BCH 207

WATER

1
 Water
-Water accounts for about 70% of a cell′s weight, and most
intracellular reactions occur in an aqueous environment.
- Water (H2O) is a chemical compound consisting of two
hydrogen atoms and one oxygen.

H2O
 Water Chemistry
All living organisms are dependent on water.

The structure of water is the basis for its

unique properties.

The most important property of water is the

ability to form hydrogen bonds.
3
Structure & Properties of Water
 STRUCTURE OF WATER
H H

1 molecule of water is
made up of 2 hydrogen atoms
bonded with 1 oxygen atom
+ +
H H

-
 POLAR MOLECULE
The positive hydrogen ends of 1 molecule
are attracted to the negative end of the
oxygen of another molecule.
H
O
H
- A water molecule is a polar molecule with opposite ends of
the molecule with opposite charges.
 POLAR MOLECULE
• A molecule that has electrically charged
areas.
• Also called a polar substance or polar
compound.

+ DURACELL _
 NON-POLAR SUBSTANCES
• Non-polar substances do not have charged
ends
DURACELL
PROPERTIES OF WATER

11
 Properties of Water
1. Water has a high specific heat.
- A large amount of energy is required to
change the temperature of water.

2. Water has a high heat of vaporization.

- The evaporation of water from a surface
causes cooling of that surface.
12
 Properties of Water
3. Solid water is less dense than liquid water.
- Bodies of water freeze from the top down.

4. Water is a good solvent.

- Water dissolves polar molecules and ions.

13
 Properties of Water
5. Water organizes nonpolar molecules.
- hydrophilic: “water-loving”
-hydrophobic: “water-fearing”
- Water causes hydrophobic molecules to
aggregate or assume specific shapes.

6. Water can form ions.

H2O  OH-1 + H+1
hydroxide ion hydrogen ion
14
Ice, water, vapor

15
 COHESION
Water molecules like to stick to
each other.

ADHESION
Water molecules like to stick
to other things.
Adhesion:
is attraction between molecules of different substances.

- The meniscus shown below forms when water adheres to the

sides of the glass container.
-Adhesion Causes Capillary Action, Which gives water the
ability to “climb” structures.
 SPECIFIC HEAT
• Specific Heat = the amount of energy needed to
increase the temperature of something 1 degree
C.

• Water has a really HIGH specific heat

• That means it takes a lot of energy for water to

increase its temperature.

• This is because of the STRONG ATTRACTION

between water molecules.
It’s POLAR!
 Ionization of water:
- Many reactions that occur in nature are reversible and do not
proceed to completion. Instead, they come to equilibrium.
- The position of equilibrium is described by equilibrium
constant, Keq .
- Ionization of water is expressed by an equilibrium constant
 Equilibrium constant (Keq):
it is a constant characteristic for each chemical reaction, that relates the
specific concentrations of all reactants and products at equilibrium at
a given temperature and pressure.
A+B C+D
[C] [D]
Keq =
[A] [B]
- The degree of ionization of water at equilibrium is small.
H2O H+ + OH-

- The reversible reaction of water is important in its properties

and in its effects on cell function.

- Pure water ionizes slightly forming equal numbers of

hydrogen ions (hydronium ions, H3O+) and hydroxyl ions.
- The extent of ionization is described by an equilibrium
constant, Keq .
[H+] [OH-]
Keq =
[H2O]
 Keq [H2O] = [H+] [OH-]
[H2O] = constant
Keq [H2O] = kw
kw = [H+] [OH-]
kw = dissociation or ionization constant for water

● In pure water [H+] = [OH-] = 10-7

kw = 10-7 × 10-7

kw = 10-14

[H+] [OH-] =10-14

WATER AND BODY
FLUIDS

23
 Water & Body Fluids
Functions of water:
1. Transport
2. Structural support for molecules
3. Participates in metabolic reactions
4. Solvent
5. Lubricant & shock absorber
6. Body temperature regulation
7. Maintains blood volume & blood
pressure
Water balance
–Intracellular fluid-within the cells
–Extracellular fluid-outside the cells
•Interstitial fluid
•Plasma

Copyright 2005 Wadsworth Group, a division of Thomson Learning

 One Cell
And Its
Associated
Fluids

Copyright 2005 Wadsworth Group, a division of Thomson Learning

Water Balance
-Intake/output = 2.5 liters/day
-Regulated by hypothalamus & kidneys
-Carefully controlled by:
1. Intake (liquid, foods, metabolic water)
affected by thirst & satiety
2. Output (losses via kidneys, skin, lungs,
feces) affected by blood salts, volume &
blood pressure
How The Body Regulates Blood Volume

Copyright 2005 Wadsworth Group, a division of Thomson Learning

Copyright 2005 Wadsworth Group, a division of Thomson Learning
FLUID & ELECTROLYTE
BALANCE

31
 DEFINITION OF SOME TERMS
Salt – in chemistry this is used to describe a whole family of
compounds such as MgSO4, FeSO4, CaCl2, etc. but colloquially
and in clinical practice it has come to mean NaCl.

