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PUBH 3100 Human Disease and Prevention

Week 2 – The Human Body – How It Works: The Endocrine System

(ON-SCREEN GRAPHIC ---Films for the Humanities & Sciences)

(ON-SCREEN GRAPHIC ---The Human Body – How It Works: The Endocrine System)

NARRATOR: The human body is a complex combination of many interrelated parts and systems. As we go about our daily lives, we rarely give a thought to how all these parts work together to meet our daily needs and ensure our survival. To coordinate all the functions our bodies perform, we have two systems for control and communication: the nervous system and the endocrine system. In this program, we'll take a look at the endocrine system and the hormones it produces, the little chemicals that run the body.

(ON-SCREEN GRAPHIC ---Introduction to the Endocrine System)

The endocrine system is a collection of organs called endocrine glands that are located in various parts of the body. These glands secrete chemical messengers called hormones directly into the bloodstream, which then carries them to their destinations throughout the body.

(ON-SCREEN GRAPHIC ---Timothy Shaw, PhD – Professor of Human Anatomy Bethel University)

TIMOTHY SHAW: You can think of the endocrine and nervous systems as two parts of a single control system. The nervous system sends and receives messages through nerve cells. It must often react very quickly to stimulate a response, such as pulling your fingers away from a hot stove so you don't get burned. In contrast to nerve impulses, hormones travel much slower. They control responses that don't have to be immediate, but do need to be more generalized and longer lasting. So they take a slower route to their destinations via the bloodstream.

(ON-SCREEN GRAPHIC ---Hormones and How They Work)

NARRATOR: Hormones can be divided into two general chemical groups: steroids and non-steroids. Steroids include all of the sex hormones produced by the ovaries and testes, estrogens, progesterone, and testosterone, and substances from the adrenal cortex of the kidney, such as cortisol and a form of vitamin D. Non-steroid hormones include proteins, such as insulin from the pancreas, growth hormone from the pituitary

gland, and molecules called amines, such as thyroid hormone from the thyroid gland. Steroids are part of a group of biological molecules called lipids, or fats. Lipids don't dissolve in water, so they're referred to as hydrophobic, which literally means water fearing. In order to be transported through water-based bodily fluids such as the blood, lipids must combine with protein molecules to form lipoproteins. Non-steroid hormones are water soluble, so they are called hydrophilic, or water loving. They can travel through the blood as they are.

Hormones act on target cells. Each target cell has a receptor protein for its specific hormone molecule. The receptor and the hormone attach to each other exclusively, like a lock and key. Because cell membranes are made primarily of lipids, all lipids can easily enter or leave cells. So the receptors for lipids are inside the cell. Water-soluble proteins and amine molecules can't cross the waterproof lipids cell membrane on their own, so their receptors are located on the outside of the cell membrane. But the receptor protein extends through the membrane and is attached to a signal protein on the inside.

TIMOTHY SHAW: Each type of cell has a different set of receptor proteins. So, the same hormone can produce different responses in different types of cells, or no response at all, depending on the specific set of receptor proteins the cells contain.

NARRATOR: Once a hormone has attached to a receptor, the receptor causes a specific change inside the cell through a process called signal transduction.

(ON-SCREEN GRAPHIC ---The Endocrine Organs)

Endocrine glands are located in various parts of the body. The hypothalamus is located near the center of the brain. It receives chemical and nervous input about sight, sound, taste, smell, temperature, and blood-glucose concentration, as well as salt and water balance. It also helps control hunger and thirst, as well as mating and sexual behavior. The hypothalamus controls the pituitary gland, which is attached to the underside of the brain.

The pituitary gland has two parts: the anterior and posterior pituitary. The anterior pituitary releases many hormones, including growth hormone, which stimulates bone and muscle growth. And endorphins, which reduce the perception of pain. The pineal gland, about the size of a pea, is located slightly above and behind the hypothalamus. It receives information about light and dark cycles and secretes melatonin, which is involved in rhythmic behavior, such as sleep cycles. The thyroid gland is a butterfly- shaped structure located in front of the trachea, or windpipe, between the larynx and the notch at the top of the ribcage.

The thyroid gland releases thyroid hormone, which plays a critical role in development and growth. It also increases the body's metabolic rate and the release of calcitonin, a hormone that lowers blood calcium. That parathyroid glands are four small tissue masses attached to the back of the thyroid. They secrete parathyroid hormone, which raises blood calcium levels, producing the opposite effect of calcitonin. The thymus gland, located in the chest region, produces thymosin, a chemical that activates lymphocytes, cells that are involved in many aspects of immunity. The pancreas, located beneath the stomach, secretes two hormones: insulin and glucagon. Insulin lowers your blood-glucose level, while glucagon raises it. Together insulin and glucagon maintain a nearly constant concentration of glucose in our blood.

The two kidneys, located at the back of the abdominal cavity just below the ribcage, produce renin, a hormone that ultimately helps regulate blood pressure and erythropoietin, a hormone that stimulates red bone marrow to produce more red blood cells. The adrenal glands sit above the kidneys. The outside of the adrenal gland, the adrenal cortex, secretes cortisol. The middle of the gland, the medulla, secretes epinephrine and norepinephrine, also known as adrenaline and noradrenaline.

