Net filtration pressure

Glomerular filtration rate (GFR) is the volume of plasma-like fluid that is filtered per unit time across the glomerular capillary membranes to enter the tubular space. Filtrate formation is driven by the net filtration pressure that is equal to the capillary hydrostatic pressure diminished by the sum of capillary oncotic... 

The sum of the outward forces (41 mmHg) exceeds that of the inward force (28 mmHg) resulting in a net filtration pressure of 13 mmHg. In other words, net movement of fluid is out of the capillary at the arteriolar end. 

Explain how the filtration coefficient and net filtration pressure determine glomerular filtration... 

The net filtration pressure is determined by the following forces (see Figure 19.3) ... 

Net filtration pressure = 60 mmHg - 28 mmHg - 15 mmHg = 17 mmHg... 

Blood is supplied to the kidneys via the renal vein, a branch of the descending vena cava, at relatively high pressure to ensure rapid filtration of plasma across the membranes of the blood vessels in the glomeruli and the epithelial cells of the Bowman s capsule. The net filtration pressure of about 5-6 kPa, is the difference between the blood pressure forcing plasma water across the filtration barrier and the opposing osmotic and... 

Assuming the capsular pressures opposing the movement of water out of the blood and into the top of the nephron are constant, the net filtration pressure is due largely to the blood pressure. Any fall in blood pressure can have a dramatic effect on the efficiency of filtration and therefore clearance of waste materials. So important is the pressure within the renal vasculature that the kidney is critical in regulating systemic blood pressure via the renin-angiotensin-aldosterone (RAA) axis, a physiological process which relies on transport mechanisms within the renal tubules. 

Net filtration pressure = Outward forces Inward forces... 

The result of the combination of myogenic mechanisms and tubuloglomerular feedback is that the net filtration pressure or Pccap is kept reasonably constant over a very wide range of systemic arterial pressures. It should be noted that renal blood flow and GFR change across this range of systemic pressures but to a significantly smaller extent than would be predicted if these autoregulatory mechanisms were not in place. 

Figure 15.7 Starling principle a summary of forces determining the bulk flow of fluid across the wall of a capillary. Hydrostatic forces include capillary pressure (Pc) and interstitial fluid pressure (PJ. Capillary pressure pushes fluid out of the capillary. Interstitial fluid pressure is negative and acts as a suction pulling fluid out of the capillary. Osmotic forces include plasma colloid osmotic pressure (np) and interstitial fluid colloid osmotic pressure (n,). These forces are caused by proteins that pull fluid toward them. The sum of these four forces results in net filtration of fluid at the arteriolar end of the capillary (where Pc is high) and net reabsorption of fluid at the venular end of the capillary (where Pc is low).

Bulk flow plays only a minor role in the exchange of specific solutes between blood and tissue cells. A far more important function of bulk flow is to regulate distribution of extracellular fluid between the vascular compartment (plasma) and the interstitial space. Maintenance of an appropriate circulating volume of blood is an important factor in the maintenance of blood pressure. For example, dehydration and hemorrhage will cause a decrease in blood pressure leading to a decrease in capillary hydrostatic pressure. As a result, net filtration decreases and net reabsorption increases, causing movement, or bulk flow, of extracellular fluid from interstitial space into the vascular compartment. This fluid shift expands the plasma volume and compensates for the fall in blood pressure. 

Glomerular capillary pressure (PGC) is a hydrostatic pressure that pushes blood out of the capillary. The blood pressure in these capillaries is markedly different from that of typical capillaries. In capillaries elsewhere in the body, blood pressure at the arteriolar end is about 30 mmHg and at the venular end is about 10 mmHg (see Chapter 15). These pressures lead to the net filtration of fluid at the inflow end of the capillary and net reabsorption of fluid at the outflow end. 

A plate-and-frame filter press contains 16 frames and operates at a constant flow rate of 30 gpm. Each frame has an active filtering area of 4 ft2, and it takes 15 min to disassemble, clean, and reassemble the press. The press must be shut down for disassembly when the pressure difference builds up to 10 psi. What is the total net filtration rate in gpm for a slurry having properties determined by the following lab test. A sample of the slurry is pumped at a constant pressure differential of 5 psi through 0.25 ft2 of the filter medium. After 3 min, 1 gal of filtrate has been collected. The resistance of the filter medium may be neglected. 

