Transcript Document
REGULATION OF FLUID AND
ELECTROLYTE BALANCE
• Body Fluids and Fluid Compartments
• Body Fluid and Electrolyte Balance –
fluid and electrolyte homeostasis
Why do we care about this?
ECF volume
Osmolarity
The Body as an Open System
“Open System”. The body exchanges
material and energy with its
surroundings.
Water Steady State
• Amount Ingested = Amount Eliminated
• Pathological losses
vascular bleeding (H20, Na+)
vomiting (H20, H+)
diarrhea (H20, HCO3-).
Electrolyte (Na+, K+, Ca++)
Steady State
• Amount Ingested = Amount Excreted.
• Normal entry: Mainly ingestion in food.
• Clinical entry: Can include parenteral
administration.
Electrolyte losses
•
•
•
•
Renal excretion
Stool losses
Sweating
Abnormal routes: e.g.. vomit and
diarrhea
Body Fluids and Fluid
Compartments
• The percentage of total body water: 45-75%
• Intracellular compartment
– 2/3 of body water (40% body weight)
• Extracellular compartment
– 1/3 of body water (20% body weight)
– the blood plasma (water=4.5% body weight)
– interstitial fluid and lymph (water=15% body
weight)
– transcellular fluids: e.g. cerebrospinal fluid,
aqueous humor (1.5% BW)
• Distribution of substances within the body is
NOT HOMOGENEOUS.
Body Water Distribution
•Individual variability (lean body mass)
–55 - 60% of body weight in adult males
–50 - 55% of body weight in adult female
–~42 L For a 70 Kg man.
RBC
PLASMA WATER
5%
CELL WATER
40%
28 L
Input
3L
INTERSTITIAL
FLUID
COMPARTMENT
15%
ECF
20% 14 L
10 L
TRANSCELLULAR WATER
1%
1L
Electrochemical Equivalence
• Equivalent (Eq/L) = moles x valence
• Monovalent Ions (Na+, K+, Cl-):
– 1 milliequivalent (mEq/L) = 1 millimole
• Divalent Ions (Ca++, Mg++, and HPO42-)
– 1 milliequivalent = 0.5 millimole
Solute Overview:
Intracellular vs. Extracellular
• Ionic composition very different
• Total ionic concentration very similar
• Total osmotic concentrations virtually
identical
Summary of Ionic composition
Protein
Organic Phos.
Inorganic Phos.
Bicarbonate
Chloride
Magnesium
Calcium
Potassium
Sodium
400
300
200
100
0
Plasma
H2O
Interstitial
H2O
Cell
H2O
Net Osmotic Force
Development
•
•
•
•
Semipermeable membrane
Movement some solute obstructed
H2O (solvent) crosses freely
End point:
– Water moves until solute concentration on
both sides of the membrane is equal
– OR, an opposing force prevents further
movement
Osmotic Pressure ()
• The force/area tending to cause water
movement.
= p
S
S
S
S
S
S
S
S
S
S S
S
S
Glucose Example
Initial
Gl
Gl
Gl
10 L
Final
Gl
Gl
15 L
Gl
Gl
10 L
Gl
5L
Osmotic Concentration
• Proportional to the number of osmotic
particles formed: Osm/L = moles x n (n, # of
particles in solution)
e.g. 1 M NaCl = 2 M Glu in Osm/L
• Assuming complete dissociation:
– 1mole of NaCl forms a 2 osmolar solution in 1L
– 1mole of CaCl2 forms a 3 osmolar solution in 1L
• Physiological concentrations:
– milliOsmolar units most appropriate
– 1 mOSM = 10-3 osmoles/L
Principles of Body Water
Distribution
• Body control systems regulate ingestion
and excretion:
– constant total body water
– constant total body osmolarity
• Osmolarity is identical in all body fluid
compartments (steady state conditions)
– Body water will redistribute itself as
necessary to accomplish this
Intra-ECF Water
Redistribution
Plasma vs. Interstitium
• Balance of Starling Forces acting
across the capillary membrane
– osmotic forces
– hydrostatic forces
Intracellular Fluid Volume
• ICFV altered by: changes in
extracellular fluid osmolarity.
• ICFV NOT altered by: iso-osmotic
changes in extracellular fluid volume.
