Monday, July 17, 2017

Water Metabolism


•Powerful solvent for ionic & neutral molecules.
•Dissociation of macromolecules of the cell along with dilute salt solutions.
•Influence on the structural & functional     components of the cell.
•Cooling of the body by evaporation of moisture in the lungs &from the skin.

•Most abundant compound in the body (65-95% of body weight)

Disorders of water and sodium homeostasis are very common problems encountered in clinical medicine. Disorders of water metabolism are divided into hyperosmolar and hypoosmolar states, with hyperosmolar disorders characterized by a deficit of body water in relation to body solute and hypoosmolar disorders characterized by an excess of body water in relation to total body solute. This article briefly reviews the physiology of hyperosmolar and hypoosmolar syndromes, then focuses on a discussion of the pathophysiology, evaluation, and treatment of specific pre- and postoperative disorders of water metabolism in patients with pituitary lesions.

Normal Physiology of Hypothalamic Pituitary Regulation.
The anterior pituitary is a complex heterogeneous gland that exerts a central role in the integration of several regulatory systems. Its six key hormones affect peripheral glands or target tissues and are essential for reproduction, growth and development, metabolism, adaptation to external environmental changes, and stress. Each of the pituitary hormones is regulated by the central nervous system through neuroendocrine pathways involving the hypothalamus, by feedback effects from peripheral target gland hormones, and by intrapituitary mechanisms. The hormones are secreted in a pulsatile manner, which is distinct for each hormone and reflects the influence of its individual neuroendocrine control mechanisms.
Water and salt metabolism.

Author(s) : STANDARD, S.
Author Affiliation : New York.
Journal article : Surg. Qynecol. Obstet. Internat. Abst. Surg. 1938 Vol.67 pp.301-320
Abstract : The literature on water and salt metabolism is reviewed. lesions of the gastro-intestinal tract are classified into pylorio obstruction producing dehydration and alkalosis, duodenal fistula producing dehydration and acidosis, and intestinal obstruction or fistula producing varying degrees of dehydration and, hypochloraemia without disturbance of acid base balance. The methods available for replacement of water and chloride are detailed and indications for their use discussed. Parenteral administration of fluids and, salt has reduced the mortality rate considerably in intestinal obstruction. A. Lyall

Chapter 1

Sodium and Water Metabolism.

Disorders of salt and water homeostasis result in a
variety of clinical syndromes such as dehydration,
oedema, hyponatraemia and hypernatraemia.
Patients with these disorders require careful
clinical evaluation prior to appropriate therapy, and
this evaluation will not be satisfactory unless the
clinician has an adequate grasp of the
physiological principles involved.

Furthermore it should be emphasised that in these
subjects the most important investigation is the
clinical examination, particularly the history and
the evaluation of the patient's hydration status.
Diagnosis is often made on clinical grounds and
laboratory tests should be used to confirm a
clinical impression and to uncover specific
abnormalities such as hypernatraemia, renal
failure and the like.

A number of definitions, which may be new to the
reader, are used throughout the chapter and are
appropriately dealt with in the body of the text. Two
terms which recur are osmolality and tonicity which
require some explanation.


The osmolality of a fluid is a measure of the total
number of particles (ions, molecules) present in
solution. It differs from the closely related term,
osmolarity, in that it is expressed in millimoles per
kilogram (concentrations per mass) of solvent
(water in the case of plasma) whilst osmolarity is
expressed in millimoles per litre (concentrations
per volume) of solution. Both osmolality and
osmolarity are units of measurement of osmotic
effects; however, osmolality is a thermo-
dynamically more precise expression than
osmolarity because:

• Solution concentrations expressed on a weight
basis are temperature independent, while those
based on volume (e.g., osmolarity) will vary with
temperature in a manner dependent on the thermal
expansion of the solution.

• It is the osmolal, not the osmolar, concentration
which exerts an effect across the cell membrane
and which is controlled by homeostatic mechanisms.

This is because of the volume occupied by
the solid phase of plasma (made up mainly of its
protein and lipid content) which in normal plasma
is about 7% of the plasma volume. Dissolved
particles are confined to the aqueous phase of

A one osmolal solution is defined to contain 1
osmole/kg water; however, the term milliosmole/kg
(mOsm/kg) which should have been used for the
relatively low osmolalities of physiological fluids is
not an SI unit and mmol/kg is recommended to be
used in its stead. The plasma osmolality is normally
295±5 mmol/kg.

As we shall see later, the distribution of water
across biological membranes separating different
compartments depends on the concentration
difference of particles between the two
compartments. On a per unit volume basis there
will be a larger number of small molecular weight
particles which will exert a greater osmotic effect,
as compared to bigger particles.

Thus the major contributors to plasma osmolality
are  Na+, K+ and their associated anions
(mainly CI" and HCCy), urea and glucose.
Plasma proteins, because of their massive sizes,
exert a relatively insignificant osmotic effect,
individually or as a group. In practice, plasma
osmolality can either be measured or calculated
(see below).

Measured plasma osmolality. This is obtained
using osmometers which measure colligative
properties such as freezing point depression or
vapour pressure. It gives a measure of the total
osmolality of the solution — the sum of the
osmotic effects exerted by all the ions and
molecules present in the solution across a
membrane which, unlike biological ones, is
permeable to water.

Calculated plasma osmolarity. As sodium is the
major cation of the extracellular fluid (interstitial fluid
or plasma) its osmolarity can be roughly estimated
from the following equation:

Calculated ECF osmolarity (mmol/L) =

2 x [Na+]* + [urea]* + [glucose]*

* (plasma analyte values expressed in mmol/L)

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