body fluids & electrolytes pt. 1

body fluids

composed of water (solvent) and electrolytes, carbohydrates, proteins, lipids, vitamins, & minerals (solutes). intracellular in RBC, WBC, & tissues. extracellular in intravascular + extravascular (interstitial & transcellular)

functions of body fluids

dissolve & deliver substances (nutrients) to cells. account for blood volume (necessary to maintain BP & tissue perfusion): high blood volume → hypertension, low blood volume → hypotension. protect & lubricate body tissues (ex: CSF)

cerebrospinal fluid

protects brain & spinal cord

amniotic fluid

protects the fetus

synovial fluid

lubricates joints

pleural fluid

lubricates lungs’ surface

pericardial fluid

lubricates the sac in which the heart beats

electrolytes

substances that will dissociate into ions in solution & so acquire the ability to conduct electricity. essential components of all living matter

electrolyte functions

regulate osmotic pressure & fluid distribution between compartments (Na+). transmit nerve signals (Na+, K+). conduct neuromuscular impulses (Mg²⁺, Ca²⁺, K⁺). maintain acid-base balance (HCO₃^-, Cl shift). activate enzymes as cofactors (Mg²⁺, Ca²⁺)

major extracellular cations/anions

Na⁺, Cl^-, HCO₃^- (2nd major)

major intracellular cations/anions

K⁺, P^-

factors that affect fluid & electrolyte balance

ion transport (Na-K ATPase pump), hormones (aldosterone, ADH), pressure (crystalloid osmotic [Na⁺], hydrostatic, colloidal osmotic/oncotic [albumin]), lymphatic system

active transport

requries energy (ATP). Na-K-ATPase is an enzyme that pumps K⁺ into cells and Na⁺ out of cells.

passive transport

diffusion, requires no energy. Cl^-, HCO₃^- move freely in & out of cells.

what happens when Na-K-ATPase loses its function?

serum K⁺ increases while Na⁺ decreases. pathologic in-vivo: occurs in energy-deficient (DM) or O₂ deprived (hypoxia) cells. a problem in vitro (lab) when specimen is chilled to decrease RBC/WBC metabolism

aldosterone

plasma volume control. released from the adrenal cortex in response to apparent low BV/BP, which triggers RAS system. action in distal & collecting renal tubule. stimulates Na+ & water reabsorption in exchange for K⁺ or H⁺, a Na⁺ saving hormone. high serum [Na⁺] → high serum osmo → high BV/BP

hypo-secretion of aldosterone

low BP

hyper-secretion of aldosterone

high BP & ECF expansion. high serum Na⁺ (high serum osmo). low urine Na⁺ (low urine osmo). serum K⁺ & H⁺ low. urine K⁺ & H⁺ high

antidiuretic hormone (ADH)

plasma osmolality control. produced by hypothalamus, stored by posterior pituitary. secreted when plasma osmo rises & when BV/BP low, regardless of osmo. action in renal collecting duct. increases permeability of tubule membrane to water (reabsorption of water from urine), a water saving hormone

hypo-secretion of ADH

occurs in diabetes insipidus (DI). if access to water → polydipsia/-uria -. urine volume high, urine [Na⁺] + osmo low, serum Na⁺ + osmo high

hyper-secretion of ADH

occurs in syndrome of inappropriate diuretic hormone (SIADH). urine [Na⁺], urine osmo, & SG high. serum [Na⁺] + serum osmo low

crystalloid osmotic pressure

osmotic pressure that depends on dissociated ions & small molecules (crystals_ as osmo-active particles. regulates the distribution of water between intracellular & extracellular spaces: maintains cell shape. expressed as plasma osmolality

plasma osmolality (serum osmo)

concentration of plasma in terms of total # of electrolytes per kg plasma (mainly Na⁺). unaffected by large molecules like glucose, urea, proteins, & chylomicrons. mOs of solutes/kg of solvent (mOs/kg).

plasma osmo of Na

concentration (mmol/L): 135

osmo: 270 (with anions)

plasma osmo of glucose (90 mg/dL) & urea (14 mg/dL)

concentration (mmol/L): 5

osmo: 5

osmolality reference range

280 - 310 mOs/kg

iso-osmolality

concentration of electrolytes is the same on either side of the cell membrane

hypo-osmolality

serum osmo < 250 → cerebral edema. caused by SIADH (high ADH), hyper-hydration

hyper-osmolality

serum osmo > 320 → brain cells shrinkage. caused by dehydration (common), DI (low ADH), DM

urine osmolality

often measured following an abnormal plasma osmo or plasma Na⁺ test. range is wide, depends on water intake. reflects amount of osmotically active constituents in urine (Na⁺, Cl^-, K⁺, urea). assesses state of hydration, concentrating ability of kidney tubules, electrolyte balance

how does urine & serum osmo help differentiate DI from SIADH?

