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Human Body

How is the human body similar to a well-tuned machine?

Many people have compared the human body to a machine. Think about some common machines, such as drills and washing machines. Each machine consists of many parts, and each part does a specific job, yet all the parts work together to perform an overall function. The human body is like a machine in all these ways. In fact, it may be the most fantastic machine on Earth.

The human machine is organized at different levels, starting with the cell and ending with the entire organism (see the figure below). At each higher level of organization, there is a greater degree of complexity.

The human organism has several levels of organization.


The most basic parts of the human machine are cells—an amazing 100 trillion of them by the time the average person reaches adulthood! Cells are the basic units of structure and function in the human body, as they are in all living things. Each cell carries out basic life processes that allow the body to survive. Many human cells are specialized in form and function, as shown in the figure below. Each type of cell in the figure plays a specific role. For example, nerve cells have long projections that help them carry electrical messages to other cells. Muscle cells have many mitochondria that provide the energy they need to move the body.

An illustration of different types of human body cells

Different types of cells in the human body are specialized for specific jobs. Do you know the functions of any of the cell types shown here?


After the cell, the tissue is the next level of organization in the human body. A tissue is a group of connected cells that have a similar function. There are four basic types of human tissues: epithelial, muscle, nervous, and connective tissues. These four tissue types, which are shown in the figure below, make up all the organs of the human body.

An illustration of the four tissue types found in the human body

The human body consists of these four tissue types.

  • Connective tissue is made up of cells that form the body’s structure. Examples include bone and cartilage.
  • Epithelial tissue is made up of cells that line inner and outer body surfaces, such as the skin and the lining of the digestive tract. Epithelial tissue protects the body and its internal organs, secretes substances such as hormones, and absorbs substances such as nutrients.
  • Muscle tissue is made up of cells that have the unique ability to contract, or become shorter. Muscles attached to bones enable the body to move.
  • Nervous tissue is made up of neurons, or nerve cells, that carry electrical messages. Nervous tissue makes up the brain and the nerves that connect the brain to all parts of the body.

Organs and Organ Systems

After tissues, organs are the next level of organization of the human body. An organ is a structure that consists of two or more types of tissues that work together to do the same job. Examples of human organs include the brain, heart, lungs, skin, and kidneys. Human organs are organized into organ systems, many of which are shown in the figure below. An organ system is a group of organs that work together to carry out a complex overall function. Each organ of the system does part of the larger job.

An overview of the organ systems that make up the human body

Many of the organ systems that make up the human body are represented here. What is the overall function of each organ system?

Your body’s 12 organ systems are shown below (see the table below). Your organ systems do not work alone in your body. They must all be able to work together. For example, one of the most important functions of organ systems is to provide cells with oxygen and nutrients and to remove toxic waste products such as carbon dioxide. A number of organ systems, including the cardiovascular and respiratory systems, all work together to do this.

Organ SystemMajor Tissues and OrgansFunction
CardiovascularHeart; blood vessels; bloodTransports oxygen, hormones, and nutrients to the body cells. Moves wastes and carbon dioxide away from cells.
LymphaticLymph nodes; lymph vesselsDefend against infection and disease, moves lymph between tissues and the blood stream.
DigestiveEsophagus; stomach; small intestine; large intestineDigests foods and absorbs nutrients, minerals, vitamins, and water.
EndocrinePituitary gland, hypothalamus; adrenal glands; ovaries; testesProduces hormones that communicate between cells.
IntegumentarySkin, hair, nailsProvides protection from injury and water loss, physical defense against infection by microorganisms, and temperature control.
MuscularCardiac (heart) muscle; skeletal muscle; smooth muscle; tendonsInvolved in movement and heat production.
NervousBrain, spinal cord; nervesCollects, transfers, and processes information.

Female: uterus; vagina; fallopian tubes; ovaries

Male: penis; testes; seminal vesicles

Produces gametes (sex cells) and sex hormones.
RespiratoryTrachea, larynx, pharynx, lungsBrings air to sites where gas exchange can occur between the blood and cells (around body) or blood and air (lungs).
SkeletalBones, cartilage; ligamentsSupports and protects soft tissues of body; produces blood cells; stores minerals.
UrinaryKidneys; urinary bladderRemoves extra water, salts, and waste products from blood and body; controls pH; controls water and salt balance.
ImmuneBone marrow; spleen; white blood cellsDefends against diseases.


  • The human body is organized at different levels, starting with the cell.
  • Cells are organized into tissues, and tissues form organs.
  • Organs are organized into organ systems such as the skeletal and muscular systems.

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