PSL201Y1 Chapter Notes -Hypercalcaemia, Tubular Fluid, Hypotension
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CHP 19 (552-553)
Ca triggers exocytosis of chemical messengers, stimulates secretion of substances, stimulates muscle
contractions, and increases the contractility of the heart and blood vessels. Important for bones.
Hypercalcemia results with muscle weakness, atrophy, lethargy, behavioral changes, hypertension, sontipatino,
Hypocalcemia causes numbness and tingling sensations, muscle cramps and spasms, exaggerated reflexes and
Ca can be added to the plasma from bone and absorbed via the digestive tract. It can be removed from the
plasma by bone and the kidneys.
The bone provides a reservoir of Ca through resorption. When plasma Ca levels are high, Ca can be deposited
RENAL HANDLING OF CA IONS
Ca is transported in blood by carrier proteins or is free in the plasma. Ca that is free in the plasma is freely
filtered at the glomerulus.
99% of filtered Ca is reabsorbed as the tubular fluid moves through the renal tubules.
70% of Ca is reabsorbed in the proximal tubules, 20% in the thick ascending limbs of the loops of Henle, and
10% in the distal tubules.
HORMONAL CONTROL OF PLASMA CA CONCENTRATIONS
EFFECTS OF PARATHYROID HORMONE
o PTH is a peptide hormone produced in the parathyroid glands and secreted in response to a decrease in
o It stimulates Ca reabsorption in the ascending lump and the distal tubules, which decreases excretion of
Ca and sustains plasma Ca levels.
o It stimulates activation in the kidneys of 1,25-dihydroxycholecalciferol, which stimulates Ca absorption
in the digestive tract, and Ca reabsorption in the kidneys.
o It stimulates resorption of bone, which increases plasma Ca levels.
EFFECTS OF 1,25-DIHYDROXYCHOLECALCIFEROL
o 1,25-(OH)2D3 is a steroid hormone that acts to increase plasma Ca levels by stimulating Ca absorption
from the digestive tract, and Ca absorption in the distal tubules.
o It is synthesized from vitamin D3, which can either be synthesized from 7-dehydrocholesterol in the skin
upon exposure of sunlight, or absorbed from the diet.
o Once in the plasma, Vit D3 travels to the liver, where it is converted to 25-OH D3. It travels in the
bloodstream to the kidneys.
o In response to low plasma Ca levels, PTH regulates the conversin of 25-OH D3 to 1,25-(OH2) D3.
EFFECTS OF CALCITONIN
o It decreases plasma Ca levels.
o It is a peptide hormone secreted from C cells of the thyroid gland, which are distinct from cells that
secrete thyroid hormone.
o Secreted when plasma Ca levels are high.
o Although its primary function is to increase bone formation by depositing Ca, it also decreases
reabsorption of Ca by the kidneys, which leads to an increase in urinary excretion of Ca.
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