FLUIDS AND ELECTROLYTES

Body Fluid Composition

• Water – largest body component– 55-65% total body weight

• Solutes –dissolved in body water

Solutes– Electrolytes

• Cations – positive– Sodium (Na+)

– Potassium (K+)

– Calcium (Ca+2)

• Anions – negative– Chloride (Cl-)

– Bicarbonate (HCO3-)

– Phosphate (HPO4-)

– Non-electrolytes Proteins • Urea

• Glucose

• Oxygen (O2)

• Carbon dioxide (CO2)

Body Maintains Charge and Osmolality

• Body fluids – Electrically neutral

– Osmotically maintained• Specific # solute molecules per volume fluid

• Homeostasis of charge, osmolality maintained by – Ion transport

– Water movement between fluid compartments

– Kidney function

Units of Solute Measurement (A Review)

• MW (molecular weight) = sum of weights of atoms • mEq (milliequivalents) = MW (in mg) / valence

– Valence = # charges of ion• Na+ valence = 1• Cl- valence = 1• Ca+2 valence = 2

– Allows comparisons of charge # of solutes in fluids, without molecular wts

• Important to monitor overall charge of body fluids

• mOsm (milliosms) = # particles in solution– Particles = atoms or molecules, charged or uncharged– Measures concent of overall # of solute particles in fluid

• Important to monitor overall concentrations of body fluids

Fluid Compartments

• ICF = IntraCellular Fluid– Inside cells– 65% total body weight

• ECF = ExtraCellular Fluid– Not inside cells– 35% total body weight– Further divided

ECF – cont’d

– IVF = IntraVascular Fluid• In blood vessels

• 8% total body weight

– ISF = InterStitial Fluid• Bathes cells + lymph

• 25% of total body weight

Movement of Body Fluids (WATER FOLLOWS SALT!)

• Solutes may be in higher concentration on one side of the cell membrane

• Fluid (water) can move to equilibrate concentration and/or charge on both sides of the membrane

Definitions

• Diffusion = net movement of particles (solutes) down concentration gradient to establish equilibrium between two sides of membrane– Passive (no energy needed)– Assisted (energy must be added by the cell)

• Osmosis = diffusion of water – Freely passes through cell membranes.

Movement of Body Fluids – cont’d

• ICF to ECF– Osmolality changes in ICF not rapid

• Cell strives to maintain fluid and ion concentrations

• Cell very dependent on relatively constant water/solute amounts

– BUT if ECF osmolality changes so water moves among compartments, both compartments affected and equilibrated over time

Movement of Body Fluids – cont’d

• IVF to ISF to IVF– Happens constantly due to changes in fluid

pressures and osmotic forces at arterial and venous ends of the capillaries

– Necessary to move oxygen and nutrients toward metabolizing cells, and wastes and carbon dioxide away from metabolizing cells

Movement of Body Fluids – cont’d• Arteriolar end of the capillary:

– Highest fluid pressure force -- Blood Hydrostatic Pressure (BHP)• Pressing out against the capillary walls• Direction toward cells• Encourages movement out • Due to fluid pressures and heart contractions

– Colloid Osmotic Pressure (COP)• Lower pressure at arteriolar end • Pulls inward from capillary wall• Encourages fluid to stay inside • Due to large proteins, cells in capillary (too large to move through capillary

walls, so remain in the capillary), can’t leave the bloodstream– Overall at arteriolar end of capillary, BHP > COP

• Greater force encouraging fluid out of capillary than encouraging fluid to stay inside capillary

• So fluid (from heart; oxygenated; w/ nutrients) encouraged to move toward metabolizing cells

Movement of Body Fluids – cont’d

• At the venous end of the capillary:– BHP decreased

• Fluid “lost” from vessel • Don’t have same high fluid pressure pushing against vessel walls

– COP stays the same• Same amt large proteins, blood cells

– Now rel higher pressure “pulling in” away from cells– Overall at venous end of capillary, COP > BHP

• Greater force encourages fluid into capillary than encourages fluid out of capillary

• Overall, fluid (from around cells; containing wastes and CO2) encouraged to move into capillary

– Now returned to lungs to excrete CO2 and to kidneys to excrete wastes

Movement of Body Fluids – cont’d

• ECF to environment– Fluid intake = fluid output

• Intake is -- water, food, beverages

• Output -- urine, feces, sweat and water vapor

Regulation of Body Water• Works through ADH (AntiDiuretic Hormone)

• If there is – Decreased amount water in the body, or– Increased amount Na+ in the body, or– Increased blood osmolality

• So the blood is too concentrated, or

– Decreased circulating blood volume

• All of these lead to:

ADH Release – cont’d

• Stim’n hypothalamic osmoreceptors, • Release ADH, and

• Stimulation of thirst response– Thirst response increased drinking

• Overall, water volume within the body increases

ADH – cont’d

• ADH works at kidney incr’d permeability of kidney tubules to water incr’d reabsorption of water from kidney

tubules back into blood vasculature• Water WOULD have been released to urine

• So water is conserved, not excreted

ADH – cont’d• Overall:

