Lecture Mosby''s paramedic textbook (4th ed) - Chapter 11: General principles of pathophysiology
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9/10/2012
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Chapter 11
General Principles of
Pathophysiology
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Lesson 11.1
Cellular Environment,
Water and Electrolyte
Balance
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Learning Objectives
• Describe the normal characteristics of the cellular
environment and the key homeostatic mechanisms
that strive to maintain an optimal fluid and
electrolyte balance.
• Outline pathophysiological alterations in water and
electrolyte balance and list their effects on body
functions.
• Describe the treatment of patients with particular
fluid or electrolyte imbalances.
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Cells
• Basic unit of higher life forms
• Components
– Cell membrane
• Holds cell together
• Separates internal cellular environment from external
– Enzymes help biochemical processes
– Internal membranes to encapsulate chemicals
– Genetic material for replication
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Cells
• Form four basic types of tissue
– Epithelial tissue
– Connective tissue
– Muscle tissue
– Nervous tissue
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Cellular Environment
• Human body cells live in a fluid environment,
consists mainly of water
– Body water essential
• Medium in which all metabolic reactions occur
• Body’s health depends on precise regulation of volume
and composition of this fluid
– Body has two fluid compartments
• Intracellular fluid (ICF)
• Extracellular fluid (ECF)
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Intracellular Fluid and
Extracellular Fluid
• Intracellular fluid (ICF)
– Found in all body cells
– 40% of body weight
• Extracellular fluid (ECF)
– Fluid found outside of cells
– 20% of total body weight
– Blood plasma composes about 1/3
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Interstitial Fluid
• Cellular fluid between cells and outside
vascular bed
• Includes cerebrospinal and intraocular fluid
• Accounts for 15 to 16% of total body weight
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Aging and Fluid Distribution
• Body water accounts for 50 to 60% of the total
weight in adults
– With age, distribution and amount decrease to
about 45 to 55%
• Increases risk of dehydration, electrolyte abnormalities
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Based on the causes of dehydration
and your knowledge of anatomy and
physiology, what two age groups do
you think are at highest risk for
dehydration?
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Water Movement Between
ICF and ECF
• Body fluids constantly move from one
compartment to another
– Remains about the same in healthy people
• To keep volume stable
– Osmosis
– Diffusion
– Mediated transport mechanism
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Osmosis
• For healthy body, molecules must be able to
move within cell/across cell membrane
• Semipermeable membranes
– Separate fluid compartments
– Allow fluid to pass freely
– Regulate flow of solutes on the basis of size,
shape, electrical charge
• Maintain homeostasis
– Channels within permit solute passage
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Osmosis
• Diffusion or spreading of water molecules
across semipermeable membrane from lower
solute concentration to higher solute
concentration
• Separates two solutions of different
concentrations by blocking transport of salts
or other solutes
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Osmosis
• Osmotic pressure
– Pressure that prevents the flow of fluid across a
semipermeable membrane
– Pressure to maintain equilibrium depends on
• Number and molecular weight of particles on each side
of the cell membrane
• Membrane permeability to these particles
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Solutions
• Hypertonic solution
– When a living cell is placed in solution with a
higher solute concentration, lower water
concentration than that inside the cell
– When a cell is in solution, the osmotic pressure
exerted produces net movement of water out of
the cell
– Causes cell to dehydrate, shrink, possibly die
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Solutions
• Hypotonic solution
– When a living cell is placed in solution with a
lower solute concentration, higher water
concentration than that inside the cell
– Osmotic pressure draws water from the solution
into the cell
• Net movement of water into the cell
• Can swell, possibly burst
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What happens to a raisin when it is
placed in a cup of water for an hour?
Why does this change occur? Is the
water hypotonic, hypertonic, or isotonic
relative to the inside of the raisin? Does
a concentration gradient exist?
