17.9 Adrenal Glands

The paired adrenal glands function in..

synthesizing and releasing numerous types of hormones.

Each gland is composed of 2 distinct regions….

Which includes the inner adrenal medulla and the outer adrenal cortex.

Adrenal glands

Paired, pyramid-shaped endocrine glands enclosed within a connective tissue capsule. These glands are anchored on the superior surface of each kidney, and like the kidneys, are retroperitoneal, which is posterior to the parietal peritoneum.

Each adrenal gland is enclosed within…

fat and fascia to minimize their environment.

Adrenal Medulla

Forms the inner core of each adrenal gland. It has a pronounced red-brown color due to its extensive vascularization, It releases the catecholamines epinephrine(adrenaline) and norepinephrine(noradrenaline), which are biogenic amines.

The stimulus for the release of epinephrine and norepinephrine is activated by…

The sympathetic division.

Approximately 80% of the hormone released is epinephrine and about 20% is norepinephrine.

Epinephrine and norepinephrine are released in response to what?

Emotional stress or physical stress and help prolong the flight-or-fight response, which is caused by the activation of the sympathetic division.

The adrenal cortex exhibits a…

distinctive yellowish color as a consequence of the stored lipids within its cell. These cells synthesize more than 25 different lipid-soluble corticosteroids.

The 3 regions of the adrenal cortex.

-(Outer)Zona glomerulosa

-(Middle)zone fasciculata

-(Inner)zona reticularis’

Zona glomerulosa

The thin, outer cortical layer composed of dense, spherical clusters of cells.

These cells synthesize mineralocorticoids.

Mineralocorticoids

A group of hormones that help regulate the composition and concentration of electrolytes (ions) in body fluid.

Principal mineralocorticoid

Aldosterone

Aldosterone

Regulates the ratio of Na+ and K+ in our blood and body fluids by altering the amounts excreted by the kidney into the urine.

Stimulates retention of both Na+ and water and the secretion of K+.

Na+ blood concentration remind the same as a result.

K+ blood concentration decreases.

Zona fasciculata

The middle layer and largest region of the adrenal cortex.

Composed of parallel cords of lipid-rich cells that have a bubbly, almost pale appearance.

This region synthesizes glucocorticoids.

The main glucocorticoids

Cortisol and corticosterone, which are released in response to ACTH.

Zona reticularis

A narrow band of small, branching cells.

They are capable of secreting minor amounts of sex hormones called gonadocorticoids.

The primary gonadocorticoids

adrenal androgens.

Gonadocorticoids include…

dehydroepiandrosterone (DHEA), DHEA-sulfate, and androstenedione.

The amount of androgen secreted by the adrenal cortex is ___composed to that secreted by the testes.

small; however, adrenal glands tumors in genetic female can result in elevated levels of androgens and varying degrees of masculinization.

Steps in the regulation of cortisol release:

1. The stimuli that are monitored by the hypothalamus for controlling the release of glucocorticoid (e.g., cortisol) levels are varied and are described shortly.

2. The stimuli are detected by the hypothalamus, which serves as the receptor.

3. In response to the various stimuli, the hypothalamus, serving also as the control center, releases corticotropin-releasing hormone (CRH)
   into the hypothalamo-hypophyseal portal system.

4. The anterior pituitary, in response to CRH, releases adrenocorticotropic hormone (ACTH).

5. ACTH stimulates the adrenal cortex (specifically the zona fasciculata) to release glucocorticoids including cortisol and corticosterone. Cortisol accounts for 95% of the glucocorticoid activity.
   Cortisol, which is a lipid-soluble molecule (see E section 17.3a), is transported within the blood by transport proteins (corticosteroid-binding globulin or CBG, also called transcortin or by albumin). Cortisol circulates in the blood, and small amounts become unbound from their transport protein and exit the blood.

6. Cortisol stimulates targets cells (effectors) as described shortly.

7. The net result is release of all nutrients into the blood to supply the energy required to deal with the stress.

8. Increased blood levels of cortisol inhibit both the release of CRH from the hypothalamus and
   ACTH from the anterior pituitary. Thus, both CRH and ACTH release are regulated by negative feedback.

