Chapter 16 – The Endocrine System

Objectives

 

1.         List the major endocrine organs and briefly describe their location in the body.

 

2.         List the 5 ways hormones affect target cells/organs.

 

3.            Describe how hormones are classified chemically.

 

4.         Define hormone.

 

5.            Describe 5 types of cellular changes hormones cause and 2 major mechanisms by which hormones exert their effects.

 

6.            Describe how hormone release is regulated.

 

7.            Describe the 3 types of hormonal stimulation.

 

8.            Describe the functional relationship between the hypothalamus and pituitary.

 

9.         Name the 6 anterior pituitary hormones and describe their effects.

 

10.       Name the 2 posterior pituitary hormones and describe their effects.

 

11.            Describe the effects of the 2 groups of hormones produced by thyroid gland.

 

12.            Describe the role of the parathyroid gland in the regulation of calcium.

 

13.       List the hormones produced by the adrenal gland and cite their effects.

 

14.            Compare and contrast the effects of the 2 major pancreatic hormones.

 

15.       Name and gonadal hormones and describe their effects.

 


The Endocrine System

 

The endocrine system affects bodily activities by releasing chemical messengers, called hormones, into the bloodstream. The system as a whole sends messages to cells in virtually any part of the body. It not only helps to regulate the activity of smooth and cardiac muscle and some glands, it significantly affects virtually all other tissues as well. Endocrine organs are widely scattered throughout the body.

 

The nervous and endocrine systems coordinate their activities and together are considered the control systems of the body.

 

The Ways Hormones Affect Targets

Although the effects of hormones are many and varied, their actions can be categorized into 5 broad areas:

1.         Help control the internal environment by regulating its chemical composition and volume.

2.            Respond to changes in the environmental conditions to help the body cope with emergency demands such as infection, trauma, emotional stress, dehydration, starvation, hemorrhage, and temperature extremes.

3.            Integration of growth and development.

4.            Contribute tot he basic processes of reproduction.

5.         Help regulate organic metabolism and energy balance.

 

Definition of Endocrine and Exocrine

There are two kinds of glands in the body:  endocrine and exocrine

1.            Exocrine glands have ducts through which their non-hormonal products are routed to a membrane surface. Examples are sweat, sebaceous, mucous, and digestive glands.

2.            Endocrine glands are ductless glands which release hormones directly into the blood and lymph. They typically have a rich vascular and lymphatic drainage. Endocrine glands include the pituitary, thyroid, parathyroid, adrenal, pineal, and thymus glands. Several organs in the body, such as the pancreas and gonads, contain discrete areas of endocrine tissue within their substance and produce hormones as well. The hypothalamus is both an integral part of the nervous system and also a producer and releaser of hormones; it is considered a neuroendocrine organ. There are pockets of hormone-producing cells elsewhere, but these are the major endocrine organs.

 

Chemical Classification of Hormones

A hormone is defined as a chemical substance, secreted by cells into the extracellular fluids, that regulates the metabolic function of other cells in the body. All hormones have in common the function of maintaining homeostasis by changing the rate of physiological activities of cells.

 

They are classified chemically into one of two large groups of biochemical molecules:

1.         amino-acid based hormones – Most hormones belong to this group. They function via the second messenger mechanism.

2.         steroid hormones – These hormones are synthesized from cholesterol. Only the gonadal hormones and the adrenocortical hormones are steroids. They function via direct gene activation.

 

Hormone-Target Cell Specificity

A hormone influences the activity of only certain tissue cells. The cell must possess specific protein receptors in order to respond to the hormone. Although hormone-receptor binding is the crucial first step, the extent of target cell activation depends equally on three factors:

1.         blood levels of hormone

2.         relative numbers of receptors on the target cell

3.         affinity of the binding between hormone and receptor (higher affinity = better activity)

Types of Cellular Changes

Hormones alter cell activity by increasing or decreasing the rates of normal cellular processes. A hormonal stimulus typically results in one or more of the following changes:

1.         Plasma membrane permeability

2.            Synthesis of proteins or other regulatory molecules within the cell

3.            Enzyme activation or deactivation

4.            Induction of secretory activity

5.            Stimulation of mitosis

 

Major Mechanisms of Action

Hormones are produced as needed. There are 2 major mechanisms by which hormone binding is harnessed to the specific intracellular machinery needed for action.

1.            Formation of one or more intracellular second messengers

2.         Direct gene activation by the hormone itself

 

Second Messenger – The hormone is called the first messenger. To give the cell its message, the hormone attaches to a specific receptor in the cell membrane. Once attachment occurs there is an increase in the synthesis of cyclic AMP inside the cell. Cyclic AMP acts as the second messenger to alter cell function by activating enzymes inside the cell which go on to catalyze specific responses, such as inducing secretion, activating protein synthesis, and altering membrane permeability.

 

Direct Gene Activation – Being lipid soluble, steroid hormones can diffuse easily into their target cells. Once inside, they bind to a receptor located in the nucleus. The activated hormone-receptor complex then interacts with another receptor on the DNA. The result of this is to cause the transcription of certain genes, which leads to synthesis of the proteins coded for by the genes. These proteins may be enzymes that promote metabolic activity, structural proteins, or proteins exported from the cell.

 

The concentration of a hormone in blood at any time reflects its rate of release and the speed of its inactivation and removal. Some hormones are rapidly degraded by enzymes within the cell, but most are removed from the blood by the kidney and liver enzymes. The time of onset for hormonal activity varies greatly. Some are immediate (second messenger); others may require hours or days before effect is seen (direct gene activation).

 

Types of Stimulation that Cause Hormone Secretion

Various endocrine glands are stimulated to manufacture and release their hormones by three main types of stimuli:

1.            Hormonal – In some cases hormones stimulate other endocrine glands to secrete other hormones. For example, the release of anterior pituitary hormones is regulated by releasing and inhibiting hormones produced by the hypothalamus. Then, the anterior pituitary hormones in turn stimulate other endocrine glands to release their hormones. As the end-result hormones increase in the blood, they inhibit the release of anterior pituitary hormones and thus shut down their own release.

2.            Humoral – Hormone release is stimulated by substances dissolved in extracellular fluids. Usually changing blood levels or certain ions or nutrients stimulate hormone release. For example, the release of parathyroid hormone is prompted by decreasing blood calcium levels. Other examples are calcitonin, insulin, and aldosterone.

3.         Neural – In some cases, nerve fibers stimulate hormone release. For example, catecholamines (epinephrine and norepinephrine) are released during times of stress by stimulation of the adrenal medulla by the sympathetic nervous system. Also, the hypothalamus stimulates the release of oxytocin and anti-diuretic hormone by neural stimulation.

 

Pituitary Gland (Hypophysis)

About the size and shape of a pea, located in the sella turcica of the sphenoid bone, attached to the hypothalamus via a stalk called the infundibulum. It has two major lobes, one neural and one glandular.

 

Posterior Lobe (Neurohypophysis) – composed of nervous tissue; releases neurohormones that it receives ready-made from the hypothalamus; oxytocin and anti-diuretic hormone (ADH); released on demand in response to nerve impulses from hypothalamic neurons.

 

Anterior Lobe (Adenohypophysis) – composed of glandular tissue; manufactures and releases its own hormones as a result of stimulation by releasing hormones from the hypothalamus; called the master endocrine gland; releases 6 hormone products which all function via second messenger systems.

 

Four of the six are hormones that regulate the functioning of other endocrine glands: thyroid stimulating hormone (TSH), adrenocorticotropic hormone (ACTH), follicle stimulating hormone (FSH), and luteinizing hormone (LH)

 

Remaining two affect non-endocrine targets:  growth hormone (GH) and prolactin.

 

Anterior Pituitary Hormones

 

1.            Thyroid Stimulating Hormone (TSH) – Stimulates the thyroid gland to secrete thyroid hormone.

 

2.            Adrenocorticotropic Hormone (ACTH) – Stimulates the adrenal cortex to release corticosteroid hormones (glucocorticoids, gonadocorticoids, and mineralocorticoids).

 

3,4.            Gonadotropins – Follicle Stimulating Hormone (FSH) and Luteinizing Hormone (LH)

Regulate the function of the gonads (ovaries and testes). In both sexes, FSH stimulates gamete production, while LH promotes the production of gonadal hormones (estrogen and testosterone).

 

5.            Growth Hormone (GH) – Stimulates most body cells to increase in size and divide. Its major targets are the bones and skeletal muscles. Induces protein synthesis and encourages the use of fats for fuel, thus conserving glucose. GH release is controlled by hormones from the hypothalamus which stimulates or inhibits its release. Abnormalities result from hypersecretion (acromegaly) and hyposecretion (dwarfism).     

 

6.            Prolactin (PRL) – Stimulates milk production by the breast.

 

Posterior Pituitary Hormones

 

1.            Antidiuretic Hormone (ADH) – Influences body water balance. Targets kidney tubules, which respond by reabsorbing more water from the forming urine and returning it to the bloodstream; less urine is produced and blood volume is increased. Ingestion of alcohol inhibits ADH secretion and causes copious urine output. Under certain conditions, such as severe blood loss, exceptionally large amounts of ADH are released. At high levels, ADH has a vasoconstrictor effect to try to maintain blood pressure.

 

2.            Oxytocin – Stimulates contraction of smooth muscle of uterus during childbirth.

 

Thyroid Gland Hormones

The thyroid is a butterfly-shaped gland located in the anterior neck, overlying the trachea. Internally, the gland is composed of hollow spherical structures called follicles with other cells located between the follicles.

