F & E Part I Lecture Notes

Fluid & Electrolytes

• Human life is suspended in a salt solution (concentration 0.9%)

• For life to continue and cells to properly function, body fluids must maintain constant composition of water and electrolytes

Homeostasis: Essential to Life

• Maintenance of stable environment for body cells

• Internal environment: narrow range of normal values

• Ongoing process: changes constantly occurring in the body

Keys to Homeostasis

• Constancy: steady state

– Prevent or compensate for unacceptable changes

– Fluid volume, electrolyte concentrations remain the same

• Equilibrium: balance

– Fluid intake = fluid output

Homeostatic Mechanisms:

• Responses to disruptive changes

– Compensatory

– Self-regulating

• Work by negative feedback:

– Stimulus (stressor):

– Response (mechanism):

• Compensates for original stimulus

• Then turns self off

Negative Feedback: Works Like a Thermostat…

Negative Feedback System: Calcium and PTH

Body and Water

• 55-60% of adult body weight is water

– Higher due to muscle (more H2O) or lower due to fat (less H2O)

– Females lower

• Fluctuations in the amount of water in the body can have harmful and even fatal consequences

Functions of Water in the Body

• Water is vital to health and normal cellular function, serving as:

– A medium for metabolic reactions within cells

– A transporter for nutrients, gases, wastes

– A lubricant

– An insulator and shock absorber

– Maintenance of body temperature

Body Fluids: Water and Solutes

• The term ‘fluid' refers to water and the components it contains

– Solution: liquid containing a dissolved substance (fluid)

– Solvent: liquid part of a solution (water)

– Solute: substance that can be dissolved in a solution (electrolyte)

Cell Membrane

• Membrane

– Permeability

– Selective permeability

– Impermeability

• Controls differences in fluid and electrolyte composition in the different body compartments

Fluid Compartments

Water…

• Most common substance in body

• 55% - 60% total adult body weight

• ~42 kg for 70 kg man

Fluid Compartments

• 2 compartments:

– Intracellular fluid (ICF)

• 60% TBW

• Body (cell) metabolism

– Extracellular fluid (ECF)

• 40% TBW

• Nutrients, gas, and wastes exchanged

ECF Subcompartments

• Interstitial (ISF)

– Tissue fluid, lymph

– 75% ECF

– Surrounds, bathes cells

• Intravascular (Plasma)

– 25% ECF

– Entrance and exit site for F & E

• Transcellular

– Fluid in transit and special spaces

Transcellular Fluid is Found Where?

Intravascular Fluid: Plasma

• Fluid in the intravascular space minus the formed elements of blood

• Consists of water, protein, nutrients, electrolytes, and waste products

• Electrolytes: narrow range

Solutes: Two Types

• Electrolytes (crystalloids)

– Solutes with an electrical charge

– Break apart into ions

• Nonelectrolytes

– Solutes without an electrical charge

– Remain intact in solution

• Glucose, urea, lipids, C02, 02

• Proteins (colloids)

– Albumin, globulin, fibrinogen

Electrolytes

• Chemical substances that break apart into electrically charged particles (ions) with positive (+) or negative (-) charges when placed in a solution

– Cation: positive charge

– Anion: negative charge

Ions

• Dissociated particles of electrolytes

• Carry either a positive or negative charge

– Cations (+): sodium (Na+), potassium (K+), calcium (Ca++), magnesium (Mg++), hydrogen (H+)

– Anions (-): chloride (Cl-), phosphorus (HPO4 2 -), and bicarbonate (HCO3-)

Electrolytes: Measurement

• Milliequivalent (mEq): chemical combining power of the ion

– Measured per liter (mEq/L)

– 1 mEq of any cation equals 1 mEq of any anion

– Sodium and chloride are equivalent since they combine equally

– Most common measurement

• Milligram or grams (mg %, g%): weight of ions in a solution

– Concentration

– mg/dL (100 ml) or g/dL

– Calcium, phosphate, magnesium, 0.9% NaCl

Combining Power vs Weight

Functions of Electrolytes

• Essential minerals

• Fluid balance

• Acid-base balance

• Transmit neuromuscular impulses

Distribution of Electrolytes

• Extracellular fluid

– Sodium Na+

– Chloride Cl-

– Bicarbonate HCO3-

• Intracellular fluid

– Potassium K+

– Magnesium Mg2+

– Phosphate HPO42-

• Electroneutrality: total sum of cations must equal total sum of anions (pluses and minuses must equal)

Movement

Exchange: Water and Solutes

• Constant movement of water and solutes between different fluid spaces

• Goal: maintain homeostasis

• Selective permeability: controls solute movement

– Small particles (ions, 02, C02, H20) pass easily

– Larger molecules (glucose, proteins) have more difficulty passing between fluid compartments

• Sodium controls water distribution (water follows sodium)

Passive vs Active Transport

• Gradient: difference in concentration, pressure, or electrical charge between two compartments

• Passive transport: no energy expended

– Flows down the concentration gradient

– Only from high concentration to low

Passive Transport

Types of Passive Transport

• Diffusion

• Facilitated diffusion

• Osmosis

• Filtration

Simple Diffusion

• Random movement of solutes across a permeable membrane down a concentration gradient

• High conc.® low conc.

