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6 Cards in this Set

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Drug molecules can move across cell layers without having to pass through any cell membranes by a process known as?
Bulk Flow.
The liquid passing across the cell layer via Bulk Flow includes?
water, electrolytes (Na, K, Cl etc) and relatively small molecules including drug molecules.
Larger molecules, such as proteins, will not normally pass across cell layers in this way although in certain pathophysiological situations movement of such molecules will occur by?
Bulk Flow.
Where are such hydrostatic pressure differences and such arrangements of cells so that Bulk Flow occurs?
The wall of blood vessels allows Bulk Flow of materials from the plasma to the surrounding extravascular space around arterioles and the arteriolar side of capillaries because the hydrostatic pressure in these vessels is greater than in the extravascular space around those vessels.

Similarly, Bulk Flow of materials occurs from extravascular space to the plasma of small veins and the venous side of capillaries because the hydrostatic pressure in the extravascular space around these vessels is greater than in these vessels.

Bulk Flow also occurs in the glomerulus of each kidney tubule where water and electrolytes and drug molecules in the plasma are filtered into the (Bowman's capsule) lumen of the tubule; this filtrate solution passes down the tubule and is processed to form urine.
Where is Bulk Flow of drug molecules across cell layers prevented?
In certain organs, the cells forming the wall of the blood vessels are tightly joined together. In this situation, movement of water, electrolytes and small drug molecules by Bulk Flow across the cell layers of the blood vessel wall is restricted and perhaps essentially absent.
Organs where bulk flow from/to the plasma is restricted, include the brain (where the wall of blood vessel forms part of what is known as the "blood brain barrier") and the placenta. At these sites, movement of drugs (by Bulk Flow) into the brain and to the developing fetus is reduced compared to movement (by Bulk Flow) into other tissues and organs.
Bulk flow is also restricted in the kidney where the cells forming the wall of the kidney tubule(s) are associated in a similar manner to prevent Bulk Flow movement of molecules, including drug molecules and of electrolytes, from the lumen of the tubule(s) back to the plasma.
Is Bulk Flow important in terms of drug delivery within the body?
Certainly. Except in those tissues and organs noted above, bulk flow is a very important mechanism for drugs to move from the plasma into the extravascular space. However, bulk flow is to some extent "ignored" as a process for drug movement from the plasma because bulk flow can not be controlled by medical intervention and because physical/chemical drug design changes are not useful to alter bulk flow movement by those drug molecules.