In this video, we will learn how to recall the types of endocrine glands in the human body. We will first learn what a hormone is before looking at the specific glands in the human endocrine system and the hormones that they release. We will also investigate some of the key scientists who contributed towards our current understanding of animal hormones and the main differences between the two types of hormones, steroid and nonsteroid hormones.
The functions that the human body carries out are generally controlled by the nervous system and the endocrine system, working in harmony to produce various useful responses in the human body. The human endocrine system, which we will be focusing on in this video, is a series of glands that produce and secrete hormones that the body can use for diverse range of functions.
Hormones are chemical messengers that travel throughout an organism’s body, usually via the blood or another liquid transport medium to cause specific effects. The word hormone comes from the Greek term meaning “activator” or to arouse activity. This is because hormones, which are shown here as these green dots, activate an effect elsewhere in the body from the endocrine gland cells from which they are secreted. The cells and the endocrine glands themselves are activated by a change in the body. For example, perhaps there is an increase in your blood glucose concentration. Or the glands might also be activated when they’re stimulated by another hormone or a nervous impulse, showing how the nervous and endocrine systems work in tandem to help our body function effectively.
Once they are released from a gland, hormones travel in a liquid transport medium, usually the bloodstream, dissolved in the blood plasma until they reach their target organs or cells. Once these hormones reach their target cells, they combine to receptors which are either presented on the cell’s surface membrane of the target cell or some hormones can actually pass through the cell’s surface membrane of the target cell and bind to receptors in the cytoplasm or in the nucleus.
Let’s take a closer look at the different endocrine glands within the human endocrine system. You can see the main glands of the human endocrine system in a typical biological male and a typical biological female in this image. Hormones are released in tiny doses by these different organs called glands that belong to the endocrine system. If we look at the brains on both biological males and females, we can see that both sexes tend to have a hypothalamus, a pituitary gland, and a pineal gland, all of which are responsible for releasing different hormones. They also both have a thyroid gland located in their neck and a thymus gland in their chest.
The abdomen of typical males and females also contains a pancreas, which forms part of the digestive system, and two adrenal glands, one of which sits above each of the kidneys. The endocrine systems of biological males and females typically differs only in their gonads. While males typically have two testes, females typically have two ovaries. And these gonads are primarily responsible for releasing sex hormones.
Now that we’ve seen an overview of the human endocrine system, let’s take a look at the types of glands it contains. There are three different types of glands: endocrine glands, exocrine glands, and mixed glands. All glands release certain substances, but the nature of the substances they release and the mechanism by which they do this differs depending on the gland type.
As we now know, endocrine glands are ones which produce hormones and secrete them directly into the bloodstream. Exocrine glands produce different substances like enzymes and secrete them via ducts onto body surfaces. These surfaces can be internal, such as the mouth into which exocrine salivary glands secrete saliva via a salivary duct. The surfaces may alternatively be external, such as the skin onto which exocrine sweat glands secrete sweat, also via ducts.
Finally, mixed glands are those which are capable of both endocrine and exocrine functions. So they can not only secrete hormones into the bloodstream but can also secrete substances via ducts onto body surfaces. In this video, as we’re focusing on the human endocrine system, we’ll be looking specifically at the endocrine glands and the mixed glands as these are the two that will have endocrine functions. So, let’s look at some specific examples of these two types of glands, starting with endocrine glands.
The adrenal glands are examples of endocrine glands. Humans typically have two adrenal glands, one above each of our kidneys. Let’s take a closer look at a cross section of one of these adrenal glands so we can see the type of hormones that they release more clearly. Each adrenal gland contains an outer section called the adrenal cortex and an inner region called the adrenal medulla. Each of these regions is responsible for releasing different hormones with very different functions. For example, while the adrenal cortex releases hormones such as glucocorticoids, mineralocorticoids, and androgens, otherwise known as sex hormones, the adrenal medulla releases catecholamines and peptide hormones.
Cortisol is an example of a glucocorticoid released from the adrenal cortex. Cortisol is sometimes called the stress hormone, as it’s responsible for regulating the metabolism of carbohydrates and the use of fats and proteins as a source of energy often in stressful situations. Adrenaline is an example of a catecholamine released from the adrenal medulla. It’s partly responsible for stimulating the fight-or-flight response in humans, which allows us to react effectively to stressful situations. It’s worth noting that small quantities of androgens, otherwise known as sex hormones, will also be released from the adrenal glands.
