THE ENDOCRINE SYSTEM

By Abukar salisu fago.

Composition of the Endocrine System

The endocrine system is composed of organs that produce and secrete hormones.

Because these organs perform mainly a secretory function, they are also referred to as glands

Two types of glands:– EXOCRINE– ENDOCRINE

EXOCRINE glands secrete their products into ducts.

The ducts transport the products into body cavities, into the spaces within organs, or into the body surface.

EXOCRINE glands include oil glands, sweat glands, mucous glands, and salivary glands.

ENDOCRINE GLANDS

ENDOCRINE glands secrete their products into the extracellular space surrounding the secretory cells.

Endocrine glands are sometimes called ductless glands.

Their products are the HORMONES, which diffuse from here to enter the bloodstream.

The primary endocrine glands of the body are the pituitary gland, the thyroid gland, the parathyroid glands, the adrenal gland, the pancreas, and the sex glands, or gonads (the testes in the male and ovaries in the female).

Endocrine glands that provide a minor role in body maintenance include the pineal gland and the thymus gland.

The stomach, kidneys, small intestine, and the placenta, all have a secondary role as endocrine glands

HORMONES

Hormones are the chemical units produced by endocrine glands.

Hormones are the means by which endocrine glands provide control of body activities to maintain homeostasis.

HORMONAL ACTION

Hormones are released in very small quantities, because they are extremely potent compounds.

Once released by secretory cells into the extracellular space, they find their way by diffusion into the bloodstream.

A given hormone will have an effect only on a particular type of cell. This is called the target cell.

The effect is limited to the target cells because only they contain special protein molecules in their plasma membrane that serve as receptors, which recognize and bind to specific hormones while rejecting others.

Cells other than target cells are not affected by a hormone, because they lack the appropriate receptors.

Once a hormone has united with the receptor on a target cell, it begins to exert its effect.

Its effect is to alter the cell’s metabolic processes.

Examples: a hormone may change the rates of enzyme activities, the rate of protein synthesis, the rates of secretion, or the rates at which materials are transported across the plasma membrane.

Although there are many types of hormones that differ chemically, hormones may be grouped into two broad categories on the basis of their solubility: those that dissolve in water, or are water-soluble; and those that dissolve in lipids, or are lipid-soluble.

WATER-SOLUBLE HORMONES

Hormones that are soluble in water include molecules that are composed of amino acids.

Because these hormones are soluble only in water, they cannot pass through the lipid plasma membrane.

Question: how can they produce an effect on the cell if they cannot penetrate the membrane?

This is done by passing the signal to a second-messenger system located within the cell. One that uses a compound called cyclic AMP (adenosine monophasphate).

The enzymatic cascade activated by the second-messenger system has an enormous amplification effect within the cell.

A single hormone molecule triggers a single enzyme, which catalyzes literally hundreds of reactions.

Water-soluble hormones that serve as first messengers in this system include epinephrine, norepinephrine (NE), antidiuretic hormone (ADH), oxytocin (OT), calcitonin (CT), and parathyroid hormone (PTH).

LIPID-SOLUBLE HORMONES

Hormones that dissolve in lipids include mainly steroid hormones.

Because the plasma membrane is composed of a bilayer of lipid molecules, steroid hormones can pass directly through it by diffusion to enter the target cell quite easily. (recall that steroids are a type of lipid also; lipids dissolve in other lipids).

Lipid-soluble hormones activate genes to synthesize new proteins and enzymes.

The protein products that are newly formed include enzymes that promote the metabolic activities specified by the hormone.

Lipid-soluble hormones that stimulate protein synthesis include aldosterone, cortisol, testosterone, estrogen, and thyroxine.

PROSTAGLANDINS

Prostaglandins are a group of chemicals that also have regulating effects on cells.

They are lipids that are produced by many different parts of the body.

Like hormones, they are extremely potent compounds and are released in very small quantities.

Specifically, prostaglandins stimulate or inhibit the formation of cyclic AMP, thereby modulating the effect of hormones that use cyclic AMP as a second messenger.

Because they do not induce their own effect but instead modify the effect of a hormone, prostaglandins as a group are not considered true hormones.

Some prostaglandins reduce blood pressure and open airways by causing smooth muscles to relax, others have the opposite effect.

Other types inhibit the secretion of HCL from the stomach wall, increase intestinal contractions, stimulate contraction of the uterus, regulate metabolism, cause inflammation, and even cause fever.

