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Structural Organization of the Human Body

Author: Sophia

what's covered

In this lesson, you will learn about how the structure of the body is organized in levels, what those levels are, and how they are ordered. Specifically, this lesson will cover:

1. Levels of Organization

1. Levels of Organization

Before you begin to study the different structures and functions of the human body, it is helpful to consider its basic architecture; that is, how its smallest parts are assembled into larger structures. It is convenient to consider the structures of the body in terms of fundamental levels of organization that increase in complexity:

Chemical
Cellular
Tissue
Organ
Organ Systems
Organism

This illustration shows biological organization as a pyramid. The chemical level is at the apex of the pyramid where atoms bond to form molecules with three-dimensional structures. An example is shown with two white hydrogen atoms bonding to a red oxygen atom to create water. The next level down on the pyramid is the cellular level, as illustrated by a long, tapered, smooth muscle cell. At this level, a variety of molecules combine to form the interior fluid and organelles of a body cell. The next level down is the tissue level. A community of similar cells forms body tissue. The example given here is a section of smooth muscle tissue, which contains many smooth muscle cells closely bound side by side. The next level down is the organ level, as illustrated by the bladder and urethra. The bladder contains smooth muscle while the urethra contains skeletal muscle. These are both examples of muscle tissues. The next level down is the organ system level, as illustrated by the entire urinary system containing the kidney, ureters, bladder, and urethra. At this level, two or more organs work closely together to perform the functions of a body system. At the base of the pyramid is the organismal level illustrated with a woman drinking water. At this level, many organ systems work harmoniously together to perform the functions of an independent organism. — Levels of Structural Organization of the Human Body - The organization of the body often is discussed in terms of six distinct levels of increasing complexity, from the smallest chemical building blocks to a unique human organism.

1a. Chemical Level

The chemical level is the lowest level of organization with the smallest units. All matter in the universe, whether solid, liquid, or gas, is composed of one or more unique pure substances called elements, some examples of which are hydrogen, oxygen, carbon, nitrogen, calcium, and iron. The smallest unit of any of these pure substances (elements) is an atom. Two or more atoms combine to form a molecule, such as the water molecules, proteins, and sugars found in living things. Molecules are the chemical building blocks of all body structures.

1b. Cellular Level

Many chemicals combine to form a cell. A cell is the smallest independently functioning unit of a living organism. Each cell contains small organs, called organelles, and other structures which provide organization to the cell. All living structures contain cells, and almost all functions of physiology are performed in cells or are initiated by cells. The human body is composed of over 200 different types of cells (Mostafa, 2021) that altogether number approximately 37 trillion total cells (Bianconi, et al., 2013).

did you know

As the smallest living structure that forms the basis for all other living organisms, cells come in a wide range of sizes, shapes, and even colors.

Cells can range in size from single-celled bacteria that are 0.1 micrometer in width (100 times thinner than a sheet of copy paper) to neurons of a large squid that reach 39 feet long. In the human body, the shortest cell is the granule cell located in a part of the brain called the cerebellum which is 4 micrometers long (25 times thinner than a sheet of copy paper). The longest cell in the human body is a neuron which can reach 1 meter.

Cells are known to span all colors of the visible spectrum of light—red, orange, yellow, green, blue, indigo, and violet. In the human body, specific cells can be clear, white, yellow, brown, red, blue, and green while producing additional colors in their surroundings.

1c. Tissue Level

Many cells combine to form a tissue. A tissue is a group of cells (generally alike though sometimes composed of a few related types) that work together to perform a specific function. A familiar example of this is muscle tissue which is composed of many cells that work together to perform types of contractions. In total, all the cells of the body combine to create four primary tissue types: epithelial, connective, muscle, and nervous tissues. You will see more on each of these types later in the course. The structure and function of the chemical, cellular, and tissue level can be evaluated using a microscope.

