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Animal Systems
Animal Systems

The Endocrine System

Communications of Yore

It's your first junior high dance. The DJ is blasting Justin Beiber, and the gym is decorated to a "Love Under the Sea" theme. Puffer fish and coral hang from the ceilings, and the tablecloths are ocean blue. You're standing next to the punch bowl with bloated Swedish Fish that have sunk to the bottom. You feel your stomach twist into knots as you see the beautiful Susie Fluffernutter across the gymnasium.

You fret over the new pimple that emerged on your forehead this morning. You worry that a recent growth spurt has left you looking goofy, as the cuff of your dress pants is now three inches above your shoe. Hey, it's not your fault. For now, blame your endocrine system for these unfortunate series of events, but be sure to make friends later, since the system controls just about every process imaginable. It's good to have on your side.

Within the endocrine system, the body is still chatting with itself, but this time it's all about special chemicals called hormones. That's right, those mysterious chemicals we blame for everything from puppy love to cranky moods to pimples on prom night. These powerful little compounds are made and released from ductless endocrine glands. Hormones are released directly into the blood stream and usually travel a decent bit to get to their final destination, which is some end organ. Once they arrive, they regulate all sorts of everyday processes like energy levels, growth and development, responding to stress or injury, and keeping all things balanced through homeostasis.

There are also exocrine glands that work in a similar way, but these are NOT a part of the endocrine system. Exocrine glands, like the salivary and sweat glands, release chemicals into the body through ducts. They've been excommunicated from the endocrine system because of those ducts. Seems silly to us, but apparently it's a big deal when you are an organ releasing chemicals. Who knew?

If all this endo/exo talk is making your head spin, it's time to return to the basics. All we're talking about is a basic messaging system; it's not all that different from any other communication method. Think of the endocrine system as the body's Pony Express, sending out messages post haste to ensure things are running smoothly. Someone ambles in to the post office and tells the man behind the counter he wants to send a message—maybe he struck oil somewhere, or his aunt got bit by a rattlesnake. The post office is an endocrine gland, and the message being sent is the actual hormone. The blood vessels are the pony, racing the message to its final destination. What about the other post office where the message is delivered? That's the end organ. While they are similar, there is one major perk the endocrine system has over the Pony Express: no sore butts.

The endocrine system is so important that some type of hormone regulates every physiological process, whether it's body temperature or reproduction. If you're body's doing it, odds are a hormone had something to say about it. In fact, most processes require activity from multiple hormones, multiple end organs, or even crosstalk between the endocrine and nervous systems. This makes this mode of conversation quite complex and, well, confusing. It's sort of like call waiting, even though it didn't certainly exist in those good old Pony Express days.

There are about 50 chemically distinct hormones, and each end organ has a slew of receptors, each recognizing and responding to just one type of hormone. What can we say? The receptors are picky. That's not to say that only one hormone can act at any organ. Each organ has a whole team of receptors waiting for the right hormone to come along and form a match made in heaven.

One quirk is that some hormones actually have a nemesis, an antagonistic partner that plays the exact opposite role. One hormone shows up at the organ and says, "Do this thing!" And then the other one shows up and says, "Knock it off!"

For example, the pancreas is an endocrine gland that releases insulin and glucagon. Both hormones act at the liver, but they have exact opposite functions. Together they regulate blood sugar levels no matter what type of situation we get into. If we are stranded in the airport with nothing to eat, glucagon responds to our low blood sugar and convinces the liver to increase it.

If we're lucky enough to find a few Twix bars in a coat pocket, we might eat them all in a hurry. Besides having a stomachache, our blood sugar levels skyrocket and the pancreas releases the insulin hormone. Insulin tells the liver to store more sugar and lower the blood sugar levels, effectively blocking any effects that glucagon might still be having. By using the two together, and adjusting the levels as needed, the body ensures it has just the right amount of sugar in the blood, keeping things all hunky dory.

Think of this relationship like riding a bicycle. The whole point of riding a bike is to keep the thing balanced—just like a host of bodily functions. If you're cruising down the street wondering if your new helmet makes your head look weird and you notice that the bike is starting to lean to the left, what do you do? You lean to the right to keep the bike from falling over and looking like an idiot. By making all those little adjustments to keep things going just as they should, both you and your body keep things on an even keel.


Before we get into any specific features of the endocrine system, we've got to go over the basics: the types of hormones and the different endocrine glands. Hey, we can't put in the granite countertops or remote-controlled toilet without getting a solid foundation down first.

