Human Body Systems
Grade 6 · Science · Study Guide
Explore the major organ systems that keep your body running every second of every day.
Your body is one of the most complex machines on Earth, and it runs 24/7 without you having to think about it. It is organized into systems — groups of organs that work together to do a specific job. There are 11 major organ systems in the human body, and they all depend on each other.
The digestive system breaks food into nutrients your cells can use. It starts in your mouth (where saliva begins breaking down carbohydrates) and ends with waste being expelled from the large intestine. The stomach uses acid and enzymes to chemically digest food, while the small intestine absorbs nutrients into the bloodstream.
The circulatory system is your body's delivery network. The heart pumps blood through a vast network of arteries, veins, and capillaries — if you laid out all the blood vessels in one adult human, they would stretch about 100,000 km, enough to circle Earth 2.5 times. Blood delivers oxygen and nutrients to cells and carries away carbon dioxide and waste.
The respiratory system brings in oxygen and removes carbon dioxide. When you inhale, air travels down the trachea into the lungs, where tiny air sacs called alveoli allow oxygen to enter the blood and CO2 to exit. The skeletal system provides structure and protection, while the muscular system works with it to produce movement. The nervous system, led by the brain, coordinates everything.
| System | Main Organs | Key Function |
|---|---|---|
| Digestive | Stomach, intestines, liver | Breaks down food into nutrients |
| Circulatory | Heart, blood vessels | Delivers oxygen and nutrients; removes waste |
| Respiratory | Lungs, trachea, diaphragm | Brings in oxygen; removes carbon dioxide |
| Skeletal | Bones, cartilage, joints | Support, protection, movement |
| Nervous | Brain, spinal cord, nerves | Controls and coordinates body functions |
Key terms
- Organ system
- A group of organs that work together to perform a specific function in the body.
- Digestive system
- Breaks down food into nutrients and absorbs them into the bloodstream.
- Circulatory system
- Pumps blood throughout the body to deliver oxygen and nutrients and remove waste.
- Respiratory system
- Brings oxygen into the body and removes carbon dioxide through breathing.
- Skeletal system
- The framework of bones that supports the body and protects organs.
- Muscular system
- Muscles attached to bones that produce movement when they contract.
- Nervous system
- The brain, spinal cord, and nerves that control and coordinate body functions.
- Alveoli
- Tiny air sacs in the lungs where gas exchange (oxygen in, CO2 out) occurs.
Flashcards
- Digestive system
- Breaks down food and absorbs nutrients; starts in the mouth, ends at the large intestine.
- Circulatory system
- Heart + blood vessels; delivers oxygen/nutrients and removes waste products.
- Respiratory system
- Lungs + trachea; brings in O2, expels CO2 through breathing.
- Alveoli
- Tiny air sacs in the lungs where gas exchange between air and blood occurs.
- Skeletal system
- 206 bones that provide structure, protect organs, and support movement.
- Nervous system
- Brain, spinal cord, and nerves; controls and coordinates all body functions.
Practice questions
Which system delivers oxygen and nutrients to body cells?
- Digestive system
- Respiratory system
- Circulatory system
- Nervous system
Show answer
Answer: Circulatory system
The heart pumps blood through arteries and veins, delivering oxygen and nutrients to every cell and carrying away carbon dioxide and waste.
Gas exchange in the lungs takes place in tiny sacs called alveoli.
Show answer
Answer: True
Alveoli are microscopic air sacs surrounded by capillaries. Oxygen diffuses into the blood and CO2 diffuses out across the thin alveoli walls.
The organ that pumps blood through the body is the ___.
Show answer
Answer: heart
The heart is the muscular pump of the circulatory system, beating about 100,000 times a day to push blood throughout the body.
Which system controls and coordinates body functions using the brain and nerves?
- Skeletal system
- Nervous system
- Endocrine system
- Muscular system
Show answer
Answer: Nervous system
The brain processes information and sends signals through the spinal cord and nerves to coordinate all body systems and responses.
The digestive system starts its work in the stomach.
Show answer
Answer: False
Digestion begins in the mouth, where teeth physically break down food and saliva begins chemically digesting carbohydrates with an enzyme called amylase.
Fun facts
- Your small intestine is about 6 meters long — coiled up inside your abdomen. That's longer than a full-size SUV.
- The human heart beats about 100,000 times per day, pumping around 7,570 liters of blood daily.
- You have about 206 bones as an adult, but babies are born with around 270 — many fuse together as you grow.
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