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

9781429208567

T h e B o d y a s a n O p e n S y s t e mT h e B o d y a s a n O p e n S y s t e m

“ O p e n S y s t e m ” . T h e b o d y e x c h a n g e s m a t e r i a l a n d e n e r g y w i t h i t s s u r r o u n d i n g s .

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

CELL WATERCELL WATER40% 28 L

RBC

INTERSTITIALFLUID

COMPARTMENT

15% 10 L

PLASMA WATER

5% 3 L

TRANSCELLULAR WATER

1% 1 L

Input

ECFECF20% 14 L

•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.

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

0

100

200

300

400

Protein

Organic Phos.

Inorganic Phos.

Bicarbonate

Chloride

Magnesium

Calcium

Potassium

Sodium

Summary of Ionic composition

InterstitialH2O

PlasmaH2O

CellH2O

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.

SS

S

S S S

S S SS

S

S S

= p

Glucose Example

Gl Gl Gl Gl

Gl Gl Gl Gl

Initial

Final

10 L 10 L

15 L 5 L

Osmotic Concentration

• Proportional to the number of osmotic particles formed: Osm/L = moles x n (n, # of particles in solution)

• 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

e.g. 1 M NaCl = 2 M Glu in Osm/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 270-290 mOSM

• Isotonic Solutions --> n.c. ICF

• Hypertonic Solutions --> Decrease ICF

• Hypotonic --> Increase ICF

SOLUTIONS USED CLINICALLY FOR VOLUME REPLACEMENT

THERAPY

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

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• 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