Homeostasis in Action

Run up a flight of stairs and your heart pounds faster, then slows again over the next several minutes. On a hot, dry afternoon, tiny pores on the underside of many leaves narrow.

The world outside you keeps changing. The air warms and cools, and you sit still or sprint. Yet the blood inside you, and the water inside a leaf, stay within fairly narrow limits. How do living things keep their insides steady?

Explore: push a feedback loop off balance

In the simulation you pick a disturbance: a burst of exercise, a hot room, or dry air around a leaf. Then you step through the loop one part at a time, from the sensor to the control center to the response. A graph shows the variable pushed away from its usual range and then pulled back. A switch lets you break the sensor.

Try this.

1. Pick exercise. Before you step through, predict whether heart rate will snap straight back to rest or come down gradually. Then test it.
2. Pick the hot room. Predict which response the control center will choose. Step through and check.
3. Break the sensor, then repeat the hot room. Predict the shape of the graph before you run it.
4. Pick dry air for the leaf. Predict what the guard cells do, and what that costs the leaf.

Explain: the negative feedback loop

Homeostasis is keeping internal conditions within a stable range while the outside changes. The word “range” matters. Your body temperature is usually near 37 °C, but it shifts a little through the day.

Most homeostasis works by negative feedback, a loop in which the response opposes the change that started it. “Negative” does not mean bad. It means the response pushes the other way.

Every negative feedback loop has the same parts:

• A change. Something pushes a variable away from its usual range.
• A sensor, a cell or organ that detects the change.
• A control center, which compares the sensed value with the usual range and sends a signal. In your body this is usually the brain.
• An effector, a muscle, gland or cell that carries out the response.
• The response, which pushes the variable back.

Here is the loop for body temperature on a hot day:

body warms → temperature sensors in skin and brain → control center in the brain → sweat glands release sweat → sweat evaporates and the skin cools

On a cold day the same control center sends different signals. Muscles shiver, which releases heat, and blood vessels in the skin narrow, so less heat is lost.

A thermostat in a house is a fair model. It senses the room, compares it with a setting, and turns the heat on or off. The model breaks down in one place: your body has many sensors and many effectors working at once, and no one sets the dial.

A few loops run the other way. In positive feedback, the response makes the change bigger until some event ends it. Contractions during childbirth are the usual example.

Explain: three examples, and the membrane

Heart rate during exercise. When you start to climb, nerve signals from your brain speed up your heart. Sensors in your working muscles, your blood vessels and your brain add information. Working muscles release carbon dioxide. Some brain sensors respond to the change this carbon dioxide makes in the acidity of the fluid around those brain cells. A faster heart moves more blood past the muscles, carrying carbon dioxide away and bringing oxygen. When you stop, the signals change and heart rate falls, but gradually. Exercise control is a little more than one feedback loop: the brain also raises heart rate as exercise begins, partly through commands sent before any large change in the blood, not only in response to such a change.

Stomata in a leaf. A leaf’s surface has tiny pores called stomata (one is a stoma). Each pore sits between two guard cells, which change shape to open or narrow it. Carbon dioxide enters through these pores and water vapor leaves through them. Experiments show that drier air can make guard cells close the pores, which slows water loss. The cost is that less carbon dioxide gets in. Temperature matters too, but not simply: in one 2020 study of Arabidopsis leaf tissue, warming opened the pores. In a whole plant on a hot afternoon, what the guard cells do depends on several signals together, including dry air and the water supply.

Roots and water. Roots grow toward wetter soil, a response called hydrotropism. It is slow compared with a heartbeat, and it changes the direction of growth rather than a number inside the plant. It still helps keep the plant’s water supply steady. Scientists usually describe this as a response to the environment, not a complete feedback loop with a control center.

The cell membrane. Every cell has its own homeostasis, and the membrane does much of the work. It is selectively permeable: it lets some substances cross and holds others back. In passive transport, substances move from where they are more concentrated to where they are less, and the cell spends no energy. In active transport, the cell spends energy to move substances against that direction, from less to more. Together these keep the inside of a cell different from the outside. Crossing the Membrane covers them in detail.

Elaborate: how fast does heart rate come back?

These are means for 52 healthy adult volunteers on the first day of a treadmill study by Tulumen and colleagues at Hacettepe University in Ankara, Turkey, published in 2011. Exercise ended with an active cool-down, and heart rates came from ECG reports. The “peak” row is the highest heart rate during exercise, not a reading taken at the exact moment exercise stopped. For comparison, the American Heart Association gives 60 to 100 beats/min as a usual resting range for adults.

1. By how much did mean heart rate fall in the first minute? By how much between 4 and 5 minutes? What does that tell you about the shape of the recovery graph?
2. At 5 minutes, was the group back to its resting mean? By how many beats per minute did it differ?
3. The same volunteers repeated the test on days 7 and 30. Their mean heart rates at 1 minute were 145, 144 and 145 beats/min on days 1, 7 and 30. What is the mean of these three values?

Design problem

Plan a safe class investigation: how does heart rate change after the same short activity, and how long does it take to come back toward rest? Name your variables, what you keep the same, how many trials you run, and how you keep it safe and fair.

Evaluate: check your thinking

When you run up the stairs, nerve signals speed your heart, and when you stop, the signals change and it slows over several minutes, back toward its resting range. On a dry afternoon, guard cells narrow a leaf’s stomata, so it loses water more slowly. Both are responses that hold conditions inside the organism within a range while the outside changes.

Science practices you used: making predictions and testing them in a model, planning a safe and fair investigation with repeated trials, reading a data table and calculating a mean, and naming the limits of a model and of one study’s data.