 

1.            Thyroid Hormone – referred to as the body’s major metabolic hormone. It comes from a precursor molecule called thyroglobulin which is produced by the cells making up the walls of the follicles. Thyroglubulin is stored inside the follicle and requires iodine atoms to function properly. When thyroid hormone is needed, it is produced by breaking down thyroglobulin into smaller pieces. Thyroid hormone is actually two active iodine-containing hormones, thyroxine (T4) and triiodothyronine (T3). T4 is secreted from the thyroid in greater amounts than T3. T3 is the most active and is formed at target tissues by conversion of T4 to T3. Except for the adult brain, spleen, testes, uterus, and the thyroid gland itself, thryoid hormone affects virtually every cell in the body. It stimulates enzymes concerned with glucose oxidation, increases basal metabolic rate and body heat production, increases adrenergic receptor in blood vessels, and helps in tissue growth. The thyroid gland is unique in its ability to make, store, and release its hormones.

 

2.            Calcitonin – produced by parafollicular cells of the thyroid gland. It is released in response to high blood levels of calcium. Its function is to lower blood calcium levels by stimulating calcium uptake by bone. It is generally more important in childhood.

 

Parathyroid Hormone (PTH)

It is the most important hormone controlling calcium balance. Calcium is critically important for nerve impulse conduction, muscle contraction, and clotting. PTH is released by the parathyroid gland in response to low blood calcium levels. Its function is to increase blood calcium levels by stimulating three target organs:

Skeleton – causes digestion of bone matrix and release of calcium

Kidneys – causes increased reabsorption of calcium from the forming urine and returns it to the blood

Small intestine – causes increased absorption of calcium by the intestinal mucosal cells.

 

Adrenal Glands

The paired adrenal glands are located on top of the kidneys where they are enclosed in a fibrous capsule and a cushion of fat. Each gland is structurally and functionally two endocrine glands in one:

1.         the inner adrenal medulla is made up of nervous tissue; part of the sympathetic nervous system

2.         the outer adrenal cortex surrounds the medulla, forms the bulk of the gland, and is organized into three layers

 

Hormones of the Adrenal Cortex

All these hormones are steroid hormones. The cortex is organized into the following layers and major products:

1.         zona glomerulosa (outermost) – produces mineralocorticoids which help control the balance of minerals and water in the blood

2.         zona fasciculata (middle) – secretes the metabolic hormones called glucocorticoids

3.         zona reticularis (innermost) – secretes the glucocorticoids and small amounts of adrenal sex hormones or gonadocorticoids

 

Mineralocorticoids

Most important function is regulation of the electrolyte concentration in extracellular fluids, particularly that of sodium and potassium ions. Aldosterone is the most potent of these. It decreases the excretion of sodium from the body. Its target is the kidney tubules where it stimulates reabsorption of sodium ions. Along with sodium retention, water is also retained (increasing blood pressure), and potassium is excreted.

 


Glucocorticoids

Influence the metabolism of most body cells and help provide resistance to stressors. Absolutely essential for life. Helps the body adapt to external changes and intermittent food intake by keeping blood sugar levels fairly constant. Maintain blood volume by preventing shift of water into tissue cells.  Main function is to help the body with long term response to stress (such as infection and trauma). An example is cortisol.

 

Gonadocorticoids

The bulk of gonadocorticoids are androgens, or male sex hormones, with testosterone as the most common. The adrenal cortex also makes small amounts of female hormones. The testosterone produced by the adrenal cortex is thought to be responsible for the sex drive.

 

Adrenal Medulla

The adrenal medulla is stimulated by the sympathetic division of the autonomic nervous system. Two powerful hormones are secreted:  epinephrine and norepinephrine, collectively known as catecholamines. When the body is activated by a short term stressor or emergency the sympathetic nervous system is mobilized resulting in blood sugar level increases, vasoconstriction, and a faster heart rate.

 

Pancreas and its Hormones

Located behind the stomach. It is a mixed gland composed of both endocrine and exocrine gland cells. The bulk of the gland is exocrine, producing digestive enzymes that are carried by a duct to the small intestine. Scattered among the exocrine part are minute clusters of cells (islets of Langerhans) that produce pancreatic hormones. The islets contain 2 major populations of hormone-producing cells:

1.         alpha – make glucagon

2.         beta – make insulin

 

Insulin and glucagon are both involved in the metabolism and regulation of blood glucose

 

Glucagon

Released in response to low blood sugar levels. Its function is to raise blood sugar to the proper level. Its major target is the liver where it promotes:

1.            glycogenolysis – breakdown of glycogen to glucose

2.            gluconeogenesis – formation of glucose from non-carbohydrate precursors such as fatty acids and amino acids.

 

Insulin

Released in response to high blood sugar levels. Its function is to lower blood sugar to the proper level by the following mechanisms:

1.         inhibits breakdown of glycogen to glucose

2.            enhances transport of glucose into cells

3.            promotes breakdown of glucose inside cells for ATP production

4.         joins glucose together to form glycogen

5.            converts leftover glucose to fat after the body’s immediate energy needs are met

 

Gonads

Produce gonadal sex hormones, identical to those produced by adrenal cortex. Sex hormones are steroid hormones. Ovaries produce estrogen and progesterone. Estrogen is responsible for maturation of female reproductive organs and secondary sex characteristics of females at puberty. During pregnancy the levels of both hormones rise. The testes produce testosterone which is responsible for maturation of male reproductive organs, secondary sex characteristics of males at puberty, and the sex drive.