BODY FLUIDS and ELECTROLYTES
Resources:
Textbook (glossary, clinical studies, sample quizzes)
Textbook (sample quizzes, exercises, cadaver practicals)
Medical Research
Learning Activities
Interactions:Foundations CD (System overview, relationships with other systems)
Marieb
CD (Electrolytes)
Tortora Grabowski CD ( Regulating Blood pH)
Interactive Physiology CD (Body Fluids, Water Homeostasis, Electrolyte
Homeostasis, Acid/Base Balance)
Lecture Notes
I. Body fluids (H2O + solutes) usually 55-60 % of body.
A. Fluids
1. locations
a. Intracellular (within cells)
 most (2/3) of body fluids
b. Extracellular (outside cells)
 1/3 of body fluids
interstitial fluid (around cells) and plasma (blood)
2. consistency
water and solutes
solutes mostly ions (electrolytes) with some dissolved organics
B. Fluid balance (movement of fluid between compartments)
1. Active transport (ions). Ion transport, increasing solute concentration, determines osmotic pressure
gradient.
2. Osmosis (H2O). Higher osmotic pressure draws more water as result of greater solute concentration.
C. Fluid Balance
1. Water gain
a. Diet (drinking and moist food) ~ 2 liters/day
b. Metabolic water manufactured due to oxidative metabolism
~ 0.2-3 l/d.
total input ~ 2 1/2 l/d
2. Water loss
a. Kidneys ~ 1.5 l/d
b. Skin ~0.6 l/d
c. Lungs ~.3l/d
d. GI tract ~.1 l/d
total output 2 1/2 l/d
3. Regulation of water gain (thirst)
if more loss than gain, then dehydration.
Dehydration results in low blood volume, decreased saliva and increased blood osmotic pressure. These
stimuli stimulate thirst center in hypothalamus, triggering increased water intake. (mostly drinking).
4. regulation of water and solute loss
most water is lost via urination due to NaCl loss- "water follows salt"
NaCl loss determines blood osmotic pressure.
Aldosterone primarily determines salt loss
ADH determines water loss
D. Dissolved substances
1. Non-electrolytes
covalently bonded molecules .
serum proteins play a role in maintaining blood OP.
2. Electrolytes
molecules that are ionically bonded (usually inorganic)
NaCl = Cl– + Na+
CaCl2 = Ca++ + Cl– + Cl–
H2O breaks ionic bonds causing increased # of particles therefore increased osmotic pressure
therefore electrolyte concentrations control water movement
a. Na+ - most abundant cation in extracellular fluids
*Very strong role in fluid movement (increased ECF osmotic pressure)
*Strong role in transmission of electrical impulses
if low [Na+] then increased [aldosterone] (more Na+ reabsorbed) and decreased ADH (more
water lost) increase ECF [Na+]
if excess [Na+] then edema results (renal failure and hyperaldosteronism)
b. Cl– most abundant anion in extracellular fluid
helps Na+ distribute fluid to ECF
if low [Cl– ] , then decreased ADH (more water lost) increases ECF [Cl– ]
c. K+ most abundant cation in intracellular fluid
major role in transmission of electrical impulses
if excess [K+], then increased [aldosterone] decreases [K+]
d. HCO3– second most abundant anion in extracellular
important in buffering pH of extracellular and plasma fluids
major mechanism for transporting CO2 gas
e. Ca++ high in extracellular, however most in bone (CaHPO4)
controlled by CT & PTH
increased [CT] causes decreased [Ca++] in blood and with Ca staying in bone
increased [PTH] causes increased [Ca++] in blood and with more Ca++ into blood
f. PO4– highest anion in intracellular fluid
important buffer in cells
most is bound in organics including ATP
much is also bound up with Ca++ as Ca2PO4 bone matrix
increased [CT] causes decreased [PO4–] in blood
increased [PTH] causes increased [PO4–] in blood
II. Acid base balance (buffering)
A. Acids & bases
1. pH = [H+] in solution goes from 0 – 14 depending on [H+]
values 0 – 7 = acid \ many H+ ions
values 7 – 14 = base \ few H+ ions
2. Ionic acids (HCl, H2CO3) dissociate (break up) in water and give off H+ ion to decrease pH (more
acidic) of solutions
3. Base (HCO3-) combines with H+ to increase pH (more basic) of solutions
B. pH balance systems
1. Chemical buffering systems
consist of weak acid and salt of acid (which serves as weak base)
H2CO3 = NaHCO3 + H, i.e.,
weak acid will give off H+ if conditions have low [H+]
weak base (salt) will receive H+ if conditions have high [H+]
therefore [H+] is kept from going too low (basic) or too high (acidic)
a. Carbonic acid -- bicarbonate buffer major interstitial fluid buffer too few H+ ions (blood basic)
(buffer acts to release +++
CO2 + H2O= H2CO3 = H+ + HCO3–
b. Protein buffer
major intracellular/plasma buffer
Amino acid acts both ways \ same molecule can act as base or acid
c. Phosphate buffer similar to bicarbonate system (Na2HPO4) unimportant in plasma but effective
buffer for urine & intracellular
result:
too few H+ ions (buffer acts as acid) and release H+
too many H+ ions (buffer acts as base and combines H+)
2. Exhalation of CO2 (Respiratory system)
increased [CO2] drives reaction toward release of H+ therefore more acidic
decreased [CO2] drives reaction toward combination of H+ therefore more basic
therefore rate of ventilation can change [CO2]
increased ventilation : increase pH (less acidic)
3. Kidney excretion of H+ (Urinary system)
H+ ions are actively transported in nephron to adjust body pH in exchange for Na+
III. Acid-base imbalances
normal blood pH range is narrow 7.35–7.45
more acid - acidosis 7.35 – 6.20 (low O2 transport)
more basic – alkalosis 7.45 – 8.00
usually these changes are moderated (compensated) by above pH balancing systems
A. Imbalances of PCO2 (respiratory)
1. Respiratory acidosis (pH low )
PCO2 too high due to CO2 buildup
increased carbonic acid and increased H+ ions (more acidic)
Cause: Respiratory disease & damage -- hypoventilation
Compensation: increased renal excretion of H+ ions
2. Respiratory Alkalosis (pH high )
PCO2 too low due to low CO2
Cause: Hyperventilation or any disease/damage that stimulates respiration
Compensation: decreased renal excretion of H+ ions
B. Imbalance of HCO3 concentration
1. Metabolic acidosis (pH low )
low [HCO3] \ H+ not bound in that buffer, resulting in excess H+ (low pH) in blood
Cause: Loss of HCO3 ( diarrhea, renal failure, ketosis)
Compensation: respiratory by hyperventilation (remove excess CO2)
low pHof blood (Bohr effect) : decreased O2 transport
2. Metabolic alkalosis (pH high )
high [HCO3]
Cause: nonrespiratory loss of acid, e.g., vomiting, endocrine problems
Compensation: respiratory by hyporventilation i.e., slow CO2 loss