body via the bloodstream and have an effect on target cells.
Hormones are, therefore, an integral part of the human endocrine system, not the nervous system. The nervous system consists of nerve cells, otherwise known as neurons, which transmit electrical signals. Two neurons meet at a junction called a synapse. Neurotransmitters, not hormones, are chemicals that are transported across the gap between two neurons to transmit the signal from one neuron to the next.
The functions of hormones are numerous.
One key function of many hormones is regulating a constant internal environment. For example, if the blood sugar level rises too high, hormones will usually be released to act upon specific tissues to return the blood sugar level to a normal, constant level.
Hormones can also be responsible for growth and reaching sexual maturity, at which point certain hormones control the development of secondary sexual characteristics like growth of body hair or breast tissue.
Some hormones are also responsible for regulating our metabolism. While the function of certain hormones is to break down the molecules we do not require into more useful forms, others can help our cells to use these molecules by building up the molecules our body needs.
Therefore, the first option—transmitting electrical signals between neurons—is not a function of hormones in the human body.
Let’s now take a look at an important gland called the pituitary gland and understand how it works. The pituitary gland is located just beneath the brain, as you can see in Figure 4.
The pituitary gland is made up of two lobes: the anterior lobe and the posterior lobe. The pituitary gland is closely linked to a part of the brain called the hypothalamus. Some hormones are produced by the hypothalamus and stored in the posterior pituitary gland to be secreted into the blood when they are needed. One such hormone is the antidiuretic hormone (ADH), which is responsible for controlling the reabsorption of water into the bloodstream by the kidneys.
The pituitary gland is sometimes called the “master gland” as it controls the activities of most other endocrine glands. For example, the anterior pituitary gland secretes the adrenocorticotropic hormone that stimulates the adrenal glands to secrete other hormones. The same gland also secretes growth hormones that regulate our development via protein metabolism. These are just two of the seven hormones secreted by the anterior pituitary gland alone.
The pancreas is an example of a mixed gland, as it has both endocrine and exocrine functions, whose location within the human digestive system you can see in Figure 5 below.
Most of the pancreas is made up of exocrine tissue, responsible for the production and secretion of the pancreatic juice. The pancreatic juice is an alkaline substance that contains a mix of enzymes to aid digestion. It is secreted via the pancreatic duct into the duodenum, the first section of the small intestine.
The endocrine function of the pancreas is to release hormones, such as insulin and glucagon, directly into the bloodstream. While insulin’s function is to lower blood glucose levels when they become too high, glucagon increases blood glucose levels when they become too low. These two mechanisms allow the pancreas to closely regulate the blood sugar concentration in the human body to keep it at a constant, normal level.
The mucous membranes that line the digestive tract carry out both endocrine and exocrine functions. The exocrine function of these membranes includes the secretion of digestive enzymes that help to break down food. These enzymes are secreted via ducts into the digestive tract, such as in the duodenum.
The mucous membranes also secrete gastrointestinal hormones into the bloodstream, which has an effect on other organs in the digestive system.
For example, gastrin is a hormone released by the G cells lining the stomach and the duodenum. Gastrin is transported via the blood back to the stomach to stimulate it to release gastric acid, sometimes called gastric juice, or simply stomach acid, among its other target tissues and functions. Gastric acid contains hydrochloric acid and protease enzymes to break down proteins in the stomach.
Secretin and cholecystokinin are hormones released by the S and I cells, respectively, both of which are found in the lining of the duodenum. These hormones stimulate the release of pancreatic juice from the pancreas.
Example 3: Comparing the Functions of Endocrine and Exocrine Glands
Which of the following best explains the key difference between an endocrine and an exocrine gland?
- Exocrine glands secrete substances via ducts, whereas endocrine glands secrete substances directly into the bloodstream.
- Endocrine glands secrete substances via ducts, whereas exocrine glands secrete substances directly into the bloodstream.
The human body contains many different glands, all of which release substances but differ in the mechanism by which they do this. Glands may either be endocrine glands, exocrine glands, or mixed glands.
