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What are hormones and what do they do?

In the previous section you learned about the changes your body goes through at puberty. But how and why do these changes occur? What makes them happen? If we were to choose one word to answer these questions, it would be hormones. In this section you will learn about hormones and what they do.

Did You Know?

Hormones . . .

  • have short lives-they remain active for only a few minutes.
  • can bind to different kinds of cells and produce different results. For example, androgen can stimulate muscle growth or hair growth.
  • of different types can send conflicting messages to a cell. What does the cell do? Compromises and listens to both-or to the one shouting the loudest.
  • cause the human body to follow rhythms, the menstrual cycle for example.

Hormones are chemical substances that the body's various systems need to perform their functions. These chemicals are produced by endocrine glands, clusters of specialized cells located in various parts of the body. Each hormone influences the functions of certain target cells in the body. Thus, a hormone acts like a chemical message sent out by an endocrine cell and received by a target cell. There are over a dozen major endocrine glands in the body. We will focus on only four of these: the hypothalamus, pituitary, adrenals, and the gonads (ovaries and testes).

Endocrine glands are not the only glands in the body. Others, called exocrine glands, do not produce hormones but release secretions to the places needed through ducts, or small tubes. For instance, saliva, a secretion from an exocrine gland in the cheek is delivered through a duct to the mouth. As you chew food, saliva mixes with the food and helps soften and digest it.

“Just as some girls make enough androgens to grow a shadow of a mustache so some boys make enough estrogen to grow minor bumps beneath their nipples.”

-The Body Book

Sara Stein

Endocrine glands have no ducts. These ductless glands release hormones directly into the bloodstream where they circulate throughout the body. Hormones play a vital role in regulating all the body's functions. They are responsible for your growth and sexual maturation, and they help keep you healthy and alive. This section will focus on the hormones directly responsible for the changes of puberty.

Word Origin: Endocrine, Exocrine Where do the terms exocrine and endocrine come from? Use a dictionary or other word reference book to find the answer.

Figure 4.1 The endocrine glands of males and females that are discussed in this unit.

Glands
Exocrine (ducts) Endocrine (no ducts)
  • secretions
  • hormones
  • delivered to specific location
  • delivered into blood stream

Figure 4.2 The difference between exocrine and endocrine glands.

How do hormones function?

As mentioned, hormones are released directly into the bloodstream. Once released and circulating in the bloodstream, they go to every cell in every corner of the body. Although every cell comes into contact with many different hormones, only target cells bind to their respective hormones. How? Before you read further, stop and make a prediction about how cells might know which hormones to select and which to reject.

A cell's selection of hormones works a little like the way people pick up their luggage at an airport baggage claim area. Suitcases come out to the carousel and go round and round. People identify and pick up their own suitcases as they go by. They do not take up the suitcases that do not belong to them.

Figure 4.3 Certain hormones bind to certain receptors the way a lock fits a key.

Did You Know?

You are aware of less than 1% of what's happening in your brain and body at any given time.

This system requires that people recognize a suitcase as theirs. How can cells recognize a hormone as theirs? They do so through receptors located either within the cell or on the cell surface that recognize and bind with hormones. A cell receptor and a hormone work like a lock and key. If the key (hormone) fits, the lock (receptor) will open-if the key (hormone) does not fit, the lock (receptor) will not open. Each hormone has its own kind of receptor. So, although hormones come in contact with cells, they do not affect a cell unless they match the cell's receptors.

Hormones can cause many different reactions, depending on the type of cell to which they attach. They can make a cell grow bigger, secrete fluids, or move. Hormones can affect a cell close to the releasing endocrine gland or influence cells far away from the gland. Glands function like a remote control device: they emit hormones to turn body functions on and off.

Hormones don't function alone, however. The endocrine (hormone) system works closely with the nervous system, consisting of the brain, spinal cord, and nerves, to integrate and control the physiological functions of the body, such as the reproductive system. You should think of them as one large system whose functions and actions are closely coordinated. They work together like a team. The team members have different positions and specific tasks, but they all work together toward a common goal: making the body function.

Why does the body need both a nervous system and an endocrine system to regulate its function?

Activity 4-1: Glands and Hormones

Introduction

Hormones from endocrine glands circulate through all parts of the body but are picked up only by certain cells. This exercise will help you learn about the way cells identify the hormones they need and the way that hormones attach themselves to particular cells.

