The human body is composed primarily of water (60% of body weight) and proteins (18% of body weight). Water constitutes a higher percentage of body weight in infants (82%) compared to elderly adults (52%). The total body water is divided into intracellular fluid (ICF, 2/3 of total) and extracellular fluid (ECF, 1/3 of total). ECF is further divided into intravascular, interstitial, and transcellular fluids. Osmotic and hydrostatic pressures regulate the exchange of fluids between blood vessels and tissues.

1. BODY FLUIDS
BODY FLUIDS
Composition of the human body:
1-Water → 60% of the body
weight.
2-Proteins → 18% of the body
weight.
3-Fats → 18% of the body
weight.
4-Minerals → 4% of the body
weight.

2. Body Water
-Water constitutes about 60% of body
weight in young adult male.
-The amount of body H2O decline with age:
-The new born has about 82%of body
weight as H2O at birth and elderly about
52%.

3. Body Water
-Loss of body H2O is a common cause of death in
children who suffer from dehydration.
-The amount of H2O in the body is affected by the
quantity of body fat..
Lean subjects have a higher percentage of H2O
than those with more body fat.
This is because adipose tissue contains far less
H2O than muscle, skin, and other soft tissues.

4. Body Water
Variation of body H2O with age and sex as
% of body weight
Male Female
At birth 82% 82%
Children&adolescence 70% 70%
18-20 years 59% 57%
20-40 years 56% 51%
40-60years 55% 47%
Over 60 years 52% 46%

5. Body Water
A man who is weighing about 65 kg, the
total body water (TBW) is equal to 40
Liters it is subdivided into:
1-Intracellular :(inside the cells)ICF: It is
about 2/3 of total body water(25 liters)
2- Extracellular:(outside the cells)ECF:It
is about 1/3 of the total body
water(15liters)

7. Body Water
The extracellular fluid is further subdivided into:
a)Intravascular(inside the blood vessels): It is
the blood plasma(3 liters).
b)Interstitial(between the cells) in the tissue
spaces(12 liters), it is similar to the plasma but
with low protein content.
c)Transcellular(500 ml):It found in special
compartments in the body such as the pleural
cavity, peritoneal cavity, spaces between the
lining of the brain and spinal cord (CSF)and the
joint cavities.

8. Body Water
There is continuous exchange between the
intravascular and interstitial fluid to supply
the nutrients needed by the cells and
removing the waste products. This
exchange keeps the internal environment
constant

9. Comparison of ECF&ICF
solutes:
ECF contains large amount of Na+ and Cl-
but only small amounts of K+,Mg++ and
PO4--
In contrast, ICF contains large quantities of
K+,PO4--,moderate amounts of Mg++
and exceedingly few Ca++.

10. Osmolality of body fluids:
Because the cell membrane is highly
permeable to water, the osmolarity of
ECF&ICF compartments are normally the
same, about 290milliosmol/L
The plasma osmolality is calculated by using
the following formula:
Osmolality =2[Na+] +0.055[Glucose] +0.36[ Urea]
mEq/L mg% mg%

11. Mechanism of water balance:
Normally ,total body water remains
constant, therefore over 24-hours period,
intake and loss of water must balance
exactly to precisely regulate ECF
osmolality and Na+ concentration.
Maintenance of TBW balance involves
regulation of both water excretion by the
kidneys and regulation of water intake by
the thirst mechanism.

12. Mechanism of water balance:
Water intake:
1-Drinks and food → supply about 2100
ml/day.
2-Oxidation of carbohydrates, produces
about 300ml/day.

13. Mechanism of water balance:
Water loss:
1-Urine: about 1.5L/day.
2-Sweat: variable amount depending on the
environmental temperature and physical activity.
The volume of sweat is normally 100ml/day.In
hot weather or during heavy exercise, water loss
may increase up to 1-2 liters/hour.
3-Insensible water loss by evaporation through
skin and respiratory system. It is termed
insensible water loss because we are not
consciously aware of it. It is about 0.5 L/day.

14. Mechanism of water balance:
Regulation of water intake:
-It is controlled by thirst sensation.
-Thirst sensation: It is the conscious desire for
water.
-The thirst center is found in the lateral nuclei of
the hypothalamus.
-This center is connected to the osmoreceptors in
the anterior nuclei of the hypothalamus.

