The active ingredients in marijuana – cannabinoids and terpenes – are delivered to the blood through the lungs (when inhaled), the digestive system (when consumed) or the skin (when applied topically). From the blood, they are available to the brain, central nervous system and immune system.
Humans are built to interact with cannabinoids and with the endocannabinoid systems of special receptor molecules embedded in our brains and along neural pathways. The receptors influence the flow of chemical signals to the brain.
To learn more about the endocannabinoid system, take some time to research Raphael Mechoulam’s work in the cannabis field.