Starling’s Law of the Capillaries
Starling’s law of the capillaries states that the movement of fluid between the capillaries and interstitial fluid is due to the net effect of all four of the pressures described. An equation can be used to calculate the NFP and determine the direction of the fluid movement.
NFP = (BHP + IFCOP) – (BCOP + IFHP) (Equation 6)
BHP and IFCOP promote the movement of fluid out of the capillaries or filtration. BCOP and IFHP promote the movement of fluid into the capillaries or reabsorption. If the pressures that promote filtration are greater than the pressures that promote reabsorption, the NFP will have a positive value. If the pressures that promote reabsorption are greater, the NFP will have a negative value.
Under normal physiological conditions, the values of IFHP, BCOP, and IFCOP do not significantly vary from the arterial end of the capillaries to the venous end. Therefore, in most tissues, the rate of filtration and reabsorption is due to BHP. To demonstrate, assume the following pressures at the arterial end of a capillary network:
BHP = 35 mmHg
IFHP = 0 mmHg
BCOP = 26 mmHg
IFCOP = 1 mmHg
NFP = (35 mmHg + 1 mmHg) – (26 mmHg + 0 mmHg)
NFP = +10 mmHg
Because the value of NFP is positive, filtration occurs at the arterial end of the capillary. Moving along the capillary, the BHP becomes lower as blood moves along the pressure gradient of the vascular system, but the other three pressures will not fluctuate. At about the midpoint of the capillary network, the pressures that promote filtration are equal to the pressures that promote reabsorption. Moving away from the midpoint toward the venous end of the capillary, the BHP dips below the BCOP. From this point forward, reabsorption occurs. Near the junction of the capillaries with the venules, the BHP is about 16 mmHg.
NFP = (16 mmHg + 1 mmHg) – (26 mmHg + 0 mmHg) or NFP = –9 mmHg.
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Describe the different modes of transport that molecules may take during capillary exchange. Define Starling's Law of the Capillaries and use it to determine the relativ e rate and direction of fluid exchange in the capillaries. Relate imbalances in capillary exchange to edema.
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This reabsorption value is close to the initial filtration value at the arterial end of the capillary. This means that most of the water that moved from the capillaries into the interstitial fluid at the arterial end will be returned to the bloodstream at the venous end. In fact, about 85 percent of the fluid filtered from the capillaries is reabsorbed. The remaining 15 percent flows through the tissues and into the lymphatic capillaries. The lymphatic system returns the excess fluids plus any plasma proteins that escape back into the bloodstream. About 22 liters of fluids are filtered from the capillaries each day. Of this, approximately 19 liters is reabsorbed back into the venous end of the capillaries, with the lymphatic system picking up the remaining 3 liters of non-reabsorbed fluid per day.