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Solutions
• Isotonic solution
– When a cell is placed in solution with the same
solute and water concentration as the solution
inside the cell
• No net movement of water molecules
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Diffusion
• Result of constant motion of all atoms,
molecules, or ions in a solution
• Passive process
– Molecules or ions move from an area of higher
concentration to an area of lower concentration
– Area of high concentration has more solute
particles than area of low concentration
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Diffusion
• Passive process
– More solute particles move from higher
concentration to lower one
– Once at equilibrium, movement of solutes in one
direction is balanced by equal movement in
opposite direction
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Diffusion
• Concentration gradient
– When the concentration of the solute is greater at
one point in the solvent than at another point
• Solutes diffuse down their concentration gradients
from high to low concentration until equilibrium
is achieved
• Some nutrients enter and some waste products leave
the cell by diffusion
– Maintenance of proper intracellular
concentrations of certain substances depends on
this process
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Mediated Transport Mechanism
• Required to move large, water‐soluble molecules,
electrically charged molecules across cell
membranes
– Some vital molecules (glucose) cannot enter by diffusion
– Some products (proteins) cannot exit by diffusion
• Use carrier molecules
– Proteins combine with solute molecules on one side of
the membrane
– Change shape, pass through the membrane, release solute
molecule on other side
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Carrier‐Mediated Transport
• Two types
– Active transport
– Facilitated transport
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Carrier‐Mediated Transport
• Active transport
– Moves substances against concentration gradient,
from areas of lower concentration to areas of
higher concentration
• Cell must expend energy to work against this
concentration gradient
• Occurs at faster rate than diffusion
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Carrier‐Mediated Transport
• Facilitated diffusion
– Moves substances into/out of cells from area of
higher concentration to area of lower
concentration
• Direction of movement is with concentration gradient
• Occurs more quickly than in normal diffusion
• Facilitated diffusion does not require cell to
expend energy
• Moving force is downhill concentration gradient
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What is the connection between
necessary cellular processes and
patient care?
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Water Movement Between
Plasma and IF
• Fluid is transferred between circulating blood
and interstitial fluid as a result of pressure
changes
– Occur at arterial and venous ends of the capillary
• Human body has about 10 billion capillaries
• Few of the body’s functional cells are farther than
5/1000 inch (20 to 30 microns) from one another
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Anatomy of Capillary Network
• Thin‐walled tube of endothelial cells
• No elastic, connective tissue, smooth muscle
that would impede transfer of water, solutes
• Blood enters from arterioles, flows through
capillary network into venules
– Capillary ends closest to arterioles are
arteriolar capillaries
– Ends closest to venules are venous capillaries
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Anatomy of Capillary Network
• Nutrient, metabolic end product exchange
takes place at the capillary level
• Arterioles give rise to capillaries, metarterioles
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Anatomy of Capillary Network
• Most tissues have distinct types
– True capillaries
– Thoroughfare channels
• From metarteriole, blood may flow into thoroughfare
channel that connects arterioles and venules directly,
bypassing true capillaries
– Blood flow through thoroughfare channel is constant
– From channel, fluid exits/reenters network of
true capillaries
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Anatomy of Capillary Network
• Sphincters
– Capillary: small cuffs of smooth muscle that
encircle proximal and distal capillary portions
– Precapillary: arterial end sphincter
– Postcapillary: venous end sphincter
– Control blood flow, open, close capillary
entrance, exit
– True capillary blood flow is not uniform, depends
on contractile state, sphincter presence
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Anatomy of Capillary Network
• Nutritional flow
– Blood flow through capillaries that provides
exchange of gases, solutes between blood, tissue
• Nonnutritional, shunt flow
– Blood bypasses capillaries traveling from arteriole
to venous side of circulation
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AV Shunts
• True arteriovenous anastomoses (AV shunts)
– Occur naturally in sole of foot, palm of hand,
terminal phalanges, nail bed
– Regulate body temperature
– Some evidence suggests presence upstream from
capillary sphincters
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Capillary Network
• Sympathetic fibers innervate all blood vessels,
except
– Capillaries
– Capillary sphincters
– Most metarterioles
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Capillary Network
• Sympathetic innervation includes
