Biomass pyramids are typically more better at displaying the concept of energy loss as you move down the food chain. Pyramids have the form they do because energy is wasted at each trophic level. Only a portion of the energy is available to be transmitted down the food chain at each trophic level. For example, only 10% of the solar energy that reaches the surface of the earth is actually used by plants for growth. The rest is lost through radiation into space or consumed by other organisms. When we talk about biomass pyramids, we are talking about the relationship between body mass and different measures of organ size. Scientists use the term "pyramid" to describe how organs scale with respect to body mass in mammals. The three main types of biomass pyramids are the metabolic pyramid, the structural pyramid, and the evolutionary pyramid. Metabolic pyramids show the relative sizes of various organs when expressed on a per unit of body mass basis. The heart is usually the largest organ in metabolic pyramids, followed by the brain and then the lungs. Other major organs include the liver, kidneys, muscles, and bones. Structural pyramids show the relative sizes of various organs when expressed in terms of total body weight. The heart is usually the largest organ in structural pyramids, followed by the lungs and then the brain. Other major organs include the liver, muscles, and bones.