Download Water and Mineral Metabolism

Water and Mineral Metabolism
Minerals are inorganic substances . Although they constitute
a relatively small amount of the total body tissues , they are
essential to many vital processes . Unlike vitamins , minerals
are not destroyed in food preparation . But as they are
soluble in water , some amount will be lost when cooking
liquids are discarded . Unlike carbohydrates , fats and
proteins , minerals do not provide energy . Seven principal
(macro) minerals required by the body are sodium m
potassium , calcium , magnesium , phosphorus , sulphur and
chlorine . These constitute 60-80 per cent of all the inorganic
material in the body . At least 7 other minerals are utilized
in traces : ( microminerals ) iron , copper , iodine ,
manganese , cobalt , zinc and molybdenum . Minerals
required for the body can be divided into 2 main groups –
macrominerals which are required in amounts more than
100 mg per day and microminerals ( trace elements )
required less than 100 mg per day . Basic fuctions
performed by the minerals are :
1. As structure components of body tissues
2. In the acid base balance
3. In the transport of gases
4. In muscle contractions
SODIUM
Sodium is the principal cation of the extracellular fluid . It is
usually associated with chloride and bicarbonate in
regulation of acid base equilibrium . Other functions are in
the preservation of normal irritability of muscles ,
permeability of cells and in the maintenance of osmotic
pressure of body fluid , thus protecting the body against
excessive fluid loss . Daily requirement is 1-3.5 gm . About
95 % of the sodium which leaves the body is excreted in the
urine as sodium chloride .Sodium is readily absorbed so that
faeces contains very little except in diarrhea . Sodium is
present in the blood mainly in the serum . Normal serum
sodium level is 133-146m Eq / L.
Hyponatremia
Low levels of sodium in serum is called hyponatremia . It
occurs in the following condition :
1. Excessive vomiting and diarrhoea resulting in excessive
loss of fluid from gastrointestinal tract .
2. Severe burns
3. Acute Addison's disease
4. Chronic renal disease with acidosis
5. In active sweating and when take plain water .
Hypematremia
It is the increased level of serum sodium . This is not very
common but may occur due to :
1. Hyperactivity of adrenal cortex as in Cushing's disease
2. Prolonged administration of cortisone , ACTH and sex
hormones .
POTASSIUM
Potassium is the principal cation of the intracellular fluid . It
is also a very important constituent of the extracellular fluid
and influences muscles activity . Potassium ions are essential
for contraction of cardiac and skeletal muscles . During
muscular contraction , potassium from the muscle cells are
lost into the ECF and after muscle contraction they are
returned to the muscle cells . Potassium is essential for the
activity of nerve cells and is present in the nerve fibres and
maintains the proper transmission of nerve impulses . It
maintains the electric potential of the cells as also their size .
It influences the acid base balance and is also related to
carbohydrate metabolism . The normal concentration of
potassium in the serum is 3.5 – 5.5 m Eq / L . Vegtables are
rich sources of potassium . The recommended amount of
potassium for adults 4 gm / day
Hyperkalaemia
This is due to increased serum potassium and occurs in the
following condition :
1. Renal failure
2. Advanced dehydration
3. Addison's disease
4. Shock
5. Intravenous administration of excess amount of
potassium.
This is due to low levels of serum potassium and occurs in
case of :
1. Diarrhoea and vomiting
2. Metabolic alkalosis
3. Familial periodic paralysis
4. Overactivity of adrenal cortex ( Cushing's syndrome )
5. Prolonged administration of diuretics because of excretion
of potassium in the urine
6. During heart failure .
CALCIUM
Calcium forms the largest amount of minerals present in the
body . Calcium accounts for 2 per cent of the body weight
and is almost entirely distributed in the bones and teeth .
Requirements
Men and women after 18 years of age = 800 mg daily
During second and third trimesters
= 1.2 -1.3g daily
of pregnancy
Infants under 1 year
= 400- 600 mg daily
Children 1 to 18 years
= 0.7- 1.4 g daily
Sources
Dietary sources of calcium include milk , cheese , egg yolk ,
beans , lentils , nuts , figs , cabbage , turnip , green ,
cauliflower and asparagus .
Blood Calcium
Calcium is present exclusively in the plasma , to the extent of
9 to 11 mg % . The calcium content of plasma during
infancy and early childhood is slightly higher . In the plasma
, calcium exists in two physiologically distinct fractions :
1. Diffusible or ionized fraction
2. Non- diffusible or unionized fraction .
A small fraction of about 0.5 mg% is present bound as an
unionized complex ion with citrate.
Absorption of Calcium
1.On a high protein diet 15 per cent of dietary calcium is
absorbed. On a low protein diet, 50 per cent of the dietary
calcium is absorbed. Phytic acid in cereal grains interferes
with calcium absorption by forming insoluble calcium
phytate in the intestine. So also the oxalates in food.
2.Calcium salts are more soluble in acidic media than in the
alkaline media.
3.When fat absorption is not proper, much free fatty acids is
present. These free fatty acids react, with free calcium to
form insoluble calcium soaps.
4.If the Ca: P ratio is high, much Ca3(PO4)2 will be formed
and absorption diminished.
5.Vitamin D promotes the absorption of calcium from the
intestine.
Functions
1.Calcium along with phosphorus is essential for bones and
teeth formation.
2.For coagulation of blood. Calcium activates the conversion
of prothrombin to thrombin.
3.For clotting and digestion of milk.
4.In muscle contraction.
5.In the normal transmission of nerve impulses. A low
calcium level is associated with increase neuromuscular
irritability.
