6
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