TIMOTHY SHAW: Epinephrine and norepinephrine are hormones that are released during periods of stress. For example, they cause the response known as fight or flight that gives you a seeming boost in energy or strength that allows you to survive a possible life-threatening situation.

NARRATOR: The gonads, which are the ovaries in women and the testes in men, produce the sex hormones. The ovaries produce estrogens, the primary sex hormones in females and progesterone. The testes produce androgens.

(ON-SCREEN GRAPHIC ---Blood Glucose Levels)

All living cells need energy to do work and grow and divide. For most cells, the sugar molecule glucose is the source of this energy.

TIMOTHY SHAW: Complex carbohydrates are digested to produce glucose molecules in the small intestine, and amino acids are also converted into glucose molecules. These molecules are then picked up by the blood and transported to all the cells of the body.

NARRATOR: The liver and muscle cells take in glucose and store it as a larger molecule called glycogen. As mentioned before, the blood-glucose level is primarily maintained by insulin and glucagon. These two hormones are produced by groups of cells in the pancreas called islets of Langerhans. There are two different kinds of islet cells: alpha cells, which secrete glucagon, and beta cells, which secrete insulin. These hormones have opposite reactions: insulin lowers blood glucose and glucagon raises it.

After a person eats carbohydrates, blood glucose levels go up. That stimulates the beta cells to release insulin into the blood. People with Type-1 diabetes have to provide additional insulin because their beta cells don't produce a sufficient quantity. The insulin travels throughout the body, enhancing the transport of glucose from the blood into cells. Insulin also increases the production of glycogen in the liver and muscle cells.

TIMOTHY SHAW: As your blood-glucose level goes down, the pancreas gradually produces less insulin until the level reaches about 80 to 85 milligrams of glucose per 100 milliliters of blood. Then, when blood glucose drops to about 50 milligrams per 100 milliliters, the alpha islet cells of the pancreas begin to secrete glucagon. This glucagon stimulates the liver and the muscles to convert glycogen to glucose, which quickly raises blood-glucose levels again.

NARRATOR: The net effect of these two hormones is to keep blood glucose levels in a nearly constant balance called homeostasis.

(ON-SCREEN GRAPHIC ---Growth and Metabolism)

How tall a human being will be as an adult depends on many factors, but first and foremost is genetics. Tall parents tend to have tall children, and short parents tend to have short children. Several hormones also affect growth and development, either directly or indirectly. The two most important are growth hormone and thyroid hormone. Growth hormone stimulates protein synthesis in bone and muscle. It also stimulates the use of fat as a fuel so that lean body mass increases.

Growth hormone, also called somatotrophin, is secreted by the anterior pituitary gland under the control of two hormones from the hypothalamus. One of these is growth- hormone releasing hormone, or GHRH. When growth-hormone levels are high enough, feedback to the hypothalamus inhibits the release of GHRH and stimulates instead the release of somatostatin. Somatostatin slows the release of growth hormone in the pituitary.

Like growth hormone, the release of thyroid hormone is controlled by the hypothalamus and pituitary. Besides being necessary for normal growth and development, thyroid hormone also stimulates the metabolism of nearly every tissue in the body.

TIMOTHY SHAW: Growth hormone and thyroid hormone are synergists, meaning that they increase each other's effectiveness to promote natural growth and development. Neither hormone alone can cause normal growth. For example, if there isn't enough thyroid hormone present during gestation in the womb or during infancy, the baby will have a form of growth and mental retardation called cretinism, even if growth hormone levels are normal.

NARRATOR: Growth is also affected by the sex hormones, testosterone and estrogen, both of which accelerate growth, especially during puberty.

TIMOTHY SHAW: Because testosterone causes protein synthesis, especially in muscles, some athletes may take various forms of testosterone called anabolic steroids to enhance muscle development and performance. There are both physical and psychological risks to using anabolic steroids, including an increased risk of heart attack, stroke, sterility, and some cancers. Many areas of the brain have receptors for testosterone, so steroid use can also lead to aggressive behavior, depression, and delusions.

(ON-SCREEN GRAPHIC ---Reproduction)

NARRATOR: Hormones are involved in every aspect of reproduction. During development, hormones determine the physical sexual characteristics and produce and maintain the physical traits that are associated with being male and female. Estrogens produce female characteristics and testosterone produces male characteristics. Hormones also control the production of eggs and sperm and pregnancy, birth, milk production, and nursing.

(ON-SCREEN GRAPHIC ---Stress)

When a person is exposed to danger, the immediate response of the central nervous system is to signal the medulla in the adrenal gland to release epinephrine and norepinephrine. These hormones are also known as adrenaline and noradrenaline.

TIMOTHY SHAW: Within milliseconds of the stimulus reaching the brain, nerves send these chemicals to the blood vessels and to virtually all the internal organs. The person is suddenly alert, the heart and breathing rates increase, blood-glucose levels rise, and blood flow to muscles increases. This is often referred to as the fight or flight response.

