Human development, the process of growth and change that takes place between birth and maturity.

Human growth is far from being a simple and uniform process of becoming taller or larger. As a child gets bigger, there are changes in shape and in tissue composition and distribution. In the newborn infant the head represents about a quarter of the total length; in the adult it represents about one-seventh. In the newborn infant the muscles constitute a much smaller percentage of the total body mass than in the young adult. In most tissues, growth consists both of the formation of new cells and the packing in of more protein or other material into cells already present; early in development cell division predominates and later cell filling.

Different tissues and different regions of the body mature at different rates, and the growth and development of a child consists of a highly complex series of changes. It is like the weaving of a cloth whose pattern never repeats itself. The underlying threads, each coming off its reel at its own rhythm, interact with one another continuously, in a manner always highly regulated and controlled. The fundamental questions of growth relate to these processes of regulation, to the program that controls the loom, a subject as yet little understood. 

Pyramid represents a hierarchical organization of human body components

Diagram of a six-layer pyramid to represent the hierarchical organization of human body components into the following, from bottom layer to top: chemicals, cells, tissues, organs, organ systems, and organism.

Simplified definitions of the various levels of organization within the body are:

Organ system  a group of organs that contribute to specific functions within the body. Examples include:

      Gastrointestinal system

      Nervous system

Organ  a group of tissues precisely arranged so that they can work together to perform specific functions. Examples include:

      Liver

      Brain

Tissue  a group of cells with similar structure and function. There are only four types of tissues:

Diagram of the hierarchy of body components. Along the left are chemicals, followed by cells, then tissues, followed by organs, and finally, organ systems. Tissues are broken into epithelia, which voer exposed surfaces, line internal passageways and chambers, and produce glandular secretions; connective, which fill internal spaces, provide structural support, and store energy; muscle, which contract to produce active movement; and nerve, which conduct eliectrical impulses and carry information.

Cell  the smallest living units in the body. Examples include:

      Hepatocyte

      Neuron

Chemicals  atoms or molecules that are the building blocks of all matter. Examples include:

      Oxygen

      Protein

The body includes nine major organ systems, each composed of various organs and tissues that work together as a functional unit. The chief constituents and prime functions of each system are summarized below.

(1) The integumentary system, composed of the skin and associated structures, protects the body from invasion by harmful microorganisms and chemicals; it also prevents water loss from the body.

(2) The musculoskeletal system (also referred to separately as the muscle system and the skeletal system), composed of the skeletal muscles and bones (with about 206 of the latter in adults), moves the body and protectively houses its internal organs.

(3) The respiratory system, composed of the breathing passages, lungs, and muscles of respiration, obtains from the air the oxygen necessary for cellular metabolism; it also returns to the air the carbon dioxide that forms as a waste product of such metabolism.

(4) The circulatory system, composed of the heart, blood, and blood vessels, circulates a transport fluid throughout the body, providing the cells with a steady supply of oxygen and nutrients and carrying away waste products such as carbon dioxide and toxic nitrogen compounds.

(5) The digestive system, composed of the mouth, esophagus, stomach, and intestines, breaks down food into usable substances (nutrients), which are then absorbed from the blood or lymph; this system also eliminates the unusable or excess portion of the food as fecal matter.

(6) The excretory system, composed of the kidneys, ureters, urinary bladder, and urethra, removes toxic nitrogen compounds and other wastes from the blood.

(7) The nervous system, composed of the sensory organs, brain, spinal cord, and nerves, transmits, integrates, and analyses sensory information and carries impulses to effect the appropriate muscular or glandular responses.

(8) The endocrine system, composed of the hormone-secreting glands and tissues, provides a chemical communications network for coordinating various body processes.

(9) The reproductive system, composed of the male or female sex organs, enables reproduction and thereby ensures the continuation of the species.