There are three different types of hormones: steroid, amino acid-derived, and peptide. Steroid hormones are fat-soluble chemicals that begin their chubby lives as cholesterol. Chemical reactions end up changing it, so each steroid hormone differs from the others based on the types, numbers, and locations of those chemical alterations. These hormones aren't normally hanging out in the body, though, and are released with constitutive secretion. That's just a fancy way of saying that they are released by endocrine glands, like the adrenal glands and gonads, whenever the brain decides they are needed. They're made-to-order.

Cholesterol Rings. The cholesterol rings are the precursor to the steroid hormones. Each hormone will have a different chemical structure at the "R" group.

Once the steroid hormone is synthesized and released, it travels through the blood stream to its appropriate end organ, where it's going to be put to good use addressing some particular issue. Steroid hormones are a bit of a VIP on the molecular level, and they get easy access to the inner sanctum of cells without breaking a sweat. Since steroids are cholesterols and cholesterols are lipids and lipids can pass through the lipid bilayer of cell membranes, these hormones pass right on through a cell's outer wall and high tail it to the nucleus where they bind to their hormone receptor. Once the steroid and receptor are linked, they travel inside the nucleus where they control the rate of protein synthesis as a team.

There are 5 different types of steroid hormones, and each has a different and specific effect on the body.
  • Glucocorticoids are released from the adrenal glands and affect metabolism, decrease inflammation, and block stress.
  • Mineralocorticoids are also secreted from the adrenal glands and, like their name implies, help to maintain mineral levels, especially the salt and water balance.
  • Estrogens come from both the adrenal gland and the ovary (the female reproductive organ that produces eggs waiting to be fertilized by some smooth-talking gentleman), and promote the development and function of female sexual organs.
  • Progesterone isanother girly steroid hormone. It's also made in the ovary, as well as the placenta (the thing that attaches to a developing fetus and provides nutrients) to maintain pregnancy and regulate the menstrual cycle.
  • Androgens (made from testosterone, which we're sure you've heard of) are the male equivalent to estrogens and develop the male sexual organs. Teenage boys eager to impress the ladies can thank their androgens for the budding five o'clock shadow they're growing on their faces—even if it's really just a wimpy little mustache.
Remember when we said that steroids don't just loiter around the body, but rather are created by the body to address particular issues as they arise (that whole constitutive secretion thing)? Take a look at the steroids we just ran down, and you'll see what we mean. These hormones don't lend a helping hand to just regular old situations. It's not like our metabolism or salt balance is constantly out of whack, and it's DEFINITELY not every day that we end up pregnant or go through puberty. Thank goodness.

Now on to amino-acid derived hormones. They are made up of the tyrosine or tryptophan amino acids. Thyroid hormones are released from the thyroid endocrine gland and are made up of tyrosine residues and iodine atoms. An animal cannot live without these thyroid hormones since they are involved in just about every bodily process imaginable: metabolism, growth, and development.

The other amino-acid hormones are unique because they are used by both the endocrine and nervous systems. Catecholamines, or neurohormones, are released from the adrenal gland and regulate various processes like blood flow to certain organs and respiration.

About 80% of all hormones are peptide hormones that are made from proteins. These hormones are usually synthesized well ahead of time, and are stored in tiny vesicles until they get released in a process called regulated secretion. The key difference between this and constitutive secretion is that these hormones are already baked and ready to go when needed, as opposed to being made on the spot.

Insulin is probably the best example of a peptide hormone since it is famous for its role in diabetes, a disorder that occurs when the body can't regulate its blood sugar levels. Insulin is made in the pancreas and stored within vesicles there until you eat a Milky Way. The body digests the candy bar, and oodles of sugar make its way into your bloodstream. Your body isn't doing backflips over you chattering your teeth and talking way too fast, so it lets insulin loose to get your blood glucose levels back in check.

Both amino-acid derived and peptide hormones are water soluble, meaning they can't pass through cell membranes like the lipid soluble steroid hormones. Luckily for them, their hormone receptors aren't located inside the cells, but hang out right on the cell membrane, where the hormone can actually reach them. When the water-soluble hormone binds to its membrane-bound receptor, it triggers a signal inside the cell that affects proteins or gene transcription. These hormones may not be able to get past the guard at the door of the cell, but they can at least make sure he passes along their message.

Endocrine Glands

There are seven different places in the body where hormones are synthesized and released:
  1. Hypothalamus
  2. Pituitary gland
  3. Thyroid
  4. Pancreas
  5. Adrenal glands
  6. Pineal gland
  7. Gonads

The Endocrine System. In one way or another, the endocrine system contributes to all physiological processes.

Each of these endocrine glands responds to specific stimuli and releases particular hormones. Some glands respond to other hormones, others respond to a signal from the brain, and others wait for other changes in the body before they jump into action.

