The cell, which is the fundamental biological unit, requires a
defined composition to function normally.
Concept of the internal environment
Homeostasis = maintenance of stable internal environment
Cells live in a liquid inter
Exchange of water between interstitial fluid and plasma
The plasma and interstitial components of the ECF are
separated by the capillary walls which are freely permeable to
both water and solutes, but only slightly permeable to
5. Changes in ECF volume are important because they
are accompanied by proportional changes in blood /
plasma volume, which in turn affects the adequacy
with which the circulatory system can perfuse vital
organs with blood (effective circulatory volume, E
Important concepts to remember:
1.Particles restricted to a compartment determine its
volume (Na+ in ECF and K+ in ICF).
2.Water cross cell membrane until the osmolality
(particle/H2O ratio) is equal on both sides of that
Case 2: Ingestion of pure NaCl (290 mmoles)
Final osmolality = New total body osmoles / TBW
(12180 + 580)/ 42 = 304 mOsm
Final ICF = ICF osmoles / New osmolality = 8120 / 304 = 26.7 L
Final ECF = TBW Final ICF = 42 26.7 = 15.3 L
Fluid exchange between different compartments
Case 1: Ingestion of 2 L of pure water
Initial TBW = 42 L ; ICF = 28 L; ECF = 14 L
Initial total body osmoles = 42 L x 290 mOsm = 12180 mosmoles
Initial ICF osmoles = 28 L x 290 mOsm = 8120
The osmolarity of the plasma (Posm) is equal to the sum
of the osmolarities of the individual solutes in the
All expressed as mmol/L
2 x plasma [Na+] + plasma [glucose] + plasma [urea]
Since urea is an ineffective osmole,
2 x p
Isotonic solutions have the same effective osmolality as
body fluids (285 290 mOsm).
Hypotonic solutions have a lower effective osmolality
than body fluids.
Hypertonic solutions have an effective osmolality greater
than that of body fluids.
If, on the other hand, the solute particles are impermeable to
the membrane they will exert an osmotic effect.
Effective osmolality high
From R4 P. 20
Water distribution depends on the number of particles
restricted to the ICF or to the ECF. These particl
Osmolality is the major factor that determines the ratio
of ICF and ECF.
If two compartments are
separated by a membrane
with water on either side
and a solute is put into one
compartment, it will only
exert an osmotic effect if
the particles stay on one
The number of particles in a solution determines the
osmotic activity of that solution and can be expressed in
terms of osmoles (osmole) or milliosmoles (mosmole).
The concentration of particles in a solution can be
expressed either as osmolarity or as
Exchange of water between ICF and ECF
Movement of water in and out of cells is driven by
osmotic gradient ONLY.
Osmosis refers to the movement of water across a
membrane from a solution of lower concentration to a
solution of higher concentration. The m
Distribution of water between ECF and ICF:
2/3 (40% of body weight) inside cells
vol. of ICF = 70 kg x 40% = 28 L
1/3 (20% of body weight) outside cells
vol. of ECF = 70 kg x 20% = 14 L
Total Body Water
From R4 P. 27
Variation between body water content in infant, male and
From R4 P. 26
The elderly have less body fluid than their younger
counterparts. With aging, there is a decrease in lean body
mass in favor of fat.
A fat man contains
proportionately less water
than does a thin man.
By and large, women
contain less water in
proportion to their body
weight than do men.
From R4 P. 25
Women are made of water!
From R4 P. 26
Body water content:
Total body water = Body weight x 60%
e.g. Body weight = 70 kg
Body water content = 70 x 60% = 42 L
Different cell types have different water content:
Therefore, different body composition
Kidney is the ONLY exit route that can be closely regulated.
Variation in urinary
excretion is the major
means by which the
body maintains salt and
From R4 P. 5
INPUT = OUTPUT
Oral intake = Urinary excretion + Non renal loss
1. How to revert enzyme inhibition?
2. Could dilution help to differentiate competitive and
non competitive inhibition?
3. Why intake of ethanol is recommended for treating
4. How irreversible inhibition is related to
Students should be able to:
list the various ways by which enzyme activity is
discuss the nature and applications of enzyme
describe the classification of enzymes.
explain how enzymes are systematically named.
Aims of Lecture:
1. Explain how enzyme activity is regulated.
2. Describe the potential therapeutic applications
of enzyme inhibitors.
3. Discuss the significance of enzyme activities in
4. Explain the EC enzyme naming system.