DI: urine osmo low, serum osmo high (excess water excretion & not responsive to water restriction).

SIADH: urine osmo > 200 mOsmol/kg, serum osmo low (excess water retention → body water high in all compartments).

how does urine & serum osmo ID failure of renal tubular urine concentrating function?

urine/serum osmo ratio → 1 (typically ~2-3) with loss of urine concentrating ability, even when urine should should be more concentrated

estimating serum osmolality

addresses issues of hydration & [Na⁺]. considers only common solute measurements from BMP (will be different from measured osmo). contributions from Ca, Mg, & their anions are ignored. can be more crudely approximated as 2 x Na

how is osmolality measured?

based on colligative properties of solns (# dissolved particles in soln): osmotic pressure, vapor pressure, boiling/freezing point. use cryo-osmometer to measure serum & urine directly. higher osmo = more its freezing point is depressed below 0C (solute reduce bonding forces b/t solvent molecules)

osmolar gap

measured plasma osmol - estimated plasma osmol. high = excess of uncounted low MW particles. endogenous: ketone bodies (DKA). exogenous: ethylene glycol (antifreeze), useful crude toxicology screen as many drugs have low MW

hydrostatic pressure

pressure exerted by a fluid within a closed system. regulates water distribution b/t intravascular & extravascular spaces

intravascular hydrostatic pressure

force of blood pressing outward against blood vessel wall (fluid-pushing pressure inside capillary). drives water out of arteriole & venule blood vessels. propels blood with heartbeat. increased with elevation

colloidal osmotic (oncotic) pressure

depends on large, colloidal particles (proteins, lipids). regulates distribution of water b/t intravascular & extravascular spaces. essential to maintain intravascular fluid volume (BV/BP). albumin is main regulator

why is albumin the main regulator of oncotic pressure?

its plasma concentration is less than its interstitial fluid concentration. its MW is less than other plasma proteins → more particles for a given weight → greater osmotic effect

edema

excessive accumulation of interstitial fluid. causes: low serum protein, heart failure, blockage of lymphatics

how does low serum protein contribute to edema?

from either inadequate protein synthesis (severe liver disease), protein loss (via renal or GI tract), or inadequate protein intake

how does heart failure contribute to edema?

high right-sided pressures cause elevated hydrostatic pressure in the capillaries, leading to more fluid movement into the interstitium

how does blockage of lymphatics contribute to edema?

typically from surgical excision of lymph nodes & associated lymph vessels: lymph system normally returns excess interstitial fluid to the general circulation

sodium (Na+)

most abundant cation in ECF, plasma Na⁺ determines > 90% of plasma osmolality. maintains electrolyte balance & thus water distribution b/t intracellular & extracellular spaces, a/w BP. required for nerve impulse transmission (CNS) & muscle contraction

what is Na⁺ regulated by?

Na-K-ATPase pump, thirst, kidney function (renin, aldosterone, ADH)

why is Na⁺ measured?

to assess hydration, and in conditions a/w water & electrolyte imbalance. must interpret [Na⁺] in context of hydration status Na⁺/BV

hyponatremia

serum [Na⁺] < 135 mmol/L. most common electrolyte D/O, w/ diverse etiologies. symptoms primarily due to hypotonicity of blood, causing cerebral edema. classified into 3 types based on measured serum osmo: equal (isotonic), low (hypotonic), high (hypertonic)

classification of hyponatremia

isotonic hyponatremia (pseudohyponatremia)

delusion: normal Na⁺ & water. only an issue when “indirect method” used to measure Na⁺, or specimen has hyperlipidemia or hyperproteinemia in multiple myeloma. error is due to predilution required by indirect method: assumes normal water content (93%), so underestimates [Na⁺]. direct method not affected