We'll start with the king and queen of endocrine glands, the hypothalamus and pituitary. Both glands are located within the brain, and although the pituitary gland controls a boatload of hormones (it rules the land as the one true King Endocrine gland), it's the hypothalamus that tells it when to do its thing (and thus is the real boss, her majesty the Queen). The hypothalamus releases either stimulating or inhibiting hormones that act at the pituitary and regulate the release of its hormones.

Growth hormone is secreted from the pituitary gland, and controls growth and development among other processes. The pituitary can't decide by itself that it wants to secrete it, and needs a permission slip from the master hypothalamus. When growth hormone is needed, the hypothalamus secretes growth hormone-releasing hormone (don't ask us where they come up with these catchy names), which activates the pituitary to release the actual growth hormone.

The pituitary also releases prolactin (it stimulates milk production in mammals—notice the words "lactose" and "lactate" kind of hiding in there), and sex hormones including follicle-stimulating hormone (FSH) and luteinizing hormone (LH). If you're a girl, these hormones regulate ovulation. Guys have them, too, and FSH and LH help them make sperm and testosterone.

The hypothalamus (by secreting thyroid-releasing hormone) and pituitary gland (with thyroid-stimulating hormone) also control the actions of the thyroid glands. These glands are located on both sides of the trachea. (That's your throat.) It's pretty shocking, but the thyroid glands release thyroid hormone. It's actually a pair of hormones: triiodothyronine (thankfully it's abbreviated T3 since it's made from the amino acid tyrosine and has 3 iodine atoms) and thyroxine (T4, since it has 4 iodine atoms). These are super important hormones since they regulate all sorts of processes like muscle tone, digestion, and reproduction. Don't hate on these hormone since they can pull some tricks out of their hormonal hat. These hormones make tadpoles lose their tadpole tail to become an adult frog. What's your talent?

The pancreas is next on our list of endocrine glands and it's responsible for releasing two types of chemicals: those that aide in digestion and those that regulate blood glucose levels. To help in digestion, the pancreas secretes enzymes like lipase (to break down fats) and amylase (to break down sugars) when it senses those nasty acidic former-foods. But technically, this is an exocrine function of the pancreas (it gets released through ducts), so we don't have to worry about those details for this chapter.

When we talk about the pancreas being an endocrine gland, we're really referring to the islets of Langerhans. That sounds like some exotic island in the Caribbean, but don't let visions of sandy beaches carry you away. These cells release both insulin and glucagon, which tag team to regulate blood sugar levels. When blood sugar levels get too high, beta cells release insulin to get word to the liver. This hormone directs the liver to pull more sugar out of the blood passing through it and set it aside in storage. That way, the levels in the blood decrease. Glucagon is released from alpha cells and acts in the opposite way: when blood sugar levels are too low, it tells the liver to take some of that stored sugar and let it loose into the blood stream. That way sugars can get to all areas of the body where they might be needed to support cellular functions.

The adrenal cortex and the adrenal medulla (which collectively make up the adrenal glands) are endocrine glands located on the kidney. You know that feeling you get when you lean back a bit too far in your chair, and almost keel over? Right after you save yourself, you might notice that you're breathing more heavily, and maybe even shaking and sweating a bit. You can thank this adrenal duo. Both glands release hormones that deal with stress, but they do so with differing stimuli and by releasing different types of hormones. Nevertheless, whether it's a deer's stress from running away from the not-so-effectively camouflaged hunter or a human's stress when thinking about an upcoming exam, the adrenal glands cover all things stressful, large and small.

Adrenal glands. The adrenal glands rest right on top of the kidneys. Like the brain and the kidney, the cortex is the outer part of the adrenal gland, and the medulla is on the inside.

The adrenal medulla responds directly to the deer's stress when he hears the hunter and releases neurohormones, like adrenaline. These are amino-derived hormones, made from tyrosine. By acting at adrenergic (notice the "adren" in adrenal, adrenaline, and adrenergic) receptors at various parts in the body, these hormones activate a "fight or flight" response, which is part of the nervous system. They dilate the deer's bronchioles in the lungs so he can suck down more oxygen when he's running for his life. These catecholamines, including epinephrine and norepinephrine, also increase his heart rate so his muscles receive more blood (carrying helpful bits of energy) so he can run even faster. We'll learn more about these two neurohormones in our later section on the nervous system, so stay tuned.

