Body Fluids
A. Body water content
1. Water is the largest single component of
the body
A) Early embryo = 97% water
B) Newborn infant = 77% water
C) Adult male = 60% water
D) Adult female = 54% water
E) Elderly adult = 45% water
B. Fluid compartments
1. Intracellular fluid compartment (ICF) = 25L
2. Extracellular fluid compartment (ECF) =15L
A) divided into two sub-compartments:
1) Plasma – fluid portion of blood = 3L
2) Interstitial fluid – the fluid in the spaces
between cells = 12L
3. Other = lymph, cerebrospinal fluid, humors
of the eye, synovial fluid, serous fluids, and
secretions of the gastrointestinal tracts
C. Major Components of Body Fluids
1. Water – 79-85%
2. Proteins – 10-20%
3. Lipids – 2% (adipose tissue itself has
~95%)
4. Carbohydrates – 1%
5. Electrolytes – substances that dissociate
into ions when dissolved in water
A) Major Cations
1) Na+, K+, Ca++
B) Major Anions
1) Cl-, HCO3-, HPO4D. Comparison of ECF & ICF
ECF
ICF
7.35-7.45
7.35-7.45
glucose
fatty acids
amino acids
Gases
higher
higher
higher
used up immediately
used up immediately
used up immediately
oxygen
carbon dioxide
Ions
higher
lower
lower
higher
Na+ & Ca++
Cl- & HCO3-
K+
HPO4-
pH
Nutrients
cations
anions
Water Balance
A. To maintain proper water volume, the
body must balance water losses (both
obligatory and nonobligatory) with water
gains over the course of the day
1. To remain properly hydrated, water
intake must be equal to water output
(~2500ml/day)
B. Water Output
1. Water output (loss) can be divided into 2
categories:
A) Nonobligatory
1) water loss beyond that created by
normal homeostatic events; not
necessary to maintain homeostasis, and
may or may not be avoidable
2) includes excess perspiration due to exercise,
strenuous work, etc.; also includes vomiting
and diarrheal illnesses
3) vary from person to person, hard to
measure, and are generally not calculated
into a person’s “normal” daily water loss
B) Obligatory
1) water loss created by normal homeostatic
events; necessary to maintain homeostasis,
and are unavoidable and necessary to
maintain life
2) fall into 1 of 2 categories:
a) Insensible losses
i) Skin (16%)
ii) Lungs (12%)
b) Sensible losses
i) Urine (60%)
ii) Sweat (perspiration) (8%)
iii) Feces (4%)
3) The kidneys are responsible for our largest
obligatory water loss each day
a) They must produce at least a small amount
of urine each day because:
i) They must remove unnecessary blood
solutes to maintain normal blood
homeostasis
ii) They must then flush those solutes out of
the body in water (as opposed to in solid
form)
b) Failure to do so would result in improper
blood composition which, in turn, could
lead to imbalances/disease/death of other
tissues in the body
c) Beyond the homeostatic minimum, the
solute concentration and volume of urine
excreted depend on fluid intake, diet, and
water loss via other mechanisms
C. Water Intake
1. Varies greatly from person to person, and
is often dependent on diet, lifestyle,
activity level, etc.
2. Major sources of water intake:
A) Liquids (60%)
B) Solid foods (30%)
C) Metabolic water (10%)
3. Regulation of Water Intake: thirst mechanism
A) Increased plasma osmolarity (high solutes)
or decreased plasma volume triggers the
thirst mechanism, which is mediated by
hypothalamic osmoreceptors
B) When osmoreceptors lose water by osmosis
to a hypertonic ECF, the hypothalamic thirst
center is stimulated motivating the
individual to drink
C) Thirst is inhibited by distention of the GI
tract by ingested water and then by osmotic
signals
1) May be dampened before the body needs
for water have been met
Homeostatic Imbalances
A. Dehydration – water loss exceeds water
intake over a period of time
1. May result from hemorrhage, severe
burns, vomiting, diarrhea, profuse
sweating, water deprivation, or diuretic
abuse
B. Hypotonic Hydration – excessive water
build up in the cells causing them to swell
1. Particularly damaging to neurons
2. May result from excessive water intake in a
short period of time or renal insufficiency
C. Edema – accumulation of fluid in the
interstitial spaces, leading to tissue swelling
1. May result from increased blood pressure
and capillary permeability, hormones,
blockage of the lymphatic vessels, or low
plasma proteins as a result of
glomerulonephritis, malnutrition, or liver
disease
Acid-Base Balance
A. Recall the definitions of acids and bases:
