Membrane permeability tends to restrict the transfer and distribution of drugs once they are delivered to the tissue. The other major factor which determines the rate of drug distribution is blood perfusion. The rate at which blood perfuses to different organs varies widely. It would be expected that total drug concentration would rise most rapidly in these organs. Thiopental is only partly ionized and passes into the brain or muscle easily.
Perfusion limits the transport.
Since brain has a higher perfusion rate the thiopental can transfer in and out more quickly. Penicillin is quite polar and is thus slowly permeable. Permeability limited transfer is faster in muscle as muscle capillaries are less restrictive. Thus transfer of penicillin is faster in muscle than brain. In brain, perfusion or membrane permeability limits drug transport or distribution.