While anatomy is the study of the structures of an organism, physiology is the science dealing with the study of the function of an organism's component structures. However, it often is not enough to know what an organ, tissue, or other structure does. Physiologists want to know how something functions. For example, physiological questions might ask: What is the function of human lung tissue? How can a seal survive under water without breathing for over ten minutes? How do camels survive so long without water? How do insects see ultra-violet light? Physiology examines functional aspects at many levels of organization, from molecules, to cells, to tissues, to orga ns, to organ systems, to an entire organism. It is the branch of biology that investigates the operations and vital processes of living organisms that enable life to exist.
Comparative physiology, then, is the comparison of physiological adaptations among organisms to diverse and changing environments. Comparative physiology, like comparative anatomy, attempts to uncover evolutionary relationships between organisms or groups of organisms. Comparative physiology seeks to explain the evolution of biological functions by likening physiological characteristics between and among organisms (usually animals.) This branch of biology constructs phylogenetic relationships (or, more loosely, evolutionary connections) between and among groups of organisms. Comparative physiology, in conjunction w ith other comparative disciplines, enables us to trace the evolution of organisms and their unique structures and to view ourselves in a broader light. By comparing the physiology among living things, scientists can gain insights into how groups of organisms have solved the adaptive problems in their natural environments over time.
Comparative physiology compares basic physiological processes like cellular respiration and gas exchange, thermoregulation, circulation, water and ion balance, nerve impulse transmission, and muscle contraction. Because it focuses on function, comparative physiology can also be referred to as functional anatomy. The form of an organ, or other biological structure, is tied to its function much in the same way a tool is linked to its purpose. For example, the function of anenzyme (a protein molecule that speeds up a chemical reaction) depends heavily upon its three-dimensional shape. If the 3-D conform ation of the enzyme molecule is altered (by heat or acid), the function of the enzyme will also be altered. If the shape of an enzyme is changed considerably, its biological activity will be lost.
A major theme dominating the topic of comparative physiology is the concept of homeostasis. The term is derived from two Greek words (homeo, meaning "same," and stasis, meaning "standing still") and literally means staying the same. Homeostasis thus refers to the ability of animals to maintain an internal environment that compensates for changes occurring in the external environment. Only the surface cells of the human body, for example, and the lining of the gastrointestinal and respiratory tracts come into direct contact with the outside surroundings (like the atmosphere). The vast majority of cells of the body are enclosed by neighboring cells and the extracellular fluid (fluid found outside of cells) that bathes them. So the body in essence exists in an internal environment that is protected from the wider range of conditions that are found in the external surroundings. Therefore, to maintain homeostasis, th e body must have a system for monitoring and adjusting its internal environment when the external environment changes. Comparative physiologists observe physiological similarities and differences in adaptations between organisms in solving identical problems concerning homeostasis.
Some of the problems that animals face in maintaining physiological homeostasis involve basic life processes. Energy acquisition from food (digestion) and its expenditure, the maintenance of body temperature and metabolic rate, the use of oxygen or the ability to live in its absence, and the way body size affects metabolism and heat loss are examples of problems that require homeostatic systems. Comparative physiologists might, for example, compare the efficiency of the relative oxygen capturing abilities of mammalian hemoglobin (in red blood cells) and insect hemolymph. Both groups of animals must maintain homeostasis and regulate the amount of oxygen reaching their tissues, yet each group solves the problem differently.
Comparative physiology makes specific measurements to obtain biologically relevant information from which to make comparisons. The kinds of processes that physiologists measure from anatomical structures to gain insight into their function include: rates (how fast something occurs), changes in rates, gradients (increasing or decreasing conc entrations of substances), pressures, rate of flow (of a fluid such as air or blood), diffusion (the act of a substance moving from an area of high concentration to one of low concentration), tension (material stress caused by a pull), elasticity, electrical current, and voltage. For example, a comparative physiologist might measure the rate of diffusion of sugar molecules across intestinal cell membranes, or the pressure exerted on the walls of blood vessels that are close to the heart. In each case, the comparative physiologist is trying to gain information that will help explain how a particular structure functions and how it compares with similar structures in other organisms in solving the same homeostatic problem. The conclusions derived, then, tell us all about our evolutionary history.