Human Body Systems
Grade 4 · Science · Study Guide
Explore the major systems that keep your body running like a well-oiled machine.
Your body is made up of trillions of cells organized into tissues, organs, and organ systems. Each system has a specific job, but all the systems work together to keep you alive and healthy. Understanding these systems helps you take better care of your body!
The skeletal system is your body's framework — 206 bones support your body, protect organs, and allow movement. Bones are living tissue that grow and repair themselves. The muscular system works with the skeleton to create movement. There are three types of muscle: skeletal (voluntary, moves your arms and legs), smooth (involuntary, in your organs), and cardiac (your heart muscle).
The digestive system breaks down food into nutrients your body can use. Food travels from your mouth through the esophagus to the stomach, then through the small intestine (where nutrients are absorbed) and large intestine. The circulatory system delivers nutrients and oxygen throughout your body. The heart pumps blood through arteries to body tissues and receives it back through veins. The respiratory system brings oxygen in and removes carbon dioxide through breathing.
| System | Main Organs | Job |
|---|---|---|
| Skeletal | Bones, joints | Support and protect body |
| Muscular | Muscles, tendons | Move the body |
| Digestive | Stomach, intestines | Break down food into nutrients |
| Circulatory | Heart, blood vessels | Move blood through body |
| Respiratory | Lungs, trachea | Exchange oxygen and carbon dioxide |
Key terms
- Skeletal system
- The framework of 206 bones that supports and protects the body
- Muscular system
- The system of muscles that move the body and its organs
- Digestive system
- The system that breaks down food into nutrients the body can use
- Circulatory system
- The heart and blood vessels that move blood throughout the body
- Respiratory system
- The lungs and airways that bring oxygen into the body and remove carbon dioxide
- Organ
- A group of tissues that work together to perform a specific function
Flashcards
- Skeletal system
- 206 bones that support, protect, and allow movement
- Muscular system
- Muscles that move bones and power body functions
- Digestive system
- Breaks food into nutrients — mouth to stomach to intestines
- Circulatory system
- Heart and blood vessels that deliver oxygen and nutrients throughout the body
- Respiratory system
- Lungs and airways that bring in oxygen and remove carbon dioxide
- Small intestine
- Where most nutrients from digested food are absorbed into the blood
Practice questions
How many bones does the adult human body have?
- 106
- 206
- 306
- 506
Show answer
Answer: 206
Adults have 206 bones. Babies are born with about 270-300 bones, but many fuse together as they grow.
Which system is responsible for bringing oxygen into the body?
- Digestive system
- Muscular system
- Circulatory system
- Respiratory system
Show answer
Answer: Respiratory system
The respiratory system includes the lungs and airways. When you breathe in, oxygen enters your blood; when you breathe out, carbon dioxide is removed.
The heart is part of the circulatory system.
Show answer
Answer: True
The heart is the central organ of the circulatory system. It pumps blood through arteries and receives it back through veins.
Nutrients from food are mostly absorbed in the stomach.
Show answer
Answer: False
Most nutrient absorption happens in the small intestine, not the stomach. The stomach mainly breaks food down into a liquid.
The system that breaks down food into nutrients your body can use is called the ___ system.
Show answer
Answer: digestive
The digestive system breaks food into nutrients through mechanical and chemical processes from the mouth all the way through the intestines.
Fun facts
- Your small intestine is about 20 feet long — even though it's folded up to fit inside you!
- Your heart beats about 100,000 times every single day.
- Bones are so strong that pound for pound, they are stronger than steel!
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