vasoconstrictor, vasodilator, vasomotor fibers
– Vasoconstrictor fibers most important in
regulating blood flow
– Normal circulation with adequate arterial BP,
arterioles are open, AV shunts closed, 20%
capillaries open at any given time
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Capillary Network
• Diffusion across capillary wall
– Tissue cells do not exchange material directly
with blood
– Interstitial fluid acts as “middle man”
– Nutrients must diffuse across capillary wall into
interstitial fluid to enter cell
– Metabolic end products (CO2, lactic acid) must
cross membrane into interstitial fluid to diffuse
into plasma
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Capillary Network
• Diffusion across capillary wall
– At capillary arteriole end, the forces moving fluid
out of capillary are greater than the forces
attracting fluid into it
– At venous end, forces are reversed, more fluid is
attracted into capillary
– Hydrostatic and osmotic pressure forces
responsible for fluid movement
– Hydrostatic pressure, created with each heart
beat, forces water out arterial end
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Capillary Network
• Diffusion across capillary wall
– Blood colloid osmotic pressure or oncotic pressure
• When osmotic pressure results from presence of
plasma proteins, mostly albumin, too large to pass
through capillary wall
– At venous end
• Hydrostatic pressure is lower
• Protein concentration increases slightly, occurs from
fluid movement out arteriolar end
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Capillary Network
• Diffusion across capillary wall
– Result is greater plasma protein concentration,
greater colloid osmotic pressure
– Nearly all fluid that leaves capillary arteriolar end
reenters venous end
– Remaining fluid enters lymphatic capillaries,
eventually returned to general circulation
– Net filtration, fluid movement back and forth
across capillary wall
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Capillary Network
• Diffusion across capillary wall
– Starling hypothesis
• Net filtration = forces favoring filtration – forces
opposing filtration
• Forces favoring filtration include capillary hydrostatic
pressure, interstitial oncotic pressure
• Forces opposing filtration are plasma oncotic pressure,
interstitial hydrostatic pressure
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Capillary Network
• Diffusion across capillary wall
– Starling hypothesis
• Fluid also exchanged across wall as a result of cyclic
dilation, constriction of precapillary sphincter
• When sphincter dilates, pressures rise in the capillary,
which forces fluid into interstitial spaces
• When precapillary sphincter constricts, pressure drops,
fluid moves into capillary
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Capillary and Membrane
Permeability
• Permeability changes may allow plasma
proteins to escape into interstitial space
– Resultant increase in interstitial oncotic pressure
changes relationship defined by Starling
hypothesis
– Leads to osmotic movement, water into interstitial
space, results in tissue edema
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Alterations in Water Movement
• Edema is fluid accumulation in interstitial
spaces
– Caused by any condition that leads to fluid
movement out of capillaries, into interstitial
tissues
– Problem of fluid distribution, does not always
indicate fluid excess
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Pathophysiology of Edema
• Factors of normal fluid flow through
interstitial spaces
– Capillary hydrostatic pressure filters from blood
through capillary wall
– Oncotic pressure exerted by proteins in blood
plasma, attracts fluid from interstitial space back
into vascular compartment
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Pathophysiology of Edema
• Factors of normal fluid flow through
interstitial spaces
– Permeability of capillaries, determines how easily
fluid can pass through capillary wall
– Presence of open lymphatic channels, which
collect some fluid forced out of capillaries by
hydrostatic pressure of blood, return fluid to
circulation
• When any factors are disturbed, changes in water
movement can develop
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Pathophysiology of Edema
• Mechanisms most often responsible for
edema
– Increase in hydrostatic pressure
– Decrease in plasma oncotic pressure
– Increase in capillary permeability
– Lymphatic obstruction
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Pathophysiology of Edema
• Increased capillary hydrostatic pressure
– Caused by venous obstruction or sodium and
water retention
– With venous obstruction, hydrostatic pressure of
fluid in capillaries can become great enough to
cause fluid to escape into interstitial spaces
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Pathophysiology of Edema
• Increased capillary hydrostatic pressure
– Conditions that can lead to venous
obstruction, edema
• Thrombophlebitis (blood clot formation, inflammation
in vein)
• Chronic venous disease
• Hepatic obstruction (hepatic veins or common bile
duct blockage)
• Tight clothing around extremity
• Prolonged standing
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Pathophysiology of Edema
• Increased capillary hydrostatic pressure
– Sodium, water retention can cause increase in
circulating fluid volume, edema
– Conditions associated with sodium,
water retention
• Congestive heart failure
• Renal failure
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Pathophysiology of Edema
