The Endocrine System

Introduction

The endocrine system is composed of the thyroid glands, parathyroid glands, pituitary glands, adrenal glands, pancreas, gonads, and pineal glands.

Unlike other body systems, the endocrine system glands aren't connected to one another.

 

Location of the Endocrine Glands

The endocrine system is composed of ductless glands scattered throughout the body. The function of these glands is to produce substances called hormones.

The word hormone comes from the Greek word meaning "impulse" or "to urge on." One of the jobs of hormones is to stimulate "target" cells to react in specific ways. For example, the hormone prolactin "urges" the mammary glands to secrete milk, and insulin "urges" blood cells to take on glucose.

Hormones act as messengers to ensure the proper development and operation of many organs, control the metabolic rate of all cells, and regulate the homeostasis of body fluids. After producing hormones, the glands release them into the bloodstream.

 

Functions

The endocrine glands are regulated by two systems: positive feedback and negative feedback.

Positive feedback is demonstrated by our example of prolactin stimulating the mammary glands. This system is activated by a stimulus, such as a baby sucking, providing a cue for the release of a hormone.

Negative feedback is a system by which the endocrine system keeps other systems in balance. Think of it as the thermostat for your body. In a building, a thermostat is set to a certain temperature. If it detects that the temperature has dropped lower than the set point, it turns on the heat to warm it back up to the set temperature. If it detects that the air is warmer than desired, it turns off the heat to allow the temperature to fall again. This is called negative feedback because the reaction is opposite to the stimulus. If there's too much stimulus, the gland stops or slows production; if there's too little stimulus, the gland increases production. Many hormones function under this type of negative feedback control.

 

Some hormones are also controlled directly by the nervous system. This allows the central nervous system to control bodily functions through the endocrine glands. 

A familiar example of this is adrenaline, a hormone released by the adrenal glands. It's released in response to outside stimuli, such as being startled by a noise or perceiving danger. This hormone is responsible for our "fight or flight" reaction. It gives us a boost of energy to act quickly.

Adrenaline is a part of a feedback loop. An increase in stimuli causes an increase in production of adrenaline. When the stimuli decrease, the body stops producing adrenaline. 

 

Each endocrine gland produces its own hormones that have specific functions in the human body. You'll learn more about these next, but here is a brief overview:

Thyroid

Thyroid Gland. 

• Thyroxine and triiodothyronine regulate metabolism in body cells.
• Calcitonin stimulates the passage of calcium from the blood into the bones.

Pancreas

• Insulin regulates the transport of glucose to the body's cells.
• Glucagon increases blood sugar by causing conversion of glycogen to glucose.

Adrenal

Cortex.

• Cortisol (glucocorticoid) regulates the quantities of sugars, fats, and proteins in cells.
• Aldosterone (mineralocorticoid) regulates the amount of salt in the body.
• Estrogen and testosterone (gonadocorticoids) maintain secondary sex characteristics. 

Medulla.

Epinephrine (adrenaline, norepinephrine) mimics the sympathetic nervous system's response.

Parathyroid

Parathyroid hormones regulate calcium in the blood.

Pituitary (Anterior)

Anterior Lobe.

• Luteinizing hormone (LH) promotes ovulation.
• Prolactin (PRL) promotes growth of breast tissue and milk secretion.
• Growth hormone (GH; somatotropin) increases bone and tissue growth.
• Melanocyte-stimulating hormone (MSH) Increases pigmentation of the skin.
• Gonadotropins (Follicle-stimulating hormone or FSH) stimulates growth of eggs and ovarian hormone secretion. 
• Adrenocorticotropic hormone (ACTH) stimulates secretion of hormones from the adrenal cortex, especially cortisol.
• Thyroid-stimulating hormone (TSH) stimulates production of thyroxine and growth of the thyroid gland.

Pituitary (Posterior)

Posterior Lobe.

• Antidiuretic hormone (ADH; vasopressin) stimulates reabsorption of water by kidney tubules. 
• Oxytocin stimulates contraction of the uterus during labor and childbirth.

Testes

Testosterone promotes growth and maintenance of secondary sex characteristics in the male.

Ovaries

• Estradiol develops and maintains secondary sex characteristics in the female.
• Progesterone prepares and maintains the uterus in pregnancy.

 

Endocrine Glands

The largest of the endocrine glands is the thyroid gland. This gland, popularly called the "Adam's apple" in men, is located in the front and sides of the neck. The bowtie-shaped thyroid gland has two lobes, one on either side of the trachea. Thyroid hormones play an essential role in body metabolism. There are three thyroid hormones:

1. Thyroxine (T4)
2. Triiodothyronine (T3)
3. Calcitonin 

 

T4 and T3 are the major thyroid hormones. When T4 and T3 are in the bloodstream, the body's cells are able to consume oxygen at an increased rate. The increased oxygen level allows the cells to use carbohydrates and break down proteins. In other words, T4 and T3 help give the body the energy needed for maintenance and growth. They're also important for bone growth, especially in children.

The T3 and T4 abbreviations come from the number of iodine atoms in one molecule of each hormone. Thyroxine has four iodine atoms, while triiodothyronine has three.

