system
Given the following post, respond as if you're commenting on Reddit:
human
When someone consumes a drug, how does it move from the blood to receptors?

I know that drugs work by binding to receptors of cells, causing them to perform certain actions. I don't understand, however, how a drug would move from the bloodstream to those receptors. Could a biologist/pharmacologist weigh in here?
gpt
There's a boatload of factors at play here. What determines a drug's ability to be effectively distributed to the target site has to do with what the target site is, how easily the drug can permeate vascular membranes, the volume of blood flowing around the target site, the local acidity of the target site, the total volume of the target organism, and the drug's binding affinity for plasma and membrane proteins, to name just a few factors.

It stands to logic that small, lipid soluble, charge-neutral molecules will have the easiest time passing through cell membranes. Ionized compounds, conversely, have the easiest time being dissolved in bodily fluids, but don't have a great time getting through cell membranes. Therefore, those lipid soluble molecules will build up in areas with a lot of lipids, and the ionized molecules will stick around areas where there is a good deal of solvent to keep them happy.

The fun doesn't stop there. Pharmacologists must also consider the route of administration of a drug. If drug X has Y physical properties, what is the most appropriate dose and route of administration so as to optimize absorption into the bloodstream? Will the drug be present in its ionized or neutral form en route to the target site, and how will that affect absorption?

So as you can see, there are many factors working here. To sum it all up, drugs are not designed at random. The answer to your question depends, by and large, on exactly what function the drug is expected to perform, and where in the body it is needed the most. How the drug gets to the target receptors is therefore determined by where those receptors are located and concentrated in the body.