Fluid management in clinical practice part 2

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 Fluid in clinical practice

Fluid:: part 2


    Discussion on physiology and function of body fluid::

   Maintenance of body fluid is a neurohormonal regulation.

      Hormonal components---

Response to tonicity: ADH, aldosterone.

      Response to pressure: ANP,BNP

    Function of these hormone,

    ADH:: when osmolarity increases, ADH is secreted from posterior pituitary, causing vasoconstriction and free water retention from distal tubule.

   Aldosterone:: when osmolarity decreases, secreted from adrenal gland, causes Na retention from collecting tubule.

    ANP &BNP :: secreted respectively from atrium and ventricle in response to stress or in other word volume expansion, causing excretion of Na from tubule and decreased absorption of Na, thus reduce blood volume.

    Response to fluid loss :: 

when blood volume decreases , sympathetic nervous system is activated due to decreased tone in vascular wall. It causes vasoconstriction, increased heart rate, activation of renin- angiotension- aldosterone system thus causing vasoconstriction, Na and water retention. Simultaneously, ADH secretion may increase, ANP& BNP secretion decreased. In this way body tries to maintain arterial filling pressure.

  

    Effect of these events::

about 84 percent of the entire blood volume of the body is in the systemic circulation and 16 percent is in the heart and lungs. Of the 84 percent in the systemic circulation, approximately 64 percent is in the veins, 13 percent is in the arteries, and 7 percent is in the systemic arterioles and capillaries. The heart contains 7 percent of the blood, and the pulmonary vessels, 9 percent. 

From this, we can see body has a reserve volume to combat hypovolemia, when it fails signs of hypovolemia develop. It's complicated neurohormonal effect may persist 4- 5 hours even after volume correction. That is vasoconstriction persist for 4 hours and during this time fluid should be infused cautiously. For this reason, we can't give bolus infusion of total amount of lost fluid at a time. If we give large amount of infusion in this mean time, then signs of hypervolemia may develop rapidly.


   Some important things::

About 80 percent of the total osmolarity of the interstitial fluid and plasma is due to sodium and chloride ions, whereas for intracellular fluid, almost half the osmolarity is due to potassium ions and the remainder is divided among many other intracellular substances.

The interstitial and plasma fluid have same ionic components, but presence of plasma protein makes plasma a special substance. This protein exerts about 20mmHg pressure difference between them and responsible for the maintenance of plasma volume.1 milliosmole osmolity difference creates about 20mmHg pressure gradient. 

Source: Guyton and Hall textbook of medical physiology,13Ed


By: Dr. Manilal Biswas

SSMC (2012-13)

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