The adrenal glands are small triangular-shaped structures located at the top of both kidneys. Their function is to produce hormones that help in the regulation of the metabolism, immune system, blood pressure, stress response, and more.
The adrenal glands, which are covered by an inner thick layer of connective tissue with an outer thin fibrous capsule, contain two sections:
- OUTER ADRENAL CORTEX – makes up the biggest part of the gland
- INNER ADRENAL MEDULLA – the core
The OUTER ADRENAL CORTEX is made up of 3 parts:
- Zona Glomerulosa – makes up 15% of the total volume (secretes mineralocorticoids)
- Zona Fasciculata – makes up the widest part of the total volume (mainly secretes glucocorticoids)
- Zona Reticularis – secretes amounts of hormones, mostly gonadocorticoids and androgens
Adrenal Cortex vs Adrenal Medulla
Mineralocorticoids
Mineralocorticoids are responsible for water and electrolyte homeostasis through control of sodium and potassium concentrations. 95% of all mineralocorticoid activity happens through Aldosterone:
- Aldosterone acts on the kidneys’ tubule cells, causing them to increase sodium reabsorption
- Sodium ions are removed from the urine and returned to the blood
- Rapid depletion of sodium from the body is prevented
Aldosterone causes:
- potassium excretion
- sodium reabsorption
- hydrogen ions elimination
- sodium, chloride, and bicarbonate ions retention
- water retention
NOTE: Aldosterone reduces potassium reabsorption, thus, large potassium amounts are lost in urine excretion.
Electrolyte balance Secondary Effects
Sodium retention and potassium excretion lead to secondary effects:
- Sodium reabsorption causes Hydrogen ions to pass into the urine to replace positive sodium ions, making the blood less acidic, thus preventing acidosis.
- Sodium ions movement creates a positively charged field in the blood vessels surrounding the kidney tubules. This causes Chloride and Bicarbonate ions to move out from urine, back into the blood.
- When ADH (antidiuretic hormone) is present, increased sodium concentration in the blood vessels causes water to move by osmosis from the urine into the blood.
Aldosterone control #1 – the raas system
Aldosterone Control #2 – Potassium Ion Concentration
- Increased potassium concentration in extracellular fluid causes the adrenal cortex to secrete aldosterone
- Aldosterone secretion causes excess potassium to be eliminated by the kidneys
- Decreased potassium concentration in the extracellular fluid causes a decrease in aldosterone production, leading to less potassium excretion by the kidneys
Glucocorticoids
Glucocorticoids promote normal metabolism by:
- increasing the rate of protein catabolism
- increasing the rate at which amino acids are removed from cells and transported to the liver to undergo protein synthesis
- releasing fatty acids from adipose tissue to be converted into glucose
- promoting gluconeogenesis
Glucocorticoids promote stress resistance:
- gluconeogenesis from amino acids causes a sudden increase in glucose availability, prompting the body to become more alert
- blood vessels become more sensitive to chemicals that cause vasoconstriction so as to allow an increase in blood pressure
Glucocorticoids are anti-inflammatory compounds:
- cause a reduction in mast cells
- stabilise lyosomal membranes, leading to the inhibition of histamine release
- decrease blood capillary permeability
- depress phagocytosis by monocytes
Glucocorticoids:
- Cortisol (hydrocortisone) – most abundant and responsible for about 95% of all glucocorticoid activity
- Corticosterone
- Cortisone
NOTE: Cortisol Serum blood test indicates adrenal function.
NOTE: Glucocorticoids slow down connective tissue regeneration, which leads to slow wound healing.
NOTE: Steroids are a synthetic form of glucocorticoids.
ACTH (Adrenocorticotropic hormone) Control
Glucocorticoid secretion is controlled through a negative feedback mechanism stimulated by stress and low blood glucocorticoid level:
- stress and low blood glucocorticoid level stimulate the hypothalamus to secrete CRF (corticotropin releasing factor)
- CRF secretion causes ACTH to be released from the anterior lobe of the pituitary
- ACTH is carried to the adrenal cortex, where it stimulates glucocorticoid secretion
Gonadocorticoids
The adrenal cortex is responsible for the secretion of both male and female sex hormones – oestrogens and androgens.
Adrenal Medulla
- The adrenal medulla is made up of chromaffin cells (hormone-producing cells) surrounding sinuses containing blood
- These chromaffin cells are considered to be postganglionic cells specialised in secretion
- Preganglionic fibres pass directly into the chromaffin cells of the gland within the adrenal medulla
- Secretion of hormones is controlled by the autonomic nervous system and innervation by preganglionic fibres that allows rapid response to a stimulus by the gland
Epinephrine and Norepinephrine
The adrenal medulla synthesises the following two hormones:
- Epinephrine (adrenaline)
- Norepinephrine (noradrenaline)
Epinephrine is stronger than norepinephrine. It:
- increases the blood pressure by increasing the heart rate and constricting the blood vessels
- increases respiration rate
- dilates respiratory passageways
- decreases digestion rate
- increases muscular contraction efficiency
- increases blood sugar level
- stimulates cellular metabolism
However, both epinephrine and norepinephrine:
- mimic the sympathetic nervous system – they are sympathomimetic
- help in stress resistance
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