Water and Electrolyte Balance

Water

• 60% - 90% of BW in most life forms• 2/3 intracellular fluid• 1/3 extracellular fluid

– plasma– lymph– interstitial fluid– gut

Dehydration

• Occurs when fluid loss exceeds intake– sweating vs time

• Fluid lost mostly from ECF– decreased circulating blood volume

inadequate tissue perfusion, inefficient transport of substrates to muscle, and elevated HR

Dehydration• Clinical Signs

– persistent elevation of HR and RR

– weak pulse

– poor capillary refill

– muscular weakness, tremors

– depression

– weak pulse, staggering

– muscle cramps

• Treatment– IV or oral fluids

and electrolytes

Estimates of Fluid Loss

• Weigh animal before and after exercise– 1 kg = 1 L– loss of 3% of BW = reduce

performance– loss of 10% of BW = serious,

possibly life threatening

Estimates of Fluid Loss

• Blood analysis– PCV and total plasma proteins

• rise indicates dehydration

– if PCV > 50% indicates hazardous fluid loss

Detecting Dehydration• Skin pinch test

– falls back instantly - normal

– 2 - 4 sec - moderate – 4 -6 sec - severe

• Capillary refill– press finger on gums

above an upper tooth• if it takes longer than 2

seconds for blood to return - dehydration

Electrolytes

• Salt that dissociates in solution into electrically charged particles (ions)– cations - positive charge– anions - negative charge

• Function in fluid balance, transmission of nerve impulses, and muscular activity

Electrolytes

• Na, K, Cl– ICF predominate cation is K– ECF predominate cation is Na– determines water distribution

between compartments

Electrolyte Replacement

• Sweat is hypertonic– 100 mEq Na:30 mEq K: 130 mEq

Cl– small amounts of Ca, Mg, and Zn

also lost– Electrolyte loss is easily replaced

by dietary means if sweat is sparse or infrequent

Electrolyte Replacement

• Electrolyte supplementation recommended if sweating is frequent– also in hot and humid weather

• Dietary electrolytes should replace ions lost in sweat in correct proportions– water or feed

Dietary Considerations

• Type of hay– legume vs grass

• DCAD - dietary cation anion balance– strong and weak cations vs

anions

Strong Ions

• Cations– Na+

– K+

– Ca2+

– Mg2+

• Anions– Cl-

– SO42-

– lactic acid

Na/K ATPase Pump

Lehninger, 1993

Stewart (1981)• Concept of electrolytes as critical

factors in acid/base balance• Strong ion difference (SID)

– sum of all strong cations minus sum of all strong anions (NA, K, CL, SO4

2-)

– anions greater = negative SID = H+ > OH-

– cations greater = positive SID = OH- > H+

Stewart (1981)

• Balance of SID is maintained by the dissociation and reassociation of water

H+ = Dependent Variable

• Three independent variables determine the value of H+:– SID

– Pco2

• H increases as Pco2 increases

– CO2 acts as an acid

– Total concentration of weak acids (plasma proteins)

• H increases as weak acids increase

Regulation of pH

• Long Term– Kidney

• regulates electrolytes

– primarily Na

– GI tract• absorption of

cations and anions

– dependent of needs

• Rapid– Lung

• releases CO2 (HCO3-)

Altering Acid Base Balance

• DCAD diets

• Sodium bicarbonate administration– IV vs GI– effect of other sodium forms

• Furosemide