The Endocrine System

What’s the Endocrine System? The endocrine system regulates body processes slower than the nervous system. If the latter body system uses nerve impulses to make immediate action, the former body regulating system acts by the use of chemical messengers called HORMONES, which are released into the blood to be transported leisurely throughout the body that may affect one or several organs. Glands make up the endocrine system and are responsible for producing and secreting hormones. The major glands of this body system are the following:

• Hypothalamus
• Pituitary gland
• Thyroid gland
• Parathyroid
• Adrenals
• Pineal body
• Reproductive organs – ovaries in females and testes in males
• Pancreas

Hormones

Hormones are chemical messengers that transmit information from one cell to another and coordinate bodily functions. The term hormone comes from a Greek word that means “to arouse.” This chemical is the key to the incredible power of the endocrine system! It may be defined as chemical substances that are secreted by cells into the extracellular fluid and regulate the metabolic activity of other cells in the body.

A certain hormone affects only specific tissue cells or organs called target cells or target organs, although the blood-borne hormone circulates to all the organs of the body. For the target cells or organs to respond to a hormone, specific protein receptors must be present on its plasma membrane or in its interior to which that hormone can attach. The chemical messengers can ONLY influence the workings or function of a cell when the binding would take place.

After hormone-binding occurs, cellular activity is altered by either increasing or decreasing its normal rate or maintaining its usual metabolic process or a new process is stimulated. Certain changes that will take place after the binding process depends on the specific hormone and the type of target cells directed. However, either of the following would usually occur:

• S – Synthesis of proteins or certain regulatory molecules, such as enzymes, in the cell
• C – Change in plasma membrane permeability or electrical state
• A – Activation or inactivation of enzymes
• M – Mitosis stimulation

Mechanism of Action

Steroid Hormones

Changes in cells are triggered by two mechanisms depending on the type of hormone. Being a lipid-soluble molecule, the steroid hormones’ mechanism of action is through direct gene activation. The order of processes involved are the following: (S-T-A-R-T-S)

• S – Starts to diffuse through the plasma membranes of their target cells
• T – Travels and enters the nucleus, once the hormone is inside the target cells.
• A – Attaches to a specific protein receptor inside the target cells.
• R – Rushes (hormone-receptor complex) to bind to specific sites on the cell’s DNA.
• T – Transcribe messenger RNA or mRNA by activating certain genes.
• S – Synthesis of new proteins is produced by translating the mRNA in the cytoplasm.

Non-steroidal Hormones

Protein and peptide hormones, otherwise known as non-steroidal hormones, are unable to enter the target cells. To produce an effect, these amino acid-based chemicals bind to the receptors situated on the plasma membrane of target cell. Thus, a second-messenger system is utilized. The following events take place during the process:

• B – Binding of hormone to the membrane receptor takes place.
• A – A series of reactions sets off that activates an enzyme.
• S – Second messenger is produced in this case called cyclic AMP or cAMP, also known as cyclic adenine monophosphate, through a catalyzing reaction.
• E – Existing cyclic cAMP then oversees additional intracellular changes that promote the typical response of the target cell to the hormone.

Release of Hormones

What prompts the endocrine glands to produce and release or not release their hormones? Let us discuss that. There are stimuli that activate the endocrine organs to release the chemicals. These stimuli are divided into three major categories namely:

• Hormonal stimulus
• Humoral stimulus
• Neural stimulus

Hormones vary only within a very narrow range or controlled levels because of the negative feedback mechanisms. This is the chief means of regulating blood levels of nearly all hormones. When the secretion of a hormone is triggered by an internal or external stimulus, the rising levels of hormone inhibit the further release of chemicals in the body.

Hormonal Stimulus

This is the most common endocrine stimulus that helps release the hormones in the body. With this case, the endocrine organs are prodded to release a hormone by the stimulation of another hormone. Again, the endocrine organs are stimulated to take action by the presence of other hormones. For example, during a woman’s menstrual cycle the following events take place:

1. The hypothalamic hormone, Follicle-Stimulating Hormone Releasing Factor (FSHRF) incites the anterior pituitary gland, an endocrine gland.
2. The anterior pituitary gland then releases Follicle Stimulating Hormone (FSH) to prod the ovaries (female gonads), an endocrine organ to release another hormone called estrogen or progesterone.
3. When levels of hormones are rising in the blood, the feedback mechanism inhibits further chemical release.

When hormones are promoted through this mechanism, the cycle is rhythmic with hormone levels in the blood rising and falling recurrently.

