Excretion of Drugs — Transcript

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1. 0:02hi there and thanks for joining us as we
2. 0:05discuss the last process of
3. 0:07pharmacokinetics which is the excretion
4. 0:09of medications
5. 0:11for our purposes right now the most
6. 0:14important thing to know about excretion
7. 0:16has to do with the half-life and the
8. 0:19half-life of a drug is the time that it
9. 0:21takes to reduce the concentration of the
10. 0:24medication to half the original amount
11. 0:26so if you had eight milligrams of
12. 0:28medication in your system at 12 o'clock
13. 0:31and that drug had a half-life of one
14. 0:33hour that would mean that there would be
15. 0:35about four milligrams of drug in your
16. 0:38system at one o'clock
17. 0:40but for most drugs it doesn't continue
18. 0:43going down at that linear Pace it's not
19. 0:46eliminated at a linear rate most drugs
20. 0:51are eliminated by first order kinetics
21. 0:54meaning that your calculations basically
22. 0:57start over at one o'clock estimating
23. 1:00half of the remaining amount so at two
24. 1:04o'clock you would have two milligrams of
25. 1:08the drug remaining in your system
26. 1:10and normally there's about five percent
27. 1:14of the drug that remains in the system
28. 1:17after about four and a half half-lives
29. 1:21and at this time you may be wondering
30. 1:22well what's the half-life of alcohol
31. 1:26and it's a good thing that you asked the
32. 1:29half-life of a drug is relevant to drugs
33. 1:32that are metabolized by first order
34. 1:34kinetics most drugs are metabolized by
35. 1:38first order kinetics but alcohol is
36. 1:41metabolized by zero order kinetics and
37. 1:45that means that we metabolize a given
38. 1:48amount per period of time rather than
39. 1:52the varying amount that is seen with
40. 1:54drugs metabolized by first order
41. 1:57kinetics
42. 1:58this happens when a drug is being taken
43. 2:01in such a large dose that it overwhelms
44. 2:05the capacity of the enzymes that degrade
45. 2:08it
46. 2:09so for alcohol with zero order kinetics
47. 2:12we estimate that approximately one
48. 2:14standard drink is metabolized per hour
49. 2:17rather than saying that it has a
50. 2:19half-life of X hours like we say with
51. 2:22most drugs
52. 2:24another interesting thing about the
53. 2:26alcohol is that it induces enzymes that
54. 2:29are necessary to metabolize the alcohol
55. 2:32in other words it increases the number
56. 2:35of enzymes that are metabolizing the
57. 2:38alcohol and therefore if you've become
58. 2:41intoxicated for the first time in a long
59. 2:43time make sure that you're extra careful
60. 2:46before getting behind the steering wheel
61. 2:47because your blood levels of alcohol
62. 2:50will stay up a lot longer than average
63. 2:55well that's all for the final process of
64. 2:58pharmacokinetics the excretion of drugs
65. 3:01make sure that you take the quiz on this
66. 3:03section and that you reinforce your
67. 3:06knowledge of this information with that
68. 3:09quiz thanks for joining us
69. 3:12I have a question for you about the
70. 3:14half-life of medications
71. 3:17if there is four milligrams of
72. 3:19medication in the system at 12 o'clock
73. 3:22noon and the medication has a half-life
74. 3:25of four hours when would there be an
75. 3:29equivalent of one milligram of
76. 3:33medication left in the system
77. 3:39and you were correct if your answer was
78. 3:43at eight o'clock P.M if four milligrams
79. 3:47of medication was in the system at noon
80. 3:49the medication would have metabolized to
81. 3:52the point that there was two milligrams
82. 3:55left in the system at four o'clock but
83. 3:59remember that it doesn't continue to go
84. 4:02down at a linear rate we need to
85. 4:05basically start the timer again we can't
86. 4:09just continue at that rate of metabolism
87. 4:12so it's like having two milligrams of
88. 4:16the medication in the system at four
89. 4:18o'clock and four hours later of course
90. 4:22there would be one milligram left
91. 4:26so that ends the summary of
92. 4:29pharmacokinetics which includes the
93. 4:32absorption distribution metabolism and
94. 4:35excretion of medications
95. 4:37at this point you don't need to know
96. 4:40everything about pharmacokinetics we've
97. 4:42just overviewed some very complex
98. 4:45concepts of pharmacology and you will
99. 4:47still have questions
100. 4:49but if you were able to understand this
101. 4:53overview of pharmacokinetics you have a
102. 4:55really good foundation to move on and
103. 4:58understand the pharmacokinetics of other
104. 5:01medications where you'll add to your
105. 5:04knowledge