Advan. Physiol. Edu. 31: 116-a-117-a, 2007;
doi:10.1152/advan.00084.2006
1043-4046/07 $8.00
ADV PHYSIOL EDUC 31:116-117, 2007
© 2007 American Physiological Society
ILLUMINATIONS
Interstitial Hydrostatic Pressure: a Manual for Students
Kurbel Sven and
Flam Josipa
Department of Physiology
Osijek Medical Faculty
J Huttlera 4
Osijek 31000, Croatia
E-mail: sven{at}jware.hr
During the learning of capillary fluid traffic, students are often puzzled about the negative values found in their textbooks for interstitial pressures in subcutaneous tissue and lungs (4, 6). Positive pressures are reported for the brain, kidneys, and liver (6) and particularly in various tumors (3). Although the reported values differ, pressures probably range from –8 mmHg in the lungs to –3 or –2 mmHg in subcutaneous tissue to 0, +1, and +2 mmHg in the liver and kidneys or +6 mmHg in the brain (4, 6). Interstitial pressure can reach very high positive values in tumors (from 20 to >40 mmHg) (2, 3). Reported findings in freshly burned tissue describe even more negative interstitial pressures that can reach –20 to –30 mmHg (8).
During seminars on capillary dynamics, teachers are often asked to clarify the whole concept in a brief explanation. One of the possible interpretations, in a form of a manual for students, is presented here. The Croatian edition of Guyton and Hall's textbook (6) was used as the main reference.
Interstitial Hydrostatic Pressure: a Manual for Students
- Reported interstitial pressures range from –8 to +6 mmHg in different tissues and from less than –20 mmHg in burned tissue or more than +30 mmHg in tumors. The negative interstitial pressures in subcutaneous tissue and other tissues are usually attributed to the action of lymphatics (1, 6). It was stated by Guyton and Barber (5) that "... the only probable source of energy that could create the negative hydrostatic pressure in the free fluid and the equivalent negative chemical potential in the intragel fluid is a lymphatic suction pump. Unfortunately, the existence of this has not yet been proved, but mechanisms by which the terminal lymphatic system could act as a suction pump have been proposed."
- Muscle contractions compress and evacuate lymph vessels. The flow is centripetal because of lymph valves. Thus, negative subcutaneous pressures might develop ex vacuo, after tissue compressions by muscles, if intact valves in lymph vessels are able to prevent lymph flow back to the uncompressed tissue.
- We can try to simplify this concept by looking at different organs and tissues in more detail (Table 1).
- All organs with positive interstitial hydrostatic pressure are more or less space confined and lack any content of compressible gases. Fluid traffic in them must be balanced, inflow volumes must equal outflow volumes, and the interstitial hydrostatic pressure can be considered to be a result of outflow resistance. \
- For instance, kidney interstitial pressure will rise in urinary tract obstruction, in lymphatic obstruction, in perirenal compression due to injury, etc. Less outflow resistance leads to lower positive values of interstitial pressure in confined organs.
- Negative values are normally found only in lungs and subcutaneous tissue. These are the only two body parts freely compressed by outside air pressure. In both tissues under normal perfusion, outflow capacities (venous blood and lymph) are greater than needed for the inflow volume and capillary permeability. Large outflow capacities drain most of the interstitial water and reduce its hydrostatic pressure to 0.
- The draining process is augmented by the physical work of neighboring muscles in subcutaneous tissue or by breathing in lung tissue. In both cases, tissue movements and pressure changes help blood and lymph to leave the tissue.
- Tissue structures are deformed by the pressure of outside air. Cellular and interstitial structures tend to obtain a shape and position of minimal energy. Their recoil forces "stretch" the well-drained interstitial space and reduce the hydrostatic pressure in it to sub-zero values, so sub-zero values of hydrostatic pressure in small tissue volumes can result from forces of recoil of the cellular and interstitial structures around it. An optimal shape and position of tissue structures can be achieved only in edematous tissue when the interstitial pressure becomes positive due to compromised fluid outflow. (7)
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REFERENCES
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- Aukland K, Reed RK. Interstitial-lymphatic mechanisms in the control of extracellular fluid volume. Physiol Rev 73: 1–78, 1993.[Abstract/Free Full Text]
- Baxter LT, Jain RK. Transport of fluid and macromolecules in tumors. I. Role of interstitial pressure and convection. Microvasc Res 37: 77–104, 1989.[CrossRef][ISI][Medline]
- Boucher Y, Baxter LT, Jain RK. Interstitial pressure gradients in tissue isolated and subcutaneous tumors: implications for therapy. Cancer Res 50: 4478–4484, 1990.[Abstract/Free Full Text]
- Ganong WF. Review of Medical Physiology (22nd ed.). New York: McGraw-Hill, 2005, p. 592–593 and 702.
- Guyton AC, Barber BJ. The energetics of lymph formation. Lymphology 13: 173–176, 1980.[ISI][Medline]
- Guyton AC, Hall JE. Human Physiology and Mechanisms of Disease (6th ed.). Philadelphia, PA: Saunders, 1997, p. 115, 116, 131–141, 319–322, and 521–523.
- Koh L, Zakharov A, Johnston MG. Integration of the subarachnoid space and lymphatics: is it time to embrace a new concept of cerebrospinal fluid absorption? Cerebrospinal Fluid Res 2: 6, 2005.[CrossRef][Medline]
- Lund T, Wiig H, Reed RK. Acute postburn edema: role of strongly negative interstitial fluid pressure. Am J Physiol Heart Circ Physiol 255: H1069–H1074, 1988.[Abstract/Free Full Text]
Copyright © 2007 by the American Physiological Society.
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REFERENCES