The Human Body

The human body is a complex collection of cells, tissues, organs and organ systems. The parts work together to keep your body running smoothly and help you function from moment to moment.

Learning a little bit about your body can help you take care of it and recognize when something seems “off.” This knowledge may even help you better describe symptoms when you see a healthcare provider.

How does the human body work?

Your body works because all its systems work together to keep everything in balance (homeostasis). For each system to do its job, it has to depend on others to do theirs.

When you’re healthy, things work smoothly. Diseases happen when one or more systems aren’t working properly. And that’s why it’s so common for people with one condition to develop others. For example, having heart disease also affects other organs that handle blood, like your kidneys. That’s why people with heart failure are more likely to have kidney disease, and vice versa.

What organ systems make up the human body?

Body organ systems are organs, tissues and other structures that work together to handle bigger jobs. Their cooperation is the secret to how they work. They help you digest food, go for a walk or just enjoy the sights and sounds around you.

Your body has many different organ systems, but the exact number varies. Some experts look at systems separately. Others group related ones together. That’s why the number of body systems varies between 10 and 12, depending on who you ask.

Without combining any organ systems, there are 12 in total. The 12 systems and what they do are:

• Cardiovascular system: This includes your heart and blood vessels, which circulate blood. Your blood transports nutrients and waste. The average adult has about 5 liters (1.3 gallons) of blood, though this varies based on body size and other factors.
• Endocrine system: This makes and regulates several types of hormones. Those are specialized signal chemicals that your body uses to communicate and run different processes.
• Exocrine system: This system makes secretions, like mucus or tears. These secretions provide moisture and lubrication to protect sensitive tissues.
• Gastrointestinal system (also known as the digestive system): Organs in this system take the foods you eat and break them down so your body can use nutrients in the food. It also gets rid of waste products, which become poop.
• Integumentary system: Tissues that act as coverings, like your hair and nails, make up this system. These tissues are protective and help regulate your body temperature.
• Lymphatic system: This system is part of your body’s immune defenses.
• Muscular system: Your body uses this system for movement and stability. Your muscles also generate heat to keep your internal temperature in the right range.
• Nervous system: This system is all about signals. Some of those signals go from your brain to your body to control movements. Others send signals to your brain, providing information about the world around you. And your brain itself uses signals for thinking, memory and more.
• Reproductive systems (female and male): These systems are all about sex, sexuality and reproduction. They also play a major role in many of the hormones your body uses.
• Respiratory system: This system exchanges different gases into and out of your blood. Oxygen goes in, and carbon dioxide comes out. This system is also part of how your body manages its internal temperature.
• Skeletal system: This system is all about structure, support and protection. Bones give your body its shape. And many bones act as a protective barrier. That keeps sensitive internal organs, like your brain, heart or lungs, safe from harm.
• Urinary system: This system helps your body manage how much fluid it holds. It also helps your body filter certain waste products out of your blood and then get them out of your body in your pee.

Some experts group systems together because of how they’re related. Examples include terms like:

• Cardiorespiratory (circulation and breathing)
• Endocrine (sometimes includes both the endocrine and exocrine systems)
• Genitourinary (reproductive and urinary)
• Musculoskeletal system (muscular and skeletal)

And there’s one system you probably know about that experts don’t count the same as the others. That’s your immune system. It’s not an organ system like the others. But all of the other body systems play a role in your immune system. So, while it’s not on the main list, it’s definitely important enough to mention.

Your body is made up of cells, tissues, organs and organ systems. Most of your body (over 90%) is made of four elements: carbon, oxygen, hydrogen and nitrogen.

Your body is organized in levels, from smallest to largest:

Each level builds on the one before it to keep your body working properly.