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