Starling's Hypothesis Relates Fluid Flow Across the Capillaries to Hydrostatic Pressure and Osmotic Pressure
An excellent practical example of how a Balanceofdriving forces is responsible for the flow of water and permeable substances across a Semipermeable membrane is the movement of water and ions across the single layer of cells (endothelial cells) that compose blood capillaries.
The single cell layer composes, in effect, a Semipermeable membrane with different transport qualities than that of a simple lipid-bilayer membrane. The junctions between cells have holes large enough for small molecules and ions to diffuse between compartments. Only large molecules, most importantly proteins, are unable to move through the holes. The difference in protein concentration between the blood and the water solution surrounding tissue cells, called the extracellular fluid (ECF) or interstitial fluid, creates an osmotic pressure for the movement of water with all its dissolved small molecules and ions. This osmotic pressure resulting from dissolved proteins has a special name: colloid osmotic pressure or oncotic pressure. Protein is more concentrated in the blood than in the interstitial fluid, producing an oncotic pressure of about 0.02 to 0.03 atm = 15 to 25 mm Hg, driving waler into the capillary. On the basis of this driving force alone, one would expect the capillaries to fill up with water, thus dehydrating the tissue spaces. However, the heart is a pump that exerts a true hydrostatic pressure on the blood, lending to drive the water (and other permeable molecules) out of the capillaries. The net driving force is the algebraic sum of the oncotic pressure difference and hydrostatic pressure difference between the capillaries and the interstitial fluid, as follows:Net driving force in capillary = (Pc I P1) I (πc I r1)
P1 = Hydrostatic pressure in the capillary.
P1 = Hydrostatic pressure in the interstitial space (usually near 0).
π = Oncolic pressure of blood plasma in capillary (-28 mm Hg).
πl - Oncolic pressure of interstitial fluid (~5 mm Hg, but depends on the particular tissue).
I his equation has enormous relevance to the function of the circulatory system. On the arterial end of capillaries the hydrostatic pressure (Pt) is high, about 35 mm Hg. Plugging this number into the equation along with the others, the net pressure in the capillary is +12 mm Hg; fluid is being driven oul of the capillary on the arterial side (capillary filtration). The flow of fluid through the resistance of the capillary causes a decline in pressure so that the hydrostatic pressure on the venous side is low, P1 = 15 mm I lg. The oncotic pressures have not changed, so the net driving force on the venous side is -8 mm Hg; there is a net absorption of fluid into the capillary on the venous side (capillary reabsorption). This arrangement achieves a major function of the circulatory system; in this way the fluid of the blood circulates among the cells and is then recycled back into the circulatory svstem.
Pathological alterations in this system emphasize the physiological importance of balance of driving forces for transport. Chronic liver disease occurs with some frequency in horses and dogs, among other mammals. I he liver is compromised in its ability to synthesize and secrete a major blood protein, serum albumin. The decline in the concentration of serum albumin lowers the oncotic pressure of the blood. As a result, there is more force to drive fluid out of the capillaries on the arterial side and less driving force for net absorption of fluid on the venous side of capillaries. This causes the tissue spaces of the diseased animals to til) with fluid, a painful and visually obvious symptom called edema. The Clinical Correlations section at the end of the chapter provides another example of edema in which increased hydrostatic pressure in the veins and capillaries causes increased capillary filtration and less capillary reabsorption.