How does the human body prevent internal energy levels from becoming too low?

Homeostasis

Explanation:

Biological systems like those of your body are constantly being pushed away from their balance points. For instance, when you exercise, your muscles increase heat production, nudging your body temperature upward. Similarly, when you drink a glass of fruit juice, your blood glucose goes up. Homeostasis depends on the ability of your body to detect and oppose these changes.

Maintenance of homeostasis usually involves negative feedback loops. These loops act to oppose the stimulus, or cue, that triggers them. For example, if your body temperature is too high, a negative feedback loop will act to bring it back down towards the set point, or target value, of 98.6\,^\circ\text F98.6

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F98, point, 6, degrees, start text, F, end text/ 37.0\,^\circ\text C37.0

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C37, point, 0, degrees, start text, C, end text.

How does this work? First, high temperature will be detected by sensors—primarily nerve cells with endings in your skin and brain—and relayed to a temperature-regulatory control center in your brain. The control center will process the information and activate effectors—such as the sweat glands—whose job is to oppose the stimulus by bringing body temperature down.

(a) A negative feedback loop has four basic parts: A stimulus, sensor, control, and effector.  (b) Body temperature is regulated by negative feedback. The stimulus is when the body temperature exceeds 37 degrees Celsius, the sensors are the nerve cells with endings in the skin and brain, the control is the temperature regulatory center in the brain, and the effector is the sweat glands throughout the body.

(a) A negative feedback loop has four basic parts: A stimulus, sensor, control, and effector. (b) Body temperature is regulated by negative feedback. The stimulus is when the body temperature exceeds 37 degrees Celsius, the sensors are the nerve cells with endings in the skin and brain, the control is the temperature regulatory center in the brain, and the effector is the sweat glands throughout the body.

Image credit: modified from Homeostasis: Figure 1 by OpenStax College, Anatomy & Physiology, CC BY 4.0

Of course, body temperature doesn't just swing above its target value—it can also drop below this value. In general, homeostatic circuits usually involve at least two negative feedback loops:

One is activated when a parameter—like body temperature—is above the set point and is designed to bring it back down.

One is activated when the parameter is below the set point and is designed to bring it back up.

To make this idea more concrete, let's take a closer look at the opposing feedback loops that control body temperature.