BODY FLUIDS, ELECTROLYTES and pH 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.
The more solute particles the greater the osmotic potential, not the size. So...a few atoms of ions increases OP more then a massive (1000s of atoms) protein.

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).


 Describe patterns of water balance.

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

5. Functions of water
hydrolysis
lubrication
transport
thermoregulation


Describe regulation and functions of water.  

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
 

Describe roles of electrolytes in body.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. 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)
decreased ventilation : decreased pH (more acidic)

3. Urinary system
 H+ ions are actively secreted to reduce blood acidity
HCO3– are reabsorbed to increase buffering reservoir and reduce acidity

III. Acid-base imbalances
normal blood pH range is narrow 7.35–7.45
more acid - acidosis lower than 7.35
 more basic – alkalosis higher than 7.45

 An imbalanced caused by one system is regulated (or compensated) by a different pH balancing system

A. Imbalances of PCO2 (respiratory) 1. Respiratory acidosis (pH low )
Cause: Respiratory disease & damage -- hypoventilation or lung disease that prevents diffusion of CO2
PCO2 too high due to CO2 buildup which increases  carbonic acid and   H+ ions (more acidic)


Compensation: increased renal excretion of H+ ions , increased reabsorption of HCO3-

2. Respiratory Alkalosis (pH high )
Cause: Respiratory disease & damage--Hyperventilation or any disease/damage that stimulates respiration
PCO2 too low due to low CO2 which decreases  carbonic acid and   H+ ions (more akaline)


Compensation: decreased renal excretion of H+ ions , decreased reabsorption of HCO3-

B. Imbalance of HCO3 concentration (metabolic)1. Metabolic acidosis (pH low )
Cause: Loss of HCO3 ( diarrhea, renal failure, ketosis) resulting in excess H+ (low pH) in blood
NOTE:l ow  pH of blood (Bohr effect) causes decreased O2 transport

Compensation: respiratory by hyperventilation (remove excess CO2)

2. Metabolic alkalosis (pH high )
Cause: nonrespiratory loss of acid, e.g., vomiting, endocrine problems, resulting in low available H+ ions (because they are buffered) (high pH) in blood

Compensation: respiratory by hyporventilation i.e., slow CO2 loss

Describe the three mechanisms of acid/base balance. Be able to demonstrate how each mechanism responds to lowering and rising body fluid pH. Relate the cause and compensatory mechanism for each pH imbalance.
Professor Thomas M. Lancraft

Human Anatomy and Physiology Courses 
at St. Petersburg College
St. Petersburg/Gibbs Campus

5/2008