Page 110 - Textbook of Pathology, 6th Edition

94 In the intracellular fluid, the main cations are potassium potassium and phosphate are high in the intracellular fluid
and magnesium and the main anions are phosphates and whereas concentration of sodium and chloride are high in
proteins. It has low concentration of sodium and chloride. the ECF. The osmotic equilibrium between the two major
body fluid compartments is maintained by the passage of
In the extracellular fluid, the predominant cation is sodium
and the principal anions are chloride and bicarbonate. Besides water from or into the intracellular compartment. The 2 main
these, a small proportion of non-diffusible proteins and some subdivisions of ECF—blood plasma and interstitial fluid, are
diffusible nutrients and metabolites such as glucose and urea separated from each other by capillary wall which is freely
permeable to water but does not allow free passage of macro-
are present in the ECF. molecules of plasma proteins resulting in higher protein
The essential difference between the two main sub- content in the plasma.
SECTION I
divisions of ECF is the higher protein content in the plasma
than in the interstitial fluid which plays an important role in ACID-BASE BALANCE
maintaining fluid balance.
Besides changes in the volume of fluids in the compartments,
The major functions of electrolytes are as follows: changes in ionic equilibrium affecting the acid-base balance
i) Electrolytes are the main solutes in the body fluids for of fluids occur. In terms of body fluids,
maintenance of acid-base equilibrium. an acid is a molecule or ion which is capable of giving off
+
ii) Electrolytes maintain the proper osmolality and volume a hydrogen ion (H ion donor); and
a base is a molecule or ion which is capable of taking up
of body fluids (Osmolality is the solute concentration per kg hydrogen ion (H ion acceptor).
+
water, compared from osmolarity which is the solute A number of acids such as carbonic, phosphoric, sulfuric,
concentration per litre solution). lactic, hydrochloric and ketoacids are formed during normal
iii) The concentration of certain electrolytes determines their metabolic activity. However, carbonic acid is produced in
specific physiologic functions e.g. the effect of calcium ions largest amount as it is the end-product of aerobic tissue
on neuromuscular excitability. The concentration of the major activity. In spite of these acids, the pH of the blood is kept
electrolytes is expressed in milliequivalent (mEq) per litre constant at 7.4 + 0.05 in health.
so as to compare the values directly with each other. In order The pH of blood and acid-base balance are regulated in
to convert mg per dl into mEq per litre the following formula the body as follows.
is used:
1. BUFFER SYSTEM. Buffers are substances which have
mg/dl
+
_____________________ weak acids and strong bases and limit the change in H ion
mEq/L = × 10
Eq weight of element concentration to the normal range. They are the first line of
General Pathology and Basic Techniques
defense for maintaining acid-base balance and do so by
+
taking up H ions when the pH rises. The most important
NORMAL WATER AND ELECTROLYTE BALANCE
(GIBBS-DONNAN EQUILIBRIUM) buffer which regulates the pH of blood is bicarbonate-carbonic
acid system followed by intracellular buffering action of
Normally, a state of balance exists between the amount of haemoglobin and carbonic anhydrase in the red cells.
water absorbed into the body and that which is eliminated 2. PULMONARY MECHANISM. During respiration, CO
from the body. The water as well as electrolytes are is removed by the lungs depending upon the partial pressure 2
distributed nearly constantly in different body fluid of CO in the arterial blood. With ingestion of high quantity
2
compartments: of acid-forming salts, ventilation is increased as seen in
1. Water is normally absorbed into the body from the bowel acidosis in diabetic ketosis and uraemia.
or is introduced parenterally; average intake being 2800 ml 3. RENAL MECHANISM. The other route by which H +
per day. ions can be excreted from the body is in the urine. Here, H +
2. Water is eliminated from the body via: ions secreted by the renal tubular cells are buffered in the
kidneys in the urine (average 1500 ml per day); glomerular filtrate by:
via the skin as insensible loss in perspiration or as sweat combining with phosphates to form phosphoric acid;
(average 800 ml per day), though there is wide variation in combining with ammonia to form ammonium ions; and
loss via sweat depending upon weather, temperature, fever combining with filtered bicarbonate ions to form carbonic
and exercise; acid.
However, carbonic acid formed is dissociated to form CO
via the lungs in exhaled air (average 400 ml per day); which diffuses back into the blood to reform bicarbonate ions. 2
and
minor losses via the faeces (average 100 ml per day) and PRESSURE GRADIENTS AND FLUID EXCHANGES
lacrimal, nasal, oral, sexual and mammary (milk) secretions. Aside from water and electrolytes or crystalloids, both of
The cell wall as well as capillary endothelium are enti- which are freely interchanged between the interstitial fluid
rely permeable to water but they differ in their permeability and plasma, the ECF contains colloids (i.e. proteins) which
to electrolytes. Capillary wall is completely permeable to minimally cross the capillary wall. These substances exert
electrolytes while the cell membrane is somewhat pressures responsible for exchange between the interstitial
impermeable. As mentioned earlier, concentration of fluid and plasma.