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Body Fluids, Water Balance, & Acid-Base Balance
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
higher
used up immediately
fatty acids
higher
used up immediately
amino acids
higher
used up immediately
oxygen
higher
lower
carbon dioxide
lower
higher
pH
Nutrients
Gases
Ions
cations
Na+ & Ca++
K+
anions
Cl- & HCO3-
HPO4-
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
D. Homeostatic Imbalances
1. Dehydration – water loss exceeds water intake over a period of time
A) May result from hemorrhage, severe burns, vomiting, diarrhea, profuse sweating,
water deprivation, or diuretic abuse
2. Hypotonic Hydration – excessive water build up in the cells causing them to swell
A) Particularly damaging to neurons
B) May result from excessive water intake in a short period of time or renal
insufficiency
3. Edema – accumulation of fluid in the interstitial spaces, leading to tissue swelling
A) 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 OHfrom the solution
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
1) causes H+ levels to rise = decreased pH
b) amine – dropping pH (increasing H+) causes excess H+ to bind to NH2  NH3
1) 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 long-term 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) HCO3D) 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