Page 58 - 86 human physiology part-2

The Endocrine System

Aldosterone is one example of a mineralcorticoid. It signals the tubules in the kidney nephrons to
reabsorb sodium while secreting or eliminating potassium. If sodium levels are low in the blood, the
kidney secretes more renin, which is an enzyme that stimulates the formation of angiotensin from a
molecule made from the liver. Angiotensin stimulates aldosterone secretion. As a result, more sodium
is reabsorbed as it enters the blood.

Aldosterone, the major moineralcorticoid, stimulates the cells of the distal convoluted tubules of
the kidneys to decrease reabsorption of potassium and increase reabsorption of sodium. This in turn
leads to an increased reabsorption of chloride and water. These hormones, together with such hormones
as insulin and glucagon, are important regulators of the ionic environment of the internal fluid.

The renin-angiotensin-aldosterone mechanism can raise blood pressure if it tends to drop. It does
this in two ways. Angiotensin is a vasoconstrictor, decreasing the diameter of blood vessels. As vessels
constrict, blood pressure increases. In addition, as sodium is reabsorbed, the blood passing through the
kidney becomes more hypertonic. Water follows the sodium into the hypertonic blood by osmosis. This
increases the amount of volume in the blood and also increases the blood pressure.

Adrebal Medulla The hypothalamas starts nerve impulses that travel the path from the
bloodstream, spinal cord, sympathetic nerve fibers to the Adrenal Medulla, which then releases
hormones. The effects of these hormones provide a short-term response to stress

Excessive secretion of the glucocorticoids causes Cushing's syndrome, characterized by muscle
atrophy or degeneration and hypertension or high blood pressure. Under secretion of these substances
produces Addison's disease, characterized by low blood pressure and stress.

Epinephrine and norepinephrine produce the "fight or flight" response, similar to the effect from
the sympathetic nervous system. Therefore, they increase heart rate, breathing rate, blood flow to most
skeletal muscles, and the concentration of glucose in the blood. They decrease blood flow to the
digestive organs and diminish most digestive processes.

The adrenal sex hormones consist mainly of male sex hormones (androgens) and lesser amounts of
female sex hormones (estrogens and progesterone). Normally, the sex hormones released from the
adrenal cortex are insignificant due to the low concentration of secretion. However, in cases of excess
secretion, masculinizing or feminizing effects appear. The most common syndrome of this sort is
virilism of the female.

Should there be an insufficient supply of cortical hormones, a condition known as Addison's
disease would result. This disease is characterized by an excessive excretion of sodium ions, and hence
water, due to lack of mineralcorticoids. Accompanying this is a decreased blood glucose level due to a
deficient supply of glucocorticoids. The effect of a decreased androgen supply cannot be observed
immediately. Injections of adrenal cortical hormones promptly relieve these symptoms.

Hormonal production in the adrenal cortex is directly controlled by the anterior pituitary hormone
called adrenocorticotropic hormone (ACTH).

The two adrenal glands lie very close to the kidneys. Each adrenal gland is actually a double gland,
composed of an inner core like medulla and an outer cortex. Each of these is functionally unrelated.

The adrenal medulla secretes two hormone, adrenalin or epinephrine and noradrenalin or

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