You met the organ systems one at a time in How Body Systems Work Together. In high school biology the focus is on how they work together to keep up homeostasis.
Key idea
The body is organized in levels: cells form tissues, tissues form organs, organs form organ systems, and the systems together make the organism. Most jobs need several systems at once.
Why it works
Each level can do something the level below cannot. One muscle cell can contract, but only a muscle (an organ, with muscle, nerve and connective tissue) can move a bone, and only the muscular and skeletal systems together can carry you up a flight of stairs. Specialization makes each part good at one job, which makes every part depend on the others.
| Level | Example |
|---|---|
| Cell | a cardiac muscle cell |
| Tissue | cardiac muscle tissue |
| Organ | the heart |
| Organ system | the circulatory system |
| Organism | a person |
Worked example: Oxygen for a running leg
Which systems work together to get oxygen to the muscles of a runner's legs?
- The respiratory system brings oxygen into the lungs.
- The circulatory system carries it in the blood to the leg muscles.
- The nervous and endocrine systems speed up breathing and heart rate.
Answer: Respiratory, circulatory, nervous and endocrine systems, at least.
Worked example: Getting rid of carbon dioxide
Muscle cells make carbon dioxide in cellular respiration. Which systems remove it from the body?
- The circulatory system carries it in the blood to the lungs.
- The respiratory system breathes it out.
Answer: The circulatory and respiratory systems.
- Digestive and circulatory: nutrients absorbed in the small intestine travel in the blood to every cell.
- Nervous and muscular: nerves signal muscles to contract.
- Excretory and circulatory: the kidneys filter wastes out of the blood.
- Endocrine and the rest: hormones carried in the blood coordinate many organs at once.
Common mistake
It is common to describe each system as if it worked alone. Almost every function needs several. Ask of any process: what brings the materials, what carries them, what controls it, and what removes the wastes? Usually that is three or four systems.