Post on 24-Oct-2014
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CALCIUM HOMEOSTASIS
New insight : BIVEK SINGH
OBJECTIVE OF SESSION• Calcium• Body distribution of calcium• Function of calcium• Absorption and excretion of calcium• Calcium homeostasis -hormonal regulation 1. Parathyroid hormone (PTH) 2. Vitamin D (Calcitriol) 3. Calcitonin
Calcium
Alkaline earth metal Atomic weight 20
Most abundant mineral in the human body
Body distribution•Total content inadult body is 25000mmol (approx. 1kg)•99% of calcium is found in bone•extracellular fluid is 22.5 mmol
Plasma calcium
1.protein bound fraction- 4-5mg/dl
2.complex with citrate and phosphate- 1mg/dl
3.ionised form- 4-5mg/dl
Major functions
•Calcification of bone and teeth
•Muscle contraction
•Role in neuromuscular transmission
•Role in blood coagulation
•Calcium as second messenger
Absorption of calcium
• Calcium is taken in diet principally as calcium-phosphate,carbonate or tartarate
• About 40% of the average daily dietary intake of calcium is absorbed in gut.
• Absorption of calcium by gut mucosa occur by energy dependent active transport process.
Excretion of calcium
• Partly through kidney and mostly through intestine
• Renal threshold for serum calcium is 10mg/dl
• Excretion is increased in vitamin D deficiency
CALCIUM HOMEOSTATSIS
• Calcium homeostasis refers to the regulation of concentration of calcium ions in extracellular matrix
• Calcium homostasis is regulated by:
1. vitamin D (calcitriol)
2. parathyroid hormone (PTH)
3. calcitonin
Vitamin D (Calcitriol)
• Vitamin D has two sources i.e. skin and diet
• 25-hydroxy cholecalciferol is the storage form of vitamin D in liver
• 1,25-hydroxy cholecalcitriol is the active form of vitamin D
• Activated by PTH and hypophosphptemia through 1-alpha hydroxylase enzyme in kidney
VITAMIN D SYNTHESIS
SKIN LIVER KIDNEY
7-DEHYDROCHOLESTEROL
VITAMIN D3
VITAMIN D3
25(OH)VITAMIN D
h25-HYDROXYLASE
25(OH)VITAMIN D
1,25(OH)2 VITAMIN D
(ACTIVE METABOLITE)
1-HYDROXYLASE
TISSUE-SPECIFIC VITAMIN D RESPONSES
Action of Calcitriol on intestine
• Calcitriol stimulates the absorption of dietary calcium and phosphate .
• Calcitriol induces the synthesis of calcium binding proteins(Calbindin D) in the enterocytes. Thus increases the uptake of calcium by intestine.
Action of calcitriol on bone
• Calcitriol along with parathyroid hormone stimulates the osteoclastic bone resorption
• thus,mobilization of calcium and phosphate from bone.
Action of Calcitriol on the Kidney
•Calcitriol reduces the excretion of calcium and phosphate through the kidney.
• Increase reabsorption.
Parathyroid hormne• 4 parathyroid glands
• Release PTH in response to drop in serum calcium
• Magnesium needed to activate PTH release
• Effects on bone, kidney and indirectly on intestines
Action of PTH on bone• PTH causes resorption of bone by osteoclastic activity
• Due to increased stimulation of pyrophosphatase and collagenase
• This causes the mobilization of calcium from bone
• Increase blood calcium level
Action of PTH on kidney
• Increased reabsorption of calcium from kidney tubule
• Reduce the excretion of calcium
• Helps in synthesis of calcitriol by 1-alpha hydroxlase
enzyme
Action of PTH on intestine
• Indirectly
• By promoting synthesis of calcitriol
CALCIUM, PTH, AND VITAMIN D FEEDBACK LOOPS
NORMAL BLOOD Ca
RISING BLOOD Ca
FALLING BLOOD Ca
SUPPRESS PTH
STIMULATE PTH
BONE RESORPTION
URINARY LOSS
1,25(OH)2 D PRODUCTION
BONE RESORPTION
URINARY LOSS
1,25(OH)2 D PRODUCTION
Calcitonin
• Secreted by parafollicular cells of thyroid gland
• Metabolism is antagonistic to PTH hormone
• Normal concentration in plasma is 10-20 pg/ml
Action of calcitonin on bone & kidney
• Mainly acts on osteoclasts
• Reduces osteoclastic activities by reducing number and size of osteoclasts.Thus, inhibits bone resorption
• On kidney, it decreases calcium reabsorption from kidney
• Causes calciuria
CALCIUM HOMEOSTASIS
DIETARY CALCIUM
INTESTINAL ABSORPTIONORGAN PHYSIOLOGY
ENDOCRINE PHYSIOLOGY
DIETARY HABITS,
SUPPLEMENTSBLOOD CALCIUM
BONE
KIDNEYS
URINE
THE ONLY “IN”
THE PRINCIPLE “OUT”
ORGAN PHYS.
ENDOCRINE PHYS.
ORGAN, ENDOCRINE
Questions !!!!!!!!!Questions !!!!!!!!! oror suggestions ??????suggestions ??????biveksingh@hotmail.com
BIVEK SINGHBIVEK SINGHKIST MEDICAL COLLEGEKIST MEDICAL COLLEGELALITPUR,NEPALLALITPUR,NEPAL