Membrane Absorption: Dependency & Factors

[Mk]

What does the absorption rate of body tissues depend on? For example, at what rates would the rectum, vagina, mouth, and skin be able to absorb a certain chemical? Does it depend on the size of the molecule? If it is water-soluble does that make it easier to absorb? In fact, what makes something good for circulating into the blood, and passing into other body-tissues, like through the blood-brain barrier?

Hope I was clear. Thank you!

 

[VeeEight]

Factors affecting absorption are the rate of drug dissolution, surface area of the membrane, blood flow, lipid solubility, pH partitioning, etc. Polar molecules have a harder time crossing a membrane.

Distributing into blood and body tissue depends on the molecules ability to bind to plasma proteins, displace other molecules, etc. For example, albumin is a plasma protein that binds acidic drugs and things like liver and kidney problems will cause decreased levels of plasma albumin, and thus increased free drug levels.

Some terms you might want to look up are: Volume of distribution, Clearance, Fick's Law of Diffusion, Active Transport, etc.

 

[Andy Resnick]

What does the absorption rate of body tissues depend on? For example, at what rates would the rectum, vagina, mouth, and skin be able to absorb a certain chemical? Does it depend on the size of the molecule? If it is water-soluble does that make it easier to absorb? In fact, what makes something good for circulating into the blood, and passing into other body-tissues, like through the blood-brain barrier?

Hope I was clear. Thank you!

You don't specifically mention it, but those tissues (and others) are epithelial tissue. Blood vessels are lined with endothelial tissue, as an alternative. I am more familiar with epithelial physiology.

Epithelial tissue has a well-defined function; to separate 'inside' from 'outside'. There are two main pathways to go from inside to outside (or vice-versa)- either through the cells or through the cell-cell (tight) junction. So "absorption" can mean transport from outside of a cell to inside of a cell, or across an epithelial layer (vectoral transport), or into and out of the blood, or even other interpretations.

Tight junctions can be thought of as a simple molecular sieve; the permeability depends only on the molecular size and charge. The transmission is passive- meaning there are no channels or transporters.

The transcellular pathway is a lot more complex due to the presence of specialized transporter molecules. Some transporter molecules (Shaker channels) only permit potassium ions to pass, for example. Others (aquaporin) allow only water. Some transport only a single species, others couple transport of multiple chemicals (examples: the sodium-glucose transporter, sodium-potassium-bicarbonate exchanger). The lipid bilayer membrane is also a diffusional barrier in itself.

Then there are alternative transport pathways: receptors can bind to large molecules (steroids, for example) and be endocytosed. Lipoproteins can carry fatty acids into the cells from the bood. Xenobiotic transporters will eject out of the cell many "non-self" chemicals.

Rates of vectoral transport depend on several variables- whether or not trasport is active or passive, and by what mechanism the material is transported. Different tissues have different transport rates, and even more, most epithelial tissue is inhomogeneous, so different parts of the tissue can absorb differently. In general, the rate of transport must be measured in order to have a physiologically relevant data point.