Let’s take a look at the glands in the human body which release the highest quantities of these sex hormones next, the gonads. Most of the endocrine glands in the human body are common to both biological males and females. However, some very specific organs differ quite considerably between the two biological sexes. These glands are collectively called the gonads, and they make up a part of the male and female reproductive systems.
The male gonads are called the testes, or singular testis. The testes release hormones called androgens. And the main androgen that’s released in biological males, such as this one, is called testosterone. One of the main functions of testosterone is to control the growth of the male sex organs. And it also stimulates the development of secondary sexual characteristics when a person reaches puberty. In males, this is typically development of things like facial hair or body hair and also deepening of the voice among several other physical characteristics that change during this time.
The female gonads are called the ovaries, or a singular ovary. As well as releasing typically male sex hormones, like testosterone, the ovaries also release estrogen, progesterone, and relaxin in biological females. Estrogen, much like testosterone in males, stimulates the development of secondary sexual characteristics when a female reaches puberty. These physical features might include growth of her breast tissue or widening of her hips.
Both estrogen and progesterone play an important role in regulating the menstrual cycle in biological females, which can help prepare the female body for a potential pregnancy. Relaxin is another hormone released from the ovaries and also from the placenta during pregnancy. As its name suggests, it’s primarily responsible for relaxing certain parts of the female reproductive tract, particularly towards the end of pregnancy to aid in the delivery of a child.
You might think of the brain as an organ that’s primarily involved in nervous communication. However, the brain also contains many regions that function as endocrine glands, for example, the pituitary gland, which you can see in this diagram in green.
By magnifying the part of the brain that contains the pituitary gland, we can see that it is made up of two lobes, the posterior lobe and the anterior lobe. The word anterior refers to the front, as this lobe is located closer to the front of the brain, while the word posterior means back, and it refers to the fact that the posterior lobe is closer to the back of the brain. A way to remember this could be that the letter A comes before the letter P in the alphabet. So just like the letter A is closer to the start of the alphabet, the anterior lobe is closer to the front of the brain.
The pituitary gland is closely linked with the part of the brain called the hypothalamus. Some hormones are produced by the hypothalamus and stored by the posterior pituitary gland. An example is a hormone called ADH, which is secreted into the blood from the posterior pituitary gland when the concentration of water in the blood is low. ADH is then transported to its target cells which are located in the kidneys, where ADH stimulates more water to be reabsorbed into the bloodstream to increase the level of water in the blood to a normal, healthy level.
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 adrenocorticotropic hormone, which is more commonly known as ACTH. ACTH will also be absorbed into the dense capillaries surrounding the pituitary gland, and the blood transports this hormone to the adrenal glands, stimulating them to release other hormones like cortisol, which we learned about earlier. The anterior pituitary gland also secretes growth hormones which regulate the development of various parts of our body via metabolism of proteins. Growth hormones and ACTH are just two of the seven hormones released from the anterior pituitary gland alone.
Let’s take a look at the hormones released from the pancreas next. The pancreas is an example of a mixed gland, as it has both endocrine and exocrine functions. You can see the location of the pancreas in the human digestive system in blue here. Most of the pancreas is made up of exocrine tissue, which plays an important function in the digestive system.
The exocrine function of the pancreas is to secrete a substance called pancreatic juice into the first section of the small intestine, which is called the duodenum. Pancreatic juice is an alkaline substance that contains a mix of enzymes to aid digestion. This pancreatic juice will be secreted from the exocrine cells of the pancreas along a duct, called the pancreatic duct, into the duodenum.
The endocrine function of the pancreas is to secrete two main hormones directly into the bloodstream, one called insulin and another called glucagon. While insulin functions to lower blood glucose levels that have become too high, glucagon functions to raise blood glucose levels that have become too low. The two mechanisms carried out by these hormones allow the pancreas to closely regulate the blood sugar level in the human body and keep it at a constant normal concentration. Several of the organs in the digestive tract have mucous membranes in their lining, which are sometimes known as mucosa.
The mucosa can be referred to as mixed glands as they have both endocrine and exocrine functions. Some of the organs that contain mucosa in their linings are the stomach and the small intestine. The exocrine function of these membranes includes the secretion of digestive enzymes that help to break down food, in addition to those enzymes that were secreted by organs like the pancreas. These enzymes are secreted via ducts onto the inner surface of the digestive tract, such as in the stomach and the duodenum of the small intestine. These mucous membranes also secrete gastrointestinal hormones into the bloodstream, which have an effect on other organs in the digestive system.
