Introduction to the Human Body

Introduction

Now that we've broken down the medical language, it's time to review the main topic of the language. Medical terms are used to label diagnoses, procedures, and treatments, all of which have a direct association with the human body. It's vital to understand the language, but it's just as important to understand the subject to which these terms apply.

 

Structures of the Body

The human body is made of different components. The structures of the human body consist of

1. Cells
2. Tissues
3. Organs
4. Systems

Each component of the human body works as an active element in all of the other parts. Cells form tissue, tissue builds organs, organs make up body systems, and body systems are the essential components of the human body.

 

Cells

A cell is the simplest of all living things, yet it's remarkably sophisticated. The most advanced computer in the world can't match the complexity of a single cell. You could devote an entire career to studying cells. 

You may recall that atoms are the smallest particles of an element (such as hydrogen or oxygen), and molecules are the smallest amount of a substance (such as water). For example, one molecule of water is made up of two atoms of hydrogen and one atom of oxygen—hence the chemical shorthand H2O. When we say that cells are the smallest structures of the body, we aren't forgetting that atoms and molecules are really smaller. Although made up of molecules, which are made up of atoms, the cell is the smallest "living" unit of function in the body.

All living things are made up of cells. The human body has trillions! Some of the bacteria and parasitic microorganisms that can make humans sick are plants and animals composed of just a single cell each.

 

Even though all cells are microscopic, they do vary in size compared to each other. The female's ovum, or egg, is a single cell that's hundreds of times bigger than a tiny red blood cell. Cells also differ in shape. The word cell may bring a block shape to mind, but some cells are round and flat, some are shaped like thread, some are shaped like bricks, and some lack a definitive shape while others are more complicated in shape.

Each cell performs a different function. Cell functions include reproduction, hormone secretion, excretion, and energy production. Muscle cells contract, and nerve cells transmit electrical impulses. Cells are vital to every component of the human body.

*Cells specialize, and their shapes reflect what they do. Muscle cells are long and skinny, while epithelial (skin) cells tend to be square-like and flat. Nerve cells look like roots or tentacles, and fat cells look like blobs.

 

There are three main cell parts: the plasma membrane, the nucleus, and the cytoplasm.

Plasma Membrane: This is also known as the cell membrane. 

The plasma membrane is the outer boundary of the cell and is made up of proteins and lipids. It's strong enough to keep the cell intact, yet it's thin enough to let substances in and out. Substances such as food and oxygen are allowed in, and waste products are let out. The plasma membrane keeps out harmful substances.

Nucleus: In the middle of the cell is its command and control center. The nucleus is in charge of everything that goes on in the cell. The nucleus contains

• Nucleoplasm
• A nucleolus
• Ribosomes

These are all surrounded by a nuclear membrane, a double-layered substance that has openings in it to let materials pass between it and the cytoplasm. The combining form KARY/O relates to the nucleus.

Cytoplasm: Let's put this word together. The combining form for cell is CYT/O, and the suffix -PLASM means "formative, formed material." Cytoplasm is the material that form cells; it fills the space between the plasma membrane and the nucleus. In the cytoplasm there are numerous structures called organelles, which are specialized parts of cells that have important jobs in the function of the cell. The cytoplasm also contains a network of structures called the endoplasmic reticulum that connects the parts of the cell and functions in transportation and storage. There are two types of endoplasmic reticulum—rough and smooth.

 

You've already learned about your body consisting of innumerable cells and about the valuable role they play within the human body. It should come as no surprise that disorders at the cellular level can result in disease and illness. This is especially evident when you consider tumors.

The terms tumor and neoplasm both indicate the abnormal and uncontrolled growth of cells. However, this doesn't mean that all tumors or neoplasms are signs of deadly diseases. Tumors can be benign or malignant. 

Some cancers also have the tendency to send forth cells into distant sites of the body in a process called metastasis. 

Benign tumors exhibit abnormal and uncontrolled cell growth, but they don't result in illness or death, and the growth is confined to one area. Sometimes they can grow to a size that causes pain or compression against nearby structures, in which case they're generally removed surgically. Examples of benign tumors include

• Benign breast tumor, which can be removed with surgery, with no chemotherapy or radiotherapy required
• Lipoma, which is a tumor of fat cells
• Nodule in the thyroid gland 

Malignant tumors (that is, true cancers) can cause serious illness and even death. Cancerous cells don't obey the laws and restrictions that usually control cell growth. Usually, a mutation during cell division causes cells to become abnormal and develop into cancer. 

