Calcium homeostasis is tightly regulated to keep plasma calcium levels within a narrow range. Three hormones - parathyroid hormone (PTH), calcitriol, and calcitonin - work together to regulate calcium levels by moving calcium between bones and extracellular fluid. The kidneys also play an important role by reabsorbing around 80% of filtered calcium in the proximal tubule and regulating further calcium reabsorption in other parts of the nephron in response to these hormones and other factors like calcium levels and pH. Disruptions to this regulatory system can lead to hypocalcemia or hypercalcemia.

1. In the name of God

2. CALCIUM
HOMEOSTASIS

3. INTRODUCTION
• Total plasma [Ca++] = 2.5mmol/L
• Range is 2.0 to 2.5 mmol/L
• Very tightly controlled
Body
content
Bone intracellular extracellular
Calcium 1300 gms 99% 1% 0.1%

4. • Within cells, Ca++ is sequestered in the
endoplasmic reticulum and mitochondria, or it is
bound to proteins.
• Approximately 50% of the Ca++ in plasma is
ionized, 45% is bound to plasma proteins (mainly
albumin), and 5% is complexed to several anions,
including HCO3
-, citrate, Pi, and SO4
2-.
INTRODUCTION

5. FACTORS AFFECTING CALCIUM
CONCENTRATION
• the plasma albumin
concentration
• pH of plasma

6. FUNCTIONS OF CALCIUM
• Nerve and muscle functions
Hypocalcemia
Hypercalcemia

7. • Neurotransmitter release
• Preserving bone and density
• important cofactor in many enzymatic
reactions
• cell division and growth
• Hemostasis
FUNCTIONS OF CALCIUM

8. CALCIUM HOMEOSTASIS
Ca++ homeostasis depends on two factors:
1.the total amount of Ca++ in the body
Total body [Ca++] is determined by the relative amounts of Ca++ absorbed
by the gastrointestinal tract and excreted by the kidneys
2.the distribution of Ca++ between bone and ECF
Three hormones ( [PTH], calcitriol, and calcitonin) regulate the distribution
of Ca++ between bone and ECF and thereby regulate plasma [Ca++].

9. • The CaSR is a receptor expressed in the plasma
membrane of cells involved in regulating Ca++
homeostasis.
• Activation of the receptor results in inhibition of PTH
secretion and calcitriol production and stimulation of
calcitonin secretion.
CALCIUM-SENSING RECEPTOR

10. PARATHYROID HORMONE (PTH)

11. CALCITRIOL

12. CALCITONIN

13. EFFECT OF PLASMA [CA++] ON PLASMA LEVELS OF PTH AND CALCITONIN

14. CALCIUM HANDLING IN
THE NEPHRON

15. PROXIMAL TUBULAR CALCIUM REABSORPTION
• approximately 80% of Ca++ reabsorption is paracellular, and
approximately 20% is transcellular in the proximal tubule.
• In the proximal tubule, calcium reabsorption usually
parallels sodium and water reabsorption and is independent
of PTH

16. LOOP OF HENLE CALCIUM REABSORPTION
• In the loop of Henle, calcium reabsorption is
restricted to the thick ascending limb.
• approximately 50% of Ca++ reabsorption is paracellular, and
approximately 50% is transcellular in the proximal tubule.

17. DISTAL TUBULE CALCIUM REABSORPTION
• Reabsorption of Ca++ by the distal tubule is exclusively
transcellular.
• In the distal tubule, where the voltage in the tubule lumen
is electrically negative with respect to blood, reabsorption
of Ca++ is entirely active

18. REGULATION OF URINARY CALCIUM EXCRETION
• PTH
• Calcitriol
• Calcitonin
• [Ca++] in ECF
• [Pi] in ECF
• Changes in ECF volume
• PH

19. REFERENCE
• Berne & Levy Physiology
• Guyton and Hall Textbook of Medical Physiology
• Physiology by Linda S. Costanzo

20. Thank you for your attention