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DavidsonX – D001x – Medicinal Chemistry Chapter 7 – Pharmacokinetics Part 2 – Clearance I Video Clip – Poolside Demonstration The water in a pool is very much like blood within a body. The water in a pool is circulated by a pump and particulates are removed by a filter. In the same way, the blood in a body is circulated by the heart and wastes are removed by the kidneys. The kidneys clear the blood, and the filter clears the pool water. We can simulate the effect of clearance with a large beaker of water (2 L), 400 plastic beads, and 1 cup scoop with small holes to remove beads but not water. A US cup has a volume of approximately 240 mL, so one scoop of the cup should clear a little more than 10% (240 mL / 2,000 mL) of the volume of the water. A common misconception is to think if one cup clears 10% of the beads, then five scoops should remove 50% of the beads and ten scoops should remove 100% of the beads. A key detail is that the beads are removed but water is not. The remaining beads are now fewer in number but in the same volume of water. In other words, their concentration has decreased, so the next scoop will remove fewer beads. Over time, the concentration continues to drop more and more slowly. This observation follows the trend in drug elimation. The slope of a Cp-time curve slowly becomes flatter and flatter as Cp decreases.

Below is the experimental data for scooping beads from the beaker. Scoop number

Beads removed

Beads remaining

0

0

400

1

35

365

2

18

347

3

17

330

4

28

302

5

37

267

6

24

241

7

25

216

8

17

199

9

17

182

10

11

171

Plotting ln (beads remaining) againsts scoop number simulates a Cp-time plot. ln (beads remaining)-scoop number simulates a ln Cp-time plot.

MOOC Medicinal Chemistry  
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