Human Body Systems

Levels of organization

Your body is built in levels, and each level is made of the one below it.

• A cell is the smallest unit of life. Your body has many kinds, such as muscle cells, nerve cells, and red blood cells.
• A tissue is a group of similar cells that work together to do one job. Heart muscle is a tissue.
• An organ is a structure made of two or more kinds of tissue. The heart is an organ. It has muscle tissue, but also nerve tissue, connective tissue, and a lining.
• An organ system is a group of organs that work together. The heart, blood vessels, and blood make up the circulatory system.
• An organism is a whole living thing. All your organ systems together make one organism: you.

The organ systems

The OpenStax Anatomy and Physiology textbook sorts the human body into eleven organ systems. Other books may combine or split some of them, so you may see a slightly different count. Every system has its own main jobs, but none of them works alone.

Books often give exact counts for body parts, but real bodies vary. Most adults have 206 bones, but some people have a few extra small ones. Babies start out with more bone pieces than adults, because some of them fuse together during childhood and the teen years. You also have more than 600 muscles. They come in three types: skeletal muscle, which moves your bones; smooth muscle, in the walls of organs like the stomach; and cardiac muscle, found only in the heart.

The nervous and endocrine systems are both control systems, but they work differently. Nerves send electrical signals that act within seconds, in one place. Hormones travel in the blood and act more slowly, over minutes, hours, or even weeks, and they can affect many parts of the body at once.

The word "excretory" means getting rid of wastes. The kidneys do much of this work, but they aren't the only organs that do. Your lungs breathe out carbon dioxide, and your skin gives off water and salt in sweat.

Heart, lungs, and blood

Every cell in your body needs oxygen to release energy from food, and every cell makes a waste gas, carbon dioxide. The respiratory and circulatory systems work as a team to bring oxygen in and carry carbon dioxide out.

Your heart is a muscle about the size of your fist with four chambers. It is really two pumps side by side. The right side pumps blood to the lungs. There, in many tiny air sacs called alveoli, oxygen moves into the blood and carbon dioxide moves out to be breathed away. This is called gas exchange. The oxygen-rich blood flows back to the left side of the heart, which pumps it out to the rest of the body. After the body's cells take the oxygen, the blood returns to the right side, and the trip starts again.

Blood leaves the heart in arteries and comes back in veins. In between are the tiniest vessels, capillaries. In the capillaries of the body, oxygen and nutrients pass out of the blood to the cells, and carbon dioxide and other wastes pass in.

The path of food

The digestive tract is one long, twisting tube from your mouth to your anus. Food is broken down by two kinds of action: mechanical digestion, such as chewing and churning, and chemical digestion, in which juices and enzymes break food into molecules small enough to pass into the blood.

Three organs help without food ever passing through them. The liver makes a juice called bile that helps digest fats. The gallbladder stores bile between meals. The pancreas makes a digestive juice that breaks down carbohydrates, fats, and proteins. All three send their juices into the small intestine.

The small intestine is where almost all nutrients are absorbed. In a living person it is roughly 3 meters (about 10 feet) long. You may see larger numbers, such as 6 meters (20 feet), because the intestine relaxes and gets longer after death, when it is usually measured.

Systems working together

Think about what happens when you start running. At rest, an adult's heart usually beats about 60 to 100 times a minute. Once you get going, almost every system joins in.

When you stop, your breathing and heart rate slowly drop back down. All six of these changes happened at the same time, and each one relied on the others.

Homeostasis: keeping things steady

Outside you, the weather swings from hot to cold. Inside you, conditions stay in a much narrower range. Keeping the body's internal conditions fairly steady is called homeostasis. The word comes from Greek words meaning "staying the same." The American physiologist Walter B. Cannon (1871–1945) gave it that name, building on the idea of the French physiologist Claude Bernard (1813–1878) that cells stay healthy only when their surroundings stay fairly constant.

"Steady" doesn't mean frozen. Body temperature, blood sugar, and blood pressure all go up and down a little, around a target called the set point. When a value moves too far away, the body reacts to push it back. This is called negative feedback, because the response reverses the change. A feedback loop has a sensor that notices the change, a control center that compares it with the set point, and an effector that responds.

Example 1: Body temperature

The usual figure for normal body temperature is about 37°C (98.6°F). That number was made famous by the German doctor Carl Wunderlich in 1868. Normal really varies from person to person and over the day, and some recent studies suggest that many people today run a little cooler than 98.6°F.

When you get too warm, blood vessels in your skin widen to let more heat escape, and you sweat. As sweat dries, it carries heat away. When you get too cold, the reverse happens: blood vessels in your skin narrow to hold heat in, and if you keep getting colder, your muscles start to shiver, which makes heat.

Example 2: Blood sugar

Your cells use a sugar called glucose as fuel. The pancreas keeps the amount in your blood in a healthy range with two hormones that work in opposite directions.

In type 1 diabetes, the immune system destroys the insulin-making beta cells in the pancreas, so the pancreas makes little or no insulin. People with it take insulin as medicine to do the job.

A few body processes use positive feedback instead, where the response makes the change bigger until a job is done. Blood clotting after a cut is one example.

Vesalius and Harvey

In the 1500s, doctors in Europe still learned anatomy mostly from the books of the ancient doctor Galen. Two scientists changed that by looking carefully for themselves.

Key words

• TissueA group of similar cells that work together to do one job.
• OrganA body structure made of two or more kinds of tissue, such as the heart or the stomach.
• Organ systemA group of organs that work together, such as the digestive system.
• HomeostasisKeeping the body's internal conditions fairly steady.
• Negative feedbackA response that reverses a change and brings a value back toward its set point.
• HormoneA chemical messenger made by a gland and carried in the blood.

Each clue names a body part and a job it does. Choose the organ system it belongs to. Watch out: some organs, like the pancreas, belong to more than one system, so read what the clue says the organ is doing.

How scoring works: each right answer is worth 1 point, so a perfect round of twelve clues is 12. Your best round is saved in this browser only.

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