Exam 4 Review:  Chapter 26:  Regulation of Electrolyte Balance

hypothalamic thirst center - The site in the hypothalamus, close to the site of production of ADH, to which osmoreceptors send visceral sensory information about hydration status in the blood and body fluids; the threshold for thirst is a plasma osmolarity > 290-295 mOsm/L and is rapid in onset; it is also the site to which baroreceptors send visceral sensory information about blood pressure which is influenced by blood volume (hydration status); it is also a site where angiotensin II interacts; all of these influences cooperate to generate a conscious desire to drink.

antidiuretic hormone (ADH) = vasopressin - the neurohypophyseal peptide hormone which stimulates contraction of smooth muscle in blood vessel walls and stimulates the kidney tubules to reabsorb water; both target responses tend to increase blood pressure.

atrial natriuretic peptide (ANP) - The peptide hormone released from special endocrine cells in the walls of the upper chambers/atria of the heart in response to the stretching of the chamber walls due to increased blood volume or increased blood pressure; ANP is the antagonist to aldosterone and thus triggers salt (NaCl) and water excretion and potassium ion reabsorption at the kidney tubules. 

aldosterone - The main steroid hormone produced by the outermost layer of the adrenal cortex, whose release is stimulated by increased potassium levels in the plasma; this mineralocorticoid  targets the kidney to actively transport sodium ions out of the urine and potassium ions into the urine (chloride ions and water passively follow the sodium ions back to the bloodstream); the net effects of this hormone are to increase blood pressure and blood volume and to adjust sodium and potassium levels in the blood.

renin-angiotensin system - A complex endocrine negative feedback control system which plays important role in regulating blood volume, arterial pressure, and cardiac and vascular function; in response to sympathetic stimulation, low blood pressure or low blood sodium levels, (1) the juxtaglomerular apparatus of the kidney will release the enzyme renin, (2) renin will catalyze the activation of an inactive precursor substance, angiotensinogen, which is made by the liver, into angiotensin I, (3) angiotensin I (which has some activity of its own) will be further activated by an enzyme found in the lungs, angiotensin-converting enzyme (ACE), into active angiotensin II, (4) angiotensin I & II stimulate these physiological responses:  (a) enlargement of the heart and blood vessels, (b) systemic vasoconstriction, (c) aldosterone release from the adrenal cortex, (d) activation of the hypothalamic thirst center, (e) ADH release from the posterior pituitary/neurohypophysis; processes (c, d, and e) cause retention of salt and water by the kidneys and as a result, increased blood volume; thus this system has a powerful hypertensive effect.

parathyroid hormone = parathormone = PTH - A protein hormone, produced by the principle or chief cells of the parathyroid gland which increases plasma calcium levels (mechanisms 1, 2, and 3) and decreases plasma phosphate levels (mechanism 3) by (1) targeting oseoclasts to increase dissolution of bone matrix, (2) targeting the intestinal lining to increase calcium ion absorption from the digested meal, and (3) targeting the kidney to reaabsorb calcium ions from and secrete phosphate ions into the urine; PTH is the antagonist to thyrocalcitonine.

calcitonin = thyrocalcitonin - A protein hormone, produced by the parafollicular cells/C cells of the thyroid gland which lowers plasma calcium and phosphate levels by targeting oseoblasts to increase formation of bone matrix; the antagonist to parathyroid hormone.  

1.  the mechanisms regulating thirst.

Osmoreceptors in the hypothalamus note changes in hydration status by checking blood volume and blood solute concentration.  Dehydration stimulates the hypothalamic thirst center creating sensations of thirst and this may be reinforced by a sensation of a dry mouth from decreased saliva secretion as a result of the increased osmotic strength (decreased water) in the plasma.  Once water has been ingested and absorbed, the improved hydration status will inhibit further activity from the hypothalamic thirst center.

Recall the additional contribution of the Renin-Angiotensis System in stimulating the hypothalamic thirst center.  See the diagram on the Renin-Angiotensis System above.