13.1 anatomy and physiology of the endocrine system

The pituitary gland plays a leading role in the endocrine system because its hor-mones regulate several other endocrine glands as well as a number of body activities, as shown in Figure 13.2. The endocrine system produces two types of hormones: ste-roid hormones and nonsteroid hormones. A steroid hormone is a messenger that passes directly into the cells of the target organ. Cortisol is one example of a steroid hormone. It is produced by the adrenal gland and released in response to stress and a low concentration of glucose in the blood.

A nonsteroid hormone does not have the ability to penetrate cells. Instead, it binds to receptors on a cell membrane to activate secondary messengers within the cell. Adrenaline, also known as epinephrine, is one example of a nonsteroid hormone. When adrenaline is released in response to an immediate stressor (e.g., the anticipa-tion of danger), it produces the fight-or-flight response, the body’s physical prepara-tion to either escape or do battle when faced with danger.

FIGURE 13.1 Endocrine System

Endocrine glands release hormones directly into the bloodstream, where they travel throughout the body to trig-ger responses in specific target tissues.

pituitary gland

hypothalamus

thyroid

parathyroid glands

thymus

adrenal glands

pancreas

or ovariestestes

pineal gland

Pharmacy Technician Series © Paradigm Education SolutionsPharmacology for Technicians, Seventh Edition: Chapter 13, Section 13.1 Li

13.1 Anatomy and Physiology of the Endocrine System

The endocrine system consists of ductless glands and other structures that secrete hormones directly into the bloodstream. A hormone is a secretion released by an endocrine gland into the circulatory system that has a specific reg-ulatory effect on organs and other tissues. Endocrine glands include the pineal, pituitary, thyroid, parathyroid, thymus, adrenal, and reproductive glands as well as the pancreatic duct and the hypothalamus (see Figure 13.1). The cell, tissue, or organ affected by a hormone is called its target. Through the work of hormones, the endocrine system maintains homeostasis within the body by regulating the physiologic functions involved in normal daily living and in times of stress.

posteriorpituitaryanterior

pituitary

antidiuretichormone

(ADH)

oxytocin(OT)

prolactin(PRL)

mammaryglands

skin

testis

thyroidgland

adrenalcortex

bone

growthhormone (GH)

thyroid-stimulating

hormone (TSH)

adrenocorticotropichormone (ACTH)

ovary

gonadotropichormones

(FSH and LH)

melanocyte-stimulatinghormone(MSH)

uterussmoothmuscle

kidney tubules

mammaryglands

nerve connection to hypothalamus

bone

growthhormone (GH)

testis

ovary

gonadotropichormones

(FSH and LH)

skin

melanocyte-stimulatinghormone(MSH)

prolactin(PRL)

ammarymmmmaglandsg

oxytocin(OT)

mammaryglands

uterussmoothmuscle

FIGURE 13.2 Pituitary Hormones

The pituitary gland plays a key role in regulation of the thyroid, adrenal glands, and hormones within the kidney.

The disorders discussed in this chapter relate to the thyroid, adrenal glands, and pancreas. Disorders that relate to reproductive and growth hormones are discussed in Chapter 14. The thyroid is located in the throat and responds to thyroid-stimulating hor-mone (TSH). The adrenal glands are located on the top of the kidneys and are responsi-ble for producing catecholamines (such as epinephrine) and steroid hormones (such as cortisol). Finally, the pancreas and its specialized cells are responsible for producing insu-lin and glucagon, key hormones that assist the body in the management of glucose.


The endocrine system secretes hormones at a rate that keeps their level in the blood almost constant. The body can do this because it receives continuous feedback from its systems, known as a feedback mechanism. Negative feedback occurs when the level of hormone in the blood or the level of a chemical it affects moves above or below a desired range. When this occurs, hormone production is lowered or raised in the opposite direction to bring the blood level of the hormone or the chemical it affects back into the appropriate range.

