Regulation of sodium & water balance;

Isotonic , hypotonic, hypertonic expansion & contraction.

INTRODUCTION

The maintenance of extracellular fluid volume and pH are closely interrelated. Body is composed of about 60-70% water. Distribution of water in different body water compartments depends on the solute content of

each compartment. Osmolality of the intra & extra-cellular fluid is the same, but there is marked difference in the

solute content. Regulation of body fluid volume and osmolality (water and electrolyte balance) is an

integrated function of various organ systems. Kidneys play a major role in regulation of electrolyte and water content in body .

OSMOLALITY OF ECF & ICF

Movement of water across the biological membranes is dependent on the osmotic pressure differences between the intracellular fluid (ICF) and extracellular fluid (ECF).

In normal state, the osmotic pressure of ECF is mainly due to Na+ ions, is equal to osmotic pressure of ICF which is due to K+ ions.

Regulation of SODIUM & WATER balance

The major regulatory factors are the hormones :- aldosterone, ADH and the renin angiotensin system.

Aldosterone secreted by the zona glomerulosa of the adrenal cortex regulates the Na+ → K+ exchange and Na+ → H+ exchange at the renal tubules.

The net effect is the sodium retention. Anti-Diuretic Hormone (ADH) When osmolality of the plasma rises, the osmoreceptors

of hypothalamus are stimulated, resulting in ADH secretion. ADH will increase the water reabsorption by the renal tubules. Therefore proportionate amounts of sodium and water are retained to maintain the osmolality. When osmolality decreases, ADH secretion is inhibited. When ECF volume expands, the aldosterone secretion is cut off.

Renin-Angiotensin System When there is a fall in ECF volume, renal plasma flow decreases and this would result in the release of renin by the juxtaglomerular cells.

The factors which stimulate renin release are: a. Decreased blood pressure b. Salt depletion c. Prostaglandins The inhibitors of renin release are: a. Increased blood pressure b. Salt intake c. Prostaglandin inhibitors d. Angiotensin-II. #Renin is the enzyme acting on the angiotensinogen (an alpha-2 globulin, made in liver)

Various mechanisms operate to maintain a normal electrolyte balance in body fluids. These include:-

1. Sodium – Potassium pump which maintains low Na+ and high K+, INTRACELLULARLY.2. Thirst may regulate electrolyte intake to some extent.3. Environmental conditions such as – excessive heat & vigorous exercise can increase the lose

of electrolytes.4. Kidneys play an important role in maintenance of electrolyte balance.

Assessment of Sodium and Water Balance

Maintenance of intake-output chart, in cases of patients on IV fluids. The insensible loss of water is high in febrile patients.

Measurement of serum electrolytes (sodium, potassium, chloride and bicarbonate). This will give an idea of the excess, depletion or redistribution.

Measurement of hematocrit value to see if there is hemoconcentration or dilution.

Measurement of urinary excretion of electrolytes, especially sodium and chloride

Salient Features of Electrolyte Imbalance

Hypo-osmolality and hypo-natremia go hand in hand.

Hypo-osmolality causes swelling of cells and hyper-osmolality causes cell dehydration.

Hypo-natremia of ECF causes symptoms which are associated with hyper-kalemia.

Dilutional hyponatremia due to glucose or mannitol increases the effects of hyper osmolality.

Fatigue and muscle cramps are the common symptoms of electrolyte depletion.

Hypo-osmolality of gastrointestinal cells cause nausea, vomiting and paralytic ileus.

DISTURBANCES IN FLUID AND ELECTROLYTE BALANCE

Abnormalities in fluid and electrolyte balance can be expressed in terms of TONICITY. When the effective osmolality is increased, the body fluid is called HYPERTONIC. When osmolality is decreased the body fluid is called HYPOTONIC.

1. Isotonic Contraction of ECF

Results from the loss of fluid that is isotonic with plasma. The most common cause is loss of gastrointestinal fluid , due to: a. Small intestinal fistulae b. Small intestinal obstruction and paralytic ileus where fluid accumulates in the lumen c. Recovery phase of renal failure. Equivalent amounts of sodium and water are lost, the plasma sodium is often normal. Hemoconcentration is seen. Hypovolemia will reduce renal blood flow and may cause

renal circulatory insufficiency, oliguria and uremia. Renin-aldosterone system is activated, and selective sodium reabsorption occurs. ADH secretion leads to reabsorption of equivalent amounts of water.

2. Isotonic Expansion

Water and sodium retention is often manifested as edema and occurs secondary to hypertension or cardiac failure.

Hemodilution is the characteristic finding. Secondary hyper-aldosteronism may result from any cause leading to a reduced plasma volume in spite of a high ECF volume

This often results from hypoalbuminemia (edema in nephrotic syndrome, protein, malnutrition, etc.).

The water retention causes ADH secretion.

3. Hypotonic Contraction

There is predominant sodium depletion. The causes are :-A. Infusion of fluids with low sodium content like dextrose . When low sodium containing fluids are infused, the hypo-osmolality will inhibit ADH secretion resulting in water loss. Since only the excess fluid is lost, the plasma sodium tends to return to normal. Thus, osmolality is restored, but at the expense of the volume. B. Deficiency of aldosterone in Addison's disease. The decreased sodium retention lowers the osmolality and inhibits ADH secretion, resulting in contraction of ECF volume. The hypovolemia stimulates ADH secretion, causing further hemodilution and hyponatremia.

4. Hypotonic Expansion

Predominant water excess results only when the normal homeostatic mechanisms fail. There is water retention either due to glomerular dysfunction or ADH excess. The water excess will lower the osmolality. Hyponatremia persists due to the inhibition of aldosterone secretion by the expanded

ECF volume. Inhibition of ADH secretion and excretion of large volumes of dilute urine can improve

the situation. Cellular overhydration can result in unconsciousness or death.

5. Hypertonic Contraction

It is predominantly water depletion :- a. The commonest cause is diarrhea, where the fluid lost has only half of the sodium concentration of the plasma. b. Vomiting and excessive sweating can also cause a similar situation. c. Diabetes insipidus is a very rare cause. d. Hypernatremia is present with a high plasma osmolality. But the volume depletion will reduce renal blood flow and stimulates aldosterone secretion leading to further sodium retention and causing hypertension. e. The increase in osmolality will stimulate thirst and increase in the water intake. ADH secretion occurs and urine volume decreases.

6. Hypertonic Expansion

It can occur in cases of Conn's syndrome and Cushing's syndrome. The excess mineralocorticoid would produce sodium retention. Resultant increase in the plasma osmolality is expected to increase the ADH

secretion, and will restore the osmolality. Continued effect of aldosterone will cause sodium retention. There is associated hypokalemia which often leads to metabolic alkalosis. Extracellular hyper-tonicity may lead to brain cell dehydration , leading to coma

and death.