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Definition: Physiology from Black's Medical Dictionary, 43rd Edition

The branch of medical science that deals with the healthy functions of different organs, and the changes that the whole body undergoes in the course of its activities. The teaching of physiology is a basic part of the medical student's initial education.

Summary Article: Physiology
From Encyclopedia of Global Health

Physiology literally means the study of nature. This broad topic spans the study of plants, animals, and humans. More specifically, physiology is concerned with how things function. This study of function can be on a molecular level or a gross level. Even though there are obvious differences between plant, animal, and human physiology, at a microscopic level, there are similarities. Often, one type of physiology can be used to better understand another.

Plant physiology has a number of unique aspects and important characteristics to apply in the field of agriculture. Some features unique to plants help us understand our environment. For example, we know the process of photosynthesis is how plants convert energy from sunlight, water, and carbon dioxide (the part of air we breathe out) into food. This process then produces oxygen, which is the air we breathe. By understanding this link, we can better appreciate the importance of our environment to our own health.

Moreover, understanding the function of plants is applicable to agriculture. In understanding how plants function, we can look for ways to alter the growth of plants in beneficial ways. Scientists have developed ways to increase crop resistance to disease and insects in order to increase crop yields.

Another area of physiology is animal physiology. This classification can include microorganisms as well as animals. The basic study of the function of microorganisms is termed microbial physiology. Microorganism physiology has additional subdivisions. For instance, some people study how microorganisms function as a group. Usually, this categorization of microorganisms is referred to as a community or biofilm. Therefore, this specialty is referred to as bio-film physiology.

The specific study of the microorganisms called bacteria is bacterial physiology. Other areas focus on how microorganisms function while interacting with other organisms. An example of this is microbe–host interactions and bacteria–host interactions. Each of these areas study how microorganisms interact with higher organisms such as humans.

The term animal physiology often refers to the study of how an animal’s body functions. This research has several aspects. First, some people study how an animal’s body operates, simply to gain a better understanding. In contrast, others study animal physiology to better grasp how everything is inter-connected within our environment. In many instances, studies of how an animal’s body functions are carried out for other reasons. Sometimes, we are using it to better understand our own physiology. Sometimes, researchers desire to determine how something can be improved as a food source. It is essential when animals are utilized as a food source that their immune system functioning be understood. In the event an animal becomes ill, a determination can be made as to what the results of the infection or disease will be. With this, effective treatments can be established. This is crucial to prevent contamination of the food supply.

Many research studies are conducted utilizing animals. Various animal physiology studies have helped medical progress. These inquiries have resulted in effective treatments for various diseases, such as rabies, anthrax, small pox, tetanus, and measles. Furthermore, animal physiology research has helped improve medical techniques. In the 1920s the use of animals helped in discovering the hormone insulin and its function in the body, particularly, as it relates to diabetes mellitus. This discovery has helped save countless people. Other medical research advances in the 20th century have been based upon the similarities animal physiology and human physiology. These outcomes have helped to develop the technique of cardiac catheterization, open heart surgery (i.e., heart transplantation, coronary artery bypass grafts), and cardiac pacemaker implantation. Appreciating and understanding the similarities between the physiology of animals and ourselves has led to numerous lifesaving advances in medicine.

The aforementioned specialties have an appreciation for the similarities between human and animal physiology, but another looks at differences. The area of comparative physiology does just as the name implies; it compares and contrasts the functional systems of all animals and humans and notes the similarities and differences.

Like other segments of physiology, human physiology can be further subdivided into various specialties. Some specialties within human physiology focus on microscopic components. Cell physiology, for instance, looks at how cells function and interact within the human body. A more focused specialty is endocrinology, which looks at the chemical messengers called hormones. Researchers examine how these messengers work throughout the body. The study to understand how molecules are exchanged and transported across cell membranes is termed membrane physiology. Other areas within human physiology focus on entire body systems. The general study of the functions of the heart and the blood vessels is called cardiovascular physiology. A similar, yet distinct, area of study called cardiology examines how the heart functions normally and in disease states. Immunology pertains to the study of immune cells’ normal function and how they respond to foreign or “non-self” material. To analyze how muscles operate is to study myophysiology. Research that is concerned with the brain’s specialized cells called neurons, the spinal cord, and the nerves is called neurophysiology. The specialty of neuroendocrinology is concerned with the complex interactions between neurophysiology and endocrinology. To understand how the kidney’s function to control the excretion of ions and metabolites is to look at renal physiology. The study of reproductive behaviors and various aspects of the reproductive cycle (puberty, menstrual cycle, pregnancy, and menopause) are studied in the field of reproductive physiology. The aspect of physiology that unwraps the mysteries of gas exchange at the microscopic and macroscopic levels is the field of respiratory physiology. There are many subdivisions within human physiology and further division of these into explicit areas of specialty.

Besides these various areas of specialty in human physiology, there are still numerous topics that examine how the human body adapts to various stimuli. First of all, exercise physiology is a wide area of study analyzing how human physiology adapts to the stress of exercise or physical activity. Depending on where you perform your physical activity, a related field of study is environmental physiology. This subspecialty focuses on how the body functions within various climactic conditions. Our bodies for instance, adapt differently to desert conditions than to the artic cold or to high-altitude atmospheres. The field that observes how our body’s functioning may change over time is termed evolutionary physiology. Due to the complex nature of our body’s functions, there are so many areas of specialized study within the broad area of human physiology.

Human physiology is composed of all of the body systems as specialties as well as other areas of specialty including exercise and environmental physiology. To understand human physiology, we often observe the physiology of animals. Additionally, we study animal physiology for its own unique characteristics as well. Finally, plant physiology is an important study that helps us to understand our food supply. Discovering how these various systems work helps us to better understand nature.

  • Biochemistry; Endocrinology; Neuroendocrinology.

  • Bryan H. Derrickson; Gerard J. Tortora. Principles of Anatomy and Physiology, 11th ed. (Wiley, 2005).
  • Stuart Ira Fox, Human Physiology, 9th ed. (McGraw-Hill, 2006).
  • William F. Ganong, Review of Medical Physiology, 22nd ed. (McGraw-Hill Medical, 2005).
  • Dee Unglaub Silverthorn, Human Physiology: An Integrated Approach, 4th ed. (Benjamin Cummings, 2006).
  • Linda E. May
    Kansas City University
    Copyright © 2008 by SAGE Publications, Inc.

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