Endocrine glands release hormones directly into the bloodstream, where they travel in blood plasma. The bloodstream then transports the hormones to their target cells to cause an effect. For example, the adrenal glands are endocrine glands that secrete hormones, such as adrenaline, directly into the bloodstream to allow humans to respond appropriately to stressful situations through the fight-or-flight response.
Exocrine glands release substances through ducts onto body surfaces. The surfaces may be internal such as the lining of the stomach onto which exocrine cells secrete hydrochloric acid and protein-digesting enzymes to aid digestion. Alternatively, the surfaces may be external such as the surface of the eye onto which lacrimal glands—otherwise known as tear glands—secrete tear fluid via tear ducts that is wiped across the eye whenever we blink to protect it and keep its surface moist.
Glands may also be mixed glands, such as the pancreas, and have both endocrine and exocrine functions.
Therefore, the best explanation of the key differences between the two glands is that exocrine glands secrete substances via ducts, whereas endocrine glands secrete substances directly into the bloodstream.
Several scientists have contributed to our understanding of the endocrine system and the endocrine and exocrine functions of the human glands. Among key players in the discovery of animal hormones were two scientists named Bernard and Starling.
Claude Bernard was a French physiologist who studied the glycogenic functions of the liver around 1855 to 1856, whose location in the human digestive system can be seen in Figure 5 above. Bernard was particularly fascinated by the secretions of the liver. He discovered that the liver stored sugar in the form of a substance called glycogen and that the glycogenic function acts as the internal secretion of the liver.
In addition to the functions Bernard discovered, the liver was also found to make an external secretion of bile that is then transported through the bile duct to the gallbladder. The bile is stored in the gallbladder before being released onto the small intestine. These secretions are the exocrine functions of the liver.
A British physiologist, Ernest Starling, studied the pancreas and is credited, along with his brother-in-law, William Bayliss, with the introduction of the term hormone. They tried to determine whether the secretion of pancreatic juice was a result of nervous stimulation, as was believed at the time, or perhaps something else.
Upon cutting off the nerve supply to the pancreas, they discovered that the pancreas still secreted pancreatic juice into the duodenum directly after food arrived there through the digestive tract. The mechanism they proposed is outlined in Figure 6 below.
Starling and Bayliss found that there were chemicals secreted by the cells in the mucous membranes that line the duodenum and jejunum, the first two parts of the small intestine. These chemicals, which they named secretin, entered the bloodstream and were transported to the pancreas. They concluded that it was not a nervous stimulation that caused the secretion of pancreatic juice but rather chemical secretions that they named hormones. They proposed that there were many other similar secretions produced elsewhere in the body. As we know today, they were correct!
Several other scientists have researched the effects of hormones on different parts of the body by studying the symptoms that emerge when certain endocrine glands enlarge or are removed. The enlargement of a certain endocrine gland tends to result in a heightened effect of the hormones it releases as these hormones are secreted in greater volumes. The removal of a certain gland should, in theory, cause any effect of the hormones they release to be diminished.
Scientists also study the chemical composition of the hormones released by the endocrine system, which varies significantly depending on the hormone type.
Let’s look in some more detail at the structure and function of two different types of hormones: steroid hormones and nonsteroid hormones.
You can see the mechanism of how steroid hormones function to cause an effect in target cells in Figure 7 below.
Steroid hormones are derived from lipids and are lipid soluble. This means that they can diffuse through the phospholipid bilayer of the cell surface membrane surrounding a target cell to move directly into the cell’s cytoplasm. Once within a target cell, steroid hormones bind to receptors in the cytoplasm or nucleus or even embedded in the plasma membrane surrounding other organelles such as the mitochondria or endoplasmic reticulum. The hormone-receptor complex this forms usually allows a specific gene to be transcribed, which leads to the production of a specific protein. The location of the hormone receptor depends on the hormone that will bind to it.
Key Term: Steroid Hormone
Steroid hormones are derived from lipids and diffuse across plasma membranes to bind with receptors usually located in the cytoplasm or nucleus of a target cell.
This hormone-receptor complex allows a specific gene to be transcribed to produce mRNA, as you can see in step 4 of Figure 7. This mRNA will leave the nucleus and enter the cytoplasm where it will eventually synthesize a specific protein. The proteins produced by each hormone will result in a response.
