In this chapter... While exploring the endocrine system that makes our hormones, we will explore how hormones are made. This will take us into gene expression – the way in which our cells read DNA to make proteins. We will also explore how this system is coordinated in the cell using the endomembrane system. Finally, we will look at how cells around the body respond to hormone signals using receptors on their surface.

A major challenge faced by multicellular organisms is making trillions of cells work together as one coordinated organism. This is not easy. Think of it like directing a band of billions of musicians to play the same piece of music in sync and in tune. The endocrine system, along with the nervous system, signals cells to adjust what they are doing so that other cells get what they need. Endocrine glands are constantly monitoring the body to make sure things are working correctly.

Why this is important…Cells need to be directed so they contribute to the body as a whole. Hormones help this happen. Imbalance of hormones can dramatically affect to body’s ability to function.

Foundations

Unlike other systems we have studied, the endocrine system is not made of physically connected organs. Each gland is separate and has its own regulatory job. Some monitor blood glucose levels while others watch potassium levels. There is also some overlap. For example, blood glucose is regulated by two glands – the pancreas regulates short-term changes while adrenal glands regulate long-term levels.

Endocrine glands don’t do any work, they just send out chemical signals (hormones) to tell other organs what to do. This can help maintain homeostasis, redirect resources, or direct reproduction.

If this sounds like a complex government bureaucracy, then you are starting to understand.

Endocrine glands labeled in male and female bodies.

FIGURE 9-1. Anatomy of the endocrine system. ("Endocrine System" by OpenStax College is licensed under CC BY-SA 4.0)

Key terms

Hormone – chemical secreted by a gland into the bloodstream to regulate the function of other organs

Gland – organ that secretes chemicals into the bloodstream or other tubes of the body. Endocrine glands secrete hormones into the bloodstream, but there are other types of glands as well.

Details

Hormones are manufactured according to genetic instructions. Most hormones are proteins. And proteins, as we know, are constructed according to genetic instructions. In this chapter, we will learn the details of how that happens. We will also talk about how proteins are packaged and exported from the cell.

Diagram of endocrine signaling. Endocrine gland releases hormones into bloodstream. Hormones exit bloodstream and bind to receptor on target cells, creating a response.

FIGURE 9-2. Endocrine signaling. Hormones are released by glands into the bloodstream. Only cells with the correct receptors respond to the signal. The diagram is highly simplified – glands actually have a rich network of capillaries running through them to collect hormones. (“Endocrine signaling” by J. Buegge is licensed under CC BY-SA 4.0)

Hormones travel all over the body and have powerful effects. They are a bit like an Amber Alert sent to all cells. However, not all cells respond. Since hormones are chemicals, they can keep signaling for a long time. How long they stay around in the bloodstream varies depending on the hormone. Some are only around for seconds while others remain for days. Like the Amber Alert, once their mission is accomplished the signal can become a distraction. Hormones are usually cleared by the kidneys or broken down by the liver. This keeps hormones from overcorrecting the problem they were sent to solve.

Stop & Think #1: Sometimes tumors affect endocrine glands, causing them to secrete too much or too little hormone. How might the body be affected if this happened to the pancreas, specifically to the insulin-producing cells?

Hormones affect specific cells in specific ways. Since hormones are released into the bloodstream, they reach every cell of the body. But in most cases, only a few organs respond. This is because only some cells have receptors for those hormones. In a way, it’s like only some cells are tuned to the right radio frequency to receive that hormone signal. We’ll return to the topic of hormone receptors in section 9e.

Below is a brief overview of some of the body functions regulated by hormones.

Water balance and electrolyte levels are controlled by hormones. This is usually done through manipulating different types of membrane transport. Here are a couple examples:

Water conservation – the kidney tubules add more aquaporins in order to allow more water to return to the bloodstream when they are signaled by anti-diuretic hormone from the posterior pituitary gland (Figure 9-3)
Potassium excretion – high potassium levels can be dangerous for the body. Kidneys can secrete more potassium (and retain more sodium) by adding sodium-potassium pumps. This is done in response to aldosterone from the adrenal glands.

