The ancient physician Galen studied animals such as monkeys to learn about the body. In 1543 Andreas Vesalius published a famous book on the structure of the human body. Vesalius did his own careful dissections of human bodies and wrote about what he saw.
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. William Harvey showed that blood moves around the body in a circle.
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. At a synapse, chemical messengers called neurotransmitters cross a tiny gap between nerve cells; nerves carry electrical signals that tell your muscles to contract.
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. Glial cells are support cells that help maintain the nervous system and its nerve cells; a human brain weighs about 3 pounds.
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; carbon dioxide in your blood is a main signal that makes you breathe, and when it builds up you breathe faster. 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 blood carries oxygen attached to a protein called hemoglobin. Red blood cells live about 120 days; white blood cells divide the work of defense: phagocytes take in and destroy invading germs, while lymphocytes do a more specific job, helping the body fight particular invaders.
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; heart valves work like one-way doors that keep blood moving in the right direction.
Your bones are always being rebuilt: special cells replace old bone with new bone. Bone is made of a framework of a protein called collagen that is hardened by minerals; compact bone is a hard, dense kind of bone.
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. A CT scanner rotates an X-ray beam around the body and takes pictures from many angles; a computer turns them into cross-sectional slices that, taken together, make a 3D image.
Trillions of tiny living things, including bacteria, viruses and fungi, make up the body's microbiome, and many of them live in the intestines; your large intestine absorbs water, and what is left of your food becomes solid waste.
Your kidneys filter about 120 to 150 quarts of blood every day, and only about 1 to 2 quarts of that leaves your body as urine. Each kidney contains about a million tiny filters called nephrons.
During sleep the body cycles between non-REM and REM sleep over and over. One cycle lasts about 80 to 100 minutes by NHLBI's count and 90 to 120 by Cleveland Clinic's, so a night of sleep holds several cycles, usually four to six.
As of November 2026, scientists who count the cells in a human body get totals in the tens of trillions of cells.
Words to know
- synapse: gap between neurons
- neurotransmitters: chemical messengers
- phagocytes: germ-eating cells
- lymphocytes: targeted defender cells
- microbiome: body's tiny life