• Decreased plasma oncotic pressure
– Decreased plasma albumin leads to decreased
plasma oncotic pressure
• Result: fluid moves into interstitial space
• Most often results from liver disease,
protein malnutrition
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Pathophysiology of Edema
• Increased capillary permeability
– Result: greater than normal fluid filtration into
interstitial space
– Associated with allergic reactions
– Linked to inflammation and immune response
triggered by trauma
•
•
•
•
•
Burns, crushing injuries
Proteins escape from vascular bed
Capillary oncotic pressure decreases
Fluid oncotic pressure increases
Result: edema
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Pathophysiology of Edema
• Lymphatic obstruction
– Proteins, fluid accumulate in interstitial space
when lymphatic channels are blocked by infection,
surgically removed
– Obstruction blocks normal pathway by which fluid
is returned from interstitial space into circulation
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Pathophysiology of Edema
• Lymphatic obstruction
– Leads to edema in region normally drained by
lymphatic channels
– Conditions that can cause obstruction
• Certain malignancies
• Parasitic infections
• Surgical removal of lymphatics
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Pathophysiology of Edema
• Clinical manifestations of edema
– Localized
•
•
•
•
•
Limited to injury site or organ system
Sprained ankle
Cerebral edema
Pulmonary edema
Can be life threatening
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Pathophysiology of Edema
• Clinical manifestations of edema
– Generalized
• More widespread
• Obvious in dependent body parts
• First noted in legs and ankles when standing/sitting,
sacrum and buttocks when lying down
• Causes weight gain, swelling, puffiness
• Linked to other symptoms caused by underlying illness
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Why does the RICE (rest, ice,
compression, elevation) treatment
for swelling from a sprained ankle
decrease tissue edema?
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In septic shock, toxins affect the cell
membrane permeability, allowing
fluids to leak out of the blood
vessels more freely. How could that
affect cardiac output?
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Pathophysiology of Edema
• Clinical manifestations of edema
– Generalized
• In industrialized countries, most often caused by heart,
kidney, liver disease
• In developing countries, most common cause is
malnutrition and parasitic disease
• When tissue is compressed, fluid is pushed aside,
leaving indentation that gradually refills, pitting edema
• Ascites: fluid accumulation in peritoneal cavity
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Pathophysiology of Edema
• Water follows osmotic gradient established by
changes in sodium concentration
– Sodium and water balance are closely related
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Water Balance
• Regulated by antidiuretic hormone (ADH)
– ADH secretion, thirst perception help regulate
• ADH release triggered by increase in plasma
osmolality, decrease in circulating blood
volume, and decline in venous and arterial
pressure
– Increase in plasma osmolality stimulates
hypothalamic neurons, osmoreceptors, causes
thirst, increases ADH release from posterior
pituitary gland
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Water Balance
• ADH release response, water is reabsorbed into
plasma from distal renal tubules, collecting ducts
of kidneys
– Reduces water amount lost in urine
– Water reabsorbed, plasma osmolality deceases,
returning to normal
– Volume‐sensitive receptors and pressure‐sensitive
receptors also stimulate release
• Vomiting, diarrhea, excessive sweating cause
fluid depletion
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Sodium and Chloride Balance
• Sodium is major ECF cation
• Sodium balance regulated by aldosterone
• Regulates osmotic forces with chloride,
bicarbonate, hence water balance
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Sodium and Chloride Balance
• Chloride is major ECF anion, provides
electroneutrality in relation to sodium
• Increases or decreases in chloride occur in
proportion to changes in sodium
• Aldosterone secretion is triggered by decrease
in sodium levels, increase in potassium levels
– Causes distal kidney tubules to increase sodium
reabsorption, potassium secretion
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Sodium and Chloride Balance
• Renin enzyme is secreted by kidney
– Occurs when circulating blood volume is reduced,
sodium–water balance is disrupted
– Stimulates formation of angiotensin I, changed to
angiotensin II
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Sodium and Chloride Balance
• Renin enzyme is secreted by kidney
– Angiotensin II
• Potent vasoconstrictor
• Stimulates ADH secretion
• Results in reabsorption of sodium and water and
increase in systemic BP
– Renin‐angiotensin‐aldosterone system
• Mechanism regulating sodium and water
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Sodium and Chloride Balance
• Natriuretic hormone
– Helps regulate sodium
– Promotes secretion of sodium in urine
– Decreases tubular sodium reabsorption
– Subsequent sodium, water loss
• Atrial natriuretic factor
– Substance released from arterial heart cells
– Helps control sodium, water balance
– Promotes renal elimination of sodium
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