17 Introduction to Tissues

Tissues in Levels of Organization

The human body is organized at several levels of scale that can each be examined. 

Learning Objectives

Characterize where tissues fall in levels of organization

Key Takeaways

Key Points

  • The human body has many levels of structural organization: atoms, cells, tissues, organs, and organ system. 
  • The simplest level is the chemical level, which includes tiny building blocks such as atoms. 
  • Cells are the smallest functional units of life. 
  • Tissues are groups of similar cells that have a common function.
  • An organ is a structure that is composed of at least two or more tissue types and performs a specific set of functions for the body.
  • Many organs working together to accomplish a common purpose is called an organ system.

Tissues in the Human Body

The human body has many levels of structural organization. The simplest level is the chemical level, which includes tiny building blocks such as atoms. Cells are the smallest functional units of life. The simplest living creatures are single cell creatures, but in complex life forms, such as human beings, cells also exist in the tissue level. 

Tissues are groups of similar cells that have a common function. The four basic tissue types are epithelial, muscle, connective, and nervous tissue. Each tissue type has a characteristic role in the body:

  1. Epithelium covers the body surface and lines body cavities.
  2. Muscle provides movement. 
  3. Connective tissue supports and protects body organs. 
  4. Nervous tissue provides a means of rapid internal communication by transmitting electrical impulses.

Organs: Made of Tissues

An organ is a structure that is composed of at least two or more tissue types and performs a specific set of functions for the body. The liver, stomach, brain, and blood are all different organs and perform different functions. Each organ is a specialized functional center responsible for a specific function of the body.

At the organ level, complex functions become possible because of the specialized activities of various tissues. Most organs contain more than one tissue type. For example, the stomach consists of smooth muscle tissue for churning movement while it is innervated, but it is also supplied by blood, which is a connective tissue. 

The next level is the organ system level. Many organs working together to accomplish a common purpose create an organ system. For example, the heart and the blood vessels of the cardiovascular system circulate blood and transport oxygen and nutrients to all the body cells. 

This illustration is in the shape of a pyramid. From the top–down in this pyramid we see how molecules form cells, cells form tissues, and tissues form organs. Each level has a picture of the tissue or organ it describes. At the top of the pyramid are hydrogen and oxygen atoms. This is the smallest level and it is labeled the Chemical level. This is where atoms bond to form molecules with three-dimensional structures. The second smallest level is the Water molecule level, followed by the Cellular level. A variety of molecules combine at the Cellular level to form the fluid and organelles of a body cell, such as a smooth muscle cell. The Tissue level is built by the cells—a community of similar cells form a body tissue, such as the smooth muscle tissue used in the stomach. The Organ level is next. This is where two or more different tissues combine to form an organ. A cutaway view of the bladder shows how it contains skeletal muscle and smooth muscle. The Organ system level is the second largest level. This is where two or more organs work closely together to perform the functions of a body system. A cutaway view of the urinary tract system shows the kidney, ureter, bladder, and urethra. At the base of the pyramid is the largest level, the Organismal level. This is where the many levels and systems are coordinated to perform the functions of daily living. The illustration shows a woman drinking a glass of water.

Levels of Organization: Molecules form cells. Cells form tissues, and tissues form organs. Organs that fulfill related functions are called organ systems. An organism is made up of interconnected organ systems.

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