The internal environment provides for the needs of all the cells that make up a complex multicellular animal. Organs and organ systems control the composition of the internal environment so as to maintain homeostasis. The activities of organs and organ systems are regulated. Regulation requires information about optimal conditions (set points) and existing conditions (feedback) to create commands to effector cells and organs.
learning outcomes
You should be able to:
Explain why homeostasis of the internal environment in a multicellular animal is critical to the animal’s survival.
Use knowledge about fluid compartments in the human body to perform analyses.
Differentiate between negative feedback, positive feedback, and feedforward control mechanisms.
Why is maintenance of the internal environment considered to be the primary focus of physiology?
Maintenance of the internal environment is the primary focus of physiology of multicellular animals because the internal environment serves the needs of all the cells of the body. Cell types are specialized to contribute something to the maintenance of the internal environment, but no cell can provide for all of its own needs and therefore cannot exist alone.
Explain the differences between negative and positive feedback and feedforward control mechanisms.
Negative feedback is information about the regulated variable that can be compared to a set point to generate an error signal. The error signal is relayed to effector organs to bring the regulated variable back to the set point. Positive feedback is information about the regulated variable that can be converted to command signals that push the regulated variable farther from its set point. Feedforward information changes the set point in anticipation of an imbalance so that the regulated variable can remain stable as conditions change.
Based on your body mass, estimate how much water is in your body. What is your intracellular fluid volume, your extracellular fluid volume, and your blood plasma volume.
You would inject known volumes (V1) of mannitol and Evans blue at known concentrations (C1) into the blood of a subject. After a sufficient amount of time for the injected solutions to circulate around the body, you would take a blood sample and measure the concentrations of Evans blue or mannitol in the blood plasma(C2). You can then use the formula given for calculating V2 for mannitol, which will be the extracellular volume, and V2 for Evans blue, which will be the blood plasma volume. Subtracting the blood plasma volume from the ECF volume will give the interstitial fluid volume.
Principles of control and regulation help organize our thinking about physiological systems. Once we understand how a system works, we can then ask how it is regulated. Part Nine of this book describes various physiological systems, how they function, and how they are regulated. We begin by extending our example of a house thermoregulatory system to that of the mammalian body.