Question
How is tissue fluid formed and absorbed?
Answer
The formation and absorption of tissue fluid in a capillary bed are primarily governed by the interplay of hydrostatic and osmotic pressures, as described by the Starling principle.
Formation of Tissue Fluid
The process begins with the filtration of fluid from the blood into the tissue spaces. This is driven by the hydrostatic pressure within the capillaries, which is higher at the arterial end. The hydrostatic pressure pushes fluid out of the capillaries into the surrounding tissue spaces, leading to the formation of tissue fluid. This process is counterbalanced by the colloid osmotic pressure of the plasma proteins, which tends to draw fluid back into the capillaries (Taylor and Starling, 1981; Michel, Woodcock and Curry, 2020; Pappenheimer and Soto-Rivera, 1948).
Absorption of Tissue Fluid
Absorption occurs when the osmotic pressure exceeds the hydrostatic pressure, typically at the venous end of the capillary bed. The colloid osmotic pressure, primarily due to plasma proteins, pulls fluid back into the capillaries from the tissue spaces. This reabsorption is crucial for maintaining fluid balance and preventing edema (Taylor and Starling, 1981; Michel, Woodcock and Curry, 2020; Curry, 2021).
Role of Hydrostatic and Osmotic Pressures
Hydrostatic Pressure: This pressure is exerted by the blood against the capillary walls and is the primary force driving fluid out of the capillaries. It decreases from the arterial to the venous end of the capillary bed (Pappenheimer and Soto-Rivera, 1948; Deen, Robertson and Brenner, 1973).
Osmotic Pressure: Also known as oncotic pressure, it is mainly due to plasma proteins and acts to retain fluid within the capillaries. It remains relatively constant along the capillary bed but becomes more significant in fluid reabsorption at the venous end (Michel, Woodcock and Curry, 2020; Curry, 2021; Taylor and Townsley, 1987).
Conclusion
The balance between hydrostatic and osmotic pressures determines the net movement of fluid across the capillary walls. While hydrostatic pressure promotes fluid filtration into the tissues, osmotic pressure facilitates fluid reabsorption back into the capillaries. This dynamic equilibrium is essential for maintaining tissue fluid homeostasis and is a fundamental aspect of the Starling principle.
References
Taylor, A., & , S., 1981. Capillary fluid filtration. Starling forces and lymph flow.. Circulation research, 49 3, pp. 557-75. https://doi.org/10.1161/01.RES.49.3.557
Michel, C., Woodcock, T., & Curry, F., 2020. Understanding and extending the Starling principle. Acta Anaesthesiologica Scandinavica, 64, pp. 1032 – 1037. https://doi.org/10.1111/aas.13603
Pappenheimer, J., & Soto-Rivera, A., 1948. Effective osmotic pressure of the plasma proteins and other quantities associated with the capillary circulation in the hindlimbs of cats and dogs.. The American journal of physiology, 152 3, pp. 471-91. https://doi.org/10.1152/AJPLEGACY.1948.152.3.471
Curry, F., 2021. Fluid Filtration in the Microcirculation. Cardiopulmonary Monitoring. https://doi.org/10.1007/978-3-030-73387-2_6
Deen, W., Robertson, C., & Brenner, B., 1973. Transcapillary Fluid Exchange in the Renal Cortex. Circulation Research, 33, pp. 1–8. https://doi.org/10.1161/01.RES.33.1.1
Taylor, A., & Townsley, M., 1987. Evaluation of the Starling Fluid Flux Equation. Physiology, 2, pp. 48-52. https://doi.org/10.1152/PHYSIOLOGYONLINE.1987.2.2.48