Some examples of steroid hormones are cortisol and some of the main sex hormones: estrogen, progesterone, testosterone, and androsterone.
You can see an outline of the mechanism of how nonsteroid hormones function to cause an effect in target cells in Figure 8 below.
Nonsteroid hormones are derived from amino acids and are, therefore, sometimes called amino-acid- derived hormones, peptide hormones, or sometimes just amine hormones. Nonsteroid hormones are hydrophilic, so they cannot pass through the phospholipid bilayer of a target cell and must, therefore, bind to receptors on the cell surface membrane to cause an effect within the cell.
Key Term: Nonsteroid Hormone
Nonsteroid hormones are peptide hormones derived from amino acids and are generally not lipid soluble, so they must bind to receptors on the surface of a target cell membrane.
The hormone-receptor complex formed on the target cell’s membrane activates a series of reactions within the cell. This chain of intracellular reactions, which is mediated by enzymes, eventually leads to a cellular response—for example, an increase in blood glucose concentration usually results in the release of insulin. Insulin causes cellular responses, such as an increased uptake of glucose and stimulating an increase in glycolysis, which is the first step in the cellular respiration responsible for breaking down glucose into pyruvate. These processes, among others that insulin stimulates, lower blood glucose levels back to a normal level.
Some examples of nonsteroid hormones are adrenaline, ADH, insulin, and glucagon.
Example 4: Describing the Differences between Steroid and Nonsteroid Hormones
Hormones can be broadly classified into steroid and nonsteroid hormones. Which of the following statements correctly describes a difference between the mechanism of action of the two types?
- Nonsteroid hormones are lipid soluble and can pass through target cell membranes, whereas most steroid hormones are not and must bind to receptors on cell surface membranes.
- Steroid hormones are lipid soluble and can pass through target cell membranes, whereas most nonsteroid hormones are not and must bind to receptors on cell surface membranes.
Hormones are chemical messengers released from endocrine glands into the bloodstream. The blood transports these hormones to target cells upon which the hormones can cause an effect.
The mechanism of action by which this effect is brought about differs depending on the type of hormone and its chemical composition. There are two main types of hormones: steroid and nonsteroid.
Steroid hormones are lipids and are lipid soluble. This means that they can diffuse directly across the phospholipid bilayer of a target cell into the cytoplasm. There, they can either bind to hormone receptors in the cytoplasm or in the nucleus, depending on the hormone. The hormone-receptor complex causes a certain gene to be transcribed, producing a strand of mRNA that translates into a specific protein to cause an effect.
Nonsteroid hormones are derived from amino acids and are not lipid soluble. This means that they cannot diffuse across the phospholipid bilayer and, instead, bind to receptors on the target cell’s cell surface membrane. This binding triggers a chain of reactions within the cell that leads to an overall cellular response.
Therefore, the statement that correctly describes the difference between the mechanism of action of the two types of hormones is that steroid hormones are lipid soluble and can pass through target cell membranes, whereas most nonsteroid hormones are not and must bind to receptors on cell surface membranes.
Despite the small quantities released, hormones can produce significant effects on many different bodily functions. Hormones help humans to control their internal environment, regulate metabolic functions, and even stimulate the development of some sexual characteristics. At the same time, hormones released from different glands in the body help humans to grow and reach sexual maturity and may even have numerous effects on the human behavioral and intellectual development.
Let’s recap some of the key points we have covered in this explainer.
- Hormones are chemical messengers released from glands to act upon target organs or cells.
- Hormones are usually transported through the bloodstream.
- Glands can be exocrine, releasing substances via a duct onto an internal or external body surface.
- Glands may also be endocrine, releasing hormones directly into the blood.
- Glands can be mixed, which means they have both endocrine and exocrine functions.
- Scientists, such as Bernard and Starling, have contributed to the discovery of animal hormones through their studies on the liver and pancreas respectively.
- Hormones can be lipid-derived steroids that bind to receptors within a target cell to result in the transcription of a gene and synthesis of a certain protein.
- Hormones can, alternatively, be amino-acid-derived nonsteroids that bind to receptors on a target cell’s membrane to cause a chain of intracellular reactions, leading to a response.