Final answer:
Substances that enter or exit the body typically do so through a cellular membrane, which controls their passage via mechanisms such as osmosis and active transport, key in processes like nutrient absorption in the intestines and waste management in the kidneys.
Step-by-step explanation:
Most substances that enter into the body or are released from the body must pass through an epithelial barrier or more specifically a cellular membrane. The structure of these membranes separates the cell from the external environment and controls the passage of organic molecules, ions, water, oxygen, and wastes into and out of the cell. Transport across membranes employs mechanisms such as osmosis, diffusion, active transport, and facilitated transport. For instance, in the intestinal lining, nutrients are absorbed and waste substances are excreted. The kidneys play a crucial role in filtering blood and maintaining the ion concentrations of body fluids while managing waste products through urine. Furthermore, biochemical transformations are essential for rendering substances like drugs and hormones suitable for excretion, involving processes like oxidation, reduction, hydrolysis, and conjugation.