Stimuli Regulating Release of Corticotropin-Releasing Hormone (CRH)

• Several are monitored by the hypothalamus in controlling cortisol blood levels

• Most significant is negative feedback by an increase in cortisol,

• Time of day: CRH released by the hypothalamus changes throughout the day, as measured by cortisol levels in the blood. Observe in L figure 17.20a, which is a graph of blood levels of cortisol throughout the day, that in a typical sleep-wake cycle, peak levels of cortisol correspond to the late stages of a sleep cycle.About half of all cortisol release occurs during sleep, with cortisol levels peaking right before waking in the morning. This rhythm of release is regulated by light and dark cycles detected by th retina as sensory input is relayed to the hypothalamus. Thus, typically an individual's highest blood level of cortisol is when getting ou of bed in the morning. (There is significant variation in cortisol levels among individuals.)

• Stress: Both emotional stress (e.g., anxiety, anger, fear) and physical stress (e.g., fever, trauma, intense exercise) are monitored by the hypothalamus, as master control of the autonomic nervous system and the center of the limbic system (the emotional brain; see section 13.4c). Stress increases the release of CRH, ACTH, and cortisol. Thus, when we are stressed, additional cortisol is being released. The influence of chronic stress in triggering increased cortisol levels in the blood is the reason that cortisol has been given the nickname of the stress hormone

Effects of Cortisol

Cortisol and corticosterone increase nutrient levels in the blood (glucose, fatty acids, and amino acids), especially in an attempt to resist stress and help repair injured or damaged tissues. Other physiologic changes are stimulated by glucocorticoids and become most evident when present in high doses.

Cortisol is a lipid-soluble steroid molecule (see L section 17.3a) and moves through the plasma membrane of its target cells by simple diffusion.

The primary target cells of cortisol include ( figure 17.19, step 6):

(a) hepatocytes of the liver, (b) adipocytes of adipose connective tissue, and (C) all cells (except the liver).

Effect of cortisol on the liver

Cortisol specifically stimulates hepatocytes of the liver to increase glycogenolysis (chemical breakdown of glycogen into glucose) and gluconeogenesis (formation of glucose from noncarbohydrate sources) and decrease glycogenesis (formation of glycogen from glucose). This additional glucose is released into the blood. Consequently, blood glucose levels rise. (Hepatocytes also function in converting some cortisol to cortisone.

Effect of cortisol on adipose connective tissue

Adipocytes of adipose connective tissue are stimulated by cortisol to increase lipolysis (breakdown of triglycerides into glycerol and fatty acids) and decrease lipogenesis (formation of' triglycerides), resulting in the release of glycerol and fatty acids into blood.

Effect of cortisol on all cells expect hepatocytes

Most cells, including skeletal muscle cells, skin cells, and bone cells, increase protein catabolism (breakdown of protein into amino acids) in response to cortisol. An exception to this occurs in hepatocytes. In liver cells, additional amino acid released into the blood is taken up by hepatocytes and provides alternative nutrient molecules for gluconeogenesis (formation of glucose from noncarbohydrate sources). Cortisol additionally stimulates most cells to decrease glucose uptake.

This is the glucose-sparing effect, which saves blood glucose for use in the brain. The primary function of cortisol is to increase blood levels of nutrients to support the body in dealing with stress. The function of releasing nutrient molecules (e.g., glucose from hepatocytes of the liver, glycerol and fatty acids from adipose connective tissue, and amino acids from all cells except liver cells) is to provide the fuel molecules required in effectively managing the stress. Because stress increases blood glucose, effectively managing stress is of special concern for individuals with impaired blood glucose regulation associated with diabetes

Therapeutic Doses of Corticosterone

Corticosterone often is used as a topical or oral treatment for chronic inflammation and selected allergic reactions (e.g., hives, eczema). The side effects of corticosterone, especially when administered orally in high doses, include retention of Nat and water; inhibited release of inflammatory agents (the anti-inflammatory effect); suppression of the immune system; and inhibited connective tissue repair. Note that suppression of the immune system increases an individual's susceptibility to infection and the risk of cancer.