Humoral Stimulus

Humoral stimuli are the prodding of hormone release through changing blood levels of certain ions and nutrients. An example of this mechanism is the release of parathyroid hormone (PTH) by cells of the parathyroid glands. The order of events occurring with this process is the following:

1. Calcium concentration in the capillary blood is decreased.
2. Low calcium levels in the blood prompt the parathyroid glands to release parathyroid hormone (PTH).
3. PTH acts by several routes to reverse the decline in the Calcium level.
4. Blood calcium levels rise soon.
5. The feedback mechanism inhibits further PTH release.

Aside from the release of parathyroid hormone, other hormones released in response to humoral stimuli are the following:

• Release of calcitonin by the thyroid gland
• Insulin released by the pancreas

Neural Stimuli

When nerve fibers stimulate the release of hormones in the target cells, neural stimuli are used. Sympathetic nervous system stimulation is a classic example of this hormone release.

1. During stress, preganglionic SNS fiber stimulates the adrenal medulla cells.
2. The adrenal medulla releases the catecholamines, epinephrine, and norepinephrine, into the blood during periods of stress.

Major Organs of the Endocrine System

Hypothalamus

Although part of the nervous system, the hypothalamus is also recognized as a major organ of the endocrine system. This is due to the fact that this organ is responsible for producing several hormones. Hormones released by the posterior pituitary gland and the release and inhibition of hormones that regulate the anterior pituitary gland are major functions of this organ via hypothalamic neurons. The releasing hormones are secreted into a capillary network that connects via portal veins to a second capillary bed in the anterior lobe of the pituitary gland.

Pituitary Gland

The pituitary gland, although the size is approximately the size of a grape, has two functional lobes namely, the anterior pituitary, the glandular tissue, and the posterior pituitary, the nervous tissue.

Hormones of Anterior Pituitary Gland (APG)

The anterior pituitary gland is referred to as the “master endocrine gland,” as it controls the activity of several endocrine glands despite its insignificant size. Though this glad might appear so powerful it cannot perform its function alone. Why? It is because of the fact that the release of each of its hormones is controlled by releasing and inhibiting hormones of the hypothalamus.

There are about 6 anterior pituitary gland hormones. Two of the six namely the growth hormone and the prolactin direct their major effects on nonendocrine targets. The other four namely, thyrotropic hormone, adrenocorticotropic hormone, and the two gonadotropic hormones are all tropic hormones. Tropic hormones trigger target organs which are also endocrine glands to secrete their hormones which in turn exert their effects on other body organs and tissues. The characteristics of ALL anterior pituitary gland hormones are as follows:

1. All anterior pituitary hormones are proteins or peptides
2. They act through second-messenger systems
3. These hormones are regulated by hormonal stimuli and in most cases negative feedback

• Growth Hormone (GH)

This is a metabolic hormone that is responsible for the growth of skeletal muscles and long bones of the body. It is a protein-sparing and an anabolic hormone that causes amino acids to be built into proteins and stimulates most target cells to grow in size and divide. Hence, GH plays an essential role in determining final body size. Aside from that, this hormone also plays a role in maintaining blood sugar homeostasis by causing fat to break down to be used as energy while it spares glucose.

• Prolactin (PRL)

Structurally similar to growth hormone, this protein hormone is only known to target the breast in humans. It comes from the words “pro” meaning for and “lact” referring to milk. Thus, after birth, this hormone is responsible for stimulating and maintaining milk production in the mother’s breast. In males, its function is not known.

• Adrenocorticotropic Hormone (ACTH)

This hormone is responsible for regulating the endocrine ability of the cortex portion of the adrenal gland.

• Thyroid-stimulating hormone (TSH)

Also termed the thyrotropic hormone, TSH is responsible for enhancing the growth and the activity of the thyroid gland.

• Gonadotropic hormones

The hormonal activity of the gonads is regulated by the gonadotropic hormones. The hormone’s function differs by gender. In females, the Follicle-stimulating hormone (FSH) stimulates follicular development in the ovaries. As the follicles mature, estrogen is produced and the eggs are matured for ovulation. In males, FSH stimulates sperm development by the testes.

Luteinizing hormone (LH) is responsible for stimulating the ovulation of an egg from the ovaries of females and causes the rupture of the follicle to release progesterone and some estrogen. It is also referred as interstitial cell-stimulating hormone (ICSH) in males, as it triggers the production of testosterone by the interstitial cells of the testes.

