Drugs can enter the body a variety of ways (Figure 1; also see Module 1). The easiest way to get a drug1 into the bloodstream is to inject it directly into a vein. If a drug is ingested by mouth, smoked, or snorted2, it must pass several barriers before reaching the bloodstream. (See Module 1 for a review of the different modes of drug administration).
Figure 4 Modes of drug administration into the body. Red is the arterial side and blue is the venous side. Adapted from Ray O. and Ksir C. (2002). Drugs, Society, and Human Behavior, pg. 154. McGraw-Hill: New York.
Once in the bloodstream, the drugs can be distributed throughout the body. The route that drugs take follows the circulatory path of the blood. The first pass throughout the body depends on the actual route of administration. Drugs that are smoked go directly with the oxygenated blood from the lungs to the heart. Then they leave the heart through the aorta, the major artery, to travel to the rest of the body. If drugs are injected or snorted, they enter the venous system and get returned to the heart with de-oxygenated blood, before traveling to the lungs and then back to the heart. If a drug is ingested orally, it diffuses into capillaries in the stomach and small intestine that connect to blood vessels that go directly to the liver. So as drugs leave the gut they travel to the liver first (this is called the portal circulation). In the liver, some of the drug is metabolized as it passes through (see discussion of metabolism below). After the drug leaves the liver, it travels through the venous system to the heart, then to the lungs and finally back to the heart to be distributed throughout the rest of the body via the arterial system (Figure 1). The circulatory system is a very efficient way to distribute drugs throughout the body. As described above, drugs leave the heart by way of the aorta. This main artery branches into large arteries as they travel to various organs. As arteries enter organs, they branch into arterioles, which branch into even smaller units, the capillaries.
Capillaries are the smallest form of blood vessels and are very numerous. In fact they are able to deliver nutrients such as oxygen and glucose to every cell in the body. [They also pick up waste such as carbon dioxide and metabolic products.] Unlike membranes of most cells, the membranes of the capillaries (made up of loosely packed endothelial cells3) have numerous pores (or fenestrae4) that allow molecules up to 25,000 daltons and charged molecules (see discussion below) to pass through without difficulty (Figure 2) (also see Module 1).
Figure 2 Left: A cross section of a non-brain capillary showing that both lipophilic or non-polar (unionized) and polar (ionized) molecules can pass through spaces between endothelial cells. The fenestrae are not shown in this picture.
1 a substance that affects the structure or function of a cell or organism
2 to breathe in a compound in solid form through the nostrils. With reference to cocaine, it is the hydrochloride salt.
3 cells that line the blood vessels and capillaries. Unlike epithelial cells, these cells have no contact with the environment outside the body.
4 small spaces or pores within endothelial cells that form the capillary membrane. These pores allow charged drugs or larger drugs to pass through the capillaries.