Drugs don't make you 'trip' by causing damage to your brain or making it bleed, though im sure the government would like you to believe that. LSD affects dopamine, adrenergic and serotonin receptors in the brain. Many hallucinogens seem to affect the serotonin system (regulates sleep/wake cycle, mood, appetite, pain etc.) and LSD has a very high binding affinity to 5-HT1 and 5-HT2 receptors (that would usually be occupied by serotonin, a neurotransmitter) meaning that it fits very well in these receptor sites (better than serotonin does). Most interesting is that LSD is excreted from the body shortly after it is ingested, within hours most of it is already gone. It is suggested then that it works by 'awakening' parts of the brain that are normally inactive.