SSRI stands for Selective Serotonin Reuptake Inhibitor. The word `selective' has special pharmacological meaning, and, in context, refers to the ability of a drug to inhibit the neural/glial uptake of serotonin, over, say, noradrenaline.
Serotonin is the trivial name for the molecule 5-hydroxytryptamine (5-HT). It has important and separate functions in the central nervous system (as a neurotransmitter) and in the cardiovascular system (as a hormone).
The SSRIs' antidepressant properties are attributed to their effects on serotonin uptake in the central nervous system*. The Custodian's assertion that Zoloft functions as an antidepressant by `blocking the reuptake of serotonin from the bloodstream into cells' is not believed to be the case*, and would appear merely to be a misunderstanding. The rest of The Custodian's article makes illuminating reading.
Halcyon&on has provided a learned write-up about serotonin from a biochemist's point of view -- it's not really light reading :-)
- Rang, H. P. et al. Pharmacology; Churchill Livingstone: London, 3rd ed.; 1995, 590.
* Here's an excerpt from the manufacturer-supplied product information for sertraline: (Roerig)
The mechanism of action of sertraline is presumed to be linked to its inhibition of CNS neuronal uptake of serotonin (5HT). Studies at clinically relevant doses in humans have demonstrated that sertraline blocks the uptake of serotonin into human platelets. In vitro studies in animals also suggest that sertraline is a potent and selective inhibitor of neuronal serotonin reuptake and has only very weak effects on noradrenaline and dopamine neuronal reuptake.