– Incr’d water consumption, and– Incr’d water conservation, so– Incr’d amt water in body

• Relieves decr’d water, decr’d circulation blood volume

– Decr’d blood osmolality• Relieves increased body Na+, increased blood osmolality

• Note: the various conditions leading to ADH release can all be caused by different dysfunctions or traumas, but are all related physiologically – If either hemorrhage (decr’d blood volume) or sweating (decr’d water in body)

decr’d fluid available decr’d IVF decr’d blood pressure• Compensation: body tries to increase fluids in body

– If endocrine disorder incr’d Na+ incr’d blood osmolality• Compensation: body tries to increase fluids to bring blood osmolality back to

normal range (since body can’t n;ormalize osmolality through solute amount)• So if sodium LOAD has doubled to 290 mEq/L: if fluid VOLUME can double, will

now have 290 mEq/2L, which = 145 mEq/L (normal sodium CONCENTRATION is 145 mg/L)

ADH – cont’d

• Note: conditions ADH release, regardless of cause, related physiologically – Hemorrhage (decr’d blood volume) or sweating (decr’d

water in body) decr’d fluid available decr’d IVF decr’d blood pressure• Compensation: body tries to increase fluids in body

– Endocrine disorder incr’d Na+ incr’d blood osmolality

• Compensation: body tries to increase fluids (body can’t normalize osmolality through solute amount)

• Ex: Na+ LOAD doubled (290 mEq/L); by doubling fluid VOLUME 290 mEq/2L= 145 mEq/L (normal sodium CONCENTRATION is 145 mg/L)

Important Cations Contributing to Body Fluid Osmolality

• Sodium (Na+)– About 90% ECF cations– Normal range = 136-145 mEq/L in ECF

– Pairs with Cl-, HCO3- to neutralize charge

– Low in ICF (~10 mEq/L)– Most impt ion in regulating water balance

Sodium – cont’d

– Regulation in ECF -- Renal tubule reabsorption, modulated by hormones:

• Aldosterone– Rel’d from adrenal gland when body senses decr’d Na+ load

– Works at renal tubule to incr renal tubule reabsorption of Na+

• Renin/angiotensin– Affects aldosterone release (also controls Na+ reabsorption)

• Natriuretic hormone– Works at kidney decr’d renal reabsorption of Na+

Potassium• Major INTRAcellular cation• ICF concentration = 150-160 mEq/L• Lower in ECF (3.5-4.5 mEq/L)

– K+ concentration INSIDE cells approximates Na+ concentration OUTSIDE

– Na+ concentration INSIDE cells approximates K+ concentration OUTSIDE

– Body keeps electrical charge constant in ICF and ECF, but uses diff cations inside/outside cells

• Cell moves two cations differently, uses each differently – Overall cells strive to maintain high K+ inside and high Na+

outside

Potassium – cont’d

• Why keep K+ high inside and Na+ high outside cells?– Resting membrane potential in neurons, muscles

• Na+, K+ move into/out of these cells depolarization action potential

• If imbalanced, can neurological, muscle contraction problems

• REMEMBER: heart is an important muscle!

Potassium – cont’d• K+ also important:

– Regulates fluid, ion balance inside the cell• Sim to Na+ regulation outside the cell

– pH regulation• K+ can move across the cell membrane for H+ when H+ is in

excess (body fluids are too acidic)

• Regulation of body K+ is through the kidney– Aldosterone regulates K+

• If body senses decr’d K+ in plasma aldosterone release• Works at kidney tubule incr’d reabsorption of K+ from tubule

back to blood (similar to Na+ mechanism)

– Insulin increased K+ taken up by cells• So K+ must be monitored in diabetic patients

Isotonic Alterations

• Volume of fluid changes, but numbers and types of electrolytes remain at normal levels (Table 4-5)– Loss volume depletion (hypovolemia); occurs

with• Hemorrhage• Severe wound drainage• Excess sweating• Burns• Third spacing

Hypovolemia – cont’d

– Fluid lost from blood vessels, but remains in body– Receptors in the vessels sensitive to pressure– Interpreted as fluid loss, so decr’d ECF volume

• Decr’d urine output

• Weight loss (through fluid weight)

• Can hypovolemic shock

– Symptoms of hypovolemia• Decr’d blood pressure

• Incr’d heart rate

Hypervolemia

• Excess body fluid; occurs with:– Excessive IV fluids – Overproduction aldosterone

• Why should this lead to hypervolemia?

