Homeostasis: keeping relatively constant internal conditions as external conditions change. Body fluids: different types of cells and tissues have different compositions of fluid because of membrane permeability, but they exchange substances by diffusion and osmosis. Osmosis: water moves from areas of low solute concentration to high solute concentration to balance concentrations. Diffusion: solute moves from areas of high solute concentration to low solute concentration to balance concentrations. All of the processes in an animal are sensitive to solute concentrations and require hydration. Water flux: animal gaining and losing water, water entering and leaving body. Losses: excretory system (urine), expulsion of waste (feces), evaporation of body surface (sweating), evaporation of respiratory system, (in females) lactation. Comparing solute concentration and solute concentration in animals. Solute concentration in environment > animal = environment is hyperosmotic to animal, animal is hypoosmotic to environment. Solute concentration in environment < animal = environment is hypoosmotic to animal, animal is hyperosmotic to environment.