Hormones of the Posterior Pituitary Gland (PPG)

Strictly speaking, the posterior pituitary gland, is not an endocrine gland because it does NOT make the peptide hormones it releases. Instead, this organ serves as a storage area for hormones made by hypothalamic neurons. The following hormones are released by the PPG:

1. Oxytocin. This hormone is only released in significant amounts during childbirth and in nursing women. The presence of this hormone, stimulates powerful contractions of the uterine muscle during labor, during sexual relations, and when a woman breastfeeds her baby. What makes this helpful during breastfeeding is the fact that it is responsible for milk ejection or let-down reflex in a nursing woman. During the postpartum period, oxytocics are used to stop uterine bleeding by causing constriction of the ruptured blood vessels at the placental site.
2. Antiduiretic hormone (ADH). This hormone is responsible for retaining more water from forming urine thus, resulting in to decrease in urine volume and an increase in blood volume. When ADH is present in large amounts, blood pressure rises by causing constriction of the small arteries. Hence, this hormone has also been called vasopressin.

Thyroid Gland

The thyroid glands produce two hormones namely the thyroid hormone and calcitonin.

Thyroid hormone

The thyroid hormone is made up of two iodine-containing hormones namely thyroxine otherwise known as T4 and triiodothyronine also termed as T3. Thyroid hormones are often referred to as the body’s metabolic hormone whereas thyroxine is the major hormone secreted by the thyroid follicles. T3 is formed in the target tissues by the conversion of T4 to T3. Functions of thyroid hormones:

• Controls the rate at which glucose is burned or oxidized and converted to heat and chemical energy.
• Normal tissue growth and development especially in the reproductive and nervous systems.

Calcitonin

Also called thyrocalcitonin, calcitonin decreases calcium levels in the blood by causing calcium to be deposited in the bones. The action of this hormone is opposite to the hormone produced by the parathyroid glands, parathyroid hormone. Calcitonin is made by the C cells or parafollicular cells which are found in the connective tissue between the follicles. It is directly released into the blood when increasing calcium levels in the bloodstream are noted.

Parathyroid Glands

The parathyroid glands are glandular tissue masses found on the posterior surface of the thyroid gland. The glandular tissues of these glands are responsible for secreting the most important regulator of calcium ions, the parathyroid hormone (PTH) or parathormone. When the levels of calcium in the blood drop, the parathyroid glands release PTH, which in turn stimulates bone destruction cells, osteoclasts, to break down the bone matrix and release calcium in the blood. Hence, parathyroid hormone is a hypercalcemic hormone since it acts to increase blood levels of calcium.

Parathormone and calcitonin operate in a negative feedback mechanism control system that influences each other. Calcitonin, a hypocalcemic hormone, works oppositely with PTH depending on the body’s demand. The main target of PTH is the skeleton. However, it also stimulates the kidneys and the intestines to release more calcium to increase its level in the blood.

Adrenal Glands

Curved over the top of the kidneys, adrenal glands are structurally and functionally two endocrine glands in one. Like the pituitary gland, the adrenals have glandular (cortex) and neural tissue (medulla) parts. The central medulla region is enclosed by the adrenal cortex which contains three separate layers of cells. Hormones of the Adrenal cortex:

Major groups of steroid hormones such as mineralocorticoids, glucocorticoids, and sex hormones are produced by the adrenal cortex. These hormones are collectively termed corticosteroids.

Mineralocorticoids

1. Produced by the outermost layer of the adrenal cortex, the mineralocorticoids, are mainly aldosterone. These hormones play an essential part in regulating the mineral or salt content of the blood particularly the concentrations of sodium and potassium ions. The target organ of the hormone is the kidneys which selectively reabsorb the minerals or allow them to be flushed out in the body in the form of urine.
2. Increased aldosterone levels in the body would result in rising amounts of reclaimed sodium ions by the kidney tubules and secreting more potassium ions into the urine. When sodium is reabsorbed water follows. Hence, the mineralocorticoids aid in water and electrolyte regulation.
3. Aldosterone is also released when the kidneys produce the enzyme, renin when blood pressure drops. Production of renin triggers a series of reactions that form angiotensin II, which is a potent stimulator of aldosterone release.
4. To prevent aldosterone release, atrial natriuretic peptide or ANP is released by the heart. The main goal of releasing ANP is to reduce blood volume and blood pressure.

Glucocorticoids

Glucocorticoids are produced by the middle layer of the adrenal cortex which includes cortisone and cortisol. The functions of this hormone are:

1. Promotion of normal cell metabolism
2. Helping the body resist long-term stressors, by increasing blood glucose levels.

