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Body Fluids
Objectives
• Outline the functions of water in the body.
• State how water content varies with age and sex.
• Differentiate between intracellular and extra-cellular
fluid.
• Explain how water moves between fluid
compartments.
• List the major sources of body water gains and losses.
• State the function and stimuli of thirst.
• Briefly explain how ADH regulates urine concentration.
• Briefly describe the role of aldosterone in fluid
balance.
Functions of water
Water is the solvent used in the
body:
to transport nutrients to cells
to remove waste products from
cells
to act as a transport vehicle for
blood gases
O
2
Functions of water
Water is also necessary ..
* for body structure (e.g.
skin turgor)
* in temperature
regulation
* as a medium for
chemical reactions
* as a lubricant
* as a cushion and
shock absorber
Body water and age and sex
The volume of total body water in a healthy adult male is
approximately 45 L (i.e.60% of their body is H2O)
Variations in water content with age
Infant
Adult
% H2O 80%
Older Adult
% H2O 55%
Infants also ingest and excrete a greater volume of water in
relation to their size ? why
Fluid Compartments and Fluid Exchange
2/3 body H2O
Inside cells
Intra-Cellular Fluid
(ICF) = 30L
1/3 body H2O
Outside cells
Extra-Cellular Fluid
(ECF) = 15L
Interstitial fluid 12L
Plasma 3L
Also … Lymph
Cerebrospinal fluid
Body space fluid e.g.
Intra-ocular fluid
Total Body Water
About 65% of Body Weight
Adult has 45 litres of water in the body:
Intracellular
30 Litres
Extracellular
15 litres
Tissue Fluid 12L
Plasma 3L
Water gains and losses
Gains
Water
1300 ml
Food
1000 ml
Metabolism 400 ml
Total
2700 ml
Losses
urine
1700 ml
insensible losses ..
faeces
200 ml
lungs
300 ml
skin
500 ml
2700 ml
Alterations in homeostasis
• If the water gained = water lost each
day the body will be in water
equilibrium
• If the water gained is less than water
lost  DEHYDRATION
What interventions may
be necessary treatment
of dehydration?
[Application to clinical practice: Dehydration]
What patients may be at risk of dehydration?
Have you ever been dehydrated and how did
you feel?
[Application to clinical practice: Dehydration]
What are the signs and symptoms of dehydration?
What is minimum urine output/hr to maintain kidney
function?
Whose role is it to ensure that patients are
adequately hydrated?
Regulation of Intake
• The sensation of thirst
ensures that water intake
is adequate
• What patients may not
be able to respond to
the thirst sensation?
• The thirst centre is in the
hypothalamus where
there are nerve cells
called osmoreceptors
Regulation of intake
Dehydration
Saliva
Mouth / throat dry
Blood osmolarity
Thirst centre stimulated
Sensation of thirst and desire to drink
Mucosa of mouth/
pharynx wetted
Initial quenching of thirst
Distension of intestine
Osmotic pressure
Major inhibition of thirst
Regulation of Fluid Volume
DEHYDRATION
Increased Osmotic
Pressure (blood)
Decreased
Flow of Saliva
Hypothalamus
Dry Mouth &
Throat
THIRST
INCREASED INTAKE
INCREASED BLOOD VOLUME
Regulation of Output
Fluid output adjusted by...
• ADH
• Aldosterone
(both regulate urine production)
• Anti Diuretic hormone (ADH)

• Increases re-absorption of
water by the kidneys
Anti Diuretic Hormone
The hypothalamus synthesizes ADH
which is transported to the posterior
pituitary and stored there
Alcohol 
Inhibits release of
ADH
Control of ADH secretion
• Changes in extra-cellular osmolality 
• Detected by Osmoreceptors in hypothalamus
• ADH production and release will be increased
or decreased
• What will the effect be of increased ADH?
• What will the effect be of decreased ADH?
Aldosterone
• Aldosterone is a hormone (a
mineralocorticoid) produced by the adrenal
cortex
Aldosterone
Aldosterone is a Mineralocorticoid produced by
the Zona glomerulosa of the adrenal cortex
Function of aldosterone is to:
• Stimulate kidneys to reabsorb Na+ and
secrete K+  sodium and water
reabsorption
Aldosterone
Tissue fluid production
Hydrostatic
pressure?
Osmotic
pressure?
Arteriole end
Structure of the
capillary?
Venule end
HP = 15mmHg
Capillary
HP = 35mmHg
OP = 25mmHg
Tissue Fluid
Lymph vessel