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Distribution of fluids within the body

-ICF: approximately 2/3 of total body water
-ECF: approximately 1/3 of total body water
-Interstitial fluid: approximately 3/4 of ECF
-Plasma Volume: approximately 1/4 of ECF


Extracellular Fluid

-plasma and interstitial fluid


Plasma broken down into

-venous compartment
-arterial compartment (effective circulating volume)


Vascular compartment

-contains blood volume which is plasma and cellular elements of blood, primarily RBCs


Effective circulating volume (ECV)

-the volume of arterial blood effectively perfusing thetissue


Transcellular fluid

-also included in ECF
-normally contains only a small amount of water such as epithelial secretions, synovial, CSF, etc
-said to occupy a "third space"


Components of ECF

-sodium, chloride, bicarbonate


Components of ICF

-potassium, magnesium, phosphate, and organic anions, proteins


Cell membrane between ECF and ICF is

-highly water-permeable
-not permeable to most electrolytes
-fluid distribution between 2 compartments is dependent on osmotic effects of Na


Capillary cell membrane

-between ECF compartments is highly permeable to small ions
-fluid distribution is due to balance between capillary hydrostatic pressure and colloid osmotic pressure


Maintenance of body fluid balance is regulated by 2 factors which over NaCl and water balance

-ECF volume and ECF osmolarity


Distribution of fluid between ECF and ICF compartments is determined primarily by:

-ion distribution (Na)
-ATPase activity


Distribution of ECF between plasma (vascular space) and interstitial (tissue) compartments is determined by

-balance of hydrostatic vs. oncotic pressures
-intravascular pressure in capillaries vs. plasma protein and solute concentration



-palpable swelling produced by expansion of interstitial fluid volume caused by:
-alteration in capillary hemodynamics (altered starling forces with increased net filtration pressure)--fluid moves from vascular space into interstitium due to decreased capillary oncotic pressure
-renal retention of dietary Na+ and water--expansion of ECF volume


Altered Starling foces role

-edema does not become apparent until interstitial volume is increased by 2.5-3L
-normal plasma volume is only 3L
-therefore, edema fluid is not derived from only plsma
-compensatory renal retention of Na+ and water to maintain plasma volume in response to underling of the vasculature must occur in this situation to cause edema
-this renal compensation is appropriate to restore tissue perfusion although it exacerbates edema (e.g. congestive heart failure)


Forces for filtration

-hydrostatic pressure and oncotic pressure


Hydrostatic pressure (blood pressure) in the capillary (Pc)

-directly related to blood flow; venous pressure; blood volume


Oncotic (osmotic) force in the interstitium

-determined by concentration of protein in the interstitial fluid
-normally the small amount of protein that leaks to the interstitium is minor and is removed by lymphatics
-thus, under most conditions this is not an important factor influencing the exchange of fluids


Forces for absorption

-oncotic pressure of plasma and hydrostatic pressure in interstitium


onctotic (osmotic) pressure of plasma

-the oncotic pressure of plasma solutes that cannot diffuse across the capillary membrane; i.e., the plasma proteins
-albumin is the most abundant plasma protein and biggest contributor to this force


Hydrostatic pressure in the interstitium

-in most cases close to zero and is not a signficant factor affecting filtration versus reabsorption
-can become significant if edema is present or it can affect glomerular filtration in the kidney (pressure in Bowman's space is analogous to interstitial pressure)


Renal retention of Na+ and water

-results in overfilling of the vascular tree
-inappropriate renal fluid retention
-usually results in elevated blood pressure, expanded plsma and interstitial volumes
-E.g. primary renal disease (glomerulonephritis, nephrotic syndrome)


Non-pitting edema

-swollen cells due to increased ICF volume--does NOT respond to diuretics


Pitting edema

-increased interstitial fluid volume
-nephrotic syndrome, CHF, pregnancy, cirrhosis
-does respond to diuretics


Primary causes of peripheral edema

-increased capillary hydrostatic pressure
-increased interstitial oncotic pressure
-decreased vasscular oncotic pressure
-increased capillary permeability (k)
-lymphatic obstruction/removal (lymphedema)


Increased capillary hydrostatic pressure (Pc)

-marked increase in blood flow, e.g., vasodilation in a given vascular bed
-increased venous pressure, e.g., venous obstruction or heart failure
-elevated blood volume (typically the result of Na+ retention), e.g., heart failure


Increased interstitial oncotic pressure

primary cause is thyroid dysfunction (elevated mucopolysaccharides in interstitium)
-act as osmotic agents resulting in fluid accumulation and a non-pitting edema
-lymphedema can also increase this


Decreased vascular oncotic pressure

-liver failure
-nephrotic syndrome


Increased capillary permeability

-circulating agents, e.g., tumor necrosis factor alpha, bradykinin, histamine, cytokines related to burn trauma, etc., increase fluid filtration resulting in edema


Lymphatic obstruction/removal (lymphedema)

-bacterial lymphangitis