6.Calcium ions are necessary for the maintenances and
regulation of acid-base balance and water balance in the
body.
7.Calcium and phosphate ions activate certain enzymes in
the intermediary metabolism of carbohydrates, fat and
proteins.
Diseases Associated with Calcium Metabolism
Rickets Deficiency of vitamin D in children causes
impairment of intestinal absorption of calcium and
phosphorus and produces rickets which is characterised by
defective calcification. The bones therefore remain soft and
deformed by body weight.
Osteoporosis It is a condition when calcium is withdrawn
from the bones. The bones become weak and porus and
hence break.
Excretion Calcium is excreted in the faeces, urine and sweat.
About 200 mg of calcium, i.e about ¼ of to total excretion is
excreted in the urine in 24 hours.
PHOSPHORUS
Phosphorus is present in every cell of the body. As inorganic
combination with calcium Ca3(Po4)2, 80 per cent of the total
is present in the bones and teeth. As organic phosphorus, it
is present along with proteins as phosphoproteins and
nucleoproteins. It is present along with carbohydrates as
hexose and triose phosphates and along with lipids as
phospholipids.
Distribution
The total amount of phosphorus in the body is about 700
gm.
More than 85 per cent is found in bones, 15 per cent in soft
tissues and 1 per cent in ECF, about 5 gm in brain and 2 gm
in blood. In blood, phosphorus is present in the erythrocytes
and plasma in two forms: (i) inorganic phosphorus and (ii)
Organic phosphorus.
Food Sources
Food rich in phosphorus are cheese, milk, nuts, organ meat,
egg, fish, cereals and vegetables. About 1.5 gm of phosphate
is required to be taken in the diet.
Functions
1.Phosphorus along with calcium is essential for bones and
teeth.
2.For the formation of high energy bond compounds like
ATP, ADP and creatinine phosphate. These are required for
phos-phorylation of sugars in carbohydrate metabolism.
3.As a constituent of phosphate buffers in the blood and
urine for acid-base balance.
4.For the formation of phospholipids, co-enzymes and
enzymes of intermediary metabolism.
5.Constituent of phospholipids, nucleotides, nucleic acids,
lipo-proteins and phosphoproteins.
Clinical Importance
Rickets and osteomalacia are important dietary deficiency
disordes of calcium, phosphorus or Vitamin D.
WATER
Water forms 60 to 70 per cent of the body weight and is
distributed throughout the body, being closely associated
with the distribution of electrolytes in the body. It is present
in the body both inside and outside the cells. There are two
water compartments in the body:
1.Intracellular water (water present inside the cell)
2.Extracellular water(water present outside the cell).
Extracellular water is further subdivided into:
a. Plasma
b. Interstitial fluid
c. Dense connective tissue, i.e water content in the bones
and cartilages
d. Transcellular fluids.
Functions
1. It acts as a carrier of nutritive elements to tissues and
removes water materials from tissues.
2. It provides the media in which chemical reactions of
the body take place.
3. The fluidity of blood is because of water.
4. It is a solvent for electrolytes and regulates the
electrolytic balance of the body. It maintains the
equilibrium of osmotic pressure extended by the solutes
dissolved in water.
5. It is a regulator of body temperature. Because of its
high specific heat, it can absorb or give off heat without
any appreciable change in temperature. Also because of
its high latent heat, it provides the mechanism for the
regulation of heat loss by sensi-ble or insensible
perspiration.
Water Balance
Water balance is maintained by a body when water
gained by the body is equal to the water lost from the
body.
Water intake in gm per day
Water intake as such = 1100
Water intake in diet = 900
Water produced during
metabolism = 200
Total intake 2200
Water output in gm per day
Water excreted in urine = 1000
Water excreted in stools = 200
Water lost through skin and
lungs = 1000
Total output
2200
Dehydration
Dehydration occurs when the fluid loss from the body is
abnormally high, e.g. excessive perspiration in hot
weather, severe diarrhoea, fever, vomiting, severe burns
and in uncontrolled diabetes with frequent urination.
In dehydration, besides changes in water balance, there
will be also changes in electrolytes. Dehydration is
corrected by electrolytes and water.
Distribution of Electrolytes
The body has two main fluid compartments of different
electrolyte composition, i.e. the intracellular compartment
where potassium (K+) is the predominant cation, and the
extracellular compartment where sodium (Na+) is the
predominant cation. The extracelluar fluid (ECF) can be
subdivided into interstitial fluid where the protein concentration is very low and plasma (intravascular fluid)
rich in protein.
Distribution of electrolytes between plasma and
interstitial fluid is given below. Electrolyte composition of
ECF expressed in mEq wt/L.
Cations
Na+
K+
Ca+
Mg++
Anions
HCO4
CI
HPO4
SO4
Organic acids
Proteins
Intravascular Interstitial
(plasma)
fluid
142
138
5
5
5
5
3
3
155
151
27
103
2
1
6
16
155
32
108
2
1
6
2
151
Intracellular
fluid
10
158
2
35
205
8
2
140
____
____
55
205
Thus the predominant cation of ECF is Na+. The high
concentration of Na+ in the plasma and the relative
impermeability of Na+ ions across the cell wall membrane
are important factors in controlling the distribution of
water throughout the body and maintaining fluid balance.
The concentration of Na+ in the plasma is controlled by
the nerve receptors in the hypothalamus . An increase in
the serum Na+ concentration will stimulate retention of
water, thus diluting the serum balance to its normal level .
A drop in the concentration of Na+ in the serum will
initiate a loss of fluid from the body so plasma control the
retention or loss of water from the body .