• ECF undergoes proportional changes
in:
– Interstitial water volume
– Plasma water volume
Primary Disturbance:
Increased ECF Osmolarity
• Water moves out of cells
– ICF Volume decreases (Cells shrink)
– ICF Osmolarity increases
• Total body osmolarity remains higher
than normal
Primary Disturbance:
Decreased ECF Osmolarity
• Water moves into the cells
– ICF Volume increases (Cells swell)
– ICF Osmolarity decreases
• Total body osmolarity remains lower
than normal.
Plasma Osmolarity Measures
ECF Osmolarity
• Plasma is clinically accessible
• Dominated by [Na+] and the associated
anions
• Under normal conditions, ECF
osmolarity can be roughly estimated as:
POSM = 2 [Na+]p
mOSM
270-290
SOLUTIONS USED CLINICALLY
FOR VOLUME REPLACEMENT
THERAPY
• Isotonic Solutions --> n.c. ICF
• Hypertonic Solutions --> Decrease ICF
• Hypotonic --> Increase ICF
Type of solutions
• Saline solutions
– Come in a variety of concentrations: hypotonic
(eg., 0.2%), isotonic (0.9%), and hypertonic (eg.
5%).
• Dextrose in Saline
– Glucose is rapidly metabolized to CO2 + H2O
– The volume therefore is distributed intracellularly
as well as extracellularly
– Again available in various concentrations
– Used for simultaneous volume replacement and
caloric supplement
• Dextran, a long chain polysaccharide
– Solutions are confined to the vascular
compartment and preferentially expand this
portion of the ECF
Body Fluid and Electrolyte
Balance
•
Water input and output
The role of the kidneys in maintaining
balance of water and electrolytes
The regulation of body water balance
thirst sensation
control of renal water excretion by ADH
Thirst centers in the hypothalamus
relay information to the cerebral cortex where thirst
becomes a conscious sensation
controls the release of ADH
Stimuli for thirst sensation
Baroreceptors and stretch receptors as detectors
impulses sent to the thirst control centers in the
hypothalamus
Effect of ADH (vasopressin)
Factors affecting
ADH release
• Sodium balance
The kidneys - the major site of
control of sodium output
Influence of dietary input on
appropriate changes in sodium
excretion by the kidneys
Effector mechanisms include
changes in:
- glomerular filtration rate
- plasma aldosterone levels
- peritubular capillary Starling forces
- renal sympathetic nerve activity
- intrarenal blood flow distribution
- plasma atrial natriuretic factor (ANF
Effects of aldosterone
The renin-angiotensin system
release of renin
action of renin on the formation of angiotensin II
effects of angiotensin II: a.blood pressure; b.
synthesis and release of aldosterone; c. stimulation
of the hypothalamic thirst centers; d. release of
ADH
Pathway of RAAS
Principal cells & aldosterone
Net reabsorption of salt and water by the proximal
convoluted tubule
peritubular capillary hydrostatic forces
colloid osmotic pressure
Decrease in renal sodium excretion by stimulation
of renal sympathetic nerves
Release of Atrial natriuretic peptide (ANP)
in response to an increase in blood volume
increase sodium excretion by increasing GFR and
inhibiting sodium reabsorption
• Atrial natriuretic peptide
• Decreased blood pressure
stimulates renin secretion
The regulated variable affecting sodium
excretion - effective arterial blood volume
Changes in effective arterial blood volume
can elicit the appropriate renal response by
three possible mechanisms
a change in blood volume glomerular blood flow
and capillary pressure GFR
a change in blood volume detected by an
intrarenal baroreceptor release of renin
a change in blood volume could change
peritubular capillary Starling forces
Other factors affecting sodium excretion
include:
glucocorticoids
estrogen
osmotic diuretics
poorly reabsorbed anions
diuretic drugs
•
Potassium balance
Potassium plays a number of important
roles in the body
electrical excitability of cells
major osmotically active solute in cells
acid-base balance
cell metabolism
The kidneys are the major site in control of
potassium balance
Factors affecting the distribution of potassium
between cells and extracellular fluid include:
activity of the sodium-potassium pump
acid-base status of body fluids
availability of insulin
cellular breakdown due to trauma, infection,
ischemia, and heavy exercise
The regulation of plasma potassium by
hormones
insulin
epinephrine
aldosterone,
Factors affecting potassium excretion include:
intracellular potassium concentration
aldosterone
excretion of anions
urine flow rate