Human Body Systems
Grade 3 · Science · Study Guide
Explore the major systems that keep your body working every second of the day.
Your body is like an incredibly complex machine made up of billions of cells organized into tissues, organs, and organ systems. Each system has a specific job, and they all work together to keep you alive and healthy.
The skeletal system is your body's framework — 206 bones give your body shape, protect organs, and work with muscles to allow movement. The muscular system works with bones at joints to create movement. Some muscles, like your heart, work automatically without you thinking about it.
The digestive system breaks food down into nutrients your body can use. Food travels through the mouth, esophagus, stomach, and small intestine, where nutrients are absorbed into the blood. The circulatory system — your heart, blood vessels, and blood — delivers those nutrients and oxygen to every cell in your body. The heart pumps about 100,000 times every day!
The respiratory system brings oxygen into the body through the lungs and removes carbon dioxide. Every breath in fills your lungs with fresh air; every breath out removes waste gases. The nervous system — your brain, spinal cord, and nerves — controls everything else, processing information and sending signals throughout the body at incredible speed.
| System | Main Organs | Job |
|---|---|---|
| Skeletal | Bones, cartilage | Support, protection, movement |
| Muscular | Muscles, tendons | Movement and posture |
| Digestive | Stomach, intestines | Break down food into nutrients |
| Circulatory | Heart, blood vessels | Transport blood, nutrients, oxygen |
| Respiratory | Lungs, trachea | Exchange oxygen and carbon dioxide |
| Nervous | Brain, spinal cord, nerves | Control and communication |
Key terms
- organ
- A body structure made of tissues that performs a specific function
- organ system
- A group of organs that work together to perform a major body function
- skeletal system
- The system of bones that supports the body and protects organs
- circulatory system
- The system of heart, blood, and vessels that transports materials through the body
- digestive system
- The system that breaks down food into nutrients the body can absorb
Flashcards
- organ system
- A group of organs working together to perform a major function
- skeletal system
- The 206 bones and cartilage that support and protect the body
- muscular system
- Muscles that move the body and maintain posture
- digestive system
- Organs that break down food into nutrients for the body
- circulatory system
- Heart, blood vessels, and blood that transport materials through the body
- respiratory system
- Lungs and airways that exchange oxygen and carbon dioxide
Practice questions
What is the main job of the circulatory system?
- Breaking down food
- Carrying oxygen and nutrients through the body
- Controlling movement
- Exchanging gases in the lungs
Show answer
Answer: Carrying oxygen and nutrients through the body
The circulatory system — heart, blood, arteries, and veins — transports oxygen from the lungs and nutrients from digestion to every cell in the body.
The human body has exactly 100 bones.
Show answer
Answer: False
Adult humans have 206 bones. Babies are born with about 270 bones, but many fuse together as they grow.
The organ that pumps blood through your entire body is the ___.
Show answer
Answer: heart
The heart is a muscular pump about the size of your fist. It contracts and relaxes about 70 times per minute to push blood through the body's arteries and veins.
Which system controls all other body systems?
- Digestive
- Skeletal
- Nervous
- Muscular
Show answer
Answer: Nervous
The nervous system — brain, spinal cord, and nerves — receives information from the senses and sends signals to coordinate all other body systems.
The lungs are part of the respiratory system.
Show answer
Answer: True
The lungs are the primary organs of the respiratory system. They have tiny air sacs called alveoli where oxygen moves into the blood and carbon dioxide moves out.
Fun facts
- Your heart beats about 100,000 times a day and pumps about 2,000 gallons of blood through your body!
- The small intestine is about 20 feet long — much longer than the large intestine, despite the name!
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