Maintaining Balance: The Homeostasis of Heart Rate

Introduction

Why does your heart race when you run to catch a bus? Why do you start to sweat on a hot summer afternoon? These aren’t just random reactions; they are part of a sophisticated biological system called homeostasis. Homeostasis is the process by which living organisms maintain a stable internal environment despite changes in external conditions. In this investigation, you will act as a physiologist investigating how the human body uses feedback mechanisms to keep core temperature and heart rate within safe limits.

Part 1: Engage (Anchoring Phenomenon)

Consider a professional athlete competing in a marathon.

1. Prediction: What happens to their heart rate as they transition from standing at the starting line to sprinting toward the finish?
2. Prediction: What happens to their skin color and sweat production if the race is held in a $35^\circ\text{C}$ desert?
3. The Question: How does the body “know” when to increase heart rate or start sweating, and how does it “know” when to stop once the race is over?

Part 2: Explore (Clinical Investigation)

Open the Homeostasis Investigation Simulation. You will plan and conduct an investigation to provide evidence of feedback mechanisms.

Your Challenge:

Determine how Activity Level and Environment Temperature interact to affect Heart Rate and Core Temperature.

Procedural Steps:

1. Baseline: Set the Environment Temp to $22^\circ\text{C}$ (Room Temp), Activity to “Resting”, and Clothing to “Medium”. Start the simulation and record the data after 5 simulation minutes.
2. Exercise Stress Test: While the simulation is running, change the Activity Level to “Running”. Observe the Heart Rate and Core Temp for at least 15 simulation minutes.
3. Environmental Stress Test: Reset the simulation. Set Environment Temp to $45^\circ\text{C}$ (Desert) and Activity to “Resting”. Observe the body’s response (sweating, skin color) and record data.
4. Recovery: After a period of “Running” in the desert, change the activity back to “Resting” and see how long it takes for the heart rate to return to baseline.

Data Collection:

Create a table to record your findings. Focus on how the dependent variables (HR, Temp) respond to changes in the independent variables.

Part 3: Explain (Sensemaking)

Analyze your data to explain the feedback loops at work.

1. The Heart Rate Loop: Why did the heart rate increase during the “Running” phase? What “need” was the body meeting by pumping blood faster?
2. The Temperature Loop: When the core temperature began to rise during exercise or in the desert, what specific responses did you observe in the “Subject State”? How did these responses help lower the temperature?
3. Negative Feedback: Explain how your data provides evidence of negative feedback. (Hint: A negative feedback loop acts to reverse a trend to return to a set point).
4. Evidence-Based Claim: Use the data from your table to support the claim that the body maintains a “set point” for core temperature.

Part 4: Elaborate/Evaluate (Refining the Investigation)

Accuracy & Limitations

1. Precision: In the simulation, you can see heart rate to the nearest beat and temperature to the nearest $0.1^\circ\text{C}$. In a real-world lab, what tools would you need to achieve this same level of accuracy?
2. Refinement: If you wanted to test how Clothing Insulation affects the efficiency of the body’s cooling mechanisms, how would you change your experimental procedure?
3. Argumentation: A student claims that “heart rate only increases when you exercise.” Based on your “Desert Heat” test results, is this claim true? Use evidence from the simulation to argue for or against this claim.

Part 5: Summary

Construct a final model (sketch or description) that shows the components of a feedback loop:

• Sensor: Detects the change (e.g., rising temperature).
• Control Center: Processes the info (The Brain).
• Effector: Creates the response (e.g., Sweat glands, Heart).
• Result: Restoration of balance.