In the name of God

CALCIUM HOMEOSTASIS

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%

• 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 SO42-.

INTRODUCTION

FACTORS AFFECTING CALCIUM CONCENTRATION

• the plasma albumin concentration • pH of plasma

FUNCTIONS OF CALCIUM

• Nerve and muscle functions

HypocalcemiaHypercalcemia

• Neurotransmitter release• Preserving bone and density• important cofactor in many enzymatic

reactions• cell division and growth• Hemostasis

FUNCTIONS OF CALCIUM

CALCIUM HOMEOSTASISCa++ 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 kidneys2.the distribution of Ca++ between bone and ECFThree hormones ( [PTH], calcitriol, and calcitonin) regulate the distribution of Ca++ between bone and ECF and thereby regulate plasma [Ca++].

• 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

PARATHYROID HORMONE (PTH)

CALCITRIOL

CALCITONIN

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

CALCIUM HANDLING IN THE NEPHRON

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 usuallyparallels sodium and water reabsorption and is independent of PTH

LOOP OF HENLE CALCIUM REABSORPTION

• In the loop of Henle, calcium reabsorption isrestricted to the thick ascending limb.

• approximately 50% of Ca++ reabsorption is paracellular, and approximately 50% is transcellular in the proximal tubule.

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

REGULATION OF URINARY CALCIUM EXCRETION

• PTH• Calcitriol• Calcitonin • [Ca++] in ECF• [Pi] in ECF

• Changes in ECF volume • PH

REFERENCE

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

Thank you for your attention