Cells, tissues, and organs
The remaining sections of the chapter focus on the basics of anatomy, physiology, and pathology. Anatomy is the study of how the body is shaped and structured. It includes the structures, levels of organization, and the relationships among different body parts. Later chapters will describe the anatomy, physiology, and pathology of each body system. Anatomy, physiology, and pathology information is used in many ways in clinical and administrative settings. From coding and billing to entering diagnostic orders, having a strong understanding of this information will help medical assistants perform their roles more efficiently.
When starting with the anatomy basics, we need to examine the makeup of the body. If we were to break apart the body, we would have many body systems. Examples include the digestive, cardiovascular, musculoskeletal, and respiratory systems. Each system is composed of different organs. For instance, the stomach and intestines are digestive system organs. Each organ is made up of combinations of tissues, and these tissues are composed of cells. So, when studying the organization of the body, it is easier to start at the level of cells and work up to the organism (human body) level.
Cells
The cell is the basic unit of life. Cells determine the functional and structural characteristics of the entire body. Cells are microscopic. They have a variety of shapes and perform a vast array of functions. A cell is covered by a plasma membrane. Organelles are suspended in the cytoplasm.
The following organelles can be found in the cell:
Cells can also have extensions or projections on the surface, including these:
- Cilia: Fine, hairlike extensions that move in coordinated waves to sweep mucus and trapped particles across the cell surface, as seen in the respiratory tract. Some cilia also help detect surroundings or chemicals.
- Microvilli: Small projections on the surface of the cell. This increases the surface area, which allows for additional absorption.
- Flagellum: Single long hairlike extension on the surface of the cell. Used to propel or move the cell.
Cell reproduction
Most human cells reproduce by mitosis.
The two cells are genetically identical to the parent cell. Prior to the mitosis process, the cell enters the interphase stage. During this stage, the genetic information, chromosomes, replicate. Each sister pair of chromosomes (called chromatids) are joined together until they are pulled apart later in the mitosis process. The point where the two chromatids are joined is called the centromere. The centromere is also the attachment point for spindle fibers, which will be involved in the mitosis process. The mitosis process consists of four phases:
- Prophase: The membrane around the nucleus starts to break down. Centrioles produce spindle fibers and start to move toward opposite sides of the cell.
- Metaphase: Each pair of chromatids lines up, and each chromatid is attached to a spindle fiber.
- Anaphase: Spindle fibers pull the sister chromatids apart.
- Telophase: The nucleus reforms around each set of chromosomes. The cell continues to separate into two daughter cells.
Tissues
The microscopic study of body tissues is known as histology. All of the body tissues are grouped into four types:
Epithelial tissue
Epithelial tissue acts as an internal or external covering for organs. Examples of epithelial tissue include the outer layer of the skin, glands, and linings of body cavities and organs. In epithelial tissue, the cells are packed so closely together that there is little to no intercellular material. Two types of epithelial tissue include simple and stratified. Simple epithelium is a single layer of the same-shaped cells. Stratified epithelium contains multiple layers of cells.
Epithelial tissue is classified according to shape:
- Squamous: Flat in appearance
- Cuboidal: Square in appearance
- Columnar: Long and narrow in appearance
- Transitional: Varying shapes that can stretch
Connective tissue
Connective tissue supports and binds other body tissues. Examples of connective tissue include bone, blood, fat, fibrous tissue, areolar tissue, and cartilage. Connective tissue is the most frequently occurring tissue in the body.
Muscle tissue
Muscle tissue produces movement. Classifications of muscle tissue include the following:
- Skeletal: Voluntary muscles that appear striated or striped. Attached to bones and produce voluntary body movements when contracted.
- Cardiac: Involuntary muscles that appear striated and form the heart muscle wall.
- Smooth: Involuntary nonstriated muscles. Line the blood vessel walls and hollow organs and cause peristalsis and vasoconstriction.
Nervous tissue
Nervous tissue includes cells that provide transmission of information to control a variety of functions. Nervous tissue controls the body’s functions to maintain homeostasis. Nervous tissue is made up of neurons (nerve cells) and supportive structures called neuroglial cells.
Organs
An organ may have one or more functions. For instance, the pancreas has two functions: it produces insulin and digestive enzymes. Organs are grouped within body systems. An organ may be part of one or more systems. For instance, the pancreas is part of the endocrine system and also part of the digestive system.
Body systems
These structures work together to perform a specific function in the body. Body systems include the cardiovascular, respiratory, nervous, endocrine, gastrointestinal, integumentary, lymphatic, immune, muscular, skeletal, reproductive, sensory, and urinary systems.
Organism
A multicellular organism is made up of many body systems. These work together to maintain a steady environment in the body called homeostasis. If the balance is off or if the environment moves out of the normal range, diseases can occur.