A human brain weighs about 3 pounds. The cortex of the brain is called gray matter. The cerebellum helps coordinate your movement.
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; the liquid part of your blood is called plasma.
Your heart is an organ that pumps blood all around your body. The heart has four chambers: two upper and two lower. A small patch of tissue in the heart called the sinoatrial (SA) node sends the signal that starts each heartbeat. Heart valves work like one-way doors that keep blood moving in the right direction.
When you breathe, your body takes in oxygen and sends out carbon dioxide; 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.
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.
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. 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.
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.
An adult skeleton has 206 bones. Bone is made of a framework of a protein called collagen that is hardened by minerals; tendons connect muscles to bones.
In 1543 Andreas Vesalius published a famous book on the structure of the human body. 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. Roentgen's X-ray images of hands showed the bones inside.
Words to know
- hemoglobin: oxygen-carrying protein
- nephrons: tiny kidney filters