Blood glucose is regulated by several hormones. Some of this is “normal regulation” or homeostasis, while others are in preparation for changing conditions where the body might need a greater supply of energy.

Insulin from the pancreas lowers glucose by signaling liver cells to store glucose as glycogen. It also signals body cells to install more transport proteins on their surfaces resulting in more facilitated diffusion of glucose.
Epinephrine (or adrenaline) from the adrenal gland raises blood glucose by causing the liver to break down glycogen and release glucose. This usually happens as part of the fight-or-flight response, when the body might need more energy.
Cortisol from the adrenal gland raises blood glucose in response to stress. Again, this is not homeostasis. This is in preparation for a heavy workload.

Stop & Think #2: What stressful situations might lead to an increase in cortisol and therefore high blood sugar? Also explain how exercise is important in management of stress, especially high blood glucose levels.

Pituitary gland. Location at base of brain was shown. Also a closeup of the pituitary gland with numerous blood vessels.

FIGURE 9-3. Pituitary gland. ("Anterior Pituitary" by OpenStax College is licensed under CC BY 3.0)

Some hormones regulate the amount of fats and proteins in the blood. This is often in preparation for something like growing, which will demand more energy and raw materials. Some examples include:

Growth hormone from the pituitary gland (Figure 9-3) increases the amount of amino acids let into cells. It also stimulates fat cells to release fatty acids. Not surprisingly, it also stimulates mitosis in the growth plates of the bones, which is supported by the increase in available energy.
Cortisol from the adrenal glands also causes fats and proteins to be broken down and circulated in the bloodstream.

The thyroid gland determines our body’s metabolic rate. An increase in thyroid hormone causes the heart and breathing rates to increase. It also causes cells to use more glucose, proteins and fats. Plus, it stimulates protein synthesis causing more growth of the skin, hair and nails. Thyroid hormone even causes cells to make more sodium-potassium pumps, which use more energy. One effect of all of this extra energy use is a higher body temperature. This regulation of our overall metabolic rate helps our body regulate temperature during different seasons.

Stop & Think #3: Why might it be important for growth hormone and thyroid hormone to increase together?

The pituitary gland secretes four different hormones to control reproductive functions. Reproductive organs (ovaries and testes) secrete hormones of their own, but the coordination comes from the pituitary gland. Hormones from the pituitary control production of sperm or eggs, control the menstrual cycle, stimulate childbirth, and cause release of milk from the breasts. None of these have to do with homeostasis, but show the wide-reaching effect of the endocrine system, and particularly the pituitary gland (often called the “master gland”).

The boundary between the nervous system and endocrine system is often fuzzy. In the middle of the brain is an area we might call the “homeostasis center” – the hypothalamus. This has a direct connection to the pituitary gland (Figure 9-3). Many of the pituitary actions described above were initiated in the brain (nervous system) but carried out by the endocrine system.

Learning objectives

Name and identify the major organs of the endocrine system
Describe several ways the endocrine system helps maintain homeostasis
Give several examples of ways the endocrine system regulates functions other than homeostasis

Stop & Think Questions Explained

Insulin is a hormone that lowers blood sugar levels. If the pancreas released too much, then blood sugar levels could drop dangerously low. If it didn’t release enough when needed, then blood sugar could build up and cause long-term damage to kidneys, blood vessels and other body systems.
Stress that could lead to increased cortisol include many forms of psychological stress (social anxiety, work deadlines, upcoming tests) and also physical stress (lack of sleep, lack of food, fatigue). Exercise helps use up the extra sugars and fats in the bloodstream so they don’t cause problems in the body.
Growth requires more energy than normal, so without an increase in thyroid hormone to increase metabolism, the body couldn’t meet the energy demands of growing signaled by growth hormone.