Lecture 8

Question 1

What is drug absorption?

Answer 1

The passage of drug from the site of administration into the general circulation (topical drugs are an exception to this)

Question 2

What is rate?

Answer 2

How rapidly does the drug get from its site of administration to the general circulation

Question 3

What is extent?

Answer 3

How much of the administered dose enters the general circulation

Question 4

What is bioavailability?

Answer 4

The area under the concentration time curve, expressed as a percentage, symbol F

Question 5

What are the enteral routes of drug administration?

Answer 5

Oral, Sublingual (placed under tounge), rectal

Question 6

What are the parenteral routes of drug administration?

Answer 6

Intravenous, subcutaneous, Intradermal, intramuscular, lungs

Question 7

What are the two most popular routes of drug administration?

Answer 7

Intravenous, Oral

Question 8

When is intravenous administration and oral administration used?

Answer 8

Intravenous is used when speed and accuracy is important
Oral is used in situations where this is less important as it is more convenient due to the lack of technical skill required to administer to it

Question 9

What are the advantages to intravenous administration?

Answer 9

Very rapid, Precise control as 100% bioavailable, avoids problems of absorption, or drug break down before entering blood, good for drugs that are too irritating to be taken by month or given by tissue injection

Question 10

What are the disadvantages to intravenous administration?

Answer 10

Skill is required, as air embolisms can form if incorrectly administered
Careful preparation of injected material is required as it must be sterile and non-particulate
Most hazardous as there is no way to stop the dose once it has been administered

Question 11

What are the advantages to oral administration?

Answer 11

Safest, most convenient and economic

Question 12

What are the disadvantages to oral administration?

Answer 12

Slow (1/2 hour - 3 hours till effect)

Unpredictable with regards to rate, extent and reproducibility as it must pass through the various barriers in the brain

Question 13

What are the four barriers that drugs must pass through if administered orally?

Answer 13

1-Disintergration by stomach acid
2-Dissolution as the drug is dissolved in gut fluids
3-absorption drug must be absorbed by drug
4- Liver metabolism where the drug may be broken down by liver enzymes

Question 14

What are the factors influencing the oral bioavailability of drugs?

Answer 14

Decomposition in acidic gastric juices
Decomposition by hydrolytic gut enzymes
Degradation by gut microorganisms
Food in the gut may alter absorption rate and amount (interactions or complex formation)
Metabolism by gut wall enzymes
Metabolism by liver enzymes prior to reaching the systemic circulation

Question 15

What is the rate limiting step in oral drug absorption?

Answer 15

The stomach emptying rate

Question 16

What are the patient factors which influence absorption?

Answer 16

Stomach emptying rate, intestinal motility, interactions with food

Question 17

How does the stomach emptying rate affect the rate of drug absorption?

Answer 17

Rate is increased by mild exercise, hunger, metoclopramide

Rate is decreased by hot meals, vigorous exercise, pain, migraine, lanour and drugs

Question 18

How does the intestinal motility change the rate of drug absorption?

Answer 18

Increased motility due to gastroenteritis and diarrhoea (decreased transit time which is important for drugs with low H2O solubility e.g. digoxin leading to a decrease in bioavailability)
Motility can be decreased by various drugs

Question 19

How can interactions with food change the rate of drug absorption?

Answer 19

Chelation with metal ions leading to a decrease in bioavailability

Question 20

What are the two most common mechanisms for excretion of a drug from the body?

Answer 20

Liver biliary secretions which lead to faecal excretion

the more common renal excretion by the kidney

Question 21

How does capillary permeability affect drug absorption?

Answer 21

Most capillaries are relatively porous resulting in drugs being able to leave the blood regardless of their polarity
However brain capillaries also have a layer of glial cells which form the blood brain barrier, only lipid soluble drugs diffuse across this barrier

Question 22

How can the solubility of a drug influence its access to bodily fluids?

Answer 22

H2O soluble drugs is restricted to extracellular fluid (14L)

While lipid soluble drugs can move into the intracellular fluid to access almost all bodily fluids

Question 23

How does drug distribution relate to drug flow?

Answer 23

The tissue that receives more blood must also receive more drug
Rate of distribution to tissues depends on relative blood flow so the heart, lungs, brain, liver and kidney receive drugs rapidly while tissues such as skin,muscle and fat which are poorly perfused receive drugs at a slower rate