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The goal of physiology is to study the phenomena and regularity of life's activities, to explain the physical and chemical factors that are responsible for the origin, development, and progression of life. In human physiology, we are concerned with the specific characteristics and mechanisms of the human body that make it a living being.
Physiological studies provide an important basis for modern medicine. At the same time, clinical studies can help us to understand physiological function. Physiology now comprises three levels of function: cell and molecule level，organ and system level and a general level. In the modern organism-psychology-society-environment medical pattern, physiological studies should investigate not only the changes of some physiology variables, but also the effects to environment, society and psychology. Methods used in physiological research include both animal and human experiments. Animal studies can be divided into chronic and acute experiments. Acute experiments consist of manipulations in vivo and in vitro. A manipulation performed on the whole body is called an experiment in vivo whereas studies on isolated tissues or organs involve experiments in vitro. Chronic experiments may be performed on a conscious subject for a long period of time.
Metabolism, excitability, adaptability and reproduction are the basic characteristics of life activity. The term "metabolism", meaning literally "change", is used to refer to all material and energy transformations that occur in the body. Excitation signifies an increase in activity, such as contraction of a muscle or acceleration of the heartbeat. An environmental factor that causes a response in a sense organ is called stimulus. The response to stimulus is called reaction. When a maintained stimulus of constant strength is applied to a receptor, the frequency of the action potential in its sensory nerve decreases over time. This phenomenon is known as adaptation. The physiology of reproduction is to produce offspring. The process of reproduction includes fertilization, pregnancy and delivery.
All cells of the body are surrounded by extracellular fluid and so extracellular fluid is called the internal environment of the body. Homeostasis is the maintenance of a constant state with special reference to the internal medium. Functional activities often changes in certain chronological orders. If the changes appear repeatedly, this is called a rhythmical change. The rhythm is called a biorhythm.
The regulation of physiological function is divided into autoregulation, humoral regulation and nerve regulation. In certain cases, a tissue or organ can respond directly to the environmental change, depending neither on nervous nor on humoral control. This form of regulation is called autoregulation. Chemical substances secreted by some tissues arrive at target organs through body fluids and regulate physiological activity of the target organ. This is called humoral regulation. Many physiological activities are regulated by the nervous system. This is called nervous regulation. A reflex is the basic unit of nervous regulation and the reflex arc is the pathway in a reflex. It is the basic unit of integrated neural activity, consisting of sense organ, afferent and efferent nerves, interneurons and effector. Feedback is a flow of information along a closed loop. Usually, a constancy of physiological variable requires a feedback mechanism that feeds the output information back to the control system so as to modify the nature of control. If the information amplifies the changes, it is called positive feedback. However, if the regulation is in a reverse direction, it is called negative feedback. Feedforward control mechanisms often sense a disturbance and can therefore take corrective action that anticipates changes.
As a subject, physiology is ancient as well as young. In the eighteenth and nineteenth centuries, physiology made great progress at the level of organs and systems . In the twentieth century, it gained knowledge rapidly at the cell and molecular levels. Now, in the twenty-first century, the challenge for physiology is to integrate information at all levels towards a deeper understanding of mechanisms and function.
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