CALCIUM ( Ca ):
About
half of calcium present in blood is in ionized from and the rest, in unionized
from. Some of unionized calcium is bound to protein, and a small amount is
bound to citrate.
Calcium
taken in the diet is in the form of calcium phosphate or carbonate.
Sources:
- Milk , eggs, fish, and vegetables,
- Cereals (wheat and rice) contain
only small amounts of calcium.
Daily requirement:
Absorption:
- Takes placed in the first and second
part of duodenum against electric and concentration gradients.
- Two mechanisms have been proposed
for absorption of calcium. They are simple diffusion and active transport
process involving energy and the Ca2+pump.
- The factor which increases the
calcium absorption are vitamin D, parathyroid hormone, acidity and amino
acids such as lysine and arginine.
- The factors which decrease the
calcium absorption are phytic acids or hexaphosphates of inositol present
in cereals, oxalates from leafy vegetables, malabsorption syndrome, and
high phosphate content in foodstuff.
Functions:
- Intracellular concentration of
calmodulin modulates the intracellular calcium levels to maintain the
various calcium-dependent reactions. Any excess calcium ions are removed
out of the cell via protein.
- Calcification of bones and teeth
- In addition, it helps in the
following biochemical and physiological activities.
- Blood coagulation. Ionic calcium
helps in the production of thromboplastin and in the conversion of
prothrombin into thrombin. Thus, calcium plays a role in blood
coagulation.
- Action of enzymes. Several enzymes
including lipase, succinic dehydrogenase, adenosine triphosphatase and
certain proteolytic enzymes are activated by calcium.
- Muscle contraction. Calcium ions
neutralize the negative charge of myosin which then combines with action,
to help contraction. Ca2+ ions also activate the myosin ATPase which in
turn hydrolyzes the ATP to supply energy required for contraction.
- Neuromuscular excitability. Calcium
is essential for excitation of nerves.
- Hormone action. Ca2+ serves as an
intracellular secondary messenger of different hormones.
- Membrane permeability. Membrane
permeability generally is increased by calcium. This effect balances the
opposite action of sodium and potassium capillary permeability.
Clinical manifestations:
- Deficiency of calcium leads to
rickets, osteoporosis, and hyperexcitability.
Hypercalcaemia:
- It may occur in the following
conditions:
- Hyperparathyroidism
- Multiple myeloma
- Metastatic carcinoma of bone
- Milk-alkali syndrome
- Treatment with drugs such as
diuretics
- Hypervitaminosis D
Hypocalcaemia:
It is
observed in:
- Tetany
- Hypoparathyroidism
- Fanconi’s syndrome (disorder of
tubular reabsorption)
- Acute pancreatitis
- Vitamin D deficiency
- Chronic renal failure.
Applied aspects:
- If the level of ionic calcium falls,
the nervous system becomes hyperirritable.
This
could lead to tetany. On the other hand, high calcium content depresses nervous
irritability. Thus, the administration of calcium salts is indicated in the
alleviation of tetany.
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Download More Information on Calcium
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