Materials

  • 1 Activity Report for each team
  • 1 pair of scissors per team
  • 1 glue stick per team

Procedure

Step 1 Reread the section in your text about the selection of hormones by cells.

Step 2 Look at the Activity Report you have been given. Cut out all the hormones from the strip at the bottom of the page, being careful to cut accurately.

Step 3 Your job is to match the hormones with the proper cells by fitting them into the receptors.

Step 4 Imagine that the hormones are circulating through the body, past the cells. Move the pieces you have cut out across the page of cells at random. If the hormone does not fit a particular cell that it is passing near or through, move it past that cell to the next one. When a hormone comes near a cell with a proper receptor, fit it into place and leave it there.

Step 5 When you have all the hormones lined up at their proper receptor sites, glue them in place.

Step 6 Then answer the questions on the Activity Report.

What Do You Think?

What do you think would happen if the body didn't begin producing more and different hormones at puberty?

The Hormones of Puberty

Now that you know how hormones work, you're ready to learn about the hormones that are specific to puberty. Interestingly, most of the hormones affecting puberty are found in both females and males, and they produce similar results in both sexes.

The pituitary gland produces many hormones, including some of the hormones of puberty. This pea-sized “master gland” is located near the base of your brain.

One very important hormone produced by the pituitary is the growth hormone (GH). It makes your body's bones and tissues grow larger. At puberty, GH is responsible for the growth spurt, although other hormones, like the thyroid hormones that affect metabolism, also influence this process. Until recently, children who lacked growth hormone failed to grow normally. Now they can be treated with injections of GH.

Unlike GH, other hormones of the pituitary gland do not work directly to change body tissues. Rather, they stimulate other endocrine glands to produce hormones that in turn change body tissues. The pituitary's effect on the reproductive glands, or gonads, will serve as a model of how this chain of events works.

What Do You Think?

Now that you know something about what hormones can do, what do you think is meant by the phrase “raging hormones”? Why is the term often used when talking about adolescents?

The female reproductive glands are the ovaries. The male reproductive glands are the testes. These reproductive glands, also called gonads, do two things. They produce sex cells (either eggs or sperm) and secrete hormones. In both the male and female, the pituitary gland produces two hormones called follicle-stimulating hormone (FSH) and luteinizing hormone (LH) that flow through the bloodstream and are picked up by the gonads (remember the lock-and-key method of how hormones work). FSH and LH are together known as gonadotropins-hormones that control the gonads. The gonads, in turn, produce their own hormones.

In the female ovaries, FSH stimulates an egg, or ovum, to mature in its follicle, or sac. While maturing, follicle cells produce the hormone estrogen. After an egg matures and leaves the follicle, LH causes follicle cells to produce progesterone. Estrogen and progesterone play an important role in sexual maturation, the menstrual cycle, and reproduction.

Figure 4.4 Female sex hormones-where they come from and what they do.

Figure 4.5 Male sex hormones-where they come from and what they do.

Although named after the functions they perform in the female body, FSH and LH also exist in the male but perform different functions. In the male, FSH stimulates the production of sperm cells in the testes, but it does not cause the testes to produce hormones. That function is performed by LH, which acts on cells that are between tiny tubes where sperm are produced, causing the cells to produce a hormone called testosterone, which helps develop pubic hair and build muscle.

Another source of testosterone in both sexes is the adrenal gland. Also controlled by the pituitary gland, the adrenals are found on top of the kidneys (Figure 4.1). While most of the testosterone in males comes from the testes, the adrenals are the main source of testosterone in females.

“I feel as if I'm going to burst, and I know that it would get better with crying; but I can't. I'm restless, I go from one room to the other; breathe through the crack of a closed window, feel my heart beating . . . ”

-Diary of a Young Girl

Anne Frank

It is customary to refer to testosterone (or androgen) as the “male” hormone and to estrogen and progesterone as the “female” hormones. Yet all three hormones exist in both sexes in differing amounts and all belong to a group of chemicals called steroids. These hormones also have some effects that are not sexual. For instance, androgens help build up muscles in both sexes.

If the pituitary controls other endocrine glands such as the testes and the ovary, what controls the pituitary? Is the “master gland” its own boss?

