Organization in the human body | Cells and organisms | Middle school biology | Khan Academy

Have you ever thought about how incredible the human body is? For example, just to name a few things that your body's already been doing today: you're using your lungs to take breaths in and out, your heart's beating without stopping, and your brain is coordinating your body's functions, and at the same time, helping you understand this video. All these body parts are made up of cells, which, as you might remember, are the smallest units of life.

It's so amazing that trillions of cells, without thoughts or intentions of their own, can come together to form something as complex as the human body. To help us understand how this occurs, we have to understand how the body is organized. The human body has a hierarchical organization, meaning it's made up of nested layers, each one more complex than the last. In this case, four different layers make up this hierarchy: cells, which make up tissues; tissues, which make up organs; organs, which make up organ systems; and finally, organ systems, which help the human body function as a whole.

So let's start with cells and tissues. Cells are the smallest unit of life and the most basic level of organization in the human body. In the human body, not all cells look or work the same; instead, they're specialized to carry out different functions. After cells, the next level of organization is tissues. A tissue is a collection of similar specialized cells, and all of the cells that make up a tissue are like a team; they work together to ensure the tissue works properly.

For instance, this image shows a section of the type of epithelial tissue that lines the inside of your mouth. This particular tissue is made up of epithelial cells, which are specialized to help protect your mouth from wear and tear. So the next time you're eating a pointy tortilla chip, remember how important this epithelial tissue is. Next up are organs and organ systems.

So organs are structures that are made up of multiple different types of tissues, which all work together to help the organ function. An organ system is a group of organs that work together to carry out complex functions for the body. For instance, the lungs, shown here in the diagram, are made up of several tissue types that help them expand and contract. These tissue types include epithelial tissue, which is the same type of tissue we talked about earlier in the mouth, and connective tissue.

These tissues work together so that the lungs can carry out their role in the respiratory system, which is the organ system that allows us to respire or breathe. The respiratory system is made up of multiple organs, including the lungs, the trachea, the nose, and the diaphragm. When we breathe, our diaphragm, which is this muscle right here, contracts. Air is pulled through our nose and trachea and into our lungs. Here, gas exchange happens, and our bodies take in oxygen and get rid of carbon dioxide.

Then our diaphragm relaxes, and we exhale. The respiratory system and other organ systems in the body carry out specific functions but work together to keep us alive, and they do this mostly without us even realizing it. So it might get a little tricky to remember how all four levels of organization—cells, tissues, organs, and organ systems—relate to each other.

I like to think of these four different levels almost like the Russian nesting dolls that fit one inside another, with cells being the smallest doll on the inside. So cells make up tissues, tissues make up organs, and organs make up organ systems. All of these components come together to make up the human body. So I hope you've learned more about how our body's organization helps us thrive and stay alive.

So now use your brain to tell your muscular system to move your arm and give yourself a pat on the back, because your body and its organized hierarchical system is really awesome.