• Result: equal solute distribution (equilibrium)

• No ATP energy

• Example: smaller, fat-soluble molecules (02, C02)

Diffusion in Action

Facilitated Diffusion

• Diffusion of large, lipid-insoluble solutes across a membrane, with the help of transport proteins

• High conc.® low conc.

• Integral membrane protein acts as carrier

• No ATP energy

• Faster than simple diffusion

• Example: glucose

Osmosis

• Diffusion of water across a selectively permeable membrane (permeable to water, not to solute)

• High water conc. ® low water conc.

• Low solute conc. ® high solute conc.

• No ATP energy

• Examples: water movement from interstitium to cells, from interstitium to plasma

Osmotic Pressure

• Fluid-pulling power

• Exerted by all particles in a solution

• Driving force for movement of water across a cell membrane

• Solute = Osmotic pressure

• Osmotic pressure = water in

• ¯ Osmotic pressure = water out

Osmolarity

• Number of solute particles per 1L of body fluid

• Measures osmotic pressure (fluid-pulling power)

• Expressed as milliosmoles per liter (mmol/L)

• Normal osmolarity = 270-300 mmol/L

• Electrolyte molecules exert greater effect on osmosis than nonelectrolytes

• Sodium is greatest determinant of ECF osmolarity

• Water follows sodium

Tonicity

• Effect of osmotic pressure on cellular volume

• Concentration of solutes determines direction of water flow

• Isotonic: 270 – 300 mmol/L

– Equal solute and water—exact same number of particles in both solutions—no net movement (same)

• Hypertonic: > 300 mmol/L

– Greater solute, less water—water pulled out of cells (shrinks)

• Hypotonic: < 270 mmol/L

– Less solute, more water—water moves into cells (swells)

Tonicity of IV Fluids

• Isotonic: same osmolarity (270 – 300 mmol/L)

– Normal saline (NS or 0.9% NaCl), Lactated Ringers (LR)

• Hypotonic: fewer solutes (< 270 mmol/L)

– Water, ½ NS (0.45% NaCl), and D5W (5% dextrose in water, after the dextrose is used up)

Filtration

• Fluid-pushing power

• LARGE AMOUNTS (bulk flow) of water & solutes together are forced through capillary membranes by pressure in the blood

• High hydrostatic pressure ® low hydrostatic pressure

Filtration

• Filters out formed elements of blood and colloid proteins (no protein in interstitial fluid)

• Opposes osmosis (fluid-pulling power)

• No ATP energy

• Examples: capillary bed, glomerulus (kidneys)

Colloids: Plasma Proteins

• Large molecules unable to pass through membrane because of their size

– Albumin, globulin, fibrinogen

• Exert osmotic pull: colloid osmotic pressure, oncotic pressure

• Pull water back into the vascular system

• Colloid IV solutions:

– Albumin, Dextran, Hetastarch

– Remain in vascular compartment

Starling’s Forces

• Pressure differences in venous and arterial ends of capillaries influence direction of fluid movement

• Filtration (arterial end): fluid out (due to higher hydrostatic pressure)

• Reabsorption (venous end): fluid back in (due to higher colloid osmotic pressure)

Capillary Dynamics

• Interplay of 4 forces:

– Capillary hydrostatic pressure (CHP): MAJOR

– ISF hydrostatic pressure (IFHP): MINOR

– Capillary oncotic pressure (COP): MAJOR

– ISF oncotic pressure (IFOP): MINOR

Capillary Dynamics

• Arterial end: fluid out

– Plasma ® ISF

– Filtration prevails

• (CHP + IFOP) – (COP + IFOP) ~ 16 mm Hg

• Venous end: fluid in

– ISF ® Plasma

– Osmosis prevails

• (COP + IFHP) – (CHP + IFOP) ~ 9 mm Hg

• More fluid out than back in

• Excess tissue fluid returned to vascular space by lymphatic vessels

Lymphatic System: Returns Excess ISF to Vascular System

Active Transport

• Movement of solutes across a cell membrane against a concentration gradient, with the help of carrier molecules (“pumps”)

• Low conc. ® high conc.

• Requires ATP energy

• Example: sodium-potassium pump

Sodium-Potassium Pump

• Transport protein “pumps” in cell membrane

• Powered by ATP

• 3 Na+ ions transported to ECF against gradient

• 2 K+ ions transported to ICF against gradient

• Maintains ECF, ICF homeostasis

Can You Name the Process?

• Diffusion, osmosis, filtration, or active transport?

Fluid Movement Between Compartments