Homeostasis is maintained when your heart can provide the rate of blood flow necessary to meet your body's increased metabolic demand for oxygen and nutrients. Exercise increases the production of cellular wastes such as carbon dioxide and lactic acid.

Lactase is essential for digesting lactose in milk. this enzyme is specific for this sugar. why? molecules and active sites vary in size; only properly sized molecules can fit. there is a precise compatibility between the active site and the lactose molecule. specificity refers to the action of the enzyme, such as hydrolysis, and relatively few molecules can be hydrolyzed. reaction-specific enzymes assume a fit by folding around the most numerous substrate molecules.