Normal Fluid Exchange
Osmosis– refers to the tendency of a solvent (e.g. water) to pass from a solution of lesser concentration to one of greater solute
concentration when the two solutions are separated by a semipermeable membrane
‘Oncotic pressure’thus refers to the pressure generated by this act of osmosis.
‘Hydrostatic pressure’– refers to the pressure of water (i.e., the fluid part of the blood), influenced by the activity of the heart
as a pump, the elasticity and recoil of blood vessels, the quantity of blood within a vessel, etc.
Normal tissues, the tissue hydrostatic and oncotic pressures are near zero, and as such contribute little to fluid Inchange.
the normal capillary, at the arteriolar end of the capillary bed vascular hydrostatic pressure (pushing fluid out of vessels) is
higher than the vascular oncotic pressure (drawing fluid into vessels). The net effect drives fluid into the tissues
-At the venular end of the capillary bed, vascular hydrostatic pressure (pushing fluid out of vessels) is lower than the vascular
oncotic pressure (drawing fluid into vessels). This drives fluid back into blood vessels, carrying away the metabolic waste
byproducts of the tissues (carbon dioxide, etc.Normally, the fluid gains and losses between the arteriolar and venular ends of
the capillary bed are balanced, such that the net gains and losses of fluid from the vasculature are equal.
- normally, tissue hydrostatic and colloid oncotic pressures are near zero, and do not really affect this fluid exchange.
How does the fluid move out of the capillary?
It moves out of the capillary primarily at the pores,the junctions between endothelial cells. These pores permit only small
molecules to move through – water, salts and small organic molecules. This fluid which has passed out of the normal capillary
is termed ultrafiltrate.The larger protein molecules and the cellular elements of the blood are retained in the capillary.
Does all of the ultrafiltrate return to the capillary?
No, although most of it does, via the “pull” of the oncotic pressure of the plasma proteins in the capillary. Any remaining fluid,
and the small protein molecules which may also have moved out, drains from the tissue via the lymphatic system. The
lymphatic vessels eventually drain into the bloodstream, so the fluid does return, just not at the level of the capillary bed.
Edema : accumulation of excess fluid in the tissue spaces (also termed the interstitium).
- This can occur in any tissue, but it is most easily seen in the skin and the associated subcutaneous tissues.
Localized edema : A spider or other insect bite, with its associated puffy swelling of the skin.
Pitting edema:Edema of the skin ; if focal pressure is applied to the edematous area with the tip of a finger, a small focal
depression or “pit” will be produced, as the pressure has forced fluid out of the area.
Generalized edema : excess fluid being seen in many tissues. An example of this is the edema of congestive heart failure.
When tissue fluid accumulates in body cavities such as the pleural space, pericardial sac, or the peritoneal cavity, the term
Effusion is used.
Ascites :An effusion into the peritoneal cavity
Anasarcais : indicate massive edema of the whole body, including the body cavities.
- “potential spaces” within the body. The pleural space is the “space” between the surface of the lung (the visceral pleura) and
the pleura lining the chest wall (the parietal pleura).
Pericardial sac is the membranous sac which surrounds the heart, separating it from the adjacent lungs.
Peritoneal cavity is the abdominal cavity, limited by the diaphragm, the abdominal walls, and the pelvic floor, and containing
the abdominal organs.
What might cause this localized edema?Let’s return to the situation in a normal capillary bed.At the arteriolar end of the
capillary bed, fluid tends to move out of the capillaries because hydrostatic pressure (HP) exceeds the oncotic pressure (OP).At
the venular end of the capillary bed, fluid tends to move back into the capillaries, because the OP in the vessels exceeds the HP. Now, what will happen if the venous drainage of the capillary bed is obstructed? Indicate whether you think the osmotic and
hydrostatic forces at the venular end of the capillary bed will increase, decrease, or remain the same.
Obstructed venous drainage of the capillary bed
- hydrostatic force at the venular end of the capillary bed would be increased, when the venous drainage of the tissue is
obstructed, fluid will leave the capillaries normally at the arteriolar end of the capillary bed (HP>OP), but can not return to the
capillaries as easily at the venular end of the capillary bed, as HP still exceeds OP. Thus, the fluid remains in the tissue spaces as
- the extent of edema which occurs with venous obstruction is variable– it will depend on the size of the vessel(s) obstructed,
how completely they are obstructed, and the extent of collateral venous circulation in the area. If there is complete failure of
venous drainage of an area, severe edema and hemorrhage may result –
- normal fluid balance, the lymphatic system is responsible for removing the small protein molecules which leave the capillaries
normally, as well as a small amount of fluid which does not return to the capillaries via the oncotic pressure of plasma colloids.
- Venous obstruction is the cause of more immediate edema, but lymphatic obstruction can also cause edema over time.
Lymphatic obstruction :initially small amounts of fluid will remain in the interstitium. The accumulation of small protein
molecules over time will lead to increased tissue colloid oncotic pressure ( normally zero):
Two important causes of localized edema are:
Lymphatic obstruction will lead to elevated tissue colloid osmotic pressure, which will favor fluid remaining in the tissue spaces.
As the “pull” of the plasma colloid osmotic pressure is not sufficient to move adequate amounts of fluid back into the capillaries,
edema will develop. If this edema persists, the edematous tissue may undergo fibrosis, causing the affected area to become firm
There are two additional causes of local edema – acute inflammation,as in the spider bite mentioned earlier, and acute
allergic reactions(such as “hives”). In both acute inflammation and allergic reactions an increase in capillary permeability
allows fluid and plasma proteins to move out of the capillaries and into the tissues.
A person in congestive heart failure may show puffy swelling of the lower legs and ankles.
Why does this occur? We noted that local venous obstruction increased the hydrostatic pressure at the venous end of the
capillary bed, so that the osmotic pressure was insufficient to “pull” fluid back into the capillaries. In congestive heart failure,
the heart is failing in its role as a forward pump. If there is right ventricular failure,venous blood “backs up” in the systemic
circulatory system– think of it as a “body-wide” venous obstruction causing a generalized increase in venous hydrostatic
pressure.Fluid thus gradually builds up in the interstitium, leading to a generalized edema.
What can cause congestive heart failure?
Heart failure is a pathophysiologic condition, not a specific disease.
- anything which decreases the cardiac output, by decreasing the contractility of the heart muscle(cardiac muscle disease, either
primary or secondary), by causing mechanical abnormality(valvular regurgitation, intracardiac shunting, restrictive heart disease,
and increased resistance to forward outflow, such as hypertension or aortic stenosis) or causing electrical
disturbance(arrhythmias) can lead to congestive heart failure.
If the left heart is failing its role as a forward pump, then blood will tend to “build up” in the pulmonary circulation (pulmonary
venous congestion). The increased pulmonary venous hydrostatic pressure means that fluid will tend to remain in the lungs, as
There is a slight difference here, as the pulmonary circulation normally functions at a low hydrostatic pressure – the HP