What do bodies do?

sameness and change

Every cell, tissue, organ, and organ system in our body has a specialized role that makes it really good at doing its one main job, which is to contribute in some way to our human unit as a whole staying alive. But what exactly are all of these cells doing? What is their goal? What does it take to stay alive? And how do we achieve those things?

These are all very big questions, and you probably already intuitively know the answers. So, take a minute to brainstorm here: What does it mean to be alive? 

Keep brainstorming.

Keep brainstorming.

I’ll wait.

What does it mean to be alive?

Awesome! Great brainstorming.

Remembering back to when we defined living things, you may have thought of some of the specific, scientific requirements of being alive:

  • Composed of one or more cells
  • Have different levels of organization
  • Use energy
  • Grow
  • Reproduce
  • Respond to the environment
  • Adapt or evolve

Humans, as living things, must do all of these things. And we do: We are composed of trillions of cells, which are organized into tissues, organs, and organ systems. We use energy, which we take in through our food and convert into usable energy via cellular respiration. We grow and develop from babies into adults, and as adults we make new babies. We respond to the environment all the time in sometimes very simple and sometimes very complex ways, and we adapt and evolve to longer-term changes in the environment.

Keeping this whole “What does it mean to be alive?” question simple, you may have also thought of some things that we need to stay alive, such as those things we get from our outside environment. For example:

  • Food
  • Water
  • Shelter/protection
  • Oxygen

You might have also thought of some other general processes that go on inside of our bodies, like:

  • Breathing
  • Heart beating
  • Digesting food
  • Removing waste
  • Fighting disease
  • Running away from bears (and other scary things, but let’s not think too hard about it and just say “bears”)

And so on. Finally, you might think of some other advanced processes that go on, which are a little more unique to complex organisms like humans. For example:

  • Thinking, learning, retaining memories, feeling emotions, etc.
  • Maturing, getting older, growing up, etc.
  • Adapting to change
  • Anything that benefits the survival of our species as a whole, like reproducing/having kids, helping others, and so on. 

These are all great answers! They can also all be distilled down into two main ideas, which are right at the core of all of the processes that are involved in staying alive: staying the same and changing.

homeostasis: staying the same

Our bodies work really hard to stay the same. This is called homeostasis (literally, homeo = “same” and stasis = “stay”). In terms of our basic life functions, it is very important that the internal environment of our bodies remains the same. This is because all of our enzymes, proteins, and chemical reactions are designed to work best under very particular conditions. They need the same temperature, the same amount of water, the same amount of salt, the same pH—basically, the same everything—in order to function properly. They also need enough energy to make this happen, which means taking in enough oxygen, enough food, and enough of the right nutrients, and keeping your blood levels of this oxygen, this food (especially glucose), and these nutrients the same. They need a heart rate that falls within a certain range, a blood pressure that falls within a certain range, and a breathing rate that falls within a certain range.

So, one of the main things that our bodies do is stay the same. When you start to use up a lot of oxygen—if you’re exercising, for example—you brain will signal to your lungs/diaphragm to start breathing faster, to take in more oxygen, to keep our blood levels of oxygen the same. When you start to run low on sugar, your endocrine system (more specifically, the pancreas) releases a hormone called glucagon that tells your body to start using up stored forms of energy to raise your blood sugar, and your brain (and hormones) will tell you to eat some food. Meanwhile, your excretory system works hard to keep your water, salt, and pH levels stable. Your various body systems are working to keep your body temperature constant. Your pacemaker is working to keep your heart rate at a good pace. All in all, your body is constantly hard at work to make sure nothing really changes.
Here’s a great video on homeostasis:

changing temporarily

That said, we don’t just have to stay the same. For one thing, life wouldn’t be very interesting if all we did was sit around making sure our internal environments were perfect. For another, that would actually be counterproductive to life. Just think, what if you were laying around in the woods next to a nice fire, casually meditating and maintaining homeostasis, and all of the sudden a tree starts falling, or a weird bug starts crawling on you, or your fire starts to get a little out of control? Our bodies have an easy solution! Move. Shake the bug off your arm. Put out the fire, or run away from it.

All of these things involve changing in response to the environment, and it is at the core of a lot of what our bodies do. We have super-fast reflex arcs and snappy muscles to help us move away from a hot fire quickly. We have advanced brain processes to help us solve problems. We have hormones (epinephrine and norepinephrine, a.k.a. adrenaline) that basically make us superheroes in response to threats. Lots of change goes into staying the same!

changing permanently

Finally, we have more long-term changes that are beneficial for the survival of our species, not just us as organisms. This especially includes the process of reproduction—which also entails all the change that goes from developing from a baby into a kid into an adult into an even older adult—which involves some hormones that we’ll discuss a little bit. This isn’t a change that keeps us the same, it’s a change that makes us different, and that’s a change that helps our species stay alive! Other changes that keep us alive include processes like learning, which helps us get better at solving problems in our environment and keeps us from doing the same dumb things over and over. Finally, species-wide changes like evolution ensure that species can change in response to longer-term environmental changes, like living on land versus water, or in warm weather versus cold weather. We’ll talk more about evolution later in this course.

summary

Now that you understand the goals of staying alive, we can start to talk about some of the more specific ways that our bodies communicate in order to stay alive! 

homeostasis

~ Staying the same
~ Most “background” physiological processes (e.g., maintaining blood pressure)
~ When a change occurs, it is corrected to restore perfect balance

temporary change

~ Examples include movement and stress responses.
~ When goal is accomplished, everything goes back to normal

permanent change

~ Examples include development (baby into adult), learning, and adaptation/evolution.
~ No return to baseline
You should understand:

  • The two main things we need to accomplish in order to stay alive are staying the same and changing.
  • Staying the same, or homeostasis, is extremely important because it keeps the internal environments of our bodies conducive to life.
  • Changing is also important because it helps us to respond to the environment, reproduce/develop, learn, and evolve as a species.

Learning Activity

Content contributors: Emma Moulton
Some images made by biorender.com