calcium: ~2% of body mass; 99% in bone; 0.1% in serum (constant); 0.9% in cells
bone: integrity; buffer for plasma Ca
plasma: coagulation; muscle and nerve excitability
intracellular: second messanger (muscle contraction, secretion, cell division, channels, receptors)
phosphate: 1% of body mass; 85% in bone; serum (0.1%) and cellular ~ 1mM (can change with pH)
bone: integrity; buffer for plasma PI
plasma: buffer
intracellular: phosphometabolites; modifyer of proteins; buffer, charge balance
Three Stores of Calcium
cellular calcium
oscillates (~50 nM)
serum calcium
(2.5 mM; 1.3 mM is free Ca)
free Ca conc causes physiological responses, not total conc
total plasma Ca may vary (albumin and globulin varies)
free Ca is kept constant throughout lifetime; small changes cause responses
bone calcium
continuously changes (buffer)
Three Stores of Phosphate
cellular (0.5 mM)
serum phosphate varies (0.8 - 1.5 mM) note same magnitude as cellular
bone phosphate continuously changes
Calcium Concentration in Blood
recommended intake: Ca 1.3g; Pi 1.0g; Mg 0.6 g
lose 0.6g Ca and 0.3g Pi in feces, urine, body fluids even if no dietary intake
bone acts as a buffer: 0.6g Ca and 0.3g Pi are exchanged between bone and plasma each day
Hypocalcemia (< 1.1 mM)
muscle stiffness, contracture
hyperexcitibility of nerves
anxiety
decreased cardiac contractility (seen in EKG)
seizures, cardiac arrest
Hypercalcemia (> 1.5 mM)
polyuria
constipation
fatigue
calcium deposit
cardiac arrhythmia
Hormones involved in Ca and Pi Homeostasis
parathyroid hormone (PTH)
release stimulated by low Ca conc
increases bone resorption (release) of Ca and Pi
increases urinary excretion of Pi; decreases Ca excretion
stimulates production of calcitriol from vit D Þ increases intestinal absorption of Ca and Pi
overall, Pi effects cancel out so get increased Ca with little change in Pi
Pi could also decrease due to enhanced Pi clearance in kidney
calcitriol
(made from vitamin D3)
stimulated by low Ca (via PTH) and low PI
increases absorption of Ca and Pi by the intestine
decreases excretion of Ca and Pi
overall effect is to increase Ca and Pi
calcitonin
stimulated by increase in Ca
blocks bone resorption and stimulates deposition of Ca into bone
overall decreases Ca in serum with little effect on Pi (opposite of PTH)
modern diet often contains high Pi and Ca; since homeostasis of both are handled together and only fine tuning is though PTH, could lead to hyperparathyroidism
Calcium Reabsorption
70% PT; 20% TAL; 9% DT; 1% CD; 1% excreted
PTH can enhance absorption in DT
normal Ca loss is compensated by removing from bone