Positive feedback is rare. In positive feedback, the presence of the hormone or chemical it affects promotes the production of more of that hormone or chemical until some other input stops it. Positive feedback does not result in keeping the hor-mone or chemical within a particular range.

hyoid bone

larynx

trachea

thyroid gland

FIGURE 13.3 Thyroid Gland

The thyroid gland is located in the neck, surrounding the trachea.

hypothalamus releasing factors

pituitary

thyroid

thyroid hormones(T3 and T4)

thyroid-stimulatinghormone(TSH)

FIGURE 13.4 Hypothalamic-Pituitary Axis

A high TSH test result means that T3 and T4 levels are low (hypothy-roidism).


Thyroid GlandThe thyroid gland, shown in Figure 13.3, produces hormones that stimulate various body tissues to increase their metabolic activity. Two of these hormones, triiodothy-ronine (T3) and thyroxine (T4), are stored as thyroglobulin, which the thyroid cells must break down before T3 and T4 can be released into the bloodstream. T3 and T4 are usually bound to protein molecules. Thyroid hormones are only active when they are not bound to protein in the blood. T3 is more physiologically active than T4. The feedback mechanism that controls the thyroid is called the hypothalamic-pituitary axis. Chemical transmitters from the hypothalmus gland located just abovee the pitui ary, called hypothalmus releasing factors, regulate the pituitary gland in a stress response. This stress response system for the body signals the pituitary gland to produce thyroid-stimulating hormone (TSH), which in turn stimulates the thyroid to produce T3 and T4 (see Figure 13.4). These hormones build up in circulating blood and slow the pituitary gland’s production and release of TSH. For a patient with signs of hormonal imbal-ance, measuring the amount of serum TSH can help determine whether the thyroid is functioning normally; in some instances, other tests for T3 and T4 may be needed. It is also important to note that some peripheral conversion of T4 to T3 occurs in the tissues.

PharmFact

Calcitonin, a hor-

mone that regu-

lates calcium and

phosporus in the

body, is also made

by the thyroid.

producing and releasing CRF. Steroid production follows a circadian rhythm (regu-lar recurrence in cycles of 24 hours). As Figure 13.5 shows, steroid production peaks in the morning, and the low point occurs close to midnight.

Pancreas The pancreas contains groups of specialized cells, called islets of Langerhans, that pro-duce insulin. Insulin helps cells burn glucose for energy and bind with cell membrane receptors to allow glucose uptake, enhances the transport of amino acids and their incorporation into proteins, increases ion transport into tissues, and inhibits fat break-down. Thus, in addition to having other metabolic roles, insulin is critical in maintain-ing blood glucose levels.

Steroidproduction

level

midnight midnightnoon8 a.m. 8 p.m.Time of day

FIGURE 13.5 Steroid Production

Steroid produc-tion follows a circadian rhythm, with a peak in the morning and a low point around midnight.


Adrenal Glands and Corticosteroids The adrenal glands are located on the top of the kidneys. The adrenal medulla, or inner portion of an adrenal gland, functions like the sympathetic nervous system and produces catecholamines such as epinephrine (adrenaline) and norepinephrine. The adrenal cortex, or outer portion, produces several types of steroid hormones. Each such hormone, known as a corticosteroid, has its own combination of glucocorticoid (involved in cholesterol, fat, and protein metabolism) and mineralocorticoid (involved in regulating electrolyte and water balance) activity. The principal adrenal steroid hormone is cortisol. It is responsible for gluconeogenesis (conversion of fatty acids and proteins to glucose), anti- inflammatory reactions, stimulation of fat deposition, and sodium and water retention (steroids are nec-essary for mineral retention). The adrenal cortex also produces various sex hormones.

As with the thyroid hormones, the production of cortisol and other steroids begins in the hypothalamic-pituitary axis. The hypothalamus produces corticotropin-releasing factor (CRF), which stimulates the pituitary gland to produce adrenocorti-cotropic hormone (ACTH), which in turn enters the bloodstream and travels to the adrenal cortex, where it stimulates the release of cortisol into the blood. Through a feedback mechanism, the rising cortisol levels slow the action of the hypothalamus in

13.1 anatomy and physiology of the endocrine system

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