Physiology Dr Abdullah Flashcards
Adrenal gland
.Adrenocortical Hormones 😎😎
The two adrenal glands, each of which weighs about 4 grams, lie at the superior poles of the
two kidneys. Each gland is composed of two distinct parts, the adrenal medulla and the
adrenal cortex. The adrenal medulla, the central 20 percent of the gland, is functionally
related to the sympathetic nervous system; it secretes the hormones epinephrine and
norepinephrine in response to sympathetic stimulation. This lecture will focus on the adrenal
cortex only.
The adrenal cortex secretes an entirely different group of hormones, called corticosteroids.
These hormones are all synthesized from the steroid cholesterol, and they all have similar
chemical formulas.
Corticosteroids: Mineralocorticoids, Glucocorticoids, and Androgens.
Two major types of adrenocortical hormones, the mineralocorticoids and the glucocorticoids,
are secreted by the adrenal cortex. In addition to these, small amounts of sex hormones are
secreted, especially androgenic hormones, which exhibit about the same effects in the body
as the male sex hormone testosterone.
The mineralocorticoids have gained this name because they especially affect the electrolytes
(the “minerals”) of the extracellular fluids, especially sodium and potassium. The
glucocorticoids have gained their name because they exhibit important effects that increase
blood glucose concentration. They have additional effects on both protein and fat metabolism
that are equally as important to body function as their effects on carbohydrate metabolism.
More than 30 steroids have been isolated from the adrenal cortex, but two are of exceptional
importance to the normal endocrine function of the human body: aldosterone, which is the
principal mineralocorticoid, and cortisol, which is the principal glucocorticoid.
Synthesis and Secretion of Adrenocortical Hormones❤❤❤❤😎😎😎
The adrenal cortex has three distinct layers:
1. The zona glomerulosa, a thin layer of cells that lies just underneath the capsule, constitutes
about 15 percent of the adrenal cortex. These cells are the only ones in the adrenal gland
capable of secreting significant amounts of aldosterone because they contain the enzyme
aldosterone synthase, which is necessary for synthesis of aldosterone. The secretion of these
cells is controlled mainly by the extracellular fluid concentrations of angiotensin II and
potassium, both of which stimulate aldosterone secretion.
2. The zona fasciculata, the middle and widest layer, constitutes about 75 percent of the
adrenal cortex and secretes the glucocorticoids cortisol and corticosterone, as well as small
amounts of adrenal androgens and estrogens. The secretion of these cells is controlled in large
part by the hypothalamic-pituitary axis via adrenocorticotropic hormone (ACTH).
3. The zona reticularis, the deep layer of the cortex, secretes the adrenal androgens
dehydroepiandrosterone (DHEA) and androstenedione, as well as small amounts of estrogens
and some glucocorticoids. ACTH also regulates secretion of these cells, although other factors
such as cortical androgen-stimulating hormone, released from the pituitary, may also be
involved.
Functions of the Mineralocorticoids—Aldosterone😎😎
Renal and Circulatory Effects of Aldosterone
Aldosterone increases reabsorption of sodium and simultaneously increases secretion of
potassium by the renal tubular epithelial cells, especially in the principal cells of the
collecting tubules and, to a lesser extent, in the distal tubules and collecting ducts. Therefore,
aldosterone causes sodium to be conserved in the extracellular fluid while increasing
potassium excretion in the urine. Aldosterone also causes secretion of hydrogen ions in
exchange for sodium in the intercalated cells of the cortical collecting tubules.
Aldosterone Stimulates Sodium and Potassium Transport in Sweat Glands, Salivary Glands,
and Intestinal Epithelial Cells. Aldosterone has almost the same effects on sweat glands and
salivary glands as it has on the renal tubules.
Regulation of Aldosterone Secretion
Potassium ion concentration and the renin-angiotensin system are by far the most potent
factors that regulate aldosterone secretion. A small percentage increase in potassium
concentration can cause a severalfold increase in aldosterone secretion. Likewise, activation
of the renin angiotensin system, usually in response to diminished blood flow to the kidneys
or to sodium loss, can increase in aldosterone secretion several fold.
Functions of the Glucocorticoids
The glucocorticoids have functions just as important to the long-continued life of the animal
as those of the mineralocorticoids. At least 95 percent of the glucocorticoid activity of the
adrenocortical secretions results from the secretion of cortisol, known also as hydrocortisone.
In addition to this, a small but significant amount of glucocorticoid activity is provided by
corticosterone. Of note, glucocorticoids can act on mineralocorticoid receptors. However,
aldosterone effects predominate in the kidney
Effects of Cortisol on Carbohydrate Metabolism
1- Stimulation of Gluconeogenesis.
a. Cortisol increases the enzymes required to convert amino acids into glucose in the
liver cells.
b. Cortisol causes mobilization of amino acids from the extrahepatic tissues mainly
from muscle.
