General physiology is the scientific study of the functions of living systems, exploring the chemical and physical mechanisms that operate within the body. It focuses on how the body works, contrasting with anatomy, which studies the body’s physical structures. The function of a body part is always dictated by its form, making the relationship between structure and function inseparable. Understanding physiology involves analyzing the coordinated actions that allow an organism to sustain life.
Organizational Levels of the Human Body
The human body is organized into a structural hierarchy that begins with the most fundamental components. The chemical level involves atoms (e.g., carbon, hydrogen, oxygen) that combine to form molecules like proteins, carbohydrates, and DNA. These molecules then aggregate to form specialized structures within cells called organelles, which perform specific tasks.
The cellular level represents the smallest independent functional unit of the body, performing all basic life processes. Groups of similar cells form tissues, such as muscle or nervous tissue, that work together for a common function. Multiple tissues then combine to construct an organ, like the heart or the stomach, each having a distinct shape and function.
Organs cooperate within organ systems (e.g., the digestive or respiratory system) to carry out broad, complex life functions. The highest level of organization is the organism, the complete living being composed of all these integrated systems. Functionality emerges from this integration, as the complex activities of the entire organism result from coordinated actions at every lower level.
Maintaining Stability: The Principle of Homeostasis
The central concept in physiology is homeostasis, the body’s ability to maintain a relatively stable internal environment despite continuous challenges. This internal stability, known as the milieu intérieur, is a dynamic state where variables like body temperature, blood glucose, and pH are kept within narrow, acceptable ranges. All physiological processes are ultimately directed toward preserving this steady state.
Homeostatic control is managed by feedback loops that contain three main components. A receptor monitors the environment and detects changes, sending information to the control center. The control center (often the hypothalamus) receives the input, compares it to a set point, and determines the appropriate response. Finally, the effector (typically a muscle or a gland) executes the response to counteract the initial change.
The vast majority of control mechanisms utilize negative feedback, which works to reverse the direction of the initial stimulus. For example, if body temperature rises, a negative feedback loop triggers sweating and vasodilation to bring the temperature back down. This mechanism ensures that fluctuations in regulated variables remain minimal, promoting overall stability.
Positive feedback loops are far less common because they intensify the original stimulus, pushing the variable further away from the set point. This mechanism is typically only activated when rapid action is required to reach a specific end point. Examples include the release of oxytocin during childbirth, which increases uterine contractions, and the chemical reactions leading to blood clotting following an injury.
Inter-System Communication and Control
The maintenance of homeostasis requires constant and rapid communication across all organ systems. This coordination is primarily managed by the nervous system and the endocrine system, which employ distinct signaling methods to regulate body functions. The difference in their approach allows the body to manage both immediate, localized events and slower, widespread processes.
The nervous system specializes in fast, targeted communication using electrical signals (action potentials) and chemical messengers (neurotransmitters). This system is structured for speed, with signals traveling directly along nerve fibers to specific cells, resulting in near-instantaneous responses, such as muscle contraction or reflexes. Its effects are typically short-lived and localized.
In contrast, the endocrine system operates more slowly, relying on hormones secreted into the bloodstream. Hormones travel throughout the body via circulation, reaching virtually every cell, but only specific target cells with appropriate receptors will respond. The endocrine response is slower to initiate but produces effects that are more prolonged and widespread, controlling long-term processes like growth, metabolism, and reproduction.
These two control systems often work together, with the nervous system providing quick initial adjustments and the endocrine system providing sustained support. The hypothalamus links the two, controlling the pituitary gland and regulating hormone release. This dual-system approach allows the body to respond to stimuli across a broad spectrum of time scales and functional requirements.
Energy Acquisition and Internal Transport
Every cell in the body requires a continuous supply of energy, provided in the form of adenosine triphosphate (ATP), the universal energy currency of the cell. Cellular metabolism is the collective set of chemical reactions that extract energy from nutrient molecules like glucose, converting it into ATP. The most efficient form of this production is aerobic respiration, which occurs inside the mitochondria and requires a steady supply of oxygen.
The integrated function of the respiratory and circulatory systems supports this cellular energy demand by managing necessary inputs and outputs. The respiratory system’s primary role is to acquire oxygen from the external environment and facilitate its diffusion into the blood within the lungs. It simultaneously removes the metabolic waste product, carbon dioxide, generated during ATP production.
The circulatory system acts as the body’s internal transport network, powered by the heart’s pumping action. It carries oxygen from the lungs and absorbed nutrients from the digestive system to every cell and tissue. At the cellular level, blood exchanges oxygen and nutrients for metabolic waste products (e.g., carbon dioxide and urea) within the capillary beds. The blood then returns carbon dioxide to the lungs for exhalation and transports other wastes to organs like the kidneys for elimination.