NARRATOR: The medulla can secrete those hormones for a few days if necessary. If stress continues beyond that, the brain signals the hypothalamus, which signals the anterior pituitary to release ACTH, adrenocorticotropic hormone.

TIMOTHY SHAW: ACTH travels through the blood to the adrenal cortex and stimulates the release of glucocorticoids, primarily the hormone cortisol. Cortisol raises blood- glucose levels and increases blood volume and blood pressure. It also suppresses the immune system and the inflammatory response that could cause swelling and pain.

NARRATOR: For example, if a person falls and sprains an ankle while hiking in the woods, epinephrine from the adrenal medulla helps prepare the person for the exertion needed to get help. Cortisol from the adrenal cortex keeps the ankle from swelling and allows the person to keep walking on it.

TIMOTHY SHAW: Suppressing the immune system can be a life-saving response, but there are risks involved. If it continues for any length of time, it can make the body more susceptible to disease-causing organisms.

(ON-SCREEN GRAPHIC ---Mineral Balance)

NARRATOR: Certain minerals are critical to the proper functioning of our body systems. Through the release of various hormones, the endocrine system ensures that the body maintains the required levels of those minerals. One of the most important is calcium. We all know that calcium is essential for such things as strong bones and teeth, but did you also know that without calcium your blood will not clot adequately, nerves and muscle cells cannot function, and many hormones and enzymes will not work? To ensure the proper functioning of all these systems, the body regulates the level of calcium within very narrow limits. Three hormones are primarily responsible for this regulation: parathyroid hormone from the parathyroid gland, calcitonin from the thyroid, and active vitamin D from the liver and kidneys.

TIMOTHY SHAW: The cells of the parathyroid gland have calcium ion receptors on them. As blood calcium levels decrease, the cells respond by secreting parathyroid hormone. This causes the kidneys to increase the amount of active vitamin D.

NARRATOR: In response, bone cells begin to release calcium from bone tissue within minutes.

TIMOTHY SHAW: If the calcium removed from the bones is not replaced by calcium in the diet, the bones will weaken. This is often what happens to older people, accounting for the stooped shoulders and the frequent fractures among this group. For this reason, dietary recommendations for calcium are fairly high, from 400 to 1500 milligrams per day, depending on age. As people get older, they need higher levels of calcium to prevent bone loss.

NARRATOR: Parathyroid hormone also acts on the kidney, to increase the re-absorption of calcium from the fluid in the kidney tubules. This puts calcium back into the blood, instead of being excreted in the urine. As blood-calcium levels increase, less parathyroid hormone is released, and the thyroid gland begins to secrete calcitonin. The primary action of calcitonin is to inhibit calcium-releasing activity in bone cells. As a result blood-

calcium levels decrease. The body is 60 to 65% water. Dissolved in the body's water are a number of chemical substances called electrolytes.

TIMOTHY SHAW: Electrolytes are charged particles called ions. These ions are necessary for nerve impulses and muscle contraction, so their concentrations in the blood are carefully regulated.

NARRATOR: Water and electrolyte balance needs to be maintained. The most important electrolytes are sodium, potassium, magnesium, and chloride. The electrolyte that requires the closest regulation is sodium. Several hormones work together to maintain normal water and electrolyte concentrations. When the brain detects an increase in sodium concentration in the blood, the posterior pituitary releases antidiuretic hormone, also called vasopressin, which acts on the kidney to reduce urine output. Less water is excreted, so more returns to the bloodstream.

When blood pressure decreases, the filtering units in the kidney release renin. Renin converts a protein made by the liver called angiontensin I into angiotensin II, which stimulates the release of aldosterone from the adrenal cortex. Aldosterone causes the kidney to preserve fluids, thereby raising blood pressure. When blood pressure increases, the right atrium of the heart releases hormones called atrial natriuretic factors. These factors stimulate the kidneys to produce more urine, which reduces fluid volume in the blood, which reduces blood pressure.

During exercise, the body can lose large amounts of water and electrolytes in the form of sweat. There are many sports drinks on the market that contain water, sugar, and salts. Besides replenishing fluids, the sugar in these drinks provides energy, and the salts replace electrolytes that have been lost. A sports drink may also help your performance and increase your endurance because you'll be better hydrated.

NARRATOR: The endocrine system is essential to the proper functioning of the human body. The hormones it produces are an indispensable part of our body's complex system of control and communication. Produced by many different organs and carried by the bloodstream to cells throughout the body, hormones affect growth, help us deal with physical and psychological stress, and keep virtually all of our body systems in balance.

(ON-SCREEN GRAPHIC ---

Based on the Book Series The Human Body – How It Works

The Immune System Chelsea House Publishers

Executive Producer – Craig Claudin Producer/Director – Christine Dean

Script Development – Christine Dean Script Consultant – Timothy Shaw, PhD – Professor of Human Anatomy Bethel University

Narrator – Erin Mathe Videographers – Tim Lewis and Matt Bjur

Editor – Matt Bjur Animator – Chris Parrish-Taylor

Graphics – Matt Bjur Illustrations – Britta Bjur

Thanks to the staff and students of Thomas Jefferson High School Bloomington, MN for

their assistance in producing this program series.

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ALL RIGHTS RESERVED