The adrenal cortex synthesizes and releases steroid hormones, called corticosteroids. They aren't released in direct response to stressful situations, and hormones from the pituitary and hypothalamus actually stimulate the adrenal cortex. There are two types of corticosteroids, and both respond to stress in a behind-the-scenes kind of way. We've already talking about glucocorticoids and mineralocorticoids, but here's a refresher: Glucocorticoids primarily affect glucose metabolism and promote the synthesis or release of glucose into the blood. It's this added sugar that gives the deer his energy to run longer and faster than the hunter. Mineralocorticoids affect mineral metabolism, and help maintain the salt and water balance. Long story short, these corticosteroid hormones get extra energy to the muscles so the deer can outrun his predator.

The pineal gland is the next stop in our tour of endocrine glands. It's teeny tiny and just above the roof of your mouth. Melatonin is the name of the hormonal game here, and it's secreted in the absence of light. At night, and during seasons where the days are short (like winter), melatonin is released and controls the circadian rhythm, certain biological processes that happen every 24 hours. In animals that have breeding seasons like sheep, melatonin blocks the release of certain sex hormones. Since humans don't have breeding seasons, melatonin is famous for making us sleepy.

Last, but certainly not least, we'll talk about the steroid sex hormones released from the gonads. In males, the testes create and release androgens (including testosterone), and in females, ovaries whip up estrogen and progesterone. Not only do these hormones aid in sperm and egg production, but they also help develop sexual characteristics when young men and women reach that time in their lives when their voices start to crack and they become terrified of the opposite sex. These microscopic folks are responsible for turning us into women and men.

The kidneys, heart, and liver are also hormone-releasing organs, but aren't usually grouped with these other endocrine glands. That's lucky for you: no test questions on them.

Hormone Cascade Pathway

Usually bodily processes don't involve just one endocrine gland and one hormone, but involve multiple glands, hormones, and end organs that talk to each other in what's known as a hormone cascade pathway. In this hormonal game of "you're it," one hormone regulates the release of another. These trophic hormones act at other endocrine glands, and decide whether another hormone should be released or not. They usually come from the hypothalamus or the pituitary gland.

We'll use the prolactin hormone as an example of this convoluted hormone cascade pathway. Prolactin is important in mammary gland development, milk production, and immune function. The pituitary gland is responsible for actual prolactin release, but other hormones control the pituitary's ability to secrete it. It doesn't make much sense for female mammals to be producing milk all the time. Waste not; want not (is what these organs would say if they had a mouth).

A chemical called dopamine that is secreted by the hypothalamus provides the main brake on prolactin release. It acts as a trophic hormone, binds to the pituitary, and blocks prolactin synthesis and release. Prolactin is a peptide hormone and has to be synthesized before release. That means anything that messes with dopamine concentrations will also affect prolactin. Our brains are actually bathed in dopamine, so there's always dopamine around to restrict prolactin release.

Prolactin Pathway. Since females don't always need prolactin, mammals use multiple hormones to regulate its synthesis and secretion.

If dopamine is always around preventing the pituitary gland from releasing any prolactin, how the heck is a baby calf supposed to get any milk from his mom? Well, here come a couple more hormones to make that happen.

Thyroid-releasing (from the hypothalamus) and gonadotropin-releasing (from the pituitary) trophic hormones actually stimulate the production and release of prolactin. They take the attention away from dopamine and put the spotlight back on prolactin. Estrogen, that steroid sex hormone, also stimulates prolactin synthesis and release. As a mother gets late in her pregnancy, estrogen levels rise and increase prolactin levels. This increased prolactin is what prepares her mammary glands to lactate and provide her baby with milk after birth. When babies stimulate their mother's mammary glands, the hypothalamus gets involved and releases prolactin-stimulating hormones to jump-start the pituitary.

In the case of prolactin, there are lots of chemicals that control the creation and release of just one hormone from the pituitary gland. Dopamine blocks it, but other hormones stimulate it. Whichever's concentration is higher wins the battle, and ultimately decide prolactin's fate.

Brain Snack

The Guinness Book of World Records is chock-full of fun facts like world's longest fingernail and the longest time someone has managed to stay awake. But none are more tantalizing than those that deal with the endocrine system. Trust us.

Meet Robert Wadlow: World's Tallest Person. He had an overactive pituitary gland, and it caused excessive growth hormone secretion. When Mr. Wadlow died in 1940 at age 22 from a blister infection, he was less than an inch shy of 9 feet tall and weighed nearly 450 pounds. He wore a size 37 shoe. That's one big dude. It was one teensy little gland stuck in overdrive that made it happen.

He was always big for his age, too. It's not like one day he woke up 6 feet taller than the night before. At the start of his awkward teenage years, he was already over 7 feet tall.
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