As of November 2026, scientists who count the cells in a human body get totals in the tens of trillions of cells. An adult skeleton has 206 bones, and the smallest bone in your body is in your ear, while tendons connect muscles to bones and ligaments connect one bone to another bone.

Humans have three different kinds of muscle.

Your skin has three layers, skin helps control your body temperature, and skin is a barrier that helps keep germs out of the body.

The liver is the largest organ inside your body, and each of your kidneys is about as big as a fist.

Bone marrow, found inside some of your bones, makes blood cells, and the liquid part of your blood is called plasma. Red blood cells live about 120 days, and platelets help your blood clot. Arteries carry blood away from the heart, and veins carry blood back to the heart. Capillaries are very tiny blood vessels that connect the smallest arteries to the smallest veins. Blood travels in two loops, the pulmonary circulation and the systemic circulation.

Your heart is an organ that pumps blood all around your body, and heart valves work like one-way doors that keep blood moving in the right direction. A small patch of tissue in the heart called the sinoatrial (SA) node sends the signal that starts each heartbeat. In the heart, the electrical signal makes the upper chambers squeeze first, then passes through the AV node, which acts as a relay station, to the lower chambers. Blood makes a trip through the heart: oxygen-poor blood enters the right atrium, goes to the right ventricle, then through the pulmonary artery to the lungs; oxygen-rich blood returns by the pulmonary veins to the left atrium, then the left ventricle. Your heart beats slower when you rest and faster when your body is working harder.

When you breathe, your body takes in oxygen and sends out carbon dioxide, and your nose warms and cleans the air you breathe. When you breathe in, your diaphragm moves down toward your belly. At the ends of the smallest airways in your lungs are tiny air sacs called alveoli. Oxygen and carbon dioxide move between the air in your lungs and your blood by simple diffusion. Red blood cells carry oxygen from your lungs to the rest of your body. Your brain controls how fast you breathe by sensing how much oxygen your body needs and how much carbon dioxide it must get rid of. Resting adults normally breathe at least 12 times each minute.

Glands in your mouth make saliva, a juice that begins to digest your food. After you swallow, waves of muscle squeezing, called peristalsis, push food down to your stomach. Your stomach muscles mix the food you swallowed with digestive juices, and the food inside your stomach is broken down into a liquid called chyme. The small intestine is where nutrients from your food pass into your blood, and your large intestine absorbs water, and what is left of your food becomes solid waste. An adult's digestive tract is a long tube at least 25 feet (about 7.6 meters) long.

Your kidneys filter your blood to take out waste and make urine, and they filter about 120 to 150 quarts of blood every day, and only about 1 to 2 quarts of that leaves your body as urine.

A neuron, or nerve cell, has three main parts: the cell body, dendrites and an axon. Signals come into a neuron through its dendrites and travel away from its cell body down the axon to another cell. Myelin is a fatty coating that wraps around axons. Glial cells are support cells that help maintain the nervous system and its nerve cells.

A human brain weighs about 3 pounds. The largest part of the brain, the cerebrum, is split into left and right halves called hemispheres, which communicate through a thick bundle of nerve fibers called the corpus callosum. The cortex of the brain is called gray matter, and the cerebellum helps coordinate your movement.

William Harvey published a book in 1628 on the motion of the heart and blood in animals. Harvey used arithmetic: he worked out how much blood the heart pumps and found it was far too much for the body to make fresh, so the body could not be using up its blood as people had thought.

German scientist Wilhelm Roentgen discovered X-rays in 1895. An X-ray picture is a shadow picture: bones absorb more X-rays than skin does, so bones leave shadows on the X-ray film while the skin shows faintly or not at all. Magnetic resonance imaging (MRI) builds detailed pictures of the inside of the body using a strong magnet, radio waves and a computer.

Words to know

• hemispheres: brain halves
• diffusion: spreading out
• arithmetic: working with numbers
• relay: passing station
• alveoli: tiny air sacs