The pituitary itself is under the control of a portion of the brain called the hypothalamus. This area of the brain lies right above the pituitary. It is both part of the nervous system as well as part of the endocrine system. This double function shows how closely the two systems are linked together. The hypothalamus produces the gonadotropin-releasing hormone (GnRH) that acts on the pituitary gland. The pituitary gland, in turn, releases FSH and LH.

How can a small gland like the pituitary produce so many hormones?

Figure 4.6 The hormones of puberty are regulated by a negative feedback system. This is a process in which the end product controls the function of activity or the starting elements.

What controls the hypothalamus and pituitary gland? The very same hormones produced by the glands they control-testosterone, progesterone, and estrogen-control the hypothalamus and pituitary gland. Look at the chain of command for the production of testosterone in Figure 4.6. The arrow from the hypothalamus shows GnRH acting on the pituitary gland. The second arrow shows LH acting on the testes. The testes then release testosterone that circulates in the bloodstream. When the level of testosterone in the blood gets above a certain level, it inhibits or limits the hormone outputs of the hypothalamus and pituitary. Both the hypothalamus and pituitary slow down their production of hormones. As a result, the testes get less LH and decrease production of testosterone, until its level in the blood falls to a certain level. This negative feedback system helps keep hormones in the bloodstream at fairly steady levels. How this works with females will be discussed in connection with the menstrual cycle.

Social Feedback Write a poem or story illustrating the concept of a social feedback system.

An everyday example of a negative feedback system is the heating system in a building. Suppose the thermostat is set at a chosen temperature of 68^\circ F. When the room temperature drops below 68^\circ F, the thermostat starts the heater. When the room temperature reaches 68^\circ F, the thermostat turns off the heater. Now you are back where you started. This way you maintain a more or less constant temperature in the room.

Glands are controlled by similar feedback systems that regulate the level of hormones in the bloodstream. For example, the testes produce testosterone (just like the heater produces heat). But testosterone is constantly being used up (just as heat is gradually lost from the building). When the level of testosterone drops below its “set point,” the testes are triggered, and they produce more testosterone. The level of testosterone then rises back to where it started. Such self-regulatory systems help maintain the delicate balance of hormones in the body.

The feedback system discussed here is a negative feedback system. Why? What might a positive feedback system be?

Thermostats are negative feedback systems.

Activity 4-2: All That Happens at Puberty

Introduction

Although it may seem very complicated at first, the way that hormones influence growth and change at puberty works very much like a relay, or a circuit. In this activity you take the role of a hormone or body part and are given a job, or message to relay to another hormone or body part. That hormone or body part will then pass its own message on to the next hormone or body part, and so on, until the message gets back to the start, which is the hypothalamus.

Materials

  • Name card
  • Role card
  • Construction paper
  • Markers
  • String, straight pins, or safety pins

Procedure

Step 1 Make sure you have read Section 4 carefully. Ask for help if you are confused.

Step 2 Your teacher will give you a name card that tells you the part you will play. With the construction paper and markers, make yourself a large name tag to pin on or to string and wear around your neck.

Step 3 Your teacher will also give you a role card. It contains background information about what you do. Read this carefully so that you understand your role. It will tell you what body part or hormone gives you the signal to do what you do.

Step 4 At the bottom of the card in boldface is your “script.” These are the lines you are supposed to say when given the signal by the hormone or body part that comes before you in the relay. Listen carefully, and read only the line that particular body part or hormone commands you to say at that time. You may have several different jobs to do, each controlled by different hormones or body parts.

Step 5 The hypothalamus will start the relay by saying its lines and “finding” GnRH. Once it says its lines and “tags” GnRH, it stands still, and GnRH says its lines and “tags” the pituitary gland. This continues until all cards have been used. Sometimes more than one thing will be happening at the same time. It is important to be quiet enough to hear the commands and act only when given the proper signal. Once you've done your job, stand still and listen carefully to the rest of the relay.

Close your eyes and imagine your hormonal system at work. Hormones are surging through your body, passing in and out of some cells, binding to others. What does it feel like? What might it look like artistically, not scientifically? Draw your impression of what's going on under your skin, or describe it with words.

Review Questions

  1. What are hormones? Name three body functions they affect.
  2. What is the difference between endocrine and exocrine glands?
  3. How do cells know which hormones to bind to?
  4. What is the difference between the pituitary gland and the hypothalamus?
  5. How do FSH and LH stimulate the production of other hormones?
  6. What do testosterone, estrogen, and progesterone do in males and females?
  7. What is a feedback system? How does it work?

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