Answer:
Key points
Humans—and other complex multicellular organisms—have systems of organs that work together, carrying out processes that keep us alive.
The body has levels of organization that build on each other. Cells make up tissues, tissues make up organs, and organs make up organ systems.
The function of an organ system depends on the integrated activity of its organs. For instance, digestive system organs cooperate to process food.
The survival of the organism depends on the integrated activity of all the organ systems, often coordinated by the endocrine and nervous systems.
Introduction
If you were a single-celled organism and you lived in a nutrient-rich place, staying alive would be pretty straightforward. For instance, if you were an amoeba living in a pond, you could absorb nutrients straight from your environment. The oxygen you would need for metabolism could diffuse in across your cell membrane, and carbon dioxide and other wastes could diffuse out. When the time came to reproduce, you could just divide yourself in two!
However, odds are you are not an amoeba—given that you're using Khan Academy right now—and things aren’t quite so simple for big, many-celled organisms like human beings. Your complex body has over 30 trillion cells, and most of those cells aren’t in direct contact with the external environment.^1
1
start superscript, 1, end superscript A cell deep inside your body—in one of your bones, say, or in your liver—can’t get the nutrients or oxygen it needs directly from the environment.
How, then, does the body nourish its cells and keep itself running? Let's take a closer look at how the organization of your amazing body makes this possible.
Multicellular organisms need specialized systems
Most cells in large multicellular organisms don't directly exchange substances like nutrients and wastes with the external environment, instead, they are surrounded by an internal environment of extracellular fluid—literally, fluid outside of cells. The cells get oxygen and nutrients from this extracellular fluid and release waste products into it. Humans and other complex organisms have specialized systems that maintain the internal environment, keeping it steady and able to provide for the needs of the cells.
Different systems of the body carry out different functions. For example, your digestive system is responsible for taking in and processing food, while your respiratory system—working with your circulatory system—is responsible for taking up oxygen and getting rid of carbon dioxide. The muscular and skeletal systems are crucial for movement; the reproductive system handles reproduction; and the excretory system gets rid of metabolic waste.
Because of their specialization, these different systems are dependent on each other. The cells that make up the digestive, muscular, skeletal, reproductive, and excretory systems all need oxygen from the respiratory system to function, and the cells of the respiratory system—as well as all the other systems—need nutrients and must get rid of metabolic wastes. All the systems of the body work together to keep an organism up and running.
