Page 109 - Textbook of Pathology, 6th Edition

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Derangements of Homeostasis
Chapter 5
Chapter 5
and Haemodynamics CHAPTER 5

HOMEOSTASIS

Many workers have pointed out that life on earth probably
arose in the sea, and that the body water which is the
environment of the cells, consisting of “salt water” is similar
to the ancient ocean. The sea within us flows through blood
and lymph vessels, bathes the cells as well as lies within the
cells. However, water within us contains several salts that
includes sodium, chloride, potassium, calcium, magnesium,
phosphate, and other electrolytes. Although it appears quite
tempting to draw comparison between environment of the
cell and the ancient oceans, it would be rather an
oversimplification in considering the cellular environment
to be wholly fluid ignoring the presence of cells, fibres and
ground substance. Derangements of Homeostasis and Haemodynamics
Claude Bernarde (1949) first coined the term internal
environment or milieu interieur for the state in the body in
which the interstitial fluid that bathes the cells and the
plasma, together maintain the normal morphology and
function of the cells and tissues of the body. The mechanism
by which the constancy of the internal environment is Figure 5.2 Body fluid compartments (ICF = intracellular fluid
compartment; ECF = extracellular fluid compartment).
maintained and ensured is called the homeostasis. For this
purpose, living membranes with varying permeabilities such (assuming average of 60%) is distributed into 2 main
as vascular endothelium and the cell wall play important compartments of body fluids separated from each other by
role in exchange of fluids, electrolytes, nutrients and membranes freely permeable to water. These are as under
metabolites across the compartments of body fluids. (Fig. 5.2):
The normal composition of internal environment consists
of the following components (Fig. 5.1): i) Intracellular fluid compartment. This comprises about
33% of the body weight, the bulk of which is contained in
1. WATER. Water is the principal and essential constituent the muscles.
of the body. The total body water in a normal adult male ii) Extracellular fluid compartment. This constitutes the
comprises 50-70% (average 60%) of the body weight and remaining 27% of body weight containing water. Included
about 10% less in a normal adult female (average 50%). Thus, in this are the following 4 subdivisions of extracellular fluid
the body of a normal male weighing 65 kg contains (ECF):
approximately 40 litres of water. The total body water
a) Interstitial fluid including lymph fluid constitutes the major
proportion of ECF (12% of body weight).
b) Intravascular fluid or blood plasma comprises about 5% of
the body weight. Thus plasma content is about 3 litres of
fluid out of 5 litres of total blood volume.
c) Mesenchymal tissues such as dense connective tissue,
cartilage and bone contain body water that comprises about
9% of the body weight.
d) Transcellular fluid constitutes 1% of body weight. This is
the fluid contained in the secretions of secretory cells of the
body e.g. skin, salivary glands, mucous membranes of
alimentary and respiratory tracts, pancreas, liver and biliary
tract, kidneys, gonads, thyroid, lacrimal gland and CSF.

2. ELECTROLYTES. The concentration of cations
(positively charged) and anions (negatively charged) is
Figure 5.1 Distribution of body fluid compartments. different in intracellular and extracellular fluids: