How Medications Get Absorbed By the Body

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00:00Medication absorption is the movement of a drug from its site of administration into the blood.

00:07Oral drugs enter the stomach, where they either dissolve

00:10and pass through the cell membranes of epithelial cells lining the stomach

00:16or travel undissolved through the stomach to the small intestine,

00:23which is the most common site of absorption.

00:26Here, drugs dissolve and pass through the intestinal wall.

00:32Oral drugs then travel through the portal venous system to the liver,

00:37where they undergo the first pass effect.

00:40During this process, the liver metabolizes some of the drug,

00:45either inactivating it

00:47or excreting it into bile for elimination from the body.

00:55The remaining amount of active drug leaves the liver

00:58and reaches general circulation and target organs.

01:04If a drug is administered via intravenous injection,

01:08it passes directly into the bloodstream,

01:12thus bypassing absorption in the GI tract.

01:15If administered through intramuscular or subcutaneous injection,

01:21the drug enters either muscle or subcutaneous tissue,

01:26where it passes through gaps between cells into capillary walls

01:31and then into general circulation or target organs,

01:35also bypassing absorption in the GI tract.

01:38Bioavailability is the net amount of a dose of a drug

01:47that is actually absorbed into the bloodstream.

01:51The bioavailability of oral drugs is less than 100%

01:56because of the first pass effect of the liver.

01:59In contrast, the bioavailability of IV drugs is 100%

02:07because they are not exposed to the first pass effect of the liver.

02:13Different drug formulations alter bioavailability

02:17because they are not absorbed at the same rate or to the same extent.

02:23For example, tablets dissolve at varying rates.

02:27Enteric-coated drugs dissolve in the small intestine, not the stomach.

02:34Because gastric emptying time differs between individuals,

02:39absorption times also vary.

02:42Sustained release formulas contain tiny spheres that dissolve at different rates,

02:48resulting in a steady drug release throughout the day,

02:52but also resulting in variable absorption.

02:57Some factors affecting drug absorption are

03:01the rate of dissolution.

03:03Drugs that dissolve faster are absorbed faster.

03:08Surface area.

03:10Since the lining of the small intestine has more surface area

03:14than the lining of the stomach,

03:16most drugs are absorbed faster in the small intestine.

03:21Blood flow.

03:22The greater the concentration gradient between the drug-filled stomach

03:27and the rapidly flowing drug-free blood,

03:31the faster the absorption.

03:34Lipid solubility.

03:36Highly lipid soluble drugs pass through the phospholipids in the cell membrane

03:41more easily than drugs of low lipid solubility.

03:45And pH partitioning.

03:49Absorption is faster when the difference between the pH at the site of administration

03:55saturation and the pH of university.

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