Explore the structural organization of the human body, starting at the level of the organism and progressing down in complexity through the organ system, organ, tissue, and cellular levels. Major features of each level are discussed.
Levels Of Organization
The human body is incredibly complex, but it does have an underlying organization. Learning this organizational structure can really help you visualize and understand how the human body is built and how it functions. The levels of organization start at the most complex level and end at the most basic level, from the whole organism to the cellular level. Each step down in level represents a decrease in complexity. The levels include:
- Organ system
- Cell levels
Let's start working our way down through these levels.
The top level is an easy one. This is just the organism, or the life form and all the systems, organs, tissues, cells, and molecules that it contains. An example of an organism: you!
Organ System Level
The first step down starts to break up the body, or the organism, into major systems that are dedicated to specific functions. Organ systems are made up of organs that work together to perform specific and vital functions within the body. You have likely heard of some of these systems, such as the digestive, urinary, muscular, and reproductive systems. You probably even have a pretty good idea of some of the major functions for these systems. In total, there are eleven organ systems in the body. In addition to those listed above, we also have the respiratory, cardiovascular, nervous, endocrine, lymphatic, skeletal, and integumentary systems.
We started with the whole organism, then the organ systems; now we get to the organs that are part of those organ systems. Imagine an airport, a great example of a system, dedicated to getting each passenger from the front entrance onto a plane. Within the system, there are multiple separate units: check-in, security, gates, boarding bridges, etc. These units are like organs: each has a dedicated function and only together do they achieve the overall goal of the system. Within an organ system, an organ is a structure composed of multiple tissue types that carries out a specific function.
There are many organs in the body, so let's just think about the organs that are part of one specific organ system. When you think of the digestive system, you think of processing food and absorbing it. That should bring to mind the stomach and intestines, which are in fact major digestive organs.
If you could shrink yourself down and take a little stroll through your organs, you would start to notice some similar landscapes. These landscapes are the tissues that make up your organs. A tissue is a group of similar cells working together to perform a function. There are four tissue types within the human body. That means that any individual organ can only be composed of at most four different tissue types; this is why on your stroll through the organs, you would certainly start to notice some similar looking tissues. Let's look at the major features of the four tissue types.
Epithelial tissue- cells are pressed up against one another and can form barriers (think skin).
Connective tissue- cells are embedded in a fluid and protein mixture that varies in consistency from completely liquid (blood) to rigid (bone).
Muscle tissue- collections of cells that are able to contract forcefully in a coordinated way.
Nervous tissue- collections of cells that carry information from one place in the body to another (nerves).
We defined our tissues as similar cells working together. If you think about a tissue as being a team, the cells making up the tissue are the teammates. While the whole team (tissue) is working towards a goal, the real action happens at the level of each individual teammate (cell).
Cells are considered the smallest living unit in the human body. Cells are considered living because they typically have all the components and machinery required to keep themselves alive, and some are even capable of copying themselves by dividing to form new cells. Some of the 'hardware' found within cells include: a nucleus to store the genetic material, the endoplasmic reticulum for making molecules, the Golgi apparatus for packaging and transporting cell materials to where they are needed, ribosomes for making proteins, and various other small compartments with dedicated functions.
The kind of molecules and compartments within each cell varies depending on the job of that particular cell. Muscle cells, for example, will have significantly more of the proteins required for muscle contractions than other types of cells. There are about 200 different cell types in the body, but some of the major cell types include blood cells, bone cells, muscle cells, fat cells, and nerve cells.
Remember, even though there are many different cell types within the human body and each cell is working alone without an awareness of the rest of the body, all the cells of the body work together. In fact, our lives depend on it!
Now that you have worked your way down through the levels of complexity, we will review by working our way back up!
Cells are the smallest living unit within the body. Similar cells can be grouped together to form tissues, of which there are four types. Tissues of the same and different types can be grouped together to form organs that carry out specific functions. Groups of organs that are carrying out different functions that support the same overall process within the body are called organ systems. We have eleven organ systems in the body; the collection of all these organ systems together makes up the human organism.