Chapter 26 - Fluid, electrolytes, and acid-base balance

Intracellular fluid (ICF)

• about 2/3 of total body water

• located within the cells

• potassium is the chief cation

• phosphate is the chief anion

Extracellular fluid

• about 1/3 of total body water

• located outside of cells

• sodium is the chief cation

• chloride is the major anion

• The ECF is divided into plasma and interstitial fluid

Plasma (ECF)

• fluid portion of the blood

Interstitial fluid (ECF)

• fluid in the spaces between cells

• other extracellular fluids are present in small amount and are considered IF

  • lymph, cerebrospinal fluid, eye humors, Synovial fluid, etc

Water

• is the universal solvent

• solutes are broadly classified into two types

Electrolytes

• charged particles

• include salts, acids, bases

• measured in mEq/L when assessing charge

• have greater osmotic power because they form particles in solution

Non-electrolytes

• do not dissociate into ions (no charge)

• ex. glucose, lipids, creatinine, urea

• measured in concentration units or osmolality

Osmosis

• water moves according to osmotic gradients

• movement is toward areas with higher solute concentration

• determined by the number of particles in solution, not change

Milliequivalents (mEq)

• reflect the amount of charge

• charge maintains electrical neutrality in body fluids

Milliosmoles/liter (mOsm/L)

• reflect the concentration of particles

• this determines osmosis

Monovalent ions (Na+,K+)

• 1mEq=1mOsm

Divalent ions (Ca2+)

• 1mEq=0.5mOsm

• Ca2+ carries more charge per particle, so fewer particles are needed to get the same electrical effect

Capillary exchange (plasma ←→ IF)

• driven by Starling forces, maintains plasma volume and prevents edema

• at the arterial end the hydrostatic pressure is greater than the oncotic pressure allowing for filtration

• at the venous end the oncotic pressure is greater than the hydrostatic allowing for reabsorption

Cell exchange (IF←→ICF)

• water shifts rapidly via osmosis, driven by solute concentration

• ions are regulated by channels and pumps (ex. Na+K+ATPase)

• nutrients diffuse or are transported into cells

• metabolic wastes diffuse out to IF then blood then excretion

• gas exchange driven by partial pressure gradients

Hypertonic (Increased ECF osmolarity)

• water leaves the cell causing it to shrivel

Hypotonic (decrease ECF osmolarity)

• water enters the cell and swells

Water intake sources

• fluids 60%

• food 30%

• metabolic production 10%

Water output

• urine 60%

• feces 4%

• lungs 28%

• skin 8%

Dehydration

• increased ECF osmolarity causes water shifts out of the cell

• Na+ is the primary determinant of ECF osmolarity

Hypotonic/Over hydration

• decreased ECF osmolarity causes dilutuinal hyponatremia

• water is driven into the cells

• cellular swelling poses a greater risk in the brain for cerebral edema and for RBC damage

• commonly caused by the impairment of ADH

Edema (interstitial fluid accumulation)

• results from imbalance of capillary forces or lymphatic drainage

• results in decreased plasma volume and blood pressure

Edema mechanisms

• increased capillary hydrostatic pressure increases filtration

• decreased plasma oncotic pressure decreases reabsorption

• lymphatic obstruction decreases the amount of fluid returned to circulation

Causes of edema

• hypoproteinemia (malnutrition and liver disease can cause low oncotic pressure)

• venous congestion from heart failure

• lymphatic blockage or removal

Thirst regulation

• hypothalamic thirst center drives water intake

Stimuli for thirst

• increased ECF osmolarity is the primary trigger

• decreased blood volume or pressure is another trigger

Inhibition of thirst

• mositening of mouth and throat

• stretches of stomach and intestines

Water regulation by ADH

• increased ADH allows for increased water reabsorption, causing concentrated urine and a high ECF volume (your body is keeping the water not peeing it out)

• decreased ADH causes decreased water reabsorption causing diluted urine and a low ECF volume (your body is peeing out the water)

Water regulation by aldosterone

• increased Na+ reabsorption, water follows, increasing ECF

• effect depends on presence of ADH

Hypernatremia

• excess Na+ or water loss

Hyponatremia

• Na+ loss or excess water

Sodium regulation by aldosterone

• increased Na+ reabsorption in the kidneys causes increased ECF volume

• ANP/BNP are released with cardiac stretch and increase Na+ excretion in order to lower ECF volume

Hyperkalemia

• too much potassium

Hypokalemia

• too little potassium

Potassium regulation and aldosterone

• stimulated by increased K+

• increases K+ secretion in the kidneys to lower the levels

Potassium and Acidosis

• H+ enters the cell as K+ exits

• causing an increase in plasma K+ ions

Potassium and alkalosis

• H+ exits the cells as K+ enters

• causing a decrease in plasma K+

Insulin and potassium

• insulin stimulates Na+K+ATPase, causing K+ to shift into cells

• decreases serum K+

• insulin can be used to treat hyperkalemia rapidly

• given with glucose often to prevent hypoglycemia

Hypocalcemia

• too little Ca2+

Hypercalcemia

• too much Ca2+

Calcium regulation

• Parathyroid hormone raised Ca2+ and is released in response to low Ca2+

• increases Ca2+ via bone resorption, renal reabsorption and intestinal absorption

• Calcitonin lowers Ca2+ by inhibiting bone resorption

Acidosis

• pH < 7.35

Alkalosis

• pH > 7.45

Carbon dioxide as a regulator

• normal: 40-45 mmHg

• forms carbonic acid which decreases pH

Bicarbonate as a buffer

• normal: 22-26 mEq/L

• buffers H+ to increase pH

Chemical buffers (seconds)

• regulated H+ concentration with an immediate response

• bind or release H+ (ex. HCO3- buffer system)

Respiratory system (minutes)

• H+ concentration is controlled by brainstem (ventilation control)

Renal system (hours)

• regulates H+ and HCO3-

Chemical buffers systems

• resist changes in pH

• bind H+ when pH is low

• release H+ when pH is high

Bicarbonate buffer (ECF)

• most important

• lungs regulate CO2, kidneys regulate HCO3-

Phosphate buffer (ICF)

• more important inside cells

Protein buffers

• proteins bind or release H+

• ex. hemoglobin

Respiratory acid base balance

• CO2 driven

Metabolic acid base balance

• HCO3-

Respiratory acidosis

• hypoventilation leads to CO2 retention

• cause: opioid overdose, falling asleep and the brain shuts down the automatic center causing CO2 to build up

• cause: COPD, asthma, pneumonia, directly impact the lungs (hypoventilation)

• compensation: urine becomes more acidic

Respiratory alkalosis

• hyperventilation leads to excessive CO2 loss

• causes: anxiety, sobbing, pain

• compensation: urine becomes less acidic

Metabolic acidosis

• increased H+ or decreased HCO3-

• causes: diabetic ketoacidosis, fatty acids causes keystones and the liver produce acidix keytones for energy

• causes: diarrhea, loss of HCO3-

• compensation: increase ventilation

Metabolic alkalosis

• increased HCO3- or decreased H+

• causes: vomiting, throw up stomach acid, stomach needs more acid so it pulls H+ out of the blood

• compensation: decrease ventilation

Lungs fix metabolic

Kidneys fix respiratory