Written By: Razaullah Khattak (DPT Medical Professional)

Student Source Reference: Guyton and Hall Textbook of Medical Physiology.

 Chapter: 29

To read more check out Chapter 10:

https://mednotesbyrazaullah.blogspot.com/2026/08/chapter-10-rhythmical-excitation-of.html

Control of Renal Calcium Excretion and Extracellular Calcium Ion Concentration

Overview of how the body regulates extracellular fluid volume and the concentration of key electrolytes, specifically calcium, phosphate, and magnesium. It explains the complex hormonal and intrarenal mechanisms that control electrolyte filtration, reabsorption, and excretion through the kidneys.

Maintaining precise electrolyte and fluid balance is vital for systemic homeostasis and biological function. Hormones like parathyroid hormone (PTH) regulate calcium and phosphate levels, while intrinsic kidney mechanisms ensure that overall sodium and water excretion matches daily dietary intake under steady-state conditions.

Control of Calcium Ion Concentration

• Extracellular calcium concentration remains tightly regulated at 2.4 mEq/L.
• Hypocalcemia increases nerve and muscle cell excitability.
• Severe hypocalcemia causes hypocalcemic tetany with muscle spasticity.
• Hypercalcemia depresses neuromuscular excitability and causes arrhythmias.
• Ionized calcium comprises 50 percent of plasma calcium.
• Biological activity occurs primarily via the ionized form.
• Plasma proteins bind 40 percent of plasma calcium.
• Anions like phosphate complex 10 percent of calcium.
• Acidosis decreases calcium binding to plasma proteins.
• Alkalosis increases calcium binding, predisposing to tetany.

Body Calcium Distribution and Balance

• Dietary calcium intake averages about 1000 mg per day.
• Feces excrete approximately 900 mg of calcium daily.
• Intestinal secretion can cause fecal loss to exceed intake.
• Bone stores 99 percent of total body calcium.
• Extracellular fluid contains only 0.1 percent of calcium.
• Intracellular fluid stores 1.0 percent of body calcium.
• Bone serves as a major reservoir during hypocalcemia.

Parathyroid Hormone Actions

• Low extracellular calcium directly stimulates PTH secretion.
• High extracellular calcium decreases parathyroid hormone secretion.
• PTH stimulates bone resorption to release calcium.
• Elevated PTH activates vitamin D to increase absorption.
• Active vitamin D increases gastrointestinal absorption of calcium.
• PTH directly increases renal tubular calcium reabsorption.

Renal Calcium Excretion and Reabsorption

• Renal excretion equals filtered calcium minus reabsorbed calcium.
• Glomerulus filters only ionized and complexed calcium forms.
• Kidneys reabsorb 99 percent of filtered calcium.
• Urine excretes 1 percent of total filtered calcium.
• Proximal tubule reabsorbs 65 percent of filtered calcium.
• Thick ascending limb reabsorbs 25 to 30 percent.
• Distal and collecting tubules reabsorb 4 to 9 percent.
• Proximal reabsorption occurs mainly via the paracellular route.
• Transcellular proximal reabsorption involves basolateral ATPase pumps.
• Transcellular proximal reabsorption utilizes sodium-calcium counter-transporters.
• Loop reabsorption occurs in thick ascending limb.
• Thick ascending limb reabsorbs 50 percent paracellularly.
• Positive luminal charge drives passive loop diffusion.
• PTH stimulates transcellular reabsorption in thick ascending limb.
• Distal tubule reabsorbs calcium through active transport.
• Calcium channels allow luminal entry into distal cells.
• Calcitriol and calcitonin stimulate distal calcium reabsorption.

Factors Regulating Renal Calcium Excretion

• Increased PTH levels decrease urinary calcium excretion.
• Decreased PTH levels promote renal calcium excretion.
• Extracellular volume expansion increases urinary calcium excretion.
• Increased arterial pressure decreases proximal calcium reabsorption.
• Volume contraction decreases calcium excretion via proximal reabsorption.
• High plasma phosphate stimulates PTH to increase reabsorption.
• Metabolic acidosis stimulates renal tubular calcium reabsorption.
• Metabolic alkalosis inhibits distal tubular calcium reabsorption.

Regulation of Renal Phosphate Excretion

• Overflow mechanisms primarily control renal phosphate excretion.
• Normal transport maximum for phosphate is 0.1 mM/min.
• Renal tubules reabsorb all phosphate below transport maximum.
• Phosphate spills into urine above 0.8 mM/L threshold.
• Normal plasma phosphate concentration stays above 1 mM/L.
• Proximal tubule reabsorbs 75 to 80 percent phosphate.
• Distal tubule reabsorbs 10 percent of filtered phosphate.
• Urine excretes about 10 percent filtered phosphate.
• Sodium-phosphate co-transporters mediate proximal luminal uptake.
• Low phosphate diet increases reabsorptive transport maximum.
• PTH promotes bone resorption to release plasma phosphate.
• PTH decreases renal tubular transport maximum for phosphate.
• Increased PTH levels promote urinary phosphate excretion.

Control of Renal Magnesium Excretion

• Bone stores over half of total body magnesium.
• Intracellular fluid stores most non-bone magnesium.
• Extracellular fluid holds less than 1 percent magnesium.
• Total plasma magnesium concentration equals 1.8 mEq/L.
• Plasma proteins bind over half of plasma magnesium.
• Free ionized magnesium concentration equals 0.8 mEq/L.
• Daily dietary magnesium intake is 250-300 mg.
• Gastrointestinal tract absorbs half of ingested magnesium.
• Kidneys excrete 125 to 150 mg magnesium daily.
• Glomerular filtrate excretes 10 to 15 percent magnesium.
• Proximal tubule reabsorbs 25 percent filtered magnesium.
• Loop of Henle reabsorbs 65 percent magnesium.
• Distal and collecting tubules reabsorb under 5 percent.
• High extracellular magnesium increases renal magnesium excretion.
• Extracellular volume expansion increases magnesium loss.
• High extracellular calcium concentration increases magnesium excretion.

Integration of Extracellular Fluid Control

• Water and salt balance determines extracellular fluid volume.
• Habits dictate dietary salt and fluid intakes.
• Kidneys adapt excretion rates to match daily intake.
• Sodium chloride content dictates extracellular fluid volume.
• ADH-thirst mechanisms maintain osmolality and sodium concentration.

Sodium Balance and Excretion Mechanics

• Steady-state conditions match sodium intake with excretion.
• Disturbed kidney function restores sodium balance within days.
• Intrarenal adjustments correct mild renal perturbations.
• Severe perturbations invoke systemic cardiovascular adjustments.
• Systemic compensations cause costly homeostatic trade-offs.
• Increased blood pressure maintains long-term sodium excretion.
• Hypertension damages systemic blood vessels over time.
• Sustained fluid imbalances cause rapid cardiovascular collapse.
• Excretion rate equals glomerular filtration minus tubular reabsorption.
• Normal GFR equals 180 liters per day.
• Normal tubular reabsorption equals 178.5 liters daily.
• Normal daily urine excretion equals 1.5 liters.
• Five percent GFR increase causes massive urine output.
• Glomerulotubular balance buffers sudden filtration increases.
• Macula densa feedback constricts afferent arterioles.
• Systemic hormones restore sodium excretion matching intake.

Status: Chapter 29 Completed & Simplified. 

Prepared By: Razaullah Khattak (DPT Medical Professional)

Disclaimer: This content is for educational purposes only and not a substitute for professional medical advice.