Edema is the swelling caused by excessive accumulation of fluid in tissues. Tissue fluid is formed by filtration of fluid from capillaries into tissues and is normally reabsorbed back into capillaries. Edema occurs when there is too much filtration, not enough reabsorption, or obstruction of lymphatic vessels draining tissue fluid. Edema can be intracellular within cells or extracellular outside of cells, and pitting edema leaves a depression when pressed due to free fluid whereas non-pitting edema does not due to cell or protein changes.

1. Edema

2. Introduction
Tissue fluid is the medium in which cells are bathed. It is otherwise known as
interstitial fluid. It forms about 20% of extracellular fluid (ECF).
FUNCTIONS OF TISSUE FLUID
Because of the capillary membrane, there is no direct contact between blood
and cells.
Tissue fluid acts as a medium for exchange of various substances between the
cells and blood in the capillary loop.
Oxygen and nutritive substances diffuse from the arterial end of capillary
through the tissue fluid and reach the cells.
Carbon dioxide and waste materials diffuse from the cells into the venous end
of capillary through this fluid

3. Formation of tissue fluid involves two processes:
1. Filtration.
2. Reabsorption.
Starling Hypothesis
Determination of net filtration pressure is based on Starling hypothesis.
Starling hypothesis states that the net filtration through capillary
membrane is proportional to the hydrostatic pressure difference across
the membrane minus the oncotic pressure difference. These pressures
are called Starling forces

4. FILTRATION
 Tissue fluid is formed by the process of filtration.
 Normally, the blood pressure (also called hydrostatic pressure) in
arterial end of the capillary is about 30mm Hg .
 Along the course of the capillary, the pressure falls gradually and it is
about 15 mm Hg at the venous end.
 Capillary membrane is not permeable to the large molecules,
particularly the plasma proteins. So, these proteins remain in the
blood and exert a pressure called oncotic pressure or colloidal
osmotic pressure. It is about 25 mm Hg.

5.  Osmotic pressure is constant throughout the circulatory system and it
is an opposing force for the filtration of water and other materials
from capillary blood into the tissue space.
 However, the hydrostatic pressure in the arterial end of the capillary
(30 mm Hg) is greater than the osmotic pressure.
 And, the net filtration pressure of 5 mm Hg is responsible for
continuous filtration

6. REABSORPTION
• Fluid filtered at the arterial end of capillaries is reabsorbed back into the
blood at the venous end of capillaries.
• Here also, the pressure gradient plays an important role.
• At the venous end of capillaries, the hydrostatic pressure is less (15 mm
Hg) and the oncotic pressure is more (25mm Hg).
• Due to the pressure gradient of 10 mm Hg, the fluid is reabsorbed along
with waste materials from the tissue fluid into the capillaries. About 10%
of filtered fluid enters the lymphatic vessels.
• Thus, the process of filtration at the arterial end of the capillaries helps
in the formation of tissue fluids
• The process of reabsorption at the venous end helps to maintain the
volume of tissue fluid.

8. EDEMA
Edema is defined as the swelling caused by excessive accumulation of
fluid in the tissues. It may be generalized or local.
• Edema that involves the entire body is called generalized edema.
• Local edema is the one that occurs is specific areas of the body such
as abdomen, lungs and extremities like feet, ankles and legs.
• Accumulation of fluid may be inside or outside the cell.

9. TYPES OF EDEMA
Edema is classified into two types, depending upon the
body fluid compartment where accumulation of excess
fluid occurs:
1. Intracellular edema
2. Extracellular edema.

10. INTRACELLULAR EDEMA
Intracellular edema is the accumulation of fluid inside the cell. It occurs
because of three reasons:
1. Malnutrition
2. Poor metabolism
3. Inflammation of the tissues

11. EXTRACELLULAR EDEMA
Extracellular edema is defined as the accumulation of fluid outside the cell.
Causes for extracellular edema
1. Abnormal leakage of fluid from capillaries into interstitial space.
2. Obstruction of lymphatic vessels that prevents fluid return from interstitium to blood.
Conditions which lead to extracellular edema
1. Heart failure.
2. Renal disease.
3. Decreased amount of plasma proteins.
4. Lymphatic obstruction.
5. Increased endothelial permeability.

12. PITTING AND NON-PITTING EDEMA
Interstitial fluid is present in the form of a gel that is almost like a semisolid substance, bound in a
proteoglycan meshwork.
It applies a slight suction effect and holds the tissues together.
However, in abnormal conditions, where the interstitial fluid volume increases enormously, the pressure
becomes positive.
Most of the fluid becomes free fluid that is not bound to proteoglycan meshwork. It flows freely through
tissue spaces, producing a swelling called edema.
This type of edema is known as pitting edema because, when this area is pressed with the finger,
displacement of fluid occurs producing a depression or pit.
When the finger is removed, the pit remains for few seconds, sometimes as long as one minute, till the
fluid flows back into that area.
Edema also develops due to swelling of the cells or clotting of interstitial fluid in the presence of
fibrinogen.
This is called non-pitting edema because, it is hard and a pit is not formed by pressing.

13. PITTING EDEMA NON PITTING EDEMA

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