HORMONAL CONTROL

FEEDBACK CONTROL--how does an endocrine gland “know” how much hormone to produce and release?

This information, or feedback, is provided by way of chemical signals that are sent to the endocrine gland.

There are two systems that operate in this manner: negative feedback systems and positive feedback systems.

NEGATIVE FEEDBACK

Negative feedback systems control the amount of hormone released by providing a response in the opposite direction to that of the stimulus.

In these systems, the secretion of a hormone that accelerates a body activity is inhibited by the negative feedback signal, and the secretion of a hormone that slows the same body activity is stimulated yet further.

Negative feedback systems are the most common method of hormone regulation in the body.

POSITIVE FEEDBACK SYSTEMS

Positive feedback systems regulate hormone secretion by providing a response in the same direction as the stimulus.

When the desired response stimulated by hormone action occurs, a chemical feedback signal causes the endocrine gland to increase its rate of hormone release and more responses are stimulated.

Positive feedback systems tend to cause extreme changes in conditions in the body and are therefore quite unstable and uncommon.

Example: the production of oxytocin by the pituitary gland during childbirth. It stimulates contractions of the uterus. Its rising levels in the blood cause the formation of products that stimulate further oxytocin production, and uterine contractions respond by gradually increasing in strength until birth is accomplished.

NERVOUS CONTROL

A second way of controlling hormone release is by the nervous system.

Nervous control is responsible for regulating only some endocrine glands, such as the adrenal medulla and secretory cells in the hypothalamus of the brain.

These glands secrete hormones when they receive nerve impulses.

THE ENDOCRINE GLANDS

Some are in the head, some are in the neck, and some in the abdominal cavity.

PITUITARY GLAND

The pituitary gland, or hypophysis, is located at the base of the brain.

It is about the size of a pea and weighs only 0.5 gram (0.02 ounce).

It is attached to the hypothalamus by a narrow stalk, called the infundibulum, and lies within a bony cavity formed by the sella turcica of the sphenoid bone.

The pituitary gland produces many hormones, some of which control the activities of several other endocrine glands. It thereby influences a wide range of body functions.

The pituitary gland consists of two portions: an anterior lobe and a posterior lobe.

ANTERIOR LOBE

Within its epithelium are five different types of secretory cells that release seven types of hormones.

The release of these hormones is controlled by chemical secretions from the hypothalamus, called regulating factors.

The seven hormones released by the anterior lobe are:

– growth hormone (GH): mainly bone

– prolactin (PRL): mammary glands

– thyroid-stimulating hormone (TSH)

– adrenocorticotropic hormone (ACTH)

– melanocyte-stimulating hormone (MSH)

– follicle-stimulating hormone (FSH):ovaries, testis

– luteinizing hormone (LH): ovaries, testis

GROWTH HORMONE

Growth hormone (GH) stimulates body cells to grow and divide.

On a more short-term basis, the nutritional status of your body affects the release of GH in order to maintain a relatively constant blood sugar level.

When sugar levels are low, a condition called hypoglycemia exists and the hypothalamus is stimulated to release regulating factors.

Once these factors reach the anterior lobe, GH is released into the bloodstream. As a result, blood sugar levels rise. (convert glycogen into glucose)

High levels of sugar in the blood, or hyperglycemia, cause the opposite effect (GH is inhibited).

Thus, your blood sugar levels are kept relatively constant by negative feedback mechanism involving Gh

MELANOCYTE-STIMULATING HORMONE

MSH--stimulates the production of melanin in the skin, causing the skin to increase in pigmentation.

PROLACTIN

In combination with other hormones, prolactin (PRL) stimulates and maintains milk secretion by the mammary glands in females.

The actual ejection of milk is controlled by a hormone released by the posterior lobe, called oxytocin.

The combined secretion and ejection of milk from the mammary glands is an activity referred to as lactation.

THYROID-STIMULATING HORMONE

The production and secretion of hormones by the thyroid gland are stimulated by TSH.

It is influenced by the body’s metabolic rate, levels in the blood of a thyroid hormone called thyroxine, and other factors.

ADRENOCORTICOTROPIC HORMONE

The production and secretion of hormones released by the outer region or, cortex, of the adrenal gland are controlled by the ACTH.

Its release is also influenced by various forms of stress.

FOLLICLE-STIMULATING HORMONE

FSH has a different effect upon the two sexes.

