The Human Body Systems
The human body runs on a set of organ systems, each with its own job, all working together to keep you alive. You do not need to memorize every organ, but knowing what each major system does — and how a few of them cooperate — covers most of the body-system questions on the test.
This lesson gives you a quick, practical map of the major systems and how they connect.
The Major Systems and Their Jobs
Here is the short version of what each major system does. The circulatory system (heart and blood vessels) moves blood, carrying oxygen and nutrients to cells and waste away. The respiratory system (lungs) brings in oxygen and removes carbon dioxide. The digestive system breaks food down into nutrients the body can absorb. The nervous system (brain, spinal cord, nerves) senses and controls the body. The skeletal system supports and protects, while the muscular system moves the body. The excretory system removes waste, and the immune system defends against disease.
Systems Work Together
The most important idea is that these systems cooperate. Breathing in oxygen (respiratory system) does no good unless the blood (circulatory system) carries that oxygen to your cells. When you exercise, the nervous system signals muscles to move, the muscles demand more oxygen, the respiratory system breathes faster, and the circulatory system pumps harder to deliver it. Test questions often describe this kind of teamwork and ask which systems are involved.
A Couple of Pairings Worth Knowing
Two partnerships come up again and again. First, the respiratory and circulatory systems work as a team: lungs load oxygen into the blood, and the heart pumps it everywhere. Second, the digestive and circulatory systems team up: the digestive system releases nutrients, and the blood carries them to cells. When a question asks how the body delivers oxygen or nutrients, the circulatory system is almost always part of the answer, because it is the body’s transport network.
Watch: A Short Video Lesson
Amoeba Sisters walks through this skill clearly in a few minutes. It is a helpful companion to the reading above:
A Routine for Body-System Questions
- Match each system to its one-line job (transport, breathing, digestion, control, support, movement, waste, defense).
- When a question involves delivery of oxygen or nutrients, think circulatory system.
- Look for teamwork: many questions involve two systems cooperating.
Practice
- Which system carries oxygen and nutrients to cells?
- Which system brings in oxygen and removes carbon dioxide?
- Which system breaks down food into nutrients?
- Which two systems work together to deliver oxygen to the body?
- Which system senses and controls the body?
- Which system defends the body against disease?
Answers
- The circulatory system.
- The respiratory system.
- The digestive system.
- The respiratory and circulatory systems.
- The nervous system.
- The immune system.
Where This Fits in Your Science Prep
The body systems build on levels of organization and connect to how the body stays balanced in homeostasis, enzymes, and nutrition. See all topics on the Science Topics Hub.
Recommended Prep Books
These study guides and practice books help you keep building momentum as you prepare:
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