September 4, 2018

Anatomy: Body Organization

The levels of organization in the human body consist of cells, tissues, organs, and organ systems. The smallest unit is the cell, and the largest is the organ system.

Source: ThoughtCo.
A cell is the basic unit of structure and function in a living thing.
The human body contains about 100 trillion cells.

The cell membrane forms the outside boundary of the cell.
Inside the cell membrane is a large structure called the nucleus. The nucleus is the control center that directs the cell's activities and contains information that determines the cell's characteristics. When the cell divides or reproduces, this information is passed on to the newly formed cells.
The area between the cell membrane and the nucleus is called the cytoplasm. The cytoplasm contains a clear, jellylike substance in which many important cell structures, called organelles, are found.

Source: BC Open Textbooks
A tissue is a group of similar cells that perform the same function.

The human body contains four basic types of tissue: muscle tissue, nerve tissue, connective tissue, and epithelial tissue.

Like the muscle cells that form it, muscle tissue can contract, or shorten. By doing this, muscle tissue makes parts of your body move.

Nerve tissue carries messages back and forth between the brain and every other part of the body. Your brain is made up mostly of nerve tissue.

Connective tissue provides support of your body and connects all its parts. Bone is one kind of connective tissue; its strength and hardness support your body and protect its internal structures. Fat is also a connective tissue. It pads parts of your body, provides insulation from cold, and stores energy.

Epithelial tissue covers the surfaces of your body, inside and out. Some epithelial tissue, such as the outermost layer of your skin, protects the delicate structures that lie beneath it. Other kinds of epithelial tissue absorb or release substances. The lining of your digestive system consists of epithelial tissue that releases chemicals used in digestion.

Organs and Organ Systems
An organ is a structure that is composed of different kinds of tissue.

The heart contains all four kinds of tissue - muscle, nerve, connective, and epithelial. Each tissue type contributes to the overall job of pumping blood.

Each organ in your body is part of an organ system, a group of organs that work together to perform a major function. Your heart is part of your circulatory system, which carries oxygen and other materials throughout the body. Besides the heart, blood vessels are organs in the circulatory system.


The systems of the body work together to maintain homeostasis, the body's tendency to keep an internal balance. Homeostasis is the process by which an organism's internal environment is kept stable in spite of changes in the external environment.


Stress is a reaction of your body and mind to threatening, challenging, or disturbing events.

  • More blood goes to the brain.
  • Hearing ability increases.
  • Sweating increases.
  • Muscles tense.
  • Blood receives more energy-producing substances.
  • Body cells release energy faster.
  • Pupils of eyes widen to take in more light.
  • Heart rate increases.
  • Digestive system slows.