Anatomy-Biological FormPhysiology- Biological FunctionAnimal form and function are correlated at all levels of organizationAn animals size and shape are fundamental aspects of form that significantly affect the way the animal interacts with its environment.The body plan of an animal is the result of a pattern of development programmed by the genome.Itself the product of millions of years of evolution.Evolution of Animal Size and ShapePhysical laws that govern strength, diffusion, movement, and heat exchange limit the range of animal forms.Example: some properties of water limit the possible shapes for animals that are fast swimmers. Water is aboutPhysical laws also influence animal body plans with regard to maximum size.As body dimensions increase, thicker skeletons are required to maintain adequate support. As bodies increase in size the muscles required for locomotion must represent an even larger fraction of the total body mass. At some point, mobility becomes limited.Exchange with the EnvironmentAnimals must exchange materials with their environment, and this requirement imposes limitation on their body plans.Exchange occurs as substances dissolved in an aqueous solution move across the plasma membrane of each cell.Rates of exchange are proportional to membrane surface area.Amount of material to be exchanges is proportional to cell volume.A single celled organism, such as the amoeba, has a sufficient membrane surface area in contact with its environment to carry out all necessary exchange.An animal is composed of many cells, each with its own plasma membrane across which exchange must occur.A multicellular organization therefore works only if every cell has access to a suitable aqueous environment, either inside or outside the animal’s body.Many animals with a simple internal organization have body plans that enable direct exchange between almost all their cells and the external environment.Example: pond dwelling hydra, which has a sac like body plan, has a body wall only two cell layers thick. Because the gastrovascular cavity opens to the external environment, both the outer and inner layers of cells are constantly bathed in pond water.Another common body plan that maximizes exposure to the surrounding medium is a flat shape.Example: a parasitic tapeworm, which can reach several meters in length.A thin, flat shape places most cells of the worm in direct contact with its particular environment (the nutrient rich intestinal fluid of a vertebrate host.The bodies of most animals are composed of compact masses of cells, with an internal organization much more complex that that of a hydra or a tape worm.For such a body plan, increasing the number of cells decreases the ratio of outer surface area to total volume.As an extreme comparison, the ratio of outer surface to volume for a whale is hundreds of thousands of times smaller that that for a water flea (Daphnia).Every cell in the whale must be bathed in fluid and have access to oxygen, nutrients, and other resources.In whales and most other animals, the evolutionary adaptations that enable sufficient exchange with the environment are specialized surfaces that are extensively branched or folded.In almost all cases, these exchange surfaces lie within the body, an arrangement that protects their delicate tissues from abrasion or dehydration and allows for streamlined body contours.In humans, the internal exchange surfaces of the digestive, respiratory, and circulatory systems each have an area more than 25 times that of the skin.Internal body fluids link exchange surfaces to body cells. The spaces between cells are filled with fluid, in many animals call interstitial fluid (from the Latin for “stand between”).Complex body plans also include a circulatory fluid, such as blood. Exchange between the interstitial fluid and the circulatory fluid enables cells throughout the body to obtain nutrients and get rid of wastes.despite the greater challenges of exchange with the environment, complex body plans have distinct benefits over simple ones.Example: an external skeleton can protect against predators, and sensory organs can provide detailed information on the animal’s surroundings. Internal digestive organs can breakdown food gradually, controlling the release of stored energy. In addition, specialized filtration systems can adjust the composition of the internal fluid that bathes the animal’s body cells. In this way, an animal can maintain a relatively stable internal environment while living in a changeable external environment. A complex body plan is especially advantageous for animals living on land, where the external environment may be highly variable.Hierarchical Organization of Body PlansCells form a functional animal body through their emergent properties.Cells are organized into tissues, groups of cells with a similar appearance and a common function.Different types of tissues are further organized into functional units called organs.Groups of organs that work together provide an additional level of organization and coordination and make up an organ system.Many organs contain tissues with distinct physiological roles.In some cases, the roles are different enough that we consider the organ to belong to more than one organ system.The pancreas, for example, produces enzyme critical to the function of the digestive system and also regulates the level of sugar in the blood as a vital part of the endocrine system.The “bottom up” view of the body’s organization (from cells to organ systems) reveals emergent properties.The “top down” view reveals the multilayered basis of specialization.Consider the human digestive system: the mouth, pharynx, esophagus, stomach, small and large intestines, accessory organs, and anus.Each organ has specific roles in digestion.Example: one function of the stomach is to initiate the breakdown of proteins. This process requires a churning motion powered by stomach muscles, as well as digestive juices secreted by the stomach lining.Producing digestive juices, in turn, requires highly specialized cell types: One cell type secretes a protein-digesting enzyme, a second generates concentrated hydrochloric acid, and a third produces mucus, which protects the stomach lining.The specialized and complex organ systems of animals are built from a limited set of cell and tissue types.Example: lungs and blood vessels have distinct functions but are lined by tissues that are of the same basic type