nurs 210: chapter 26 - fluid, electrolyte, and acid-base balance

major fluid compartments

body fluids make up between 55-65% of total body mass; women = 55% fluids, men = 60% fluids

• two main compartments:

  • inside cells (2/3)

    • intracellular fluids is cytosol

  • outside cells (1/2)

    • extracellular fluid is interstitial fluid (80%) and blood plasma (20%)

exchange of water and ions between major fluid compartments

• plasma membrane of cells separates intracellular fluid from interstitial fluid

• blood vessel walls divide interstitial fluid from blood plasma

• capillary walls thin enough to allow exchange of water and solutes between blood plasma and interstitial fluid !!

mechanisms of exchange for water

filtration, reabsorption, diffusion, and osmosis allow continues exchange of water and solutes among body fluid compartments; balance of inorganic compounds that dissociate into ions (electrolytes) is closely related to fluid balance

• filtration & reabsorption: pressure driven moment (mostly at capillaries)

• diffusion: ions moving from high to low conc

• osmosis: water moving to balance out solute conc

pathways by which water enters and leaves body

• body gains water by: ingestion and metabolic synthesis

  • metabolic water (200 mL), ingested foods (700 mL), ingested liquids (1600 mL)

• body loses water by: urination (main way ~1500 mL a day to regulate volume), perspiration, exhalation, in feces

  • GI tract (100 mL), lungs (300 mL), skin (600 mL), kidneys (1500 mL)

differences in electrolyte and protein concentrations in plasma, interstitial fluid, and intracellular fluid

• plasma vs. interstitial fluid

  • similar since they’re both extracellular fluid; all have different concentrations of electrolytes and protein ions (blood plasma, interstitial fluid, and intracellular fluid)

  • difference: blood plasma contains many protein ions whereas interstitial contains few since proteins are generally too large to easily cross blood vessel walls

• extracellular vs. intracellular fluid

  • most “abundant ions” are almost complete opposites

role of electrolytes

ions formed when electrolytes dissociate and disolve; functions:

• control osmosis of water btwn fluid compartments

• help maintain acid-base balance

• carry electrical current

• serve as cofactors

major electrolytes in extracellular fluids

• sodium (Na+): most abundant cation in ECF

  • used for impulsive transmission, muscle contraction, fluid and electrolyte balance

  • sodium lvl controlled by aldosterone and ANP

• chloride (Cl-): most abundant anion in EFC

  • helps regulate osmotic pressure btwn compartments

  • forms HCI in stomach

  • regulation of Cl- balance controlled by aldosterone

major electrolytes in intracellular fluids

• potassium (K+): most abundant cation in ICF

  • involved in fluid volume, impulse conduction, muscle contraction, regulating pH

  • mineralcorticoids (mainly aldosterone) regulate plasma lvl

• magnesium (Mg^2+): an intracellular cation

  • activates enzymes involved in carbs and protein metabolism

  • used in myocardial function, transmission in CNS, and operation of sodium pump

• phosphate: occurs as calcium phosphate salt (an anion)

  • used in buffer system

  • regulated by parathyroid hromone and calcitrol

other key electrolytes in body fluids

• calcium (Ca²+): most abundant mineral in body

  • structural component of bones/teeth

  • used for blood coagulation, neurotransmitter release, muscle tone, excitability of nerves/muscles

• bicarbonate (HCO3-): important plasma ion

  • major member of plasma acid-base buffer system

  • kidneys reabsorb or secrete it for final acid-base balance

hormones involved in regulation of water and solute homestasis

hormones that control homeostasis of Na+, Cl-, and water: angiotensin II, aldosterone, atrial natriuretic hormone peptide (ANP)

major hormone that regulates water loss is: antidiuretic hormone (ADH)

hormones involved in controlling homeostasis of Na+, Cl-, and water

• angiotensin II:

  • when: blood pressure or volume too low

  • trigger: kidneys release renin, starts RAAS pathway

  • action: potent vasocontrisctor (narrows vessels to raise BP)

    • stimulates release of aldosterone

  • result: saves water + raises BP

• aldosterone:

  • when: VP is low or plasma K+ is too high

  • trigger: stimulated by angiotensin II or high K+

  • action: increases reabsorption of sodium and chloride in kidneys

  • result: water follows salt = leads to more water reabsorption, increasing blood volume and decreasing K+ lvls

• ANP:

  • when: blood vol or BP too high

  • trigger: stretching atria of heart

  • action: promotes natriuresis (excreting Na+ into urine)

  • result: water follows salt out of body, increasing urine output and lowering BV/BP

hormone involved in regulating water loss

• ADH:

  • when: dehydrated or blood osmolarity too high (too salty)

  • trigger: increased osmolairty of ECF sensed by hypothalamus

  • action: makes kidney collecting ducts more permeable to water

  • result: reabsorbs water into blood, making urine more concentrated + reducing urine volume

movement of water btwn body fluid + compartments

• when ECF is isotonic to cells of body = not shrink or swell

• changes in osmolarity of EFC (dehydration or overhydration) can cause celsl of body to shrink or swell

normal pH range of blood + mechanism that maintains range

• pH of arterial blood ranges from 7.35 to 7.45

• maintenance of range:

  • buffer systems

  • exhalation of carbon dioxide

  • kidney excretion of H+

    • proximal convoluted tubules and collecting ducts of kidneys secrete H+ into tubular fluid (urine)

how kidneys work to maintain normal pH

it physicall removes or adds ions to body via urine; slow but most powerful/permanent fix

• mechanism: renal tubules secrete H+ into urine and reabsorb HCO3-, not lost in urine (keeps bicarbonate base in blood)

how respiratory system works to maintain normal pH

regulates pH by changing rate and depth of breathing to control CO2 lvls; fast - 1 to 3 mins

• mechanism: acidic blood; breathe faster to blow off CO2

  • alkaline blood: breathe slower to retain CO2, forming acid

• basically: incr exhalation of CO2 = pH rises (less H+); decr exhalation of CO2 = pH falls (more H+)

how buffers work to maintain pH

most consistent of a weak acid and its salt; functions as a weak base; prevent drastic changes in body fluid pH; instantaneous (seconds)

• proteins: most abundant buffers

• carbonic acid-bicarbonate: important regulator of blood pH, most abundant buffer in ECF

• phosphate: important buffer in ICF + urine

• mechanism: act as chemical “sponges” that temporarily bind to excess H+ ions to prevent rapid pH shifts

acidosis vs alkalosis

• acidosis: blood pH is below 7.35

• alkalosis: blood pH is above 7.45

respiratory imbalances

• respiratory acidosis: blood pH drops due to excessive retention of CO2 = leads to excess H2CO3 = pH drops

  • from emphysema or hypoventilation

• respiratory alkalosis: blood pH rises due to excessive loss of CO2 as in hyperventilation = pH rises

  • during hyperventilation or high altitude

metabolic imbalances

• metabolic acidosis: arterial blood levels of H+ increases, HCO30 falls = pH drops

  • causes: common in severe diarrhea or renal dysfunction

• metabolic alkalosis: arterial blood levels of H+ falls, HCO3- rises = pH rises

  • often caused by excessive vomiting (loss of stomach acid) or alkaline drugs