15. Regulation of water intake:
Water intake is stimulated by:
1-Inceased effective osmotic pressure of the plasma:
It acts via osmoreceptors located in the anterior
hypothalamus. When the Na+ concentration increases
about 2 mEq/L above normal, the thirst mechanism is
activated causing a desire to drink water .This is called
the threshold for drinking.
2-Decreased extracellular fluid volume:
Hemorrhage or hypotension caused increased drinking
even though there is no change in osmolality of the
plasma

16. Regulation of water intake:
Mechanism:
a)Renin-Angiotensin System:
Hypovolemia stimulates Rennin secretion→
resulting in rise of Angiotesin II in the
circulation.
Angiotensin II stimulates thirst & acts on the
kidney to reduce fluid and electrolyte excretion.
b)Baroreceptors in the heart and blood vessels:

17. Regulation of H2O excretion by
kidney:
-Urine output is regulated and is one of the
primary contributions to maintaining water
balance.
-Daily urine output can be as high as 20L or
as low as 500mL depending on the water
intake

18. Regulation of water intake:
Mechanism:
3-Other factors:
-Dryness of the pharyngeal mucous membrane→
thirst sensation.
-Prandial drinking: The intake of liquids is
increased during eating:
a-Increased plasma osmolality as food is
absorbed.
b-Gastro-instinal hormones may stimulate
the thirst.

19. Regulation of H2O excretion by
kidney
a-In conditions of H2O excess: as a result of
excess fluid intake: The renal response is to
increase water excretion. In conditions of
maximal water excretion urine volume can
increase to 18-20L/day.
b-Conditions of fluid depletion: as a result of
restricted intake or inappropriate losses, the
renal response is to reduce water excretion. The
urine volume can be reduced to as little as
500ml/day .

20. Regulation of H2O excretion by
kidney
-The ability of the kidney to excrete urine of
variable volume and osmolality is
absolutely dependent on the action of
ADH.

22. Regulation of H2O excretion by
kidney
ADH secretion is triggered by:
1-Rise in plasma osmolality,which are detected by
hypothalamic osmoreceptors.
2-Decrease in ECF volume: This is detected by
baroreceptors.
The osmoreceptors mechanism is the most
important for minute to minute control of ADH
secretion.
ADH increases water reabsorption by the collecting
duct and therefore minimizes water loss.

23. FORCES AFFCTING EXCHANGE
OF BODY FLUIDS
1-Diffusion:
It is the net movement of particles through the cell
membrane from high concentration to low
concentration. Also from positively charged
ions to negatively charged ions i.e. diffusion
governed by either chemical or electrical
gradient.
-Net movement of particle /unit time =flux.

24. FORCES AFFCTING EXCHANGE
OF BODY FLUIDS
II-Filtration:
It is forced passage of fluid through a
membrane due to difference in the
hydrostatic pressure on the two sides.
-The rate of filtration depends on the
pressure difference, surface area of the
membrane and its permeability

25. FORCES AFFECTING
EXCHANGE OF BODY FLUIDS
III-Osmosis:
It is the movement of solvent molecules
across a membrane to the other side in
which there is higher concentration of
solute to which the membrane is
impermeable.

26. FORCES AFFECTING
EXCHANGE OF BODY FLUIDS
IV-Active transport:
It is the transport of particles from low
concentration to high concentration across
the cell membrane. This type of transport occurs
against the laws of physical chemistry(uphill) it
requires energy derived from adenosine
triphosphate(ATP).The particles may be
transported against chemical, electrical or
pressure gradient. good example for active
transport is Na+-K+ pump and calcium pump.

27. FORCES AFFECTING
EXCHANGE OF BODY FLUIDS
Mechanisms of fluid exchange:
The forces which govern this exchange of
fluid between the plasma and the
interstitial fluid are:
1-Hydrostatic pressure due to fluid tension
within the circulation.
2-The colloid osmotic pressure of the
plasma proteins.

28. FORCES AFFECTING
EXCHANGE OF BODY FLUIDS
The hydrostatic pressure is much higher in
the capillaries than in the tissue spaces
and tends to drive fluid out of the
capillaries by filtration. The osmotic
pressure is much higher in the blood
plasma than in the interstitial fluid and
tend to draw back into the capillaries by
osmosis.
These two forces act in opposite directions.

29. FORCES AFFECTING
EXCHANGE OF BODY FLUIDS
While the osmotic pressure is uniform throughout
the capillary length, the hydrostatic pressure
falls from the arteriolar to the venular end. At
the arteriolar end of the capillary the hydrostatic
pressure is greater than the colloid osmotic
pressure and, therefore, fluid tends to pass out
of the capillaries. At the venous end of the
capillaries, the hydrostatic pressure is less than
the colloid osmotic pressure and therefore, H2O
is reabsorbed into the capillaries at this end of
the capillary.

30. FORCES AFFECTING
EXCHANGE OF BODY FLUIDS
APPLIED PHYSIOLOGY OF TISSUE FLUID
EXCHANGE
Edema: Edema is defined as the abnormal
collection of fluid in the interstitial spaces. There
are three main causes which occur frequently in
clinical practice:
1-Increased capillary hydrostatic pressure
2-Decrease in the plasma colloid osmotic pressure
3-Obestruction of lymph vessels.