Calcitonin (also called thyrocalcitonin) is the third thyroid hormone. It's important in calcium metabolism. Calcium is one of the electrolytes necessary for the proper function of every human cell. Calcium is particularly important to the health of bones and teeth and to the function of muscles. Calcitonin prevents calcium loss in the bones, lowering the amount of calcium in the bloodstream. It also plays a part in knowing when you've had enough to eat or drink.

 

The parathyroid glands are located next to the thyroid gland. They're embedded in the connective tissue that holds the thyroid in place. The main function of these glands is to regulate calcium levels by exerting the opposite effect from calcitonin.

Parathyroid hormone (PTH) increases the level of calcium in the blood. Each affect of PTH is balanced by an opposite effect from calcitonin. This system allows the body to maintain healthy levels of calcium.

 

The two adrenal glands are sometimes called suprarenal glands because they're located on top of the kidneys. The adrenal glands are composed of an outer adrenal cortex and an inner adrenal medulla.

The adrenal cortex helps regulate your body's metabolism and stress responses by releasing three different types of steroid hormones, or corticosteroids.

• Cortisol (glucocorticoid)
• Aldosterone (mineralocorticoid)
• Estrogen and testosterone (gonadocorticoids)

The adrenal medulla develops from nervous tissue. It's sometimes discussed in relation to the central nervous system. The adrenal medulla is nicknamed "the emergency gland" because it secretes two types of catecholamine—hormones that help the nervous system handle any type of alarm or danger to the body.

• Epinephrine, or adrenaline
• Norepinephrine, or noradrenaline

 

The most important glucocorticoids are cortisone (or hydrocortisone) and cortisol. Glucocorticoids have an anti-immunity or anti-allergy effect. They're given to patients to stop the body's natural defense to allergens or irritants.

Cortisol is the principle steroid. Some of its main functions include

• Regulation of carbohydrate, protein, and fat metabolism throughout the body
• Stimulation of gluconeogenesis in the liver (conversion of amino and fatty acids to glucose)
• Increase of blood sugar concentration

 

Aldosterone regulates the balance of electrolyte (mineral) salts in blood and body fluids. It triggers the kidney tubules to reabsorb sodium and excrete potassium.

The adrenal glands produce small amounts of androgens (male sex hormones, testosterone) and even smaller amounts of the female sex hormone estrogen. These steroids are weak and have little effect on the male body since the testes already produce so much testosterone, but adrenal androgens stimulate the sex drive in females.

 

The presence of epinephrine, or adrenaline, in the bloodstream causes an increase in heart rate, blood pressure, and cardiac output. It also causes dilation of the bronchial tubules to increase oxygen intake, which explains why a shot of epinephrine can control a severe asthma attack and treat anaphylactic shock from a severe allergic reaction.

Adrenaline elevates the blood-sugar level, too. All of these physiological effects combine to give the body an extra spurt of energy and strength.

 

Norepinephrine, or noradrenaline, reinforces many of the effects of epinephrine. It dilates vessels to the brain, muscles, and heart and constricts blood vessels leading to organs nonessential in an emergency reaction (such as the skin).

This explains why superficial wounds don't bleed as profusely when adrenaline is flowing, and why there's a sensation of blood draining from your face when you're upset, angry, or scared.

Norepinephrine also acts as a neurotransmitter, thus heightening the ability to think quickly and clearly during an emergency.

 

The pancreas is an organ located behind the stomach that serves as an accessory organ of digestion. It secretes pancreatic juice, which contains enzymes responsible for the digestion of protein, carbohydrates, and fats.

Scattered along its surface, like tiny islands in the ocean, are clusters of microscopic cells called pancreatic islets, or the islets of Langerhans. These cells have a specialized function: to secrete insulin and glucagon, two hormones that play an important role in keeping balanced levels of blood sugar in our body.

Insulin

Insulin helps push glucose from blood into our cells, causing decreased levels of blood sugar. Carbohydrates, proteins, and lipids are organic compounds used for energy in your body. Insulin affects the way your body uses these compounds. Insulin allows body cells to take up glucose for energy. 

Glucagon

Glucose is stored in the liver as glycogen. Glucagon speeds up the conversion of glycogen stored in the liver into glucose, thus increasing the blood sugar level.

 

Key Points

• The adrenal glands are composed of two different layers, called an outer adrenal cortex and an inner adrenal medulla. Both layers produce completely different hormones. 
• Insulin and glucagon have opposite functions in your body. While insulin pushes glucose into the cells, glucagon breaks down glucose stores in the liver to release glucose into your blood.
• The pituitary gland is also called the "master gland" due to its function in controlling hormone secretions from other glands. The pituitary gland, on the other hand, is stimulated by the hypothalamus. 
• The adrenal cortex releases cortisol, aldosterone, and sex hormones that play an important role in regulating a wide variety of processes such as metabolism of organic compounds, maintenance of electrolyte balance, and secondary sex characteristics.
• The adrenal medulla releases hormones called epinephrine and norepinephrine that trigger your body's "fight or flight" response.