Video: Homeostasis and feedback loops

All right, let's get to work.

Ugh! It's too hot. I can't do it.
Fine, back to it.
Hey, I need oxygen.
Where's my sodium?
Thanks!
Now it's too cold.
Okay, back to work.

The cells in our body need to be in a stable environment to work effectively. Luckily, we have mechanisms in place to ensure that our cells have everything they need to function at their best.

Let's explore together how homeostasis and feedback loops work.

Homeostasis is the tendency of the body to maintain a stable internal environment despite changes in the external environment. When we are outside on a freezing winter day or during a very hot summer day, homeostasis ensures that our body temperature remains relatively stable, so the extracellular fluid is at the perfect temperature for our cells to function effectively.

Homeostasis is fundamental for the function and survival of our cells, and therefore, our entire body. Disruptions of the homeostatic processes often result in illness or disease, and could even lead to death.

Homeostasis ensures that variables fundamental for cell survival are maintained at a level optimal for cell function. The ideal value is sometimes referred to as a setpoint. However, homeostasis does not maintain a variable at a specific value, but rather it keeps it within a normal range around the setpoint. The key variables maintained within a normal range by homeostatic processes are called regulated variables.

To be able to regulate a variable homeostatically, our body must be able to measure it. For example, blood pressure and blood glucose levels can be measured and therefore can be regulated within a set range. Instead, variables that are not directly sensed cannot be homeostatically regulated. Some variables like heart rate are generally maintained within a typical range, but are not directly sensed and therefore are not under homeostatic regulation.

Something important about setpoint and normal range is that they are not fixed, but can change. For instance, the setpoint of the body temperature is higher in the afternoon than in the morning, and when someone has a fever.

One of the main ways an organism can regulate a variable is via feedback loops. In a feedback loop, a change in a variable produces a response and the response feeds back into the variable, meaning that it is able to change it. This creates a cycle that can help keep a variable stable or induce instability depending on the type of feedback loop.

Let's start by discussing negative feedback loops.

Negative feedback loops oppose changes in a regulated variable. This means that when a variable leaves its normal range, an event sometimes called a stimulus or perturbation, the negative feedback loop brings it back toward its setpoint. This is how blood pressure, body temperature, acid-base balance, and iron concentrations are regulated.

Three components are necessary for a negative feedback loop: a sensor, a control center, and an effector. Before we dive into the complexities of the human body, let's see how these components interact in a simple example.

Let's say you're baking a cake at 180 degrees Celsius. The oven has an internal sensor that measures the current temperature and sends the information to a control center. The control center compares the current oven temperature to the setpoint; in this case, 180 degrees. If the temperature drops below the setpoint, the controller activates the effector. The heater turns on until the temperature returns above the setpoint of 180 degrees.

This feedback system keeps the oven temperature somewhat stable, so our cake will be baked to perfection. Pretty smart, eh? Well, our bodies are even smarter.

For instance, the controller of most ovens has only two states: off if the temperature is at or above the setpoint, or full-power on if the temperature drops.

In humans, homeostatic feedback loops are not simple on/off switches. Instead, they continuously monitor and fine-tune the activation of effectors in small steps, improving the stability of the regulated variable.

In the human body, the sensors are usually receptors, like chemoreceptors or mechanoreceptors. Just like in the example of the oven, these sensors monitor the regulated variable and send that information to the control center.

The control center is usually located in the central nervous system, or the endocrine system, and has two main functions. First, it detects how much a regulated variable deviates from its normal range. Second, it sends the effector a signal proportional to the error detected. So if the regulated variable is just outside its normal range, the activation of the effector will be small, but larger deviations from the normal range will result in stronger activation of the effector.

When we mention effectors, what are we talking about exactly? They are cells, tissues, and organs that can change the internal environment. Effectors adjust the regulated variable in one of two ways, either directly, or indirectly by changing the activity of variables not directly sensed called non-regulated or controlled variables.

For instance, when blood pressure drops, the brainstem increases heart rate to restore blood pressure within its normal range. In this case, blood pressure is regulated by changing heart rate, which is a non-regulated variable.

Let's look at a physiological example. Here's how a negative feedback loop regulates the blood oxygen concentration.

The sensors are chemoreceptors in the carotid bodies. They monitor the partial pressure of arterial oxygen and sends that information to the control center. The control center is in the respiratory centers of the medulla oblongata. It calculates the difference between the current oxygen partial pressure and the normal range and produces an output that depends on how large the difference is. This information is sent to the effectors.

The effectors are the inspiratory muscles. They affect breathing frequency, which is a non-regulated variable to change the arterial pressure of oxygen, which is the regulated variable.

If the amount of arterial oxygen sensed by the chemoreceptors drops below the normal range, the respiratory centers increase the breathing frequency to restore it back to the normal range. This is how negative feedback loops oppose changes in a variable.

However, sometimes the change in a variable is helpful for the body in the short term. So, it needs to be amplified quickly. This involves positive feedback loops.

Positive feedback loops reinforce a change in a variable, initiating a cycle of progressively larger responses until a definite endpoint is reached. This endpoint is often an event outside of the positive feedback loop.

Positive feedback loops may indirectly contribute to homeostasis in the long term by escalating body responses rapidly in the short term. Examples of positive feedback loops include labor, where a baby needs to be delivered; blood clotting, where a torn blood vessel must be repaired quickly to avoid drops in blood pressure; and the generation of action potentials, where the flow of sodium ions into the neuron opens channels that let even more sodium inside the cytoplasm.

Let's explore the positive feedback loop that occurs during labor.

When ready to be delivered, the fetus descends lower in the uterus, pushing against the uterine cervix. Stretch-sensitive cells detect stretching of the cervix and send this information to the brain and the pituitary gland, which releases oxytocin. Oxytocin stimulates contractions of the uterus, pushing the fetus forward and further stretching the cervix. This self-reinforcing loop helps labor progress until it reaches its endpoint, which is when the baby is delivered.

Homeostasis is one of the core concepts in physiology that explains much of how the human body works in health and pathology.

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