Physiology: how the body keeps working
Physiology is concerned with how biological structures function. A diagram of a heart names its chambers and valves, but a physiological description tells us how blood flows under pressure through them. First describe a process you can explain. Next, add the cells, signals and measurements required to understand this process. These four topics connect several body systems.
Stability takes continuous work
Homeostasis refers to maintaining a relatively constant internal environment despite changes around us. It does not imply that all measurements remain at the same level. Your temperature, fluid status and levels of numerous chemicals exist within certain ranges. When examining an example, define the variable being controlled and the response to a change. Sweating is a response involved in regulating temperature; it is not the temperature we are trying to control.
A regulated variable and a measurement of that variable are related but not identical. For example, a temperature reading is a sample taken at a particular time and place. When studying regulation, ask what is being kept relatively stable, what disturbance occurred and what response followed. A changing measurement is not automatically evidence that a regulatory process has failed.
Follow the direction of the response
Negative feedback loops involve a response that works against a change in the regulated variable. If you get too hot, sweat evaporates off your skin, resulting in the loss of heat. Record the initial change, identify what detects it and describe what happens next. The term negative describes the direction of the feedback mechanism rather than whether the response is harmful. Positive feedback amplifies a change and needs an endpoint.
A feedback diagram becomes clearer when each arrow is explained in words. State what changed, how that change was detected and how the response alters the original variable. An arrow labelled only with the name of an organ leaves part of the explanation missing. The loop describes an interaction between components, rather than the action of a single isolated structure.
Trace a feedback loop
- Change
- Detection
- Coordination
- Response
Signals coordinate distant tissues
Both nerve impulses and hormones allow coordination among different parts of the body. Nerve impulses typically produce quick, targeted responses. Hormones travel via the circulatory system and act on cells that possess appropriate receptors. Responses depend on both the hormone and its target. Do not assume that all hormonal activities occur slowly or that every exposed cell will be affected. Identify where the signal originated and which tissue responded.
The same signal can have different effects in different tissues because the targets and their responses differ. A hormone circulating in blood does not act on every cell in the same way. When following a pathway, keep the source of the signal, the route it takes and the responding tissue separate. These distinctions connect molecular activity with a whole-body response.
Explain a response across several organs
When you engage in physical exercise, your muscles require more oxygen and produce more carbon dioxide. Circulation and breathing assist in providing oxygen and removing carbon dioxide. This illustrates how body systems interact, although textbooks may teach them separately. Before drawing a graph to illustrate a sequence, describe it verbally. Label the axes and units, and determine whether the recorded change represents a cause, a response or another association.
An exercise example can connect demand, delivery and removal. Working muscles change their activity, circulation transports substances and the lungs exchange gases with the environment. None of those descriptions alone explains the whole response. A useful account follows the sequence and identifies the contribution of each system, while avoiding the assumption that two changes occurring together prove a direct cause.
Check your understanding
A short quiz, with an explanation after each answer.
What does homeostasis maintain?
Select one common example, such as warming up while exercising. Describe the variable, identify the signal and explain how the body responded. Finally, describe how another body system contributed. Use the quiz to find any part of your explanation that needs another look.