|MadSci Network: Medicine|
Thank you for your question. You are correct that caffeine is a methylxanthine and that it blocks adenosine receptors. In fact, it is caffeine's blockade of the A1 adenosine receptor in the heart that causes the heart to pound after a significant caffeine dose. An 8oz cup of coffee contains about the 100mg of caffeine that you mentioned. The effects of drinking a cup of coffee can be felt within 15-45 min of ingestion, and include an increase in heart rate, basal metabolic rate, urine production, gastric acid secretion and blood pressure. However, at moderate doses, the increase in blood pressure due to caffeine intake is no greater than that caused by routine daily activities.
Caffeine's maximal effects are achieved 30-60 minutes after ingestion, and caffeine is completely cleared from the body within 3-4 hours of consumption. Caffeine clearance is enhanced by smoking. So after the blood pressure increase caused by caffeine, blood pressure will drop back to normal due to release of the adenosine receptors from their blockade.
I hope I've answered your question! If you have any more questions, or if I haven't completely answered your question, please feel free to e-mail me.
Have a good day.
Ingrid Dodge :)
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