Stimulate active absorption of calcium and phosphate by the intestine (major role), stimulates the renal-tubular reabsorption of calcium and phosphate by increasing the synthesis of the protein components in the transport pathway.
Describe the role of vitamin D in calcium balance.
Vitamin D (active form) and parathyroid hormone (PTH)
Name the two major hormones involved in calcium and phosphate regulation.
calcium; parathyroid glands
PTH secretion is controlled on a moment-to-moment basis by the ____ concentration of the ECF bathing the cells of the ____.
Decreased; increased
____ plasma calcium concentration stimulates PTH secretion, and ____ plasma concentration inhibits secretion.
G protein-linked; divalent cations; inhibits; removing a tonic inhibition
Extracellular calcium concentration acts directly on the parathyroid glands by binding to a novel class of ____ receptors whose ligands are ____. The calcium receptor couples via an intracellular G protein to a signaling cascade that ____ the secretion of
stimulates; stimulating
Elevated phosphate ____ PTH secretion by ____ the capacity of the parathyroid gland to synthesize PTH.
By PTH, which is responsible for stimulating the 2nd hydroxylation of the precursor at the proximal tubule.
How is synthesis of the active form of vitamin D regulated?
Distal convoluted tubule and connecting tubule
Identify the nephron segments which respond to parathyroid hormone by increasing calcium reabsorption.
Stimulate release of calcium across bone membrane; stimulate resorption by osteoclasts; stimulate activation of vitamin D; increase tubular Ca2+ reabsorption; increase synthesis of all components of transport pathway; decrease proximal tubular reabsorptio
State the (six) major actions of PTH.
proximal tubule; sodium
Approximately 75% of filtered phosphate is actively reabsorbed, almost entirely in the ____ (in symport with ____).
tubular maximum-limited (Tm); higher; most; some
Phosphate is reabsorbed by a ____ system, and the normal filtered load is just a little ____ than the Tm. Thus, ____ filtered phosphate is reabsorbed, but ____ spills into the urine.
inhibited; stimulated
Whenever PTH is increased or decreased, tubular phosphate reabsorption is powerfully ____ or ____, respectively.
Proximal tubule
Identify the nephron segment which responds to increased plasma [Pi].
10 mg/dL (2.5 mmol/L or 5 mEq/L); 1/2
State the normal total plasma calcium concentration and the fraction that is free.
bone; extracellular fluid (ECF)
The vast majority of calcium exists as a structural component of ____. A much smaller, but critical fraction is dissolved in the ____.
bone
Moment-to-moment regulation of extracellular calcium is achieved by shifting calcium in and out of ____.
vitamin D
Besides the rapid exchange of calcium into or out of bone, there is also a long-term regulation of total calcium in bone that is important for bone growth during childhood and bone integrity in adult life. Here, the kidneys play an important but indirect
GI tract
The dominant regulation of total-body calcium balance is less focused on output and more focused on input from the ____. Here, it is both reabsorbed and secreted.
majority
A ____ of the calcium we eat simply passes through the GI tract to the feces.
secretory; absorbed; absorption; absorption
The ____ rate of calcium is more or less constant, but the fraction ____ is regulated to produce anywhere from small to moderate net ____. Some net ____ is essential for the long-term maintenance of adequate bone calcium.
passively; calbindins; actively; Ca-ATPase; Na-Ca antiporter
Calcium enters from the intestinal lumen ____ through calcium-selective channels, binds reversibly to mobile cytosolic calcium-binding proteins (called ____), and is then ____ transported out the basolateral side via a ____ and ____.
proximal tubule; thick ascending limb of Henle's loop, distal convoluted tubule, and collecting-duct system
Most calcium reabsorption occurs in the ____ (about 60% of the filtered load) and the remainder in the ____.
passive; paracellular; sodium; active; transcellular
Calcium reabsorption in the proximal tubule and thick ascending limb is largely ____ and ____, and the electrochemical forces driving it are dependent directly or indirectly on ____ reabsorption as they are for so many other substances. In contrast, calci
filtered; reabsorbed; secreted
Calcium that reaches the kidneys is ____ and ____, but not ____.
Rapid buffering system that involves movement of calcium across the bone membrane.
What protects the blood plasma from short-term swings in calcium concentration?
bone remodeling; osteoclasts; osteoblasts
The second flux process involving calcium is called ____ and affects calcium stores on a slower time scale. It involves the paired actions of giant, multinucleated cells called ____ that erode little pits in the bone matrix and their partners, nearby ____
hydrogen ions; acidic; hydroxyapatite
The osteoclasts pump ____ and create ____ microspace directly underneath them that solubilizes ____. The calcium and phosphate freed up by this process is then transported transcellularly to the ECF.
less
The daily flux of calcium via remodeling is much ____ than that associated with rapid flux across the bone membrane.
liver; proximal tubular cells; 3
Circulating vitamin D is hydroxylated at the 25 position by the ____ and then hydroxylated again at the 1 position by ____ to yield a cholesterol derivative containing ____ hydroxyl groups, the active form of vitamin D.