– Some drugs (ex: cortisol)– With incr’d ECF volume

• Weight gain (fluid weight)

• Diluted urine

• Incr’d blood pressure

• Can also edema

Edema -- ECF Isotonic Volume Excess

• Accum’n isotonic fluid in interstitial space (incr’d ISF)

• Forces that favor incr’d ISF also favor edema:– Incr’d BHP, if

• Chronic hypertension• Venous obstruction• Water retention

– Decr’d COP if• Not enough proteins/cells in the blood:

– Protein synthesis disorders of liver– Blood cell disorders decreased # of blood cells

Edema – cont’d– Increased capillary permeability, which can occur with

• Trauma

• Inflammation

• Decreased lymph drainage with– Blocked lymph node

– Surgical removal of lymph vessels

– REMEMBER: ISF drains into lymph vessels, which returns ISF to the bloodstream

Edema – cont’d

• Clinical– Pitting – Weight gain (water weight)– Neck vein distension– Incr’d blood pressure

• Treatment– Treat underlying conditions (tumor, blood cell

disorder, etc.)– Relieve symptoms

Electrolyte Imbalances (Table 4-6): Sodium

• Hypernatremia (a hypertonic imbalance)– Plasma Na+ > 145 mEq/L– Too much Na+ or too little water– “Tonicity”: # of solute particles in solution

• Hypertonic -- high amt solute

• Hypotonic = dilute

– Characteristics of hypernatremia: movement of water from ICF to ECF, so

– Cells dehydrate – Overall incr’d ECF vol (at expense of the cell vol)

Sodium Imbalances – cont’d

• Hypernatremia – cont’d– Due to

• Admin hypertonic IV sol’ns

• Oversecretion aldosterone

• Loss of pure water

• Long term sweating w/ chronic fever

• Respiratory infection water vapor loss

• Diabetes polyuria

• Insufficient water ingested

Sodium Imbalances – cont’d

• Hypernatremia – cont’d– Clinical

• Thirst

• Lethargy

• Neurological dysfunction (dehydration of brain cells)

– Treatment• Lower serum Na+

• Use isotonic salt-free IV fluid (5% glucose) to replace body water; returns Na+ concentration to normal levels

Sodium Imbalances – cont’d

• Hyponatremia – Overall Na+ decr ECF– Two types

• Depletional: Too little Na+• Dilutional: Too much water

– Causes of each type• Depletional (Na+ lost out of body or insufficient)

– Diuretics– Chronic vomiting– Chronic diarrhea

» Though electrolytes loss – Decr’d aldosterone– Decr’d Na+ intake

Sodium Imbalances – cont’d

• Hyponatremia – cont’d• Dilutional

– Renal dysfunction w/ incr’d hypotonic fluid intake– Excessive sweating increased thirst intake excessive pure water

• Syndrome of Inappropriate ADH (SIADH) impaired renal excretion of water

• Excessive beer drinking (???)– Clinical

• Neurological symptoms– Now improper Na+ concentration outside cell improper

depolarization/action potential/neuron conduction– Seizures possible

– Treatment• Restrict water, or• Administer Na+

Electrolyte Imbalances: Potassium

• Hypokalemia– Characteristics

• Serum K+ < 3.5 mEq/L

• Beware if diabetes:– Insulin plays a role in K+ into the cell

– Ketoacidosis incr’d H+ in ECF (so ECF too highly + charged)

» H+ enters cells

» Body tries to equilibrate + charges by moving K+ out of cells

» Now ECF high in K+ (lost through urine)

» Overall whole body K+ deficit

Potassium Imbalances – cont’d• Hypokalemia – cont’d

– Causes• Decr’d K+ intake (rare)• Incr’d K+ loss

– With chronic diuretics, g.i. disturbance– Acid/base imbalance K+ out of cells into ECF, then lost through

urine

• Clinical– Neuromuscular disorders– Cardiac arrest

• REMEMBER: Na+/K+ gradient for proper action potentials in neurons/muscles

– Treatment• Increase K+ intake• BUT slowly to avoid abrupt Na+/K+ gradient change

Potassium Imbalances – cont’d

• Hypokalemia – cont’d• Clinical

– Neuromuscular disorders– Cardiac arrest– REMEMBER: body maintains Na+/K+ gradient for proper

action potentials in neurons/muscles so proper neuron/muscle function

– Treatment• Increase K+ intake• BUT must be increased slowly to avoid abrupt Na+/K+ gradient

change

Potassium Imbalances – cont’d• Hyperkalemia

– Serum K+ > 5.5 mEq/L

– Importance/causes/when to check• Renal disease (kidney regulates K+)

• Massive cellular trauma– High intracellular K+ is released into ECF

• Insulin deficiency– Insulin plays a role in K+ uptake into cells

• Addison’s disease altered aldosterone secretion

• Decr’d blood pH (high blood acidity) H+ into cells K+ out of cells ECF High K+ in ECF

– BUT over time K+ will be lost to urine overall decrease in body K+

Potassium Imbalances – cont’d

• Hyperkalemia – cont’d– Clinical

• Muscle weakness, paralysis

• Change in ECG pattern

– Treatment• Insulin + glucose K+ taken into the cells (out of

ECF)

• Bicarbonate – buffers H+ ions if hyperkalemia due to acidosis

• Ca+2 counteracts K+ effects on heart