Electrolyte – a substance whose components dissociate in aqueous

solution into positively (cation) and negatively (anion) charged
ions. For example, sodium chloride in solution (saline), dissociates
into Na+ and Cl-.

Total body water (TBW) – percentage of body composition

consisting of water, approximately 60% of body weight, less in
obesity and more in infants. It is the sum of the intracellular and
extracellular water in the body.
32
Intracellular fluid (ICF) volume – that part of the TBW contained
within the cells, approximately 40% of body weight and 2/3rds of
TBW. Muscle cells contain 75% water and fat cells have <5% water.

Extracellular fluid (ECF) volume – that portion of the TBW

outside the cells, approximately 20% of body weight and 1/3rd of
TBW..
Interstitial fluid volume – that portion of the ECF outside the
circulation and surrounding the cells.
Intravascular fluid volume
- the total blood volume consisting of red and white cells and
plasma. May be estimated at approximately 6-7% of the body weight.
- the plasma volume is that part of the ECF contained within the
circulation and supported oncotically by the plasma proteins,
separated from the interstitial fluid by the capillary membrane.
Comprises approximately 3-4% of the body weight.
Fluid & Electrolyte Balance

• Electrolytes – salts that dissolve in

water, forming ions
• Dissociation of salt in water
–Ions
•Cations- + charged particles
•Anions- - charged particles
–Electrolyte solution

Copyright 2005 Wadsworth Group, a division of Thomson Learning

-Water also dissolves many simple organic compounds having
carboxyl or amino groups, which tend to ionize by
interaction with water.

• A second class of substances readily dissolved by water

includes many neutral organic compounds with polar
functional groups, such as sugars, alcohols:
- Their solubility is due to the propensity of water molecules
to form hydrogen bonds with the hydroxyl groups of sugars
and alcohols or the carbonyl groups of aldehydes and
ketones.
36
• Electrolytes attract water - affects fluid
distribution in the body by osmosis
• Water follows electrolytes
– Solutes
– Osmosis
– Osmotic pressure
• Electrolyte concentration regulated by
kidneys
• Electrolytes & proteins act as buffers to
maintain the pH of body fluids – acid/base
balance

Copyright 2005 Wadsworth Group, a division of Thomson Learning

Copyright 2005 Wadsworth Group, a division of Thomson Learning
Fluid & Electrolyte Imbalance
• Replacing lost
fluids and
electrolytes
–Oral
rehydration
therapy (ORT)-
a simple
solution of
sugar, salt and
water
Copyright 2005 Wadsworth Group, a division of Thomson Learning
Fluid & Electrolyte Imbalance

• Sodium and chloride most easily

lost
• Different solutes lost by different
routes
• Rapid fluid/electrolyte loss, i.e.
vomiting, diarrhea, heavy
sweating, fever, burns, wounds =
medical emergency
Copyright 2005 Wadsworth Group, a division of Thomson Learning
Dehydration – the term ‘dehydration’ strictly means lack of water
(hypertonic dehydration), yet it is also used colloquially to mean lack of
salt and water (e.g. isotonic dehydration) or even more loosely to
describe intravascular volume depletion.

Salt and water depletion – this is one of the commonest problems in

hospital practice, arising from such
conditions as diarrhoea and vomiting, ketotic and non-ketotic diabetic
decompensation, and diuretic
excess.

Intravascular volume depletion – this signifies a deficit in plasma or

total blood volume, as in burns or
haemorrhage, or a reduction in circulating volume secondary to a
reduction in total ECF due to salt and
water loss.
Salt and water excess – this is most commonly iatrogenic,
resulting from excessive administration of saline, but is, of course,
a feature of congestive heart failure and other oedema producing
conditions. It takes 2-3 litres of salt and water excess before the
extracellular fluid is expanded sufficiently for oedema to become
clinically apparent.

Osmosis – this describes the process by which water moves across

a semi-permeable membrane (permeable to water but not to the
solutes) from a weaker to a stronger solution until the concentration
of solutes are equal on the two sides.
This force is termed osmotic pressure.It is proportional to the
number of atoms/ions/molecules in solution and is expressed as
mOsm/l (osmolarity) or mOsm/kg of solution (osmolality). E.g.
the osmolarity of 0.9% saline is 308 mOsm/l, but the osmolality is
305 mOsm/kg (308 mOsm in 1.009 kg).
SOME RISK FACTORS FOR SALT AND WATER DEPLETION

1. Limited access to fluids from physical and mental disability

2. Cognitive impairment
3. Deliberate limitation of intake due to incontinence
4. Polypharmacy (e.g. diuretics, sedatives, antidepressants, etc.)
5. Lower socioeconomic background
6. Living alone and social deprivation
7. Pre-existing comorbidities
8. Gastrointestinal disease (e.g. diarrhoea, vomiting, stomas)
9. Impaired renal function
10. Impaired thirst sensation
11. Hot weather
SOME RISK FACTORS FOR SALT AND WATER OVERLOAD

1. Congestive heart failure

2. Impaired renal function
3. Liver disease
4. Malnutrition
5. Iatrogenic (e.g. surgery, intravenous fluid therapy)
6. Obesity