A toxic xenobiotic is less likely to enter and damage critical cells if it is rapidly excreted. The main routes of excretion are via urine, feces, and exhaled air. Three processes are involved in urinary excretion: filtration, secretion, and reabsorption. Filtration takes place in the glomerulus, the very vascular beginning of the nephron. Small molecules (both polar and lipid-soluble), including water, readily pass into the renal tubule. Secretion transports certain molecules