1. Acids (pH 1-6.9) – release H+ when in
solution; often called hydrogen donors
2. Bases (pH 7.1-14) – release OH- when in
solution; often called hydrogen
acceptors
B. The homeostatic pH range of arterial
blood is 7.35 to 7.45
1. Higher pH = alkalosis
2. Lower pH = acidosis
C. Abnormalities of Acid-Base Balance
1. Respiratory acidosis is the result of an
increase in CO2 in the blood
A) may be caused by hypoventilation (for
any reason), when there is airway
obstruction (ex. asthma), or due to
alveolar dysfunction (ex. pulmonary
edema)
B) Increased CO2 = increased H+ =
decreased pH
2. Respiratory alkalosis is the result of a
decrease in CO2 in the blood
A) May be caused by hyperventilation (for
any reason) or mechanical ventilation
B) Decreased CO2 = decreased H+ =
increased pH
3. Metabolic acidosis is due to a decrease in
HCO3- which lowers pH
A) May be caused by excessive alcohol
consumption, prolonged diarrhea, renal
dysfunction, hyperkalemia
B) Decreased HCO3- = decreased pH
4. Metabolic alkalosis is due to an increase in
HCO3- which increases pH
A) May be caused by excessive vomiting,
hypokalemia, or excessive NaHCO3
(sodium bicarbonate; ex baking soda &
some antacids) consumption
B) Increased HCO3- = increased pH
D. Chemical Buffering Systems
1. Work by replacing a strong acid with a
weak one or a strong base with a weak
one to minimize the pH change in the
body fluid
2. Responsible for immediate changes to pH
3. 3 examples
A) Bicarbonate buffer system – ECF; utilizes
NaHCO3 (sodium bicarbonate) and H2CO3
(carbonic acid)
1) NaHCO3 functions as a weak base
2) H2CO3 functions as a weak acid
3) when a strong acid is added to the solution,
NaHCO3 dissociates to form HCO3 and Na+
a) HCO3 binds with excess H+ create H2CO3
(weak acid) eliminating large amounts of
H+ from the solution and preventing a
drastic drop in pH
4) when a strong base is added to the solution,
H2CO3 dissociates to form HCO3- and H+
a) HCO3 binds with the Na+ to form
NaHCO3 (weak base) and preventing a
drastic rise in pH
b) H+ binds with excess OH- to create H2O
eliminating large amounts of OH- from
the solution and preventing a drastic rise
in pH
B) Phosphate buffer system – urine & ICF;
utilizes Na2HPO4 (disodium monohydrogen
phosphate) and NaH2PO4 (sodium
dihydrogen phosphate)
1) Na2HPO4 functions as a weak base
2) NaH2PO4 functions as a weak acid
3) when a strong acid is added to the solution,
Na2HPO4 dissociates into NaHPO4 and Na+
a) NaHPO4 binds with excess H+ to create
NaH2PO4 (weak acid)
4) when a strong base is added to the solution,
NaH2PO4 dissociates into NaHPO4 and H+
a) NaHPO4 binds with the Na+ to form
Na2HPO4 (weak base)
b) H+ binds with excess OH- to create H2O
C) Protein buffer system – plasma & ICF;
utilizes carboxyl and amine side groups on
amino acids
1) Most abundant chemical buffering
system in the body
2) Utilizes the carboxyl group or amine
group from an amino acid
a) alkalosis – rising pH (decreasing H+) results
in the release of H+ from -COOH
i) causes H+ levels to rise = decreased pH
b) amine – dropping pH (increasing H+)
causes excess H+ to bind to NH2  NH3
i) causes H+ levels to drop = increased pH
E. Physiological Buffering Systems
1. Respiratory Control
A) responsible for minute-to-minute
changes in pH
B) Utilizes bicarbonate reaction
1) recall CO2 + H2O  H2CO3 
HCO3- + H+
C) Driven by CO2 levels
1) Decreased pH (acidosis) causes increased
ventilation; pushes the reaction to the left
(decreased CO2 = decreased H+ = increased
pH)
2) Increased pH (alkalosis) causes decreased
ventilation; pushes the reaction to the right
(increased CO2 = increased H+ = decreased
pH)
2. Renal Control
A) The kidneys provide the major longterm mechanism for controlling acid-base
balance
B) In addition, metabolic acids (phosphoric,
uric, lactic, and keto) can only be
eliminated by the kidneys
C) Works by creating/reabsorbing or
secreting (excreting) HCO3-
D) Utilizes bicarbonate reaction
1) Tubule cells are impermeable to HCO3 on
their tubule borders but not on their
vascular borders
a) Therefore, HCO3- is continually lost in
urine
b) Blood HCO3- levels are controlled by
the bicarbonate reaction within the
tubule cells
i) To counteract acidosis, HCO3- is produced
and reabsorbed resulting in more H+
secretion
ii) To counteract alkalosis, HCO3- is produced
and secreted resulting in more H+
reabsorption