12.1: Introduction

Learning Objectives

• Describe the process of negative feedback and explain its role in homeostasis.
• Compare and contrast the methods used by ectotherms and endotherms for maintaining body temperature.

Homeostasis is the process by which living organisms maintain stable internal conditions despite changes in their external environment. This stability is essential for proper cellular function and overall survival. To achieve homeostasis, the body relies on sensory receptors that detect changes, or stimuli, in the internal or external environment, such as fluctuations in temperature, blood pressure, or pH. These receptors send information to a control center (often the brain or a specific gland), which processes the data and determines the necessary action to restore balance. The control center then sends signals to effectors, such as muscles or glands, which carry out the appropriate response to counteract the disturbance. Most homeostatic regulation occurs through negative feedback loops, in which the response reverses or reduces the original change, bringing the system back toward its normal set point. Once the organism returns to the set point, the response that was initiated will be shut down (hence the term negative feedback).

The ability to maintain homeostasis is no small feat considering some of the significant environmental challenges that organisms can face that would threaten such a steady state. Consider, for example, that the average body temperature of a polar bear is 37°C (like humans) and that their surroundings can drop well into the -30°s. There are several physical and behavioral adaptations that allow for this including having insulating layers of blubber and fur, having a high metabolic rate, and exhibiting behavioral changes such as seeking shelter or basking in the sun. Also consider that the body temperature of polar bears will be close to the same regardless of whether they are sunbathing on the ice or actively chasing down their prey. To maintain such a stable body temperature in the face of such different physiological needs, there are several mechanisms at play.

Like humans, polar bears are endotherms and, as such, the mechanisms used for regulating body temperature (also known as thermoregulation) relies on internal processes. In humans, this includes changes to the individuals blood vessels, muscular activity and metabolic rate. For example, when faced with cold temperatures, superficial blood vessels undergo vasoconstriction (narrowing of the blood vessels) to divert heat away from extremities and towards the core where visceral organs are located. Another reaction to the cold is to increase metabolism to generate heat from metabolic processes (this is known as thermogenesis). The involuntary muscular contractions that we know as shivering can also generate body heat to help keep someone warm when faced with a cold environment. In contrast, when our bodies need to release heat to avoid becoming too hot, they can use sweat to carry away the hottest water molecules in our bodily fluids so that it can cool our body surface (this process is also known as evaporative cooling). In the opposite effect of vasoconstriction, blood vessels near the surface of our skin can expand, known as vasodilation, to bring heat to the surface of our skin to be released as well.

In this lab, we will evaluate some of these physiological responses that our body has to changes in temperature.

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