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# Measuring kidney function

## Problem

The glomerular filtration rate (GFR) is used to determine how well the kidneys are functioning. The glomerulus in the kidney is a high-pressure system that is optimal for fluid filtration. In addition to differences in hydrostatic pressure (P) between the glomerular capillary and Bowman’s capsule, disparities in oncotic pressure (π) can also direct fluid movement. Oncotic pressure is determined by the concentration of protein present such that protein pulls fluid into the space it inhabits. The formula in Figure 1 defines the relationship between hydrostatic and oncotic pressure and how they determine the glomerular filtration rate.
Figure 1. The effects of hydrostatic and oncotic pressure on GFR.
GFR = k [(start text, P, end text, start subscript, G, end subscript + start text, π, end text, start subscript, B, end subscript) - (start text, P, end text, start subscript, B, end subscript + start text, π, end text, start subscript, G, end subscript)
where,
k = Filtration constant
start text, P, end text, start subscript, G, end subscript = Hydrostatic pressure in the glomerular capillary
start text, π, end text, start subscript, B, end subscript = Oncotic pressure in Bowman’s capsule
start text, P, end text, start subscript, B, end subscript = Hydrostatic pressure in Bowman’s capsule
start text, π, end text, start subscript, G, end subscript = Oncotic pressure in the glomerular capillary
If a serum marker could be injected into an animal model to calculate the GFR, an appropriate control would be a type of molecule that is: