That causes(patho) a person to get dizzy or drowsy in anthicholinergic drugs?

Because of the blockage of acetylcholine in the CNS.

What causes the dizziness? Low bp? Low volume of blood?

In some cases they can be used to treat HBP, so I'd apply a phrase from HURST I learned, "less volume=less pressure." There is less vascular tone because the arteries are relaxing which decreases the pressure. I hope that made sense. Is that what you were asking?

I understand the less volume less pressure concept in general. But with anti-cholinergic, doesn't that increase heart rate. Were does the less volume come in. With an increase in heart-rate there would be more cardiac output not less. Anticholinergic would stop the rest and digest system, parasympathetic, and increase the sympathetic system. What is the something that is causing the person to be dizzy?

I understand the less volume less pressure concept in general. But with anti-cholinergic, doesn't that increase heart rate. Were does the less volume come in. With an increase in heart-rate there would be more cardiac output not less. Anticholinergic would stop the rest and digest system, parasympathetic, and increase the sympathetic system. What is the something that is causing the person to be dizzy?

If the heart rate is increased to a point that it's pumping before it has time to adequately fill then the output would be decreased.

I see. As this is a side effect of anti-cholinergics. Anticholinergics would make the heart pump faster, and if it pumped to fast there would be less output. So that's where the less volume less pressure eventually comes in. With less output, the person would become dizzy. Thanks. I understand. :)