Net filtration pressure (NFP) in the glomerulus is a critical factor that drives the filtration of blood plasma into Bowman’s capsule, initiating urine formation. It reflects the balance between forces promoting and opposing fluid movement across the filtration barrier.

Pressures Influencing Glomerular Filtration

• Glomerular hydrostatic pressure (GHP): The blood pressure within glomerular capillaries that pushes fluid out into Bowman’s capsule (provided as 75 mm Hg).

• Glomerular capillary colloid osmotic pressure (GCOP): Osmotic pressure exerted by plasma proteins in the capillaries that pull fluid back into the capillaries (given as 40 mm Hg).

• Hydrostatic pressure in Bowman’s capsule (BCHP): Pressure exerted by fluid already in Bowman’s capsule opposing filtration (given as 20 mm Hg).

Formula for Net Filtration Pressure

$$\text{NFP}=\text{GHP}-(\text{GCOP}+\text{BCHP})$$

Calculation with Given Values

$$\text{NFP}=75\text{mm Hg}-(40\text{mm Hg}+20\text{mm Hg})=75-60=15\text{mm Hg}$$

The positive value indicates a net outward pressure favoring filtration from the glomerular capillaries into Bowman’s capsule.

Significance

• NFP determines the glomerular filtration rate (GFR), the volume of filtrate formed per unit time.

• Abnormalities in any pressure components can affect kidney function, leading to fluid retention or excessive filtration.

Conclusion

The net filtration pressure under the given healthy kidney conditions is:

15 mm Hg

Correct Answer: (2) 15 mm Hg