The precise numbers you choose to use are not as important as the concept that, under normal circumstances, the net filtration and absorptive forces are the same. Anything which alters these component pressures such as venous congestion (Pc increased) or dehydration loss (rcc increased) will, in turn, shift the... 

Other features An increase in venous pressure owing to venous congestion will increase venous hydrostatic pressure. If the pressure on the arterial side of the capillaries is unchanged, this moves the venous end of the hydrostatic pressure line upwards and the gradient of the line decreases. This increases area A and decreases area B, again leading to net filtration. 

Pressure, Flux, Frequency, and Duration of Backwash Backwashing conditions such as pressure, flux, frequency, and duration, in practice, are usually obtained by trial and error. Kennedy et al. (1998) studied the backwash conditions in order to maximize the net flux per filtration cycle. They found that increasing the backwash to filtration pressure ratio (Ph/Pf) above 2.5 did not result in any significant increase in flux restoration within the range of backwash pressures tested (0.2—1.6 bars) (Fig. 6.15). Their conclusion that applying high backwash pressures (8 limes the filtration pressure) cannot restore irreversible flux decline was in agreement with membrane suppliers recommendations. 

Arrow b This represents an increased Pc. If only the arteriolar pressure rises, the gradient of the line will increase, whereas if the venous pressure rises in tandem the line will undergo a parallel shift. The net result is again filtration. This occurs clinically in vasodilatation. The opposite scenario is seen in shock, where the arterial pressure at the capillaries drops. This results in net reabsorption of fluid into the capillaries and is one of the compensatory mechanisms to blood loss. 

A hyperfiltration process developed by Mobil Oil, now ExxonMobil, for this separation is illustrated in Figure 5.28(b). Polyimide membranes formed into spiral-wound modules are used to separate up to 50 % of the solvent from the dewaxed oil. The membranes have a flux of 10-20 gal/ft2 day at a pressure of 450-650 psi. The solvent filtrate bypasses the distillation step and is recycled directly to the incoming oil feed. The net result is a significant reduction in the refrigeration load required to cool the oil and in the size and energy consumption of the solvent recovery vacuum distillation section. 

Tetrahydrofuran (3.2 ml) and S-(+)-3-chloro-l,2-propanediol (0.299 ml, 3.58 mmol, 1.19 eq) are mixed. The mixture of THF (3.2 ml) and S-(+)-3-chloro-1,2-propanediol (0.299 ml, 3.58 mmol, 1.19 eq) is cooled to -16°C and potassium t-butoxide (3.2 ml, 1.0 M) in THF (3.2 mmol, 1.07 eq) is added at less than -10°C. The resulting slurry is stirred at -14-0°C for 1 hour. Then added to the lithium anion mixture while maintaining both mixtures at 0°C, then rinsed in with THF (2 ml). The resultant slurry is stirred at 20-23°C for 2 hour and then cooled to 6°C and a mixture of citric acid monohydrate (0.4459 g, 2.122 mmol, 0.705 eq) in water (10 ml) is added. The resultant liquid phases are separated and the lower aqueous phase is washed with ethyl acetate (12 ml). The organic layers are combined and solvent is removed under reduced pressure until a net weight of 9.73 g remains. Heptane (10 ml) and water (5 ml) are added and solvent is removed 4-nitrobenzenesulfonyl chloride y reduced pressure until a total volume of 5 ml remains. The precipitated product is collected by vacuum filtration and washed with water (7 ml). The solids are dried in a stream of nitrogen to give (R)-[N-3-(3-fluoro-4-(4-morpholinylphenyl)-2-oxo-5-oxazolidinyl]methanol. 

In summary, virtually all anesthetic agents and techniques are associated with reductions in glomerular filtration rate and urine output. These changes are usually readily reversed in the immediate postoperative period and represent the net effect of complex interactions between direct actions of the anesthetics on the kidney and indirect changes in cardiac output, blood pressure, and neuroendocrine function. 

---