Calcium and Electrolyte Regulation

Study Notes

Introduction

Calcium, phosphate, and magnesium are essential multivalent cations involved in various biological functions.
The kidneys regulate homeostasis of these ions, balancing gastrointestinal absorption with renal excretion.
Depletion of these ions triggers increased gastrointestinal absorption, bone resorption, and tubular reabsorption to restore normal levels.
Renal regulation impacts plasma concentrations through mechanisms of glomerular filtration and tubular reabsorption/secretion.
Urinary excretion adjustments are key to maintaining overall body balance of these ions.

Clinical Implications

Imbalances in calcium, phosphorus, and magnesium can lead to serious complications such as arrhythmias, seizures, and respiratory issues.
The kidneys play a critical role in maintaining serum levels of these electrolytes.

Calcium Overview

Total body calcium ranges from 1000 to 1200 g, with approximately 99% stored in bones and 1% in extracellular/intracellular spaces.
Ionized calcium plays critical roles in nerve transmission, muscle contraction, blood coagulation, hormone secretion, and cell adhesion.
Normal serum calcium levels are approximately 8.9–10.1 mg/dl (2.2–2.5 mmol/l).
Decreases in serum albumin concentration lead to proportional decreases in total serum calcium levels; a 1.0-g/dl decrease in albumin correlates to a 0.8 mg/dl decrease in serum calcium.

Renal Transport of Calcium

Approximately 10 g of calcium is filtered daily in individuals with a glomerular filtration rate (GFR) of 170 liters per 24 hours.
Urinary calcium excretion ranges from 100 to 200 mg per day, indicating a reabsorption rate of 98%–99% in renal tubules.
Calcium reabsorption occurs primarily in:
- Proximal convoluted tubule (60%–70%)
- Loop of Henle (20%)
- Distal convoluted tubule (10%)
- Collecting duct (5%)

Proximal Convoluted Tubule

Calcium reabsorption is linked with sodium and water reabsorption, predominantly through passive diffusion and solvent drag.
The paracellular pathway accounts for about 80% of calcium reabsorption in this segment.
Active transport occurs through:
- Calcium entry from tubular fluid across apical membrane.
- Calcium exit through basolateral membrane.
This active transport process constitutes approximately 10%–15% of total calcium reabsorption, regulated by parathyroid hormone (PTH) and calcitonin.

Loop of Henle

No calcium reabsorption occurs in the thin segment of the loop of Henle.
In the thick ascending limb, 20% of filtered calcium is reabsorbed primarily through paracellular and transcellular pathways.
The majority of calcium reabsorption in the thick ascending limb occurs via the paracellular route, influenced by the osmotic gradient established in this nephron segment.

Description

Explore the roles of calcium, phosphate, and magnesium in biological functions and their regulation by the kidneys. This quiz delves into the implications of electrolyte imbalances and how they can affect overall health. Test your knowledge on the mechanisms of renal regulation and its importance in maintaining homeostasis.