Title: The Parathyroid Gland, Adrenal Gland, Pancreas and their Secretions
1- The Parathyroid Gland – The parathyroid gland secretes parathyroid
hormone, which regulates calcium, magnesium and phosphate levels
a- The parathyroid glands are small (usually four) little glands embedded in
the posterior surface of the thyroid gland
b- Each gland weighs about 0.04 grams, and contains two types of cells
1- the more numerous Chief cells that produce the parathyroid
hormone
2- and the less common oxyphil cells whose function is not known,
however they do not regularly appear until after pituitary
2- Parathyroid Hormone
a- The parathyroid hormone is the major regulator of calcium, magnesium
and phosphate ions in the blood
b- Blood calcium levels directly controls the secretion of calcitonin and
parathyroid hormone through negative feedback loops.
c- The two hormones have opposite effects on the blood calcium
1234567- PTH increases the number and activity of osteoclasts which releases
calcium and phosphate in the bone into the blood. PTH also acts on the
kidneys by slowing the rate at which calcium and magnesium are lost in
the urine. It also increases the loss of phosphate in the urine. This
decreases blood phosphate levels and therefore increases the levels of
calcium and magnesium in the blood. PTH also stimulates the production
of calcitriol which increases the rate of absorption of calcium and
magnesium from the GI tract into the blood.
8- Diagram
3- The adrenal cortex secretes mineralcorticoids and androgens. The adrenal
medulla secretes epinephrine and norepinephrine
a- The paired adrenal glands (suprarenal) glands lie on top of the kidney’s
b- The adrenal gland is divided into two portions: the adrenal cortex and the
adrenal medulla
c- The adrenal gland, like the thyroid gland is highly vascularized
d- The adrenal cortex produces steroid hormones that are essential to life.
Complete loss of the adrenal gland leads to death due to dehydration and
electrolyte imbalance in several days unless replacement therapy is
started right a way. The adrenal medulla produces epinephrine and
norepinephrine
4- The Cortex
a- The cortex is divided into three zones, each of which secretes a
distinctively different type of hormone
1- The outer zone produces hormones called mineralcorticoids that
effect mineral homeostasis
2- The middle zone produces the glucocorticoids which effect glucose
metabolism
3- The inner zone producers small amounts of androgens, which are
steroid hormones that have masculinizing effects
4- Diagram
b- Mineralcorticoids –
1- Aldosterone is the major mineralcorticoid it regulates the metabolism
of sodium and potassium ions.
2- Aldosterone increases the reabsorption of sodium from the urine into
the blood and stimulates the secretion of potassium into the urine
3- Aldosterone also adjusts blood pressure and blood volume
4- Secretion of aldosterone is controlled by the angiotensin-aldosterone
pathway. This can be summarized as such. Dehydration, low blood
sodium or hemorrhage all of which lead to low blood volume and
decreased blood pressure activate this system
a- Decreased blood pressure stimulates the kidneys to release
Renin into the blood
b- in turn this activates angiotensin I, which as it passes
through the lungs is converted to angiotensin II
c- Angiotensin II causes contraction of smooth muscle,
resulting in vasoconstriction and an increase in blood
pressure
d- Angiotensin II Stimulates the adrenal cortex to release
aldosterone. This causes sodium to move from the urine
into the blood. And causes potassium to move from the
blood into the urine
e- This causes more water to return to the blood from the
urine and raises blood volume. This in association with the
constriction of the smooth muscle returns blood pressure
back to normal
c- Glucocorticoids –
1- The glucocorticoids regulate metabolism and resistance to stress. The
glucocorticoids include cortisol, hydrocortisone, corticosterone and
cortisone.
2- The glucocorticoids have the following effects
a- Protein breakdown
b- Glucose formation
c- Breakdown of Triglycerides
d- Resistance to stress
e- Anti-inflammatory effects
f- Depresses immune response