Distribution

What structure plays important role for distribution?
How circulatory system structured?
Is there any barrier for distribution?

What structure plays important role for distribution?
Absorption into and out to bloodstream occurs in the circulatory system which is the capillaries, inhaled toxicants diffuse accross the alveoli.

How circulatory system structured?
It is composed of the heart along with an extensive system of blood vessels that transport oxygen and nutrient-rich blood to the cells. The blood carries oxygen, waste products, nutrients and toxic substances into and out of the bloodstream by active and passive mechanisms.

Is there any barrier for distribution?
There are several mechanisms that oppose distribution to target tissue:
1. binding to plasma protein
2. distribution to storage sites
3. specialized barriers

Toxic substances that enter the bloodstream may bind with plasma proteins such as albumin, globulin and lipoproteins. Most toxicant will bind with albumin. However, there is always a portion which not bound and is in equillibrium with the bound portion. The length of time a toxic substance remains bound to the plasma protein is dependent on the amount of toxicant that exists free in the blood (not bound) and the rate at which the nonbound toxicant passes through the capillaries to the surrounding tissues. As the unbound toxicant pases through capillary into the extravascular space, the bound toxicant separates from the protein to maintain an equillibrium with the amount of free toxicant in the blood.

Toxic subtances are stored in several types of tissues. Toxicant may be stored in the target tissue (causing adverse effect), or be stored in other tissue types which may not be affected readily. DDT is targeting nerve tissue, however, since DDT is fat soluble, thus it is easily absorbed into fatty deposits throughout the body and stored for long period of time.

Reference:
Kent, C. 1998. Basics of Toxicology. John Wiley & Sons, Inc. New York