2- Decreased Glucose Utilization by Cells.
3- Elevated Blood Glucose Concentration and “Adrenal Diabetes.” A possible explanation for
insulin resistance in situations of high levels of glucocorticoids is that high levels of fatty
acids, caused by the effect of glucocorticoids to mobilize lipids from fat depots, may
impair insulin’s actions on the tissues.
Effects of Cortisol on Protein Metabolism
1- Reduction in Cellular Protein. This is caused by both decreased protein synthesis and
increased catabolism of protein already in the cells (except those of the liver). In the
presence of great excesses of cortisol, the muscles can become so weak that the person
cannot rise from the squatting position. The immunity functions of the lymphoid tissue
can be decreased to a small fraction of normal.
2- Cortisol Increases Liver and Plasma Proteins. It is believed that this difference results from
a possible effect of cortisol to enhance amino acid transport into liver cells (but not into
most other cells) and to enhance the liver enzymes required for protein synthesis.
3- Increased Blood Amino Acids, Diminished Transport of Amino Acids into Extrahepatic
Cells, and Enhanced Transport into Hepatic Cells.
Effects of Cortisol on Fat Metabolism
Mobilization of Fatty Acids. In much the same manner that cortisol promotes amino acid
mobilization from muscle, it also promotes mobilization of fatty acids from adipose tissue.
This increases the concentration of free fatty acids in the plasma, which also increases their
utilization for energy.
Cortisol Is Important in Resisting Stress and Inflammation
Almost any type of stress, whether physical or neurogenic, causes an immediate and marked
increase in ACTH secretion by the anterior pituitary gland, followed within minutes by
greatly increased adrenocortical secretion of cortisol.
Some of the different types of stress that increase cortisol release are the following: Trauma
of almost any type, infection, intense heat or cold, surgery or almost any debilitating disease.
A possible explanation of the reason of glucocorticoids secretion during stressful situations
is that the glucocorticoids cause rapid mobilization of amino acids and fats from their cellular
stores, making them immediately available both for energy and for synthesis of other
compounds, including glucose, needed by the different tissues of the body.
Anti-Inflammatory Effects of High Levels of Cortisol
(a) it can block the early stages of the inflammation process before inflammation even
begins.
(b) if inflammation has already begun, it causes rapid resolution of the inflammation
and increased rapidity of healing. These effects are explained further as follows.
1. Cortisol stabilizes the lysosomal membranes.
2. Cortisol decreases the permeability of the capillaries.
3. Cortisol decreases both migration of white blood cells into the inflamed area and
phagocytosis of the damaged cells.
4. Cortisol suppresses the immune system, causing lymphocyte reproduction to decrease
markedly. The T lymphocytes are especially suppressed.
5. Cortisol attenuates fever mainly because it reduces the release of interleukin-1 from the
white blood cells, which is one of the principal excitants to the hypothalamic temperature
control system.
Other Effects of Cortisol
Cortisol Blocks the Inflammatory Response to Allergic Reactions. cortisol effectively prevents
shock or death in anaphylaxis, which otherwise kills many people.
Effect on Blood Cells and on Immunity in Infectious Diseases. Cortisol decreases the number
of eosinophils and lymphocytes in the blood. (This ability of cortisol and other glucocorticoids
to suppress immunity makes them useful drugs in preventing immunological rejection of
transplanted hearts, kidneys, and other tissues). Cortisol increases the production of red
blood cells by mechanisms that are unclear.
Glucocorticoids increase bone resorption by stimulating osteoclastogenesis(osteoporosis risk)
Regulation of Cortisol Secretion by Adrenocorticotropic Hormone from the Pituitary Gland.
Secretion of cortisol is controlled almost entirely by ACTH secreted by the anterior pituitary
gland. This hormone, also called corticotropin or adrenocorticotropin, also enhances the
production of adrenal androgens.
ACTH Secretion Is Controlled by Corticotropin- Releasing Factor from the Hypothalamus.
In the same way that other pituitary hormones are controlled by releasing factors from the
hypothalamus, an important releasing factor also controls ACTH secretion. This is called
corticotropin releasing factor (CRF). The anterior pituitary gland can secrete only minute
quantities of ACTH in the absence of CRF
Physiological Stress Increases ACTH and Adrenocortical Secretion
As pointed out earlier in this lecture, almost any type of physical or mental stress can lead
within minutes to greatly enhanced secretion of ACTH and consequently cortisol as well,
often increasing cortisol secretion as much as 20-fold.
Inhibitory Effect of Cortisol on the Hypothalamus and on the Anterior Pituitary to Decrease
ACTH Secretion
.
Cortisol has direct negative feedback effects on
(1) the hypothalamus to decrease the formation of CRF (long loop) and
(2) the anterior pituitary gland to decrease the formation of ACTH (short loop).
