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Edema
 Excess accumulation of fluid in the interstitium (extra vascular space)
Homeostatic Factors
(Anti-edemous)
1. Action of lymphatic capillaries (fluids exuded from the blood vessels into the
tissue and return to the blood via the lymphatics)
 more fluid leaves the capillaries than can be reabsorbed at venous end of the
capillary network. The excess fluid returns to the heart via the lymphatic
vessels.
 Unidirectional openings at the ends of lymph vessels allow excess fluid from
the interstitium to the lymphatic vessels, one way valves prevent it from
flowing back into the interstitium.
 diaphragmatic breathing, movement of surrounding musculature and smooth
muscle contraction facilitate lymph flow.
 obstruction in lymph flow can lead to lymphedema
2. Colloid osmotic pressure in the blood (particles [albumin] in blood draw fluids
into the vessels) {remember albumin = one of the plasma proteins in blood}

Colloids are the large molecular weight particles present in a solution.

the term colloid osmotic pressure refers to the osmotic effect of particles in
the colloidal solution as apposed to dissolved crystaloids such as sodium

Osmosis is the passage of water from a dilute solution through a semipermeable membrane to a more concentrated solution.
or stated another way

Osmosis is the passage of water from a region of high water concentration
through a semi-permeable membrane to a region of low water concentration
When two water (or other solvent) volumes are separated by a semi permeable
membrane, (very thin layers of material [cell membranes are semi-permeable] which
allow some things to pass through them but prevent other things from passing through.)
water will flow from the side of low solute concentration, to the side of high solute
concentration.
Figure-1 shows connected vessels separated by a semi permeable membrane. If there is
only water in the device, the level will be the same at both sides. When solute molecules
are added to one side, water will start to flow into it, so that its level will go up on that
side, and down on the other side. (The system will stabilize when the hydrostatic pressure
generated by the difference in the water levels balances the osmotic pressure.)
In normal plasma, the plasma proteins are the major colloids. The capillary membrane is
almost impermeable to plasma proteins (albumin, globulins, fibrinogen, with albumin
being the most abundant at 70%), thus they draw fluid into the capillaries and offset the
pushing force of the hydrostatic pressure within the capillaries..
Note: it is the number of particles in a solution not the size of the particles that controls
the osmotic pressure. (magnitude of the gradient) (albumin is 1/6th the weight of
fibrinogen, or 1 gram of albumin contains 6x more solutes than 1 gram of fibrinogen)
3. Healthy normal capillary walls and heart, liver, and kidney function.
 capillary wall damage or increased capillary hydrostatic pressure will promote
edema
 ineffective heart pumping creates a “back up” of blood fluids in the system
(congestive heart failure and cor pulmonale)
 conditions such as liver disease cause a decrease in plasma protein production
which in turn decreases the amount of plasma proteins (colloids) within the
blood stream, thus promoting edema
 kidney conditions such as nephrotic syndrome will cause excessive amounts
of plasma proteins to be removed from the blood stream and excreted.

If profound hypoproteinemia is allowed to develop and colloid pressure falls below
hydrostatic pressure, infused fluid will no longer be effectively held in the circulation.
4. Tissue density (serous sacs like the pericardium, pleural lining)
5. Normal musculoskeletal mobility (action of muscles help to push fluid out of
region)
Factors promoting Edema
1. Colloid osmotic pressure of the interstitium (stronger colloids in the interstitium than
in the blood will pull fuid into the interstitium = edema)
2. Increased capillary wall permeability (e.g. inflammatory process)

plasma proteins are allowed to leak into the interstitium and fluid
osmololically accumulates in the interstitial space.
3. Increased hydrostatic pressure of the blood (increased blood pressure causes fluid to
exit the vessels) (e.g. venous obstruction/stasis)

common in the legs as gravity has to be overcome to return deoxygenated
blood to the heart and lungs.
4. Nutrition factors (e.g. malnutrition)

amino acids are used to produce plasma proteins (if no amino acids from the
diet, no plasma proteins will be synthesized)

When nutritional status deteriorates to a point that circulating proteins can no
longer sustain a colloid pressure capable of holding fluid in the circulation,
edema will develop and can be clinically significant.

Malnutrition can be manifested as either the hypoalbuminemic type of
malnutrition, adult kwashiorkor-like syndrome (kwashiorkor - severe protein
deficiency type of malnutrition found in children) or marasmus (caloric
deficiency secondary to acute disease).
5. Imbalances of sodium (sodium act like a solute in the plasma, excessive amounts or
decreased amounts can both be detrimental)