1d. Organ Level

Tissues combine to form an organ. An organ is an anatomically distinct structure of the body composed of two or more tissue types that work together to perform one or more functions. Examples include the skin, bones, muscles, brain, lungs, stomach, and kidneys.

1e. Organ System Level

Organs work together to form an organ system. An organ system is a group of organs that work together to perform major functions or meet physiological needs of the body. The human body has eleven organ systems. For example, one of these is the skeletal system composed of bones which all have their individual functions yet work together as the skeleton to provide structure and protection.

1f. Organism Level

Lastly, organ systems work together to form an organism. The organism level is the highest level of organization with the largest sized unit. For all of biology, an organism is a living being that has a cellular structure and that can independently perform all physiologic functions necessary for life. In multicellular organisms, including humans, an organism is a group of organ systems that work together to maintain life.

reflect

The levels of organization are a framework that show how individual building blocks can be grouped together to form increasingly complex structures, including a human being or any other multicellular organism. While the terms for each level may be new, the concept of using small building blocks to construct something larger and more complex is quite familiar.

Consider creating an essay for example:

The smallest unit is composed of markings such as lines, dashes, and dots.
Markings can be grouped together to create letters.
Letters can be grouped together to create words.
Words can be grouped together to create sentences.
Sentences can be grouped together to create a paragraph.
Paragraphs can be grouped together to create the essay.

The order in which the levels are organized indicates how each level increases in complexity as it is constructed.

It is important to note that the structure of any one item depends on the unique organization of its parts.

EXAMPLE

In order for a sentence to have a meaning, it must contain specific words in a specific order. Consider the two word organizations shown below. Which one creates a meaning and which does not?

Version 1: The organization is this levels about lesson of.

Version 2: This lesson is about the levels of organization.

The second sentence has meaning and functions correctly due to the correct organization of its parts. Each level or organization, similarly, must contain specific components (i.e., chemicals, cells, tissue, organs, and/or organ systems) in specific arrangements or it will not function correctly.

terms to know

Chemical Level

The chemical level is the lowest level of organization with the smallest units.

Cell

The smallest independently functioning unit of a living organism.

Tissue

A group of cells that work together to perform a specific function.

Organ

An anatomically distinct structure of the body composed of two or more tissue types that work together to perform one or more functions.

Organ System

A group of organs that work together to perform major functions or meet physiological needs of the body.

Organism

A group of organ systems that work together to maintain life.

summary

In this lesson, you learned the levels of organization in a biological organism and their order of complexity— chemical, cellular, tissue, organ, organ system, and organism. You also learned the importance of organization for the function of any level.

Source: THIS TUTORIAL HAS BEEN ADAPTED FROM OPENSTAX “ANATOMY AND PHYSIOLOGY 2E.” ACCESS FOR FREE AT HTTPS://OPENSTAX.ORG/DETAILS/BOOKS/ANATOMY-AND-PHYSIOLOGY-2E. LICENSE: CC ATTRIBUTION 4.0 INTERNATIONAL.

REFERENCES
Bianconi, E., Piovesan, A., Facchin, F., Beraudi, A., Casadei, R., Frabetti, F., Vitale, L., Pelleri, M. C., Tassani, S., Piva, F., Perez-Amodio, S., Strippoli, P., & Canaider, S. (2013). An estimation of the number of cells in the human body. Annals of human biology, 40(6), 463–471. doi.org/10.3109/03014460.2013.807878

Mostafa H. (2021). Different Cells of the Human Body: Categories and Morphological Characters. Journal of microscopy and ultrastructure, 10(2), 40–46. doi.org/10.4103/jmau.jmau_74_20

Terms to Know

Cell: The smallest independently functioning unit of a living organism.
Chemical Level: The lowest level of organization with the smallest units.
Organ: An anatomically distinct structure of the body composed of two or more tissue types that work together to perform one or more functions.
Organ System: A group of organs that work together to perform major functions or meet physiological needs of the body.
Organism: A group of organ systems that work together to maintain life.
Tissue: A group of cells that work together to perform a specific function.