Cancer cells essentially divide and multiply at the expense of normal, healthy cells. The cells may be concentrated in one place, forming a tumor.

Metastasis A cancer will generally progress until it overwhelms the victim or is eradicated by medical treatment. Just as normal cells are specialized, most cancer cells can be traced back to a specific type of tissue.

As an example, let's say a patient with a history of breast cancer is found to have a tumor in her lung. A pathologist can look at a biopsy sample from her lung and determine from the cells whether the patient now has a primary lung cancer or if the tumor is metastasized breast cancer.

 

Tissues and Membranes

Tissues and membranes represent the next level of organization in the human body. A tissue is a body of cells organized to perform a certain function. You can see an example of tissue by looking at yourself. Your own skin is a tissue—epithelial tissue. Note that tissue may also contain nonliving substances produced by the cells. These substances make up the extracellular matrix, which is a complex network of proteins, fluid, and various molecules that support the cells. 

There are four different tissue types:

1. Epithelial tissue covers the inner and outer surfaces of the body. Skin is the most visible example.
2. Connective tissue connects or supports other body structures.
3. Muscle tissue relaxes and contracts to allow body parts to move and certain organs to function.
4. Nerve tissue extends throughout the body to carry messages to and from the brain.

 

Epithelial tissue also covers organs, lines body cavities (the spaces that contain organs), and forms portions of some ducts and glands. This tissue functions to protect the body and to absorb, secrete, and excrete substances.

• Squamous epithelial cells form the outer layer of skin, but they serve much more than a cosmetic role. Your skin protects against invasion from bacteria and other infectious organisms. Throughout life, you collect a thin film of bacteria on your skin. If these squamous epithelial cells didn't form a tough barrier, the bacteria would penetrate the skin and invade your body. 

Disorders in which this barrier is lost—such as in burns—can place an individual at great risk for infection.

• Columnar cells. The lining of the digestive tract, including the stomach and intestines, is also a type of epithelium consisting of columnar epithelial cells. It covers the external surface of organs just as skin covers the outside of the body.                                                                                 Columnar epithelial cells are so named because of their column-like shape. Besides the digestive tract, they also line ducts, glands, and parts of the respiratory tract.
• Cuboidal epithelial cells form the tissue that protects the kidney tubules and covers the ovaries and certain glands.
• Endocrine gland cells form a kind of epithelial tissue that secretes its products directly into the bloodstream (for example, the thyroid gland). Exocrine gland cells don't secrete into the bloodstream but rather secrete into some other compartment of the body or outside it (for example, sweat glands).

 

Connective tissue has a number of functions and comes in many different forms, but basically it connects or supports other body structures. It can be soft and rubbery or hard and rigid, depending on the job it has to do. 

Fat, cartilage, bone, blood, and scars are all specialized forms of connective tissue.

1. Fascia is a connective tissue that forms a membrane that surrounds muscles or organs to separate and support them.
2. Cartilage is the rubbery, smooth material that lines the surfaces of the joints.
3. Adipose tissue is a form of connective tissue that can also provide energy in the form of lipid molecules.
4. Blood transports nutrients and wastes to and from cells.
5. Lymph is interstitial fluid that has been filtered by lymph vessels.

 

Muscle tissue has many different functions in the body. In general, it relaxes and contracts to allow body parts to move and certain organs to function. There are three types of muscle tissue: voluntary, involuntary, and cardiac. The specific locations and roles of muscle tissue will be covered later in your program.

 

Nerve tissue extends throughout the body to carry messages to and from the brain. Nerve tissue makes up the brain, spinal cord, and nerves. This remarkable network of tissue coordinates the functioning of the entire body. The fundamental unit of nervous tissue is the nerve cell (or neuron).

Neurons are specially designed to conduct signals from one end of the cell to another. These signals, racing along the network of nervous tissue, make it possible for you to feel pain, smell odors, see sights, hear sounds, and taste food. In your brain, these signals culminate in conscious thought and memory. Almost every part of the body is regulated in part by neurons. Even blood vessel diameter is influenced by the action of tiny nerves that course along the vessels.

 

*You may have heard that some lizards can regenerate a whole tail after it's broken off. Although humans can't duplicate that feat, our bodies are able to heal most wounds. 

Wound healing