Physiology

Physiology is the scientific study of the functions and mechanisms within living systems. As a subdiscipline of biology, it investigates how organisms, organ systems, and individual cells perform essential tasks. These tasks include chemical and physical functions that allow life to persist. Physiology examines life at many levels, from tiny biomolecules to entire populations. It is a vital field because it explains how life operates and maintains itself. When a body functions normally, it is in a physiological state. A pathological state occurs when these functions become abnormal, often resulting in disease.

To understand how life works, physiology relies on several foundational sciences. Anatomy is the study of the structure and organization of living things. Because structure and function are closely linked, anatomical knowledge is essential for physiologists. Biochemistry provides the study of chemical processes and substances inside organisms. This helps explain the molecular processes necessary for cellular function. Biophysics explores the physical properties of organisms and their environment. This field explains how creatures sense stimuli like light, sound, or temperature. Genetics studies heredity and how genes interact with the environment to influence an organism's traits. Finally, evolutionary biology explains how physiological processes originated and adapted over time.

Physiological processes often center on the concept of homeostasis. Homeostasis is the maintenance of a stable internal environment despite external changes. This stability is achieved through complex control mechanisms and communication between cells. Cells communicate using both electrical and chemical signals to integrate functions. In humans, the endocrine and nervous systems play major roles in receiving and transmitting these signals. This integration allows the body to respond to its surroundings. Without these regulatory processes, a living system could not maintain the steady states required for survival. This concept was championed by Walter B. Cannon in 1929, building on Claude Bernard's earlier work.

Scientists categorize physiology in several different ways. One method is by the type of organism studied, known as taxa. This includes human, animal, plant, microbial, and even viral physiology. Another way to group the field is by the level of biological organization. This includes cell physiology, molecular physiology, systems physiology, and organismal physiology. Researchers also look at the cause of physiological variation. This includes developmental, environmental, and evolutionary physiology. Finally, the field is divided by research goals. Applied physiology, such as medical physiology, seeks practical uses. Non-applied physiology, like comparative physiology, focuses on understanding diversity across different species.

Each subdiscipline focuses on specific biological mechanisms. Cell physiology examines processes like cell division, signaling, growth, and metabolism. Plant physiology is a branch of botany that studies how plants function. It looks at processes like photosynthesis, respiration, and nutrient absorption. It also studies how plants respond to light through tropisms or circadian rhythms. Animal physiology focuses on the complex systems of creatures. Human physiology is a major area of study within this group. It examines how the nervous, cardiovascular, and respiratory systems work together. Understanding these human systems is essential for diagnosing and treating health conditions.

The history of physiology spans thousands of years, beginning in classical Greece. Hippocrates, in the late 5th century BC, proposed the theory of humorism. This theory suggested that four substances—black bile, phlegm, blood, and yellow bile—governed health. Aristotle later emphasized the critical relationship between an organism's structure and its function. Galen, who lived until 200 AD, was the first to use experiments to probe body functions. He argued that humoral imbalances could be located in specific organs. For 1,400 years, Galenic physiology remained a dominant influence in medical practice. During the early modern period, the term "physiology" was introduced by Jean Fernel.

Significant discoveries continued to shape the field through the modern era. In the 1610s, Santorio Santorio used a device to measure pulse rates. In 1791, Luigi Galvani discovered that electricity plays a role in the nerves of frogs. In the 1820s, Henri Milne-Edwards compared living things to machines or factories. He suggested that organs act like workers in a division of labor. In 1858, Joseph Lister studied blood coagulation and inflammation. His work with antiseptics helped decrease death rates from surgery. In 1891, Ivan Pavlov conducted famous research on conditional responses in dogs. These historical milestones helped transform physiology into the rigorous science it is today.