AUSTRALIAN
NATUROPATHIC NETWORK
Serving
the community since 1998
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
| Departments
» Medical Sciences » A&P » Swollen Ankles by Peter Stevenson |
Edema is the accumulation of excess extra-vascular interstitial fluid that occurs as a result of the breakdown in the normal balance of forces across tissue capillary walls1. In order to understand the causes of edema it is first necessary to understand the mechanism of the exchange of fluid from capillaries into the extra-cellular space.
Most extra-cellular fluid moves from capillaries into interstitial space (filtration) through the mechanism called ‘bulk flow’. Bulk flow is effected by pressure differences in the capillaries and the interstitial space.
Filtration is promoted by:
- Blood hydrostatic pressure (BHP) – generated and maintained by the mechanisms discussed in another paper; and
- Interstitial fluid colloid osmotic pressure (IFOP) – a pressure gradient between the inside of the capillary and the interstitial fluid caused by the protein contained in the extra-cellular fluid (ECF). Its value is negligible
These pressure are greater at the arterial end of the capillary.
Under normal circumstances forces at the venous end of the capillary cause fluid to flow back into the capillary (reabsorption). The main force associated with reabsorption is blood colloid osmotic pressure (BCOP). This is a pressure gradient caused by the presence of greater amounts of protein in the blood than the ECF.
The difference between the outbound and inbound pressure is called the Net Filtration Pressure (NFP). Overall, the fluid volume expelled from the capillary almost equals the volume reabsorbed. The near equilibrium is called Starlings Law of the capillaries2.
In summary then, fluid leaves the capillary at the arterial end to enter interstitial space because the sum of BHP and the IFOP is greater than the BCOP. Fluid enters the capillary at the venous end of the capillary because the sum of BHP and the IFOP is less than the BCOP. Non-absorbed interstitial fluid returns to the blood via the lymphatic system. Edema results when more fluid leaves the capillary than can be carried by the venules and lymphatics.
There are numerous conditions that can cause an imbalance in the pressures and result in edema. The primary reasons are presented in table 1.
| Cause | Description | Contributing Factors |
| Increased capillary hydrostatic (filtration) pressure | The BHP overwhelms the BCOP at the venous end of the capillary thus preventing reabsorption | |
| Increased capillary permeability | This may lead to the leaking of proteins into the interstitial space which increases IFOP. | This condition may result from the release of chemical mediators, including histamine, leukotrienes, complement, and cytokines. External factors that can cause such chemical response include trauma, ischemia, burns, chemical injury, or introduction of a foreign protein or antigen1 |
| Lowered blood colloid osmotic pressure (BCOP) | Lowered BCOP results in less fluid being reabsorbed | This is related to a loss of protein from the blood, which may result from burns, malnutrition, kidney and liver diseases1. |
| Lymphatic obstruction | This causes a failure to remove excess fluid from interstitial space | This condition is known as lymphedema and may be caused by parasitic infection, burns, or other injuries that damage lymphatic vessels1. |
Table 1 Major causes of edema.
- Willis Hurst, J (Ed.), Medicine for the Practicing Physician – 3rd Edition, Butterworth-Heinemann, USA, 1992.
- Tortora, G.J., Grabowski, S.R., Principles of Anatomy and Physiology - 8th Edition, Harper Collins, NY, 1996.
- Porth, C.M., Pathophyisology: Concepts of Altered Health States 4th Edition, JB Lippincott Company, Philadelphia, 1994.