the absorption of unionized particles (particles which have neither a positive nor a
negative charge).
c. Transport Mechanisms. The body either in an active or in a passive way
(1) Passive transport. Passive transport (diffusion) follows a concentration
gradient. That is, if there is a high concentration of a substance on one side of the
barrier and a low concentration of that substance on the other side of the barrier, nature
tries to balance the two concentrations so that one is equal to the other. The two
concentrations can be equalized in one of two ways. One way is for the liquid
containing the substance to move from the side with fewer particles to the side with
more particles. This process, called osmosis, will ultimately result in the two sides
having the same concentration. The second option is for the drug particles to move
from the side of higher concentration to the side of the lower concentration. This
process, called diffusion, will also ultimately result in the two sides having the same
concentration. Most drugs are absorbed in this manner of diffusion. With diffusion, the
drug particles move from the side of higher concentration through the cell membrane
into the side of lower concentration.
(2)
Active transport.
(a) A ride in a roller coaster would give you a background to
understand this section on active transport. You have probably observed that a roller
coaster car does not have an engine. Common sense would tell you that the car does
not need an engine to go down the hills, but up those hills--that is a different story. You
have probably observed that a mechanism exists for pulling the car up the hill.
(b) Active transport works in much the same way. Proteins (in the
cells) make up the linings of the cells. Some of these proteins have a particular affinity
(attraction) for a selected drug. When the drug molecule meets the cell wall, the protein
called a "carrier molecule" attaches itself to the drug, carries it across the cell
membrane, and releases the drug on the other side. The drug then enters the
circulation and is distributed throughout the body. Active transport can move against a
concentration gradient to move a substance to a place of higher concentration. Vitamin
B-12 is an example. Very little of this vitamin can pass through the intestinal wall of the
gut by diffusion; however, a carrier molecule, often called "intrinsic factor" transports the
vitamin across the gut. Once in circulation, the vitamin is stored in the liver. The
concentration of drug in the liver is several hundred times higher than in circulation.
Therefore1 if a drug is unionized, water soluble, and fat-soluble, it may pass through the
cells of the gut if taken orally. Once in circulation, the drug must pass through the fatty
layer of the individual cell in order to have an effect. So, even injected medications
(3) Illustration of concepts. Perhaps some insight can be gained about this
whole topic of drug transport mechanisms if a diagram depicting the concepts is shown
and discussed. Figure 3-3 is provided for this purpose. In the figure, several concepts
are illustrated:
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