unit 4: fluid balance

body water content

depends on age, sex, lean/fat mass

• adipose tissue → abt 20% h2o

• skeletal muscle → abt 75% h2o

fluid compartments

areas separated by selectively permeable membranes and differing in chemical composition

2% transcellular fluid

• ex: cerebrospinal fluid, pleural/pericardial fluid

composition of body fluids: electrolytes

dissociate into ions in h2o

in ecf: mostly na+ and cl-

in icf: mostly k+ and HPO4

composition of body fluids: nonelectrolytes

bonds prevent dissociation

ex: urea, glucose

composition of body fluids: all solutes contribute to osmosis

electrolytes have greater osmotic draw

water able to move more freely than solutes

• changes in solute concentration will induce movement of water

water gain and loss

fluid balance: daily gains= loss

water gains

preformed water = ingested

metabolic water = cellular metabolism

water loss

sensible water loss = urine, feces, sweat

insensible water loss = expired breath, cutaneous transpiration

hypothalamic thirst center activated by:

osmoreceptors, decreased blood volume/pressure, dry mouth

produces conscious sensation of thirst

osmoreceptors

detect ECF osmolarity via plasma membrane stretch receptors

decreased blood volume/pressure

baroreceptors activate thirst center via angiotensin 2

dry mouth

increased blood osmolarity reduces salivation

regulation of intake

water absorbed from small intestine

• restores blood osmolarity/volume

• 30+ minute delay

regulation of intake: short term response

cooling and moistening of mouth

distension of stomach and small intestine

prevent overhydration

regulation of output

obligatory water loss

• insensible loss + at least ~500 mL/day of sensible loss

additional H2O loss dependent on intake, climate activity

kidneys modulate rate of water loss

• urine concentrated through adjustments in Na+ and H2O reabsorption

• excess H2O excreted ~30+ min after intake

influence of ADH

low ADH→ dilute urine, reduced blood volume/pressure

high ADH → small volume of concentrated urine

large changes in blood volume also impact ADH secretion

• via baroreceptors or RAA mechanism

• ADH will act as a vasoconstrictor

fluid deficiency

intake < output

dehydration and volume depletion

dehydration (negative water balance)

from profuse sweating, insufficient water intake

loss of water leads to increased blood osmolarity, electrolyte imbalances

• impacts osmotic gradients between fluid compartments

infants more vulnerable than adults

volume depletion(hypovolemia)

proportionate amounts of water and electrolytes are lost

circulatory shock due to loss of blood volume

fluid excess

hypotonic hydration and volume excess

hypotonic hydration (water intoxication)

overhydration from extreme fluid intake, renal insufficiency, ADH abnormality

• dilution of ECF causes electrolyte imbalances

vomiting, cramps, pulmonary, and cerebral edema

treated by administering hypertonic saline IV to reverse osmotic gradient

volume excess

both Na+ and water retained

fluid sequestration

buildup of fluid in a particular location

edema + pleural effusion

internal hemorrhage

anaphylactic shock

edema

abnormal accumulation of fluid in interstitial spaces

pleural effusion

accumulation of fluid in pleural cavity

are kidneys better are compensating for fluid excess or deficiencies

fluid excesses