The Human Body
The human body has 11 major organ systems that work together to keep you alive and functioning. Each system has specialized organs with distinct roles.
Circulatory System
Heart, blood, and blood vessels (arteries, veins, capillaries). Pumps oxygen and nutrients to cells, carries waste away. Heart beats ~100,000 times/day.
Respiratory System
Lungs, trachea, bronchi, diaphragm. Takes in oxygen (O₂), expels carbon dioxide (CO₂). Breathing rate: ~15 breaths/minute at rest.
Digestive System
Mouth → esophagus → stomach → small intestine → large intestine → rectum. Breaks food into nutrients absorbed into the bloodstream.
Nervous System
Brain, spinal cord, nerves. Controls voluntary actions (movement) and involuntary ones (heartbeat). Neurons transmit signals electrically and chemically.
Other Key Systems
- Skeletal: 206 bones — support, protection, movement
- Muscular: voluntary (skeletal) and involuntary (smooth, cardiac) muscles
- Endocrine: glands releasing hormones (thyroid, pancreas, adrenal)
- Immune: white blood cells fight pathogens
FAQ
What is the largest organ? Skin — it is the body's first defense barrier.
How many bones does an adult have? 206 (babies have ~270; some fuse over time).
Quick Quiz
Test what you just learned. Choose the best answer for each question.