Human Body Systems
Grade 5 · Science · Study Guide
Explore the major organ systems of the human body and how they work together to keep you alive.
Your body is made up of about 37 trillion cells, and those cells are organized into tissues, tissues into organs, and organs into systems. Each body system has a specific job, but no system works alone — they are all interconnected and dependent on each other. Understanding these systems helps us understand health, disease, and what keeps us alive every second of every day.
The digestive system breaks food into nutrients small enough to enter the bloodstream. It starts in the mouth and continues through the esophagus, stomach, small intestine, and large intestine. The small intestine is where most nutrient absorption happens — it is about 20 feet long and lined with tiny finger-like projections called villi that dramatically increase surface area for absorption.
The circulatory system is a delivery network. Your heart pumps blood through about 60,000 miles of blood vessels. Arteries carry oxygen-rich blood away from the heart; veins return oxygen-poor blood to the heart. Red blood cells carry oxygen using a protein called hemoglobin. The average adult heart beats about 100,000 times per day without ever taking a break.
The respiratory system gets oxygen into the blood and removes carbon dioxide. When you inhale, air travels through the trachea and into the lungs, where tiny air sacs called alveoli transfer oxygen into the bloodstream. The nervous system controls and coordinates all other systems, sending electrical signals at speeds up to 268 miles per hour. The musculoskeletal system provides support and movement.
| System | Main Organs | Primary Function |
|---|---|---|
| Circulatory | Heart, blood vessels, blood | Transports oxygen and nutrients |
| Digestive | Stomach, intestines, liver | Breaks down food into nutrients |
| Respiratory | Lungs, trachea, diaphragm | Exchanges oxygen and CO2 |
| Nervous | Brain, spinal cord, nerves | Controls and coordinates body |
| Skeletal | Bones, cartilage, joints | Support, protection, movement |
| Muscular | Muscles, tendons | Movement and posture |
Key terms
- Organ system
- A group of organs working together to perform a major body function.
- Circulatory system
- The system of heart and blood vessels that transports blood, oxygen, and nutrients throughout the body.
- Digestive system
- The system that breaks down food into nutrients that can be absorbed into the bloodstream.
- Respiratory system
- The system that brings oxygen into the body and removes carbon dioxide.
- Nervous system
- The system of brain, spinal cord, and nerves that controls and coordinates all body functions.
- Alveoli
- Tiny air sacs in the lungs where gas exchange (oxygen in, carbon dioxide out) occurs.
Flashcards
- Circulatory system
- Heart and blood vessels that transport blood, oxygen, and nutrients throughout the body.
- Artery
- Blood vessel that carries oxygen-rich blood away from the heart.
- Vein
- Blood vessel that returns oxygen-poor blood to the heart.
- Alveoli
- Tiny air sacs in the lungs where gas exchange occurs.
- Small intestine
- 20-foot-long organ where most nutrient absorption from digested food occurs.
- Nervous system
- Brain, spinal cord, and nerves that control and coordinate all body functions.
Practice questions
What is the main job of the circulatory system?
- Break down food
- Transport oxygen and nutrients through the body
- Filter waste from blood
- Control movement
Show answer
Answer: Transport oxygen and nutrients through the body
The circulatory system's main job is transportation — moving oxygen, nutrients, hormones, and waste products via blood throughout the body.
Arteries carry oxygen-poor blood back to the heart.
Show answer
Answer: False
Arteries carry oxygen-RICH blood away from the heart. Veins carry oxygen-poor blood back to the heart. The one exception is the pulmonary artery, which carries oxygen-poor blood to the lungs.
Tiny air sacs in the lungs where oxygen enters the blood are called ___.
Show answer
Answer: alveoli
The alveoli are surrounded by tiny capillaries; oxygen diffuses across their thin walls into the blood while CO2 moves in the opposite direction.
Where does most nutrient absorption take place during digestion?
- Stomach
- Large intestine
- Mouth
- Small intestine
Show answer
Answer: Small intestine
The small intestine's enormous surface area (from its length and villi) makes it the primary site of nutrient absorption into the bloodstream.
Body systems work independently and do not affect each other.
Show answer
Answer: False
All body systems are interdependent. For example, the circulatory system depends on the respiratory system for oxygen, and both depend on the muscular system to keep the heart beating and lungs breathing.
Fun facts
- Your small intestine is about 20 feet long — yet it fits in your abdomen because it is coiled up tightly.
- The human heart beats about 100,000 times per day and over 2.5 billion times in an average lifetime without ever stopping to rest.
Get a personalized course on Human Body Systems built around your child.