Human Body Systems
Grade 7 · Science · Study Guide
Explore the major organ systems of the human body and how they work together to keep you alive.
Your body is not just a collection of individual parts — it is a collection of systems, each made of organs and tissues that work together for a shared purpose. There are 11 major organ systems, and they are all interconnected. Understanding each one and how they communicate is key to understanding life itself.
The digestive system breaks food down into nutrients small enough to enter the bloodstream. Starting in the mouth, food travels through the esophagus to the stomach (where acids and enzymes break it down further), then through the small intestine (where most nutrients are absorbed into the blood), and finally through the large intestine (where water is reabsorbed and waste is compacted). The circulatory system then carries those nutrients — along with oxygen — to every cell in the body. The heart pumps blood through a network of arteries (carrying oxygenated blood away from the heart), capillaries (tiny vessels where exchange with cells occurs), and veins (carrying deoxygenated blood back to the heart).
The respiratory system brings in oxygen and removes carbon dioxide. Air enters through the nose/mouth, travels down the trachea, and reaches the alveoli in the lungs — tiny air sacs where gas exchange occurs between air and blood. The skeletal system provides the structural framework (206 bones in adults) and protects organs, while the muscular system uses over 600 muscles to create movement. Together they make up the musculoskeletal system.
The nervous system is the body's electrical communication network, with the brain and spinal cord as the central nervous system (CNS) and nerves branching everywhere else as the peripheral nervous system. It controls everything from conscious movement to your heart rate. The endocrine system uses hormones as chemical messengers to regulate growth, metabolism, and reproduction. These systems do not work in isolation — when you exercise, the muscular, skeletal, circulatory, respiratory, and nervous systems all coordinate simultaneously.
| System | Main Organs | Primary Function |
|---|---|---|
| Digestive | Stomach, small intestine, large intestine | Break down food; absorb nutrients |
| Circulatory | Heart, arteries, veins, capillaries | Transport blood, oxygen, nutrients |
| Respiratory | Lungs, trachea, diaphragm | Gas exchange (O₂ in, CO₂ out) |
| Skeletal | Bones, cartilage | Support, protection, movement |
| Muscular | Skeletal muscles, cardiac muscle | Movement and posture |
| Nervous | Brain, spinal cord, nerves | Coordination and communication |
Key terms
- Organ system
- A group of organs that work together to perform a major function in the body.
- Digestive system
- System that breaks down food into nutrients the body can absorb and use.
- Circulatory system
- System that pumps blood to deliver oxygen and nutrients to cells and remove waste products.
- Respiratory system
- System that exchanges gases — bringing in oxygen and removing carbon dioxide through the lungs.
- Nervous system
- System of the brain, spinal cord, and nerves that controls and coordinates all body activities.
- Alveoli
- Tiny air sacs in the lungs where gas exchange between air and blood takes place.
Flashcards
- Organ system
- A group of organs that cooperate to perform a major body function.
- Circulatory system
- The heart, blood vessels, and blood — delivers oxygen and nutrients throughout the body.
- Artery
- Blood vessel that carries oxygenated blood away from the heart to the body.
- Vein
- Blood vessel that carries deoxygenated blood back toward the heart.
- Alveoli
- Microscopic air sacs in the lungs where O₂ and CO₂ are exchanged with the bloodstream.
- Nervous system
- The brain, spinal cord, and nerves that sense the environment and coordinate body responses.
Practice questions
Where does most nutrient absorption from food occur?
- Stomach
- Large intestine
- Small intestine
- Esophagus
Show answer
Answer: Small intestine
The small intestine, lined with finger-like projections called villi, has an enormous surface area designed specifically to absorb digested nutrients into the bloodstream.
Arteries carry deoxygenated blood back to the heart.
Show answer
Answer: False
Arteries carry OXYGENATED blood AWAY from the heart. Veins carry deoxygenated blood back TO the heart. (The pulmonary artery is the exception — it carries deoxygenated blood to the lungs.)
The tiny air sacs in the lungs where gas exchange takes place are called ___.
Show answer
Answer: alveoli
The alveoli are surrounded by a dense network of capillaries. Oxygen diffuses from air into the blood, and carbon dioxide diffuses from blood into the air.
Which system controls and coordinates all body activities through electrical signals?
- Endocrine system
- Circulatory system
- Nervous system
- Digestive system
Show answer
Answer: Nervous system
The nervous system uses electrical impulses transmitted through neurons to rapidly communicate and coordinate all body functions, from reflex actions to conscious thought.
The human skeleton is made up of about 206 bones in an adult.
Show answer
Answer: True
Adults have 206 bones, but babies are born with around 270 — many fuse together during childhood and adolescence.
Fun facts
- Your small intestine is about 20 feet long — coiled up inside your abdomen to fit.
- The human heart beats approximately 100,000 times every single day.
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