Important information about glucocorticoids:

1. Fats and even proteins are broken down by body cells and converted to glucose when blood levels of glucocorticoids are elevated in the blood. Hence, these hormones are said to be hyperglycemic hormones.
2. Unpleasant effects of inflammation are also controlled by glucocorticoids as they reduce the effects of edema and they reduce the pain by inhibiting some pain-causing molecules called prostaglandins. Thus, because of their anti-inflammatory properties, glucocorticoids are often prescribed as drugs to suppress inflammation in patients with arthritis.
3. Glucocorticoids are released from the adrenal cortex in response to the rising blood levels of ACTH.

Sex Hormones

The adrenal cortex produces the sex hormones regardless of one’s gender. Production of sex hormones takes place throughout a person’s life but the amount formed is relatively small. It is the innermost layer of the cortex that produces a large amount of androgens, male sex hormones, and some estrogen, female sex hormone.

Hormones of the Adrenal Medulla

Like the knot of nervous tissue where the posterior pituitary gland develops, the adrenal medulla has the same development. When this structure is stimulated by the sympathetic nervous system neurons, epinephrine, also called adrenaline, and norepinephrine, otherwise known as noradrenaline, are released into the bloodstream. Collectively, these hormones are called catecholamines.

1. Physical or emotional stress and threat would bring about the fight-or-flight response. The response is necessary to help a person cope with a stressful situation. One of the organs stimulated in these situations is the adrenal medulla which is responsible for pumping catecholamines or hormones into the bloodstream to enhance and prolong the effects of neurotransmitters in the sympathetic nervous system.
2. The presence of catecholamines has the following effects: increased heart rate, elevated blood pressure, and rising blood glucose levels. Also, small passageways of the lungs are dilated with the presence of these hormones to cater more oxygen in the blood and a faster circulation to the organs most importantly to the brain, heart, and muscles. Increased glucose and oxygen would make the body fit to fight or deal with short-term stressors.

Pancreatic Islets

The best hidden endocrine glands in the body are the pancreatic islets. These are tiny masses of hormone-producing tissue scattered among the enzyme-producing acinar tissue of the pancreas. Although these million clumps of cells are scattered they manufacture hormones and works like an organ. Two essential hormones produced by the islets are glucagon and insulin.

Insulin

Insulin is released from the beta cells of islets. The secretion of this hormone is triggered by the increasing levels of glucose in the blood. Insulin has the unique ability of transporting glucose across plasma membranes and acts on just about all body cells. Once inside the cells, glucose is oxidized for the energy to be converted to glycogen or fat for storage. The presence of insulin speeds up activities like this and glucose is swept out of the blood. Hence, insulin is a hypoglycemic hormone. The release of insulin ends when blood glucose levels decrease.

What makes insulin such an important hormone is the fact that it is the ONLY hormone that decreases blood glucose levels! Without it, no glucose would be used by the body since no other chemical can oxidize them.

Glucagon

If insulin decreases blood levels of glucose, glucagon does the opposite. Glucagon is released by the alpha cells of the islets and its secretion is triggered by a lowered level of glucose. Hence, its action is hyperglycemic. The primary target organ of glucagon is the liver. It is in this organ where the glucagon stimulates the process of breaking down stored glycogen to glucose and to prompt its release to the circulation.

Pineal Gland

Also called the pineal body, the pineal gland is found in the roof of the brain’s third ventricle. This organ secretes substantial amounts of melatonin.

• Melatonin plays an important role in the day-night cycle of the body. Believed to be a “sleep trigger,” melatonin levels fluctuate throughout the day and night. It reaches its peak level at night, thus, making a person drowsy. The lowest level occurs during daylight around noon.
• Another role of melatonin in the body is how it coordinates the hormones of fertility and inhibits the reproductive system so that sexual maturation is prevented before adult body size has been reached.

Thymus Glands

The thymus glands produce the hormone thymosin. Aside from that, the thymus glands also acts as an incubator for the maturation of special groups of white blood cells namely the T cells or T lymphocytes which are important in the immune response.

Gonads

Hormones of Ovaries

The female gonads or ovaries produces female sex cells known as the ova or eggs and at the same time produce two steroid hormones which are:

• Estrogen – produced by the Graafian follicles of the ovaries. Triggers the development of the secondary sex characteristics in females. In addition, it works with progesterone to prepare the uterus to receive the fertilized egg.
• Progesterone – promotes growth of the uterine lining. It acts with the estrogen to bring about the menstrual cycle.

Hormones of Testes

The testes produce the male hormones, androgens, of which testosterone is the most important. Androgens support sperm formation and are an important hormone in the development and maintenance of male secondary sex characteristics.

References:

1. Anatomy and Physiology by Marieb
2. Brunner and Suddarth’s Textbook on Medical-Surgical Nursing

–Nursing Crib