The endocrine system encompasses both endocrine and exocrine glands.

• Endocrine glands release hormones directly into the bloodstream.
• Exocrine glands are secretory glands that release hormones/substances onto the surface of the skin or into the intestinal lumen through ducts.

There are also (which are unimportant for the national registry exam):

• paracrine,
• holocrine, and
• autocrine glands,
•  

Hormones fall into three major categories:

1. Protein hormones bind to cell membrane receptors that are located on the surface of the cell, causing a signaling cascade, allowing them to cause changes in the cell without ever entering the cell.
2. Steroid hormones are able to diffuse through the lipid bilayer of the cell membrane, activating and modifying functions of cells without having to send a signal through surface receptors.
3. Amine hormones are the simplest hormones and have a primary role in the regulation of metabolism, like protein hormones they work through cell surface receptors.

 

Major Endocrine Glands

The major endocrine glands include the

• pineal gland,
• pituitary gland,
• thyroid gland,
• parathyroid gland,
• hypothalamus,
• adrenal glands;
• ovaries in females and the
• testes in males.

 

 

The functions of these specific glands will be reviewed in other sections.

 

Prostaglandins

Prostaglandins, unlike endocrine hormones, are created by most cells throughout the body and not just by specific glands.

Prostaglandins and endocrine hormones are functionally similar, they are both agonists or inhibitors of certain functions in target organs and tissues, essentially functioning as a means of transmitting signals through the body. A single prostaglandin can also have different or opposite effects in differing tissues.

PROSTAGLANDINS: hormone-like lipid compounds that are responsible for a wide range of bodily functions, such as

• contraction and relaxation of smooth muscle,
• dilation and constriction of blood vessels,
• control of blood pressure, and the
• start of the inflammation process, which includes platelet aggregation to control bleeding.

Some other specific functions of prostaglandins include sensitizing spinal neurons to pain, inducing labor, decreasing intraocular pressure, regulating inflammation, regulating calcium movement, controlling cell growth, inducing fever, and are released during menstruation causing the uterus to contract.

 

Hormone Response Mechanisms

The regulation of hormone secretion includes both positive and negative feedback mechanisms. Some of the key hormone response patterns are the permissive, synergistic, and antagonistic effects.

• PERMISSIVE EFFECT: occurs when the presence of one hormone allows another hormone to act. This is seen in the relationship between thyroid hormone affecting the action of reproductive hormones, and explains how the absence of one hormone can have far-reaching effects.

• SYNERGISTIC EFFECT: occurs when two hormones with similar effects produce an amplified response greater than the sum of their individual effects. The classic example is the maturation of female egg cells through the interaction between follicle-stimulating hormone and estrogen.

• ANTAGONISTIC EFFECT: occurs when two hormones have opposing effects; antagonistic hormones often act to keep bodily concentrations of vital electrolytes and compounds in a narrow range. The classic example is the regulation of blood sugar by opposing actions of insulin and glucagon.

 

Positive and Negative Feedback Loops

As exemplified in the hypothalamic-pituitary-thyroid axis:

1. Low T3 and T4 → increased hypothalamic TRH: NEGATIVE FEEDBACK.
2. Higher TRH → increased pituitary thyroid-stimulating hormone (TSH): POSITIVE FEEDBACK.
3. Higher levels of TSH → increased production of thyroid T3 and T4: POSITIVE FEEDBACK.
4. Higher levels of T3 and T4 suppress the release of hypothalamic thyrotropic releasing hormone (TRH), and the pituitary passes on releasing TSH, which results in decreased thyroid activity until the T3 and T4 levels drop again (to be noticed by the hypothalamus): NEGATIVE FEEDBACK. 

As exemplified in the hypothalamic-pituitary-ovarian axis:

1. Low estrogen is sensed by the hypothalamus, which is thereby signaled to put out gonadotropin-releasing hormone (GnRH). This is negative feedback--high provokes less action; low provokes more action. "Negative" feedback is best described as "opposite" feedback.
2. Hypothalamic GnRH stimulates the pituitary to release FSH (follicle-stimulating hormone) and LH (luteinizing hormone), which cause the maturation of ovarian follicles and their release (ovulation), respectively. This is both positive feedback AND the permissive effect. 
3. This causes the production of estrogen and progesterone during each of these cycles. Increased estrogen/progesterone is sensed by the hypothalamus to pause in its production of GnRH, which means LH and FSH aren't made (negative feedback).  

 

Hormones Involved In Growth

Hormones that are especially important to normal growth and development include

• growth hormone (GH) from the pituitary gland,
• somatomedins,
• thyroid hormones,
• androgens, estrogens,
• glucocorticoids, and
• insulin.

Somatomedins are small proteins produced in the liver in response to stimulation by growth hormone and include insulin-like growth factor I and II that cause increased cartilage growth and collagen formation peaking at age 13, and increases in protein synthesis, DNA, and RNA synthesis, respectively.