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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