Human Body

The human body is the physical substance of the human organism, composed of living cells and extracellular materials and organized into tissues, organs, and systems. Water is the body's principal chemical constituent, accounting for about 60 percent of body weight. The body contains some 200 different types of cells, which can be grouped into four basic classes that with their extracellular materials form the fundamental tissues.

The human body can be understood as a series of levels of increasing complexity, beginning with chemical building blocks and ending with a unique human organism. The cell is the smallest independently functioning unit of the body, and a tissue is a group of similar cells working together. An organ is an anatomically distinct structure composed of two or more tissue types, while an organ system consists of organs working together to carry out major functions. The body has eleven distinct organ systems, though assigning organs to systems can be imprecise because most organs contribute to more than one.

There are four basic classes of tissue. Epithelial tissues cover surfaces and line organs, cavities, and passageways; muscle tissues are capable of contraction; and nerve tissues conduct electrical impulses. Connective tissue is implied by extracellular materials. The heart is an example of an organ composed of all four tissue types, and it pumps blood throughout the body.

Deoxyribonucleic acid carries the body's hereditary master code, the set of instructions according to which each cell operates. James Watson and Francis Crick determined the structure of the DNA molecule in 1953 — a long double helix whose pairs of nitrogen bases allow it to function like a code. Humans have more than 20,000 genes, some of which direct how the brain grows and works.

The adult human body contains 206 bones, while a baby is born with about 300 that eventually fuse together. These are organized into the axial skeleton of 80 bones in the head, neck, back, chest, and pelvis, and the appendicular skeleton of the shoulders, arms, and legs. The spine consists of 33 ring-shaped vertebrae, and the rib cage of 12 pairs protects the heart, lungs, and liver. The wrists, hands, and fingers together contain 54 bones.

The femur, or thigh bone, is the longest and strongest bone, around 18 inches (46 centimeters) long in most adults, and it can support up to 30 times body weight. The smallest bone is the stapes, at about 0.1 to 0.13 inches (2.5 to 3.3 millimeters). The kneecap protects the knee joint. Bone tissue is constantly renewed: osteoblasts form new bone tissue, while osteoclasts break down old bone tissue to make room for new tissue. During childhood, cartilage is replaced by bone with calcium, and growth is complete by about age 25. Bone marrow produces red blood cells, white blood cells, and platelets.

The human body contains more than 600 muscles, which carry out functions ranging from pumping blood to lifting heavy objects. These muscles fall into three types: smooth, cardiac, and skeletal. Skeletal muscles are voluntary and striated, working together with bones to provide power and strength. Tendons are tough cords that connect muscle to bone, so that when a muscle contracts it moves the tendon and, through it, the bone.

The heart is a fist-sized organ that pumps blood throughout the body and serves as the main organ of the circulatory system. Its rhythm and pace are governed by the heart's electrical conduction system, with signals beginning at the sinoatrial (SA) node.

Blood is a specialized body fluid composed of four main components: plasma, red blood cells, white blood cells, and platelets.

The brain is the most complex part of the human body. As a three-pound organ, it is the seat of intelligence, the interpreter of the senses, the initiator of body movement, and the controller of behavior. Some 100 million Americans suffer from devastating brain disorders at some point in their lives.

The digestive system is made up of the gastrointestinal tract plus the liver, pancreas, and gallbladder. The GI tract is a series of hollow organs joined in a long, twisting tube from the mouth to the anus, including the esophagus, stomach, and intestines. After food enters the stomach, the stomach muscles mix it with digestive juices and slowly empty the contents, called chyme, into the small intestine.

An international team of scientists created the first comprehensive index of human cells, mapping the sizes and abundance of all cell types across the entire body. The researchers compiled a dataset of over 1200 distinct cell groups across 60 tissue systems in three reference humans. Cell counts in the body are dominated by white blood cells, with approximately 3.4 trillion cells, most of which are located in tissues.

• 1543
  Vesalius publishes De corporis humani fabrica libri septem
  Vesalius published De corporis humani fabrica libri septem, or Seven books on the fabric of the human body, gaining both fame and notoriety.^{8Historical Anatomies on the Web: Vesalius — Author & Title Description · U.S. National Library of Medicine ↗}
• 1628
  Harvey describes the circulation of the blood
  William Harvey published De Motu Cordis, the book in which he recognized the full circulation of the blood and supported the idea with experiments and arguments.^{9William Harvey · Encyclopaedia Britannica ↗}
• 1796
  Jenner creates the first successful vaccine
  Edward Jenner created the smallpox vaccine — the first successful vaccine to be developed — after observing that milkmaids who had caught cowpox did not catch smallpox.^{10Smallpox · World Health Organization (WHO) ↗}
• 1895
  Röntgen discovers X-rays
  While studying cathode radiation, Wilhelm Röntgen noticed a penetrating, previously unknown type of radiation that became a powerful tool for examining the body's interior.^{11Wilhelm Conrad Röntgen – Facts · Nobel Prize Outreach (Nobel Prize in Physics 1901) ↗}
• 1901
  Landsteiner explains the human blood groups
  Karl Landsteiner showed that people have different types of red blood cells — different blood groups — a discovery that led to safe blood transfusions.^{12Karl Landsteiner – Facts · Nobel Prize Outreach (Nobel Prize in Physiology or Medicine 1930) ↗}
• 1921
  Banting and Best extract insulin
  In John MacLeod's laboratory, Frederick Banting and Charles Best treated dogs so that they no longer produced trypsin; insulin could then be extracted and used to treat diabetes.^{13Frederick G. Banting – Facts · Nobel Prize Outreach (Nobel Prize in Physiology or Medicine 1923) ↗}
• 1928
  Fleming observes penicillin
  Alexander Fleming noticed that a mold fungus growing in a bacteria culture had impeded the bacteria's growth; the substance was named penicillin and became the basis of antibiotics.^{14Sir Alexander Fleming – Facts · Nobel Prize Outreach (Nobel Prize in Physiology or Medicine 1945) ↗}
• 1954
  First successful kidney transplant
  Joseph Murray avoided rejection using radiotherapy and immunosuppressants and successfully transplanted a kidney between identical twins, paving the way for other organ transplants.^{15Joseph E. Murray – Facts · Nobel Prize Outreach (Nobel Prize in Physiology or Medicine 1990) ↗}
• 1956
  First stopped-heart surgery
  The world's first stopped-heart surgery took place in 1956 at Cleveland Clinic.^{1630 Facts About Your Heart · Cleveland Clinic (Health Essentials) ↗}
• the 1960s
  Computed tomography is developed
  Godfrey Hounsfield developed an apparatus in which clusters of X-ray beams sent through the body from different angles are registered and converted by computer calculations into images of the body's cross sections.^{17Godfrey N. Hounsfield – Facts · Nobel Prize Outreach (Nobel Prize in Physiology or Medicine 1979) ↗}
• October 1990 – April 2003
  Human Genome Project
  The Human Genome Project was launched in October 1990 and completed in April 2003; the project generated the first sequence of the human genome.^{18The Human Genome Project · National Human Genome Research Institute (NIH) ↗}

Heart disease is the greatest single threat to health and the leading cause of death globally. Stroke was the second leading cause of death worldwide in 2023 and was responsible for approximately 11% (6.8 million) of total deaths. In 2023 eight of the ten leading causes of death worldwide were noncommunicable diseases, accounting for 47% of all deaths.