In females, FSH stimulates the development of eggs, or ova, each month within the ovaries.

It also stimulates the cells in the ovaries to secrete estrogens, the female sex hormone.

In the male, FSH stimulates the production of sperm by the testes.

FSH production is controlled by regulating factors released from the hypothalamus in response to estrogens in the female and to testerone in the male, in the manner of a negative feedback system.

LUTEINIZING HORMONE

LH also plays a different role in each of the two sexes.

In females, it works together with estrogens to stimulate the ovary to release an ovum (a process called ovulation). and prepare the uterus for implantation of the fertilized ovum.

In males, LH stimulates cells within the testes to produce and secrete testosterone.

LH secretion is controlled by the hypothalamus by way of negative feedback.

OXYTOCIN

OXYTOCIN (OT) stimulate contraction of smooth muscle in the wall of the uterus.

It also stimulates cells around mammary ducts to contract, thereby causing milk to eject.

ANTIDIURETIC HORMONE

ADH regulates fluid balance in the body.

ADH causes a decrease in urine output and an increase in body fluid volume.

THYROID GLAND

The THYROID GLAND is the prominent organ in the neck.

Located slightly below the larynx in front of the trachea.

Its follicles contain a clear liquid called colloid.

The three primary hormones are: thyroxine. also known as T4; triiodothyronine (T3), and calcitonin.

Thyroxine and Triiodothyronine play important roles in metabolism and growth.

CALCITONIN

Calcitonin reduces the calcium and phosphate levels in the blood.

Calcium concentrations must be kept within narrow limits for normal nerve and muscle function, and both ions are essential mineral components of bone.

PARATHYROID GLANDS

The parathyroid glands are four or five pea-shaped masses of glandular epithelium.

They secrete one hormone, called parathyroid hormone (PTH).

Parathyroid Hormone (PTH)

Parathyroid hormone (PTH) plays an important role in maintaining the calcium and phosphate levels in the blood.

ADRENAL GLANDS

The Adrenal Glands are paired, triangular masses that lie atop each kidney.

Like the kidneys, they are located behind the membrane that encloses the abdominal cavity, which is called the peritoneum

Adrenal Medulla

The adrenal medulla is composed of modified nerve tissue.

It secretes two hormones , epinephrine and norepinephrine

Adrenal Cortex

It occupies the larger portion of the adrenal gland.

The secretory cells in each one of the adrenal cortex secrete steroid hormones.

These hormones are synthesized from colesterol. They include three classes of compounds: mineralocorticoids, glucocorticoids, and sex hormones.

Mineralocorticoids

The primary mineralocorticoid is aldosterone.

This steroid hormone maintains body fluid balance.

Sex Hormones

The two classes of sex hormones released by the adrenal cortex are androgens, which have a masculinizing effect, and estrogens, which have feminizing effects.

Pancreas

The pancreas is a soft, oblong organ located in the abdominal cavity behind the stomach.

It is actually two body systems, since it performs two distinct functions.

It is an endocrine gland, since it secretes two important hormones into the blood stream.

It is also a digestive organ, because of its secretion of digestive enzymes into ducts that empty into the small intestine.

The endocrine cells of the pancreas form clusters called the islets of Langerhans.

The hormones play important roles in providing body cells with sufficient amounts of energy. They do this by regulating the amount of sugar in the blood.

GLUCAGON--stimulates the conversion of glycogen into the simple sugar glucose.

INSULIN--has the opposite effect to that of glucagon on liver cells: it stimulates the formation of glycogen from glucose.

Gonads

The gonads are the sex organs; that is, they are the organs that produce the sex cells and secrete the primary sex hormones.

In females, the ovaries , they secrete estrogens, which are the primary female sex hormone

In males, the testes secrete testosterone.

Pineal Gland

The pinal gland is a small structure within the cranial cavity associated with the brain.

It is sometimes called the epithalamus because it is attached to the upper margin of the thalamus.

It secretes one hormone, melatonin the pineal gland in reptiles and birds, has

been shown to regulate reproduction cycles, hibernation cycles, and migration patterns

Thymus Gland

The thymus gland is a prominent structure in infants and young children but diminishes in size with advancing age.

It is a soft, irregularly shaped structure that lies in the mediastinum on top of the heart.

It secretes a hormone known as thymosin, which stimulates the production of certain white blood cells called T lymphocytes

The thymus gland plays an important role in immunity.