Physiology

Physiology is the study of how living things function. It encompasses the most basic unit of living things, the cell, and the most complex organs and organ systems, such as the brain or endocrine system.

The word "physiology" was first used by the Greeks around 600 B. C. E. to describe a philosophical inquiry into the nature of things in general. Around the sixteenth century, the word began to be used with specific reference to the vital activities of healthy humans. By the nineteenthcentury, curiosity and medical necessity stimulated research concerning the physiology of all living things. Discoveries of similar structures and functions common to living things resulted in the development of the concept of general physiology. Since the mid-nineteenth century, physiology has used experimental methods, as well as techniques and concepts of the physical sciences, to investigate the causes and mechanisms of the activities of living things. Today there are many specialized areas of study within the field of physiology including cellular, vertebrate, and invertebrate physiology, as well as medical specialties such as endocrinology.

Scientists who study physiology are called physiologists. They investigate how different parts or organs of a living thing work together to perform a particular function. In humans, for example, the circulation of blood in the body involves the action of the heart and other structures such as veins, arteries, and capillaries. Special nerve centers known as nodes trigger the ventricles of the heart to contract in a predictable rhythm, which causes the blood to flow in and out of the heart. By learning how organs such as the heart function normally, physiologists (and physicians) can better understand what happens when organs function abnormally and learn how to treat them. In their studies, physiologists pay close attention to structure, information transfer, metabolism, regulation, and transport.

The structures of living things are often related to their function. For example, the shape and structure of a bird's beak is related to how it uses the beak. Eagles have a large, sharp beak for ripping and tearing prey. Hummingbirds have long, slender beaks for sipping nectar from flowers. Physiologists often study and compare animal structures such as appendages (projecting structures or parts of an animal's body that are used in movement or for grasping objects) to determine similarities, differences, and evolutionary etiology (origin) among species.

Animals react quickly to external stimuli such as temperature change, touch, light, and vibration. Information from an organism's external environment is rapidly transferred to its internal environment. In vertebrates, nerve impulses initiated in sensory neurons, or nerve cells, are transferred to the center of the brain or spinal cord. Sensory neurons are nerve cells that transmit impulses from a receptor such as those in the eye or ear to a more central location in the nervous system. From the brain or spinal cord, impulses initiated in motor neurons (nerve cells that transmit impulses from a central area of the nervous system to an effector such as a muscle) are transferred to muscles and induce a reflex response. The brain and spinal cord receive incoming messages and initiate, or trigger, the motor neurons so that animals, including humans, can move.

Metabolism is the processing of matter and energy within the cells, tissues, and organs of living organisms. There are four major questions to be answered in the study of metabolism: How do matter and energy move intothe cells? How are substances and forms of energy transformed within the cell? What function does each transformation serve? What controls and coordinates all the processes?

All animals require the atoms and molecules from food to build their bodies. Animals also require the energy released when chemical bonds are broken and new bonds are formed. This energy is required to do work and to maintain body temperature. Plants manufacture their own food by harvesting the energy of sunlight and storing the energy in the chemical bonds of carbohydrates, fats, and proteins. Animals cannot make their own food, so they obtain the energy of sunlight indirectly by eating plants or other animals.

The bodies of animals are composed of many different chemical compounds, including specialized proteins found in muscle tissue and in red blood cells. These proteins are not present in the food animals eat, so metabolism is the process of disassembling the proteins found in plant tissue into amino acids, then reassembling those amino acids in to the proteins that animals need.

Animals must use energy to assemble new molecules. Animals also require energy to pump blood, contract muscles, and maintain body temperature. This energy comes from the carbohydrates, lipids, and proteins animals eat. A complex series of reactions called the Kreb's cycle is the primary mechanism for the controlled release of energy from these molecules.

Animals maintain their internal environments at a constant level. This process, called homeostasis, depends on the action of hormones. In humans, metabolic functions and hormone interactions expend energy and help to maintain a constant body temperature of 37°C (98.6°F). Comparative studies of neurosecretory cells, special nerve cells capable of secreting hormones, indicate that the cells are also important in the developmental and regulatory functions of most animals. In insects and crustaceans, hormones control the cycles of growth, molting, and development. By identifying the hormones that regulate these cycles in insects, scientists may be able to control insect pests by interfering with hormone production and thus, with the insect's processes of growth and development.

Most animals have a transport or circulatory system that involves the movement of oxygen and carbon dioxide through blood. In vertebrates and a few invertebrates, notably annelids and cephalopod mollusks, blood flows entirely in closed channels or vessels. In most other invertebrates, blood flows for part of its course in large sinuses (cavities or opening), or lacunae, and comes directly into contact with tissues.

The reflex response is an automatic reaction, such as your knee jerking when the tendon below the knee cap is tapped; the impulse provoked by the tap, after travelling to the spinal cord, travels directly back to the leg muscle.

Biomechanics.

Alexander, R. McNeil. Animal Mechanics. Seattle, WA: University of Washington Press, 1998.

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