Chapter 27

Fluid, Electrolyte and Acid-Base

Balance

ECF ICF

different ion compositionsdifferent fluid compartments

same osmotic concentration

interstitial fluidblood plasma

cytoplasm

≠

~1/3 ~2/3

Fluid basics

Fluid basics

1. all homeostatic mechanisms monitor the ECF, not the ICF

2. receptors can’t monitor [ion]but can monitor:

plasma volumeosmotic concentration

3. cells cannot actively move H2O“water follows salt”

Fluid basics

4. [water] and [electrolyte]

will rise if gains exceed losses

will fall if losses exceed gains

hormone basics

1. ADH(antidiuretic hormone)

high blood [osmotic]

release

H2O conservation in kidneystimulates thirst

hormone basics

2. Aldosterone

rising [K+]falling [Na+]renin release

released in response to:

[Na+], Cl-, H2Oreabsorption in kidney

hormone basics

3. Natriuretic hormones

block ADH releaseblock aldosterone release

released cardiac/brain cells

[Na+] and H2Olost to urine

interstitial fluid

plasma

Fluid movements

ECF

collo

idosm

oti

cp

ress

ure

hyd

rost

ati

cp

ress

ure

Fluid movements

fig 21-12

interstitial fluid

plasma

Fluid movements

ECF

collo

idosm

oti

cp

ress

ure

hyd

rost

ati

cp

ress

ure

ICF

rapid movement between ICF and ECF is called fluid shift

If [osmotic]

hypertonic

H2O

Fluid shift

ECF ICF

If [osmotic]

hypotonic

H2O

Fluid shift

ECF ICF

Fluid shift

net loss of water dehydration

sweating, vomiting, diarrhea

ICF and ECF become more concentrated

hypernatrimia

ADH, renin secretion thirst, …

give hypotonic fluids (H2O)

Fluid shift

net gain of water

into ECFinto ICF

reduce ADH secretion

increase fluid loss

Fluid shift

net gain of water

water excess (overhydration)

drinking a lotinjection of hypotonic solutionkidney or liver failureexcess ADH production

hyponatrimia

effects on CNS H2O intoxication

Electrolyte balance

electrolyte balance:

will affect H2O balancewill affect cell functions

Na+

K+

most common electrolytebalance problems

less commonmore serious

Electrolyte balance

Na+

too much ADH, retain H2O

renin-angiotensinogen retain Na+, H2O

too little ADH, lose H2O

ANP, BNP lose Na+, H2O

Electrolyte balance

K+

98% is in ICF

balance in ECF is smallmaintained by secretion in kidney

Electrolyte balance

K+

too much severe cardiacarrhythmias

too little and

100 Keys pg. 1007

“Fluid balance and electrolyte balance are interrelated. Small water gains or losses affect electrolyte concentrations only temporarily. The impacts are reduced by fluid shifts between the ECF and ICF, and by hormonal responses that adjust the rates of water intake and excretion. Similarly, electrolyte gains or losses produce only temporary changes in solute concentration. These changes are opposed by fluid shifts, adjustments in the rates of ion absorption and secretion, an adjustments to the rates of water gain and loss.”

100 Keys pg. 1019

“The most common and acute acid-base disorder is respiratory acidosis, which develops when respiratory activity cannot keep pace with the rate of carbon dioxide generation in peripheral tissues.”