The adrenal gland consists of two parts - an outer cortex and an inner medulla.
Adrenal cortex and adrenal medulla behave as independent organs and secrete into bloodstream different hormones.
Besides to the other its functions, adrenal gland plays the major role in response to stress. It generates and secrets such stress hormones as adrenaline, noradrenaline, and cortisol.
Adrenal cortex and medulla hormones
Adrenal cortex hormones
The adrenal cortex responsible for the production of three types of steroid hormones.
The first type - the glucocorticoids:
The primary glucocorticoid – cortisol stimulates the production of carbohydrates and related metabolic functions.
The second type - the mineralocorticoids:
The primary mineralocorticoid - aldosterone takes part in control of salt and water balance, which in turn affects blood pressure.
The third type - sex hormones:
- male hormones - androgens
- female hormones - estrogens
The glucocorticoid and the mineralocorticoid hormones add to the long-term stimulation of the immune system when the body is under stress.
A regulation of cortisol and aldosterone levels (a negative feedback loop)
The adrenocorticotropic hormone (ACTH, a polypeptide) of the pituitary gland controls the synthesis of cortisol and aldosterone.
The synthesis of adrenocorticotropic hormone, in turn, is managed by a peptide substance - corticotropin-releasing factor (CRF) - synthesized by the hypothalamus.
High levels of aldosterone and cortisol induce a negative feedback effect on the hypothalamus and anterior pituitary, which suppresses production and secretion of the adrenocorticotropic hormone.
Cortisol is secreted in “spurts” by the adrenal cortex.
Besides to this way of the regulation, unlike cortisone, the production of aldosterone is mainly controlled by shifts in blood pressure and the production of angiotensin in the kidney.
In healthy individuals, the secretion of corticotropin-releasing factor in the hypothalamus exhibits a diurnal pattern, reaching its lowest levels late at night (around midnight) and rising to a peak in early morning hours before awakening. This pattern is also reflected in the production of adrenocorticotropic hormone, aldosterone, and cortisol.
Cortisol – stress hormone. Cortisol effects on the body.
Cortisol triggers the transformation of protein into amino acids in muscles and the release of amino acids into the blood.
In the liver, cortisol triggers the intake of amino acids and stimulates the production of enzymes active in glucogenesis. This increase in glucose synthesis leads to increased glycogen stores in the liver.
Subsequently, under the influence of other hormones such as glucagon and adrenaline, this stored carbohydrate can then be converted back to glucose when needed (such as between meals).
In addition, cortisol prompts the breakdown of lipids in fat tissues (for use as an alternative energy source in other tissues), slows down metabolism, and inhibits protein synthesis in most organs in the body (with the exception of the brain and muscle tissues).
Cortisol also has strong anti-inflammatory properties.
In general, cortisol decreases the buildup of fluids in the place of inflammation by decreasing the permeability of capillaries in affected tissues. This hormone also suppresses production of T cells and antibodies, as well as other immune system responses that might cause further inflammation.
Cortisol appears to play a major role in the body’s physiological response to stress.
The extra cortisol may help to relieve some possible negative physiological effects of stress.
During extended periods of stress, cortisol can interact with insulin to increase food intake and redistribute stored energy from muscle to fat tissues, primarily in the abdominal region.
Excessive cortisol production in times of stress may also depress immune function by reducing the availability of proteins needed for synthesis of antibodies and other substances produced by the immune system. Over time, depressed immune system function may increase the body’s susceptibility to infection and the onset of some forms of cancer.
Functions of aldosterone and blood pressure.
Two primary (and linked) functions of aldosterone are osmoregulation (the process of regulating the amounts of water and mineral salts in the blood) and blood pressure regulation.
In the kidneys, aldosterone acts increasing of sodium ion absorption and secretion of potassium ions, primarily in the collecting ducts of nephrons.
Aldosterone also stimulates sodium reabsorption in the colon.
This process raises sodium concentration in the blood. This action prompts the hypothalamus to release anti-diuretic hormone (ADH), which in turn increases the absorption of water, leading to an increase in blood pressure.
Aldosterone production is mainly controlled by changes in blood pressure.
A decline in blood pressure will stimulate the kidneys to secrete the enzyme renin.
The secretion of this enzyme, in turn, causes the activation of the protein – angiotensin produced from blood proteins by the kidney. Angiotensin raises blood pressure by triggering the constriction of arterioles and by stimulating the secretion of aldosterone from the adrenal cortex.
Sex hormones of the adrenal glands
The adrenal cortex also takes part in the synthesis of male sex hormones (androgens) and female sex hormones (estrogens).
These hormones are found in both sexes, but males synthesize more androgens while females produce higher levels of estrogens.
Because the testes in males produce high levels of androgens, the quantity of this hormone secreted by adrenal glands has only a slight effect on body functions. The androgen hormones produced by the adrenal glands account for 50 percent of the total androgen output in females.
Androgens promote muscle and skeletal development in both males and females. The synthesis of estrogen by the adrenal glands remains insignificant until after menopause when the ovaries discontinue production of these hormones.
Two non-steroid hormones produced by the adrenal gland are adrenaline (also called epinephrine) and noradrenaline (also called norepinephrine).
Adrenaline is often called the “stress hormone” because it is the major hormone secreted in response to stress.
The adrenal medulla consists of modified neurons of the sympathetic nervous system. The production of adrenaline and noradrenaline is under the control of the hypothalamus via this direct connection with the sympathetic nervous system.
The hormones - adrenaline and noradrenaline also serve as excitatory neurotransmitters in the sympathetic nervous system.
The adrenal medulla secretes a mixture of 85 percent adrenaline and 15 percent noradrenaline.
Adrenaline and noradrenaline act to increase heart rate and blood pressure, and cause vasodilation (widening) of blood vessels in the heart and respiratory system.
These hormones also stimulate the liver to break down stored glycogen and release glucose into the blood. When the body is “at rest,” these two hormones sufficiently stimulate cardiovascular function to maintain adequate blood pressure without additional input by the sympathetic nervous system.
Vudeo: Human endocrine system: adrenal glands - epinephrine(adrenaline) and aldosterone
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