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 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.

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. Bone marrow, found inside some of your bones, makes blood cells.

Oxygen and carbon dioxide move between the air in your lungs and your blood by simple diffusion; 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.

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. An adult skeleton has 206 bones.

Thin skin has four layers in its epidermis, and thick skin on the palms and soles has a fifth; the dermis lies beneath the epidermis and holds blood vessels, hair follicles and sweat glands. The outer layer of skin constantly flakes off and is replaced by new cells from below.

Your kidneys filter your blood to take out waste and make urine; each kidney contains about a million tiny filters called nephrons. 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.

After you swallow, waves of muscle squeezing, called peristalsis, push food down to your stomach; 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; 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.

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.

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. The cerebellum helps coordinate your movement; a human brain weighs about 3 pounds.

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.

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

• dissections: careful body study
• epidermis: outer skin layer
• marrow: soft bone center
• scanner: imaging machine
• diffusion: spreading out