Question

Describe bulk flow and include the capillary wall structure and the Starling forces which determine rate and direction of bulk flow?

Answer in dot points

Answer 1

Bulk Flow: Filtration and Reabsorption

• Bulk flow is a passive process in which large numbers of ions, molecules, or particles in a fluid move together in the same direction.
• The substances move at rates far greater than can be accounted for by diffusion alone.
• Bulk flow occurs from an area of higher pressure to an area of lower pressure, and it continues as long as a pressure difference exists.
• Diffusion is more important for solute exchange between blood and interstitial fluid, but bulk flow is more important for regulation of the relative volumes of blood and interstitial fluid.
• Pressure-driven movement of fluid and solutes from blood capillaries into interstitial fluid is called filtration.
• Pressure-driven movement from interstitial fluid into blood capillaries is called reabsorption.
• Two pressures promote filtration: blood hydrostatic pressure (BHP), the pressure generated by the pumping action of the heart, and interstitial fluid osmotic pressure.
• The main pressure promoting reabsorption of fluid is blood colloid osmotic pressure. The balance of these pressures, called net filtration pressure (NFP), determines whether the volumes of blood and interstitial fluid remain steady or change.
• Overall, the volume of fluid and solutes reabsorbed normally is almost as large as the volume filtered. This near equilibrium is known as Starling’s law of the capillaries.
• How these hydrostatic and osmotic pressures balance- Within vessels, the hydrostatic pressure is due to the pressure that water in blood plasma exerts against blood vessel walls. The blood hydrostatic pressure (BHP) is about 35 millimeters of mercury (mmHg) at the arterial end of a capillary, and about 16 mmHg at the capillary’s venous end (Figure mentioned down below).
• BHP “pushes” fluid out of capillaries into interstitial fluid.
• The opposing pressure of the interstitial fluid, called interstitial fluid hydrostatic pressure (IFHP),“pushes” fluid from interstitial spaces back into capillaries.
• However, IFHP is close to zero. (IFHP is difficult to measure, and its reported values vary from small positive values to small negative values.) For our discussion we assume that IFHP equals 0 mmHg all along the capillaries.
• The difference in osmotic pressure across a capillary wall is due almost entirely to the presence in blood of plasma proteins, which are too large to pass through either fenestrations or gaps between endothelial cells.
• Blood colloid osmotic pressure (BCOP) is a force caused by the colloidal suspension of these large proteins in plasma that averages 26 mmHg in most capillaries.
• The effect of BCOP is to “pull” fluid from interstitial spaces into capillaries.
• Opposing BCOP is interstitial fluid osmotic pressure (IFOP),which “pulls” fluid out of capillaries into interstitial fluid.
• Normally, IFOP is very small—0.1–5 mmHg— because only tiny amounts of protein are present in interstitial fluid. The small amount of protein that leaks from blood plasma into interstitial fluid does not accumulate there because it passes into lymph in lymphatic capillaries and is eventually returned to the blood.
• For discussion, we can use a value of 1 mmHg for IFOP. Whether fluids leave or enter capillaries depends on the balance of pressures. If the pressures that push fluid out of capillaries exceed the pressures that pull fluid into capillaries, fluid will move from capillaries into interstitial spaces (filtration). If, however, the pressures that push fluid out of interstitial spaces into capillaries exceed the pressures that pull fluid out of capillaries, then fluid will move from interstitial spaces into capillaries (reabsorption). The net filtration pressure (NFP), which indicates the direction of fluid movement, is calculated as follows:

• Thus, at the arterial end of a capillary, there is a net outward pressure of 10 mmHg, and fluid moves out of the capillary into interstitial spaces (filtration).

• At the venous end of a capillary, the negative value (-9 mmHg) represents a net inward pressure, and fluid moves into the capillary from tissue spaces (reabsorption).
• On average, about 85% of the fluid filtered out of capillaries is reabsorbed. The excess filtered fluid and the few plasma proteins that do escape from blood into interstitial fluid enter lymphatic capillaries.
• As lymph drains into the junction of the jugular and subclavian veins in the upper thorax, these materials return to the blood. Every day about 20 liters of fluid filter out of capillaries in tissues throughout the body. Of this fluid, 17 liters are reabsorbed and 3 liters enter lymphatic capillaries (excluding filtration during urine formation).

KEY FOR THE DIAGRAM-

• BHP = BLOOD HYDROSTATIC PRESSURE
• IFHP = INTERSTITIAL FLUID HYDROSTATIC PRESSURE
• BCOP = BLOOD COLLOID OSMOTIC PRESSURE
• IFOP = INTERSTITIAL FLUID OSMOTIC PRESSURE
• NFP = NET FILTRATION PRESSURE