The Human Body: Anatomy, facts & functions

An illustration of the human body
(Image credit: Getty Images)

The human body is everything that makes up, well, you.  What decides and regulates the physical form and function of the human body is our genetic information, however, external environments and behaviours can alter the way our body’s look and how well they function, according to Human Growth and Developments.  

The human body is made up of all the living and nonliving components that create the entire structure of the human organism, including every living cell, tissue and organ. 

On the outside human anatomy consists of the five basic parts, the head, neck, torso, arms and legs. However, beneath the skin there are countless biological and chemical interactions that keep the human body machine ticking over. 

Related: Body changes during pregnancy

What are the different systems in the human body?

Our bodies consist of a number of biological systems that carry out specific functions necessary for everyday living.

The job of the circulatory system is to move blood, nutrients, oxygen, carbon dioxide, and hormones, around the body. It consists of the heart, blood, blood vessels, arteries and veins. According to the Cleveland Clinic, the human body's network of blood vessels, veins and capillaries is over 60,00 miles (around 96,560 kilometres) long. 

The digestive system consists of a series of connected organs that together, allow the body to break down and absorb food, and remove waste. It includes the mouth, esophagus, stomach, small intestine, large intestine, rectum, and anus. The liver and pancreas also play a role in the digestive system because they produce digestive juices filled with enzymes to break down the components of your food, such as carbohydrates, fats and proteins, according to the National Institute of Diabetes and Digestive and Kidney Diseases.

The endocrine system consists of a network of glands that secrete hormones into the blood. These hormones, in turn, travel to different tissues and regulate various bodily functions, such as metabolism, growth and sexual function, according to Johns Hopkins Medicine. For example, one of the glands in the pancreas - called the endocrine gland - releases hormones called insulin and glucagon to regulate blood sugar

The immune system is the body's defense against bacteria, viruses and other pathogens that may be harmful. The immune system is activated when antigens (proteins on the surface of bacteria, fungi and viruses) bind with receptors on immune cells, alerting the body to their presence and kicking the immune system into gear, according to Institute for Quality and Efficiency in Health Care (IQWiG). The system is made up of I lymph nodes, the spleen, bone marrow, lymphocytes (including B-cells and T-cells), the thymus and leukocytes, which are white blood cells.

(Image credit: Getty Images)

The lymphatic system includes lymph nodes, lymph ducts and lymph vessels, and also plays a role in the body's defenses. Its main job is to make and move lymph, a clear fluid that contains white blood cells, which help the body fight infection, according to the Cleveland Clinic. The lymphatic system also removes excess lymph fluid from bodily tissues, and returns it to the blood.

The nervous system controls both voluntary action (like conscious movement) and involuntary actions (like breathing), and sends signals to different parts of the body. The central nervous system includes the brain and spinal cord. The peripheral nervous system consists of nerves that connect every other part of the body to the central nervous system, according to the National Institute of Child Health and Human Development.

Related: Fight or flight: The sympathetic nervous system

The body's muscular system consists of about 650 muscles that aid in movement, blood flow and other bodily functions, according to the Library of Congress. There are three types of muscle: skeletal muscle which is connected to bone and helps with voluntary movement, smooth muscle which is found inside organs and helps to move substances through organs, and cardiac muscle which is found in the heart and helps pump blood.

The reproductive system allows humans to reproduce. The male reproductive system includes the penis and the testes, which produce sperm. The female reproductive system consists of the vagina, the uterus and the ovaries, which produce eggs. During conception, a sperm cell fuses with an egg cell, which creates a fertilized egg that implants and grows in the uterus. 

Our bodies are supported by the skeletal system, which consists of between 206 and 213 bones in an adult human body, which are all connected by tendons, ligaments and cartilage, according to the journal StatPearls. As infants, humans have 270 bones, before some fuse together during growth. The skeleton not only helps us move, but it's also involved in the production of blood cells and the storage of calcium. The teeth are also part of the skeletal system, but they aren't considered bones.

The respiratory system allows us to take in vital oxygen and expel carbon dioxide in a process we call breathing. It consists mainly of the trachea, the diaphragm and the lungs.

Illustration of lungs

An illustration of the lungs and the small air sacs inside them called alveoli. This is where the exchange of oxygen and carbon dioxide occurs.   (Image credit: Getty Images)

Related: How to increase lung capacity

The urinary system helps eliminate a waste product called urea from the body, which is produced when certain foods are broken down. The whole system includes two kidneys, two ureters, the bladder, two sphincter muscles and the urethra. Urine produced by the kidneys travels down the ureters to the bladder, and exits the body through the urethra.

The skin, or integumentary system, is the body's largest organ. It protects us from the outside world, and is our first defense against bacteria, viruses and other pathogens. Our skin also helps regulate body temperature and eliminate waste through perspiration. In addition to skin, the integumentary system includes hair and nails.

What are the body's vital organs?

Fast facts

  • The human body contains nearly 37.2 trillion cells.
  • It’s estimated that the microbial biome of our bodies, including bacteria and fungi is around 39 trillion cells.
  • The average adult takes around 22,000 breaths a day.
  • Each day, the kidneys process about 200 quarts (50 gallons) of blood to filter out about 2 quarts of waste and water.
  • Adults excrete about a quarter and a half (1.42 liters) of urine each day.
  • The human brain contains about 100 billion nerve cells.
  • Water makes up more than 50 percent of the average adult's body weight.

What are vestigial organs?

Unlike the vital organs, there are some other organs and structures within the human body that no longer serve a purpose - known as vestigial organs. That's not to say that they didn’t once play a role in human survival, according to the Department of Anatomy at Midwestern University.

Over the course of human evolution, some organs and anatomical structures have lost their function, for example, the coccyx or tailbone. It was Charles Darwin that first proposed that humans descended from primates with tails, noting the tiny set of vertebrae we all share adjacent to the pelvis - called the coccyx - is what remains of our ancestral tail, according to the New York Times.

Another well-known example of a seemingly functionless organ is the appendix. The appendix is a narrow pouch attached to the lower abdominal cavity of the human body. It’s generally considered to be redundant in the function of the human body, however, in 2017 research conducted by Midwestern University suggested that the appendix may serve as a reservoir for several types of beneficial gut bacteria.

Additional resources

Rachael Rettner

Rachael is a Live Science contributor, and was a former channel editor and senior writer for Live Science between 2010 and 2022. She has a master's degree in journalism from New York University's Science, Health and Environmental Reporting Program. She also holds a B.S. in molecular biology and an M.S. in biology from the University of California, San Diego. Her work has appeared in Scienceline, The Washington Post and Scientific American.