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Phys3 Ca & Phosph
Phys3 Renal Ca+ & Phosphorus
Question | Answer |
---|---|
Ca+ exists in 3 forms | 1.40% is non-filterable b/c it is protein bound. 2.15% of Ca+ is complexed with small anions (ex: carbonate, citrate, phosphate, & sulfate). 3.45% is ionized Ca+ (this is what is tightly regulated by the body). |
How is moment to moment extracellular Ca+ regulation achieved? | By shifting Ca+ in and out of BONE |
How many hydroxylations does Vit D undergo? | TWO: 1.Liver. 2.Kidney (stimulated by PTH). |
Vit D in gut facilitates the absorption of what? | 1.Ca+ (entry, buffering, absorption into plasma). 2.Phosphate. |
Vit D is critical in the synthesis of what in the GI Enterocytes? | Intracellular Ca-Binding proteins. **these aid Ca+ entry into the cell through apical membrane & minimize the free Ca+ within the cell. |
2 ways Ca+ exits the GI enterocyte's basolateral membrane? | 1.Ca ATPase. 2.Na/Ca exchanger when Ca ATPase is saturated. |
What stimulates the Ca-ATPase on the basolateral membrane of the enterocytes? | Vitamin D |
Dec Plasma [Ca] | Inc PTH: 1.Kidney: Inc hydrolysis of Vit D, Inc Ca+ reabsorption. 2.GI: Inc Ca+ absorption. 3.Bone: Inc osteoclast activity |
Where is the Renal tubule does PTH cause an Inc in Ca+ reabsorption in response to Dec plasma [Ca+]? | 1.Thick Ascending limb (25%). 2.Distal convoluted tubule (. 3.Connecting tubule/Collecting Ducts (1.5%). |
What are the 3 sites of Ca+ regulation? | 1.GI enterocytes. 2.Bone. 3.Kindey |
Where is 65% of filtered Ca+ reabsorped? is it subject to hormonal control (ex: PTH)? | Proximal Tubule via paracellular reabsorption. NO. **tied to Na+ & water reabsorption, Inc when they Inc b/c water leaving creates an Inc concentration gradient. |
How much of the filtered load of Ca+ to you actually excrete? | 0.5 to 1% of FL. |
How is Ca+ reabsorbed in the Thick Ascending Limb? | 1. 50% passive & paracellular (from + voltage w/in tubule, this is caused by ADH). 2. 50% active & transcellular (PTH influences). |
What is major regulatory site of Ca+ in the kidney | Distal Convoluted Tubule (DCT). 1.PTH stimulates Ca channel on apical mem causing Ca+ to move from the tubule into the cell. 2. Vit D buffers Ca+ in the cell and Activates Ca-ATPase on basolateral mem. |
Can plasma Ca+ have a direct effect on Ca+ reabsorption in the kidneys? | YES. Ca+ sensing receptor on basolateral mem which will block the tri-transporter on apical membrane which with cause the lumen + voltage to disappear and Dec Ca+ reabsorption. |
How do Loop diuretics affect Ca+ reabsorption? | DECREASE b/c they will interfere with the lumen + voltage driving paracellular reabsorption. |
Are loop diuretics used to treat hypercalcemia? | NO |
How do thiazide diuretics affect Ca+ reabsorption? | INCREASE it. **They block the Na/Cl symporter in DCT, Dec Na w/in the cell, Inc activity of Na/Ca exchanger on the basolateral membrane. |
How does volume contraction affect Ca+ reabsorption in PT? | Inc Ca+ reabsorption by Inc Na+ & water in PT due to SNS and AngII response. |
What is the important role of Phosphate in Acid/Base balance? | Buffering H+ secreted in tubule lumen and excreted as titratable acid (H2PO4). |
Where does most Phosphate reabsorption occur? | 80% occurs in the Proximal Tubule via Na/phosphate symporter. |
Does Phosphate have limited reabsorption? | YES, any Filtered Load above Tm will be excreted |
Does PTH Inc or Dec phosphate reabsorption? | DECREASES it by promoting renal phosphate excretion. **PTH inhibits apical sodium/phosphate uptake |
What 2 things stimulate PTH secretion? | 1.Dec plasma [Ca]. 2.Inc plasma [Phosphate]. |
Acidosis | Causes Inc Ca+ and Phosphate release from bone, which is good b/c filtered phosphate can absorb and buffer H+ ions to raise pH. |
Primary Hyperparathyroidism | Inc PTH!! causes: 1.Inc bone resorption. 2.Inc plasma Ca+. 3.Dec plasma phosphate. 4.Inc Urinary phosphate excretion. 5.Inc Urinary Ca+ excretion (Ca+ is so high it exceeds ability to absorb). |