For example, gastrin is a hormone that’s released by aptly named G cells that line the stomach and also the cells that line the duodenum of the small intestine, some of which you can see magnified here. Gastrin is then transported via the blood back to the stomach. Once in the stomach, gastrin stimulates the cells lining the stomach to secrete gastric juice.
Gastric juice, which is sometimes called gastric acid or simply stomach acid, contains these digestive enzymes that will be secreted from the mucosa. These will function to help aid in the breakdown of proteins in the stomach. The mucosa of the duodenum also contains S cells and I cells. While the S cells produce a hormone called secretin, the I cells produce and secrete a hormone called cholecystokinin. Both of these hormones stimulate the secretion of pancreatic juice from the pancreas.
Many different scientists have contributed to our current understanding of animal hormones, the endocrine system, and the endocrine and exocrine functions of the human glands. Claude Bernard was a French physiologist who studied the glycogenic functions of the liver around 1855 till 1856. You can see the location of the liver in the digestive system in the diagram here. Bernard was especially fascinated by the secretions of the liver. He discovered that the liver internally secretes some stored sugar. This sugar was called glycogen. And Bernard was discovering one of the exocrine functions of the liver, another of which is to secrete bile into the small intestine to help with lipid digestion.
A British physiologist named Ernest Starling and his brother in law William Bayliss studied the pancreas together and are credited with the introduction of the term hormone. These two scientists tried to determine whether secretions 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 of the small intestine directly after food arrived there through the digestive tract.
Starling and Bayliss found that there were chemicals secreted by the cells in the mucosa that line the duodenum in the jejunum, the first two parts of the small intestine. These chemicals, which they named secretin, which you may remember learning about earlier, entered the bloodstream and were transported to the pancreas, stimulating the secretion of pancreatic juice into the duodenum. They concluded there was not nervous stimulation that causes discretion of pancreatic juice, but instead these chemical secretions, which they named hormones. They proposed that there were many other similar secretions produced elsewhere in the body. And as we know today, they are correct, as the cells that are making these secretions were the S and I cells.
Let’s have a look at the two different types of hormones that function in the human body, their structures, and how they function slightly differently to have an effect on target cells.
Let’s begin with steroid hormones. Steroid hormones are derived from lipids, and so they are lipid soluble. The fact of they are lipid soluble means that steroid hormones can directly pass the phospholipid bilayer in the plasma membrane of target cells, moving directly into the cell’s cytoplasm. Once within a target cell, the steroid hormones can bind to steroid hormone receptors, which are often found in the cytoplasm. Alternatively, some steroid hormones may travel directly through the cytoplasm of the target cell and into its nucleus, where steroid hormone receptors may also be located. Steroid hormone receptors can even be found embedded in some other organelles, such as the endoplasmic reticulum or the mitochondria.
The steroid hormone receptor complex that’s formed allows a specific gene to be transcribed, forming a strand of mRNA that can leave the nucleus and be translated into a useful protein, which will have a specific function depending on which hormone triggered its production. The location of the steroid hormone receptor partly depends on which hormone will bind to it. Some examples of steroid hormones are cortisol and some of the main sex hormones, such as estrogen, progesterone, and testosterone.
Let’s see the mechanism of how nonsteroid hormones function to cause an effect in target cells next.
Nonsteroid hormones are derived from amino acids and so are sometimes called amino acid-derived hormones, peptide hormones, or amine hormones. Nonsteroid hormones are hydrophilic and are insoluble in lipids, so they cannot pass directly through the phospholipid bilayer of the target cell. Nonsteroid hormones must therefore bind to nonsteroid hormone receptors on the cell surface membrane of target cells in order to cause an effect. The hormone receptor complex formed on the target cells’ membrane activates a series of reactions within the cell.
This cascade of intracellular reactions, which is mediated by enzymes, eventually leads to a cellular response. For example, an increase in the blood glucose concentration usually results in the release of insulin from the pancreas. Insulin causes several cellular responses that function to lower blood glucose back to a normal range. Some other examples of nonsteroid hormones are adrenaline, ADH, and glucagon.
Despite the small quantities which are released, hormones can produce significant effects on many different bodily functions. Hormones help humans to control their internal environment, regulate our metabolic functions, and even stimulate the development of some of our sexual characteristics.
Let’s review the key points that we’ve covered in this video. We’ve learned that hormones are chemical messengers released from glands transported via the blood to have an effect on target cells. We’ve also learned how glands can be exocrine glands, endocrine glands, or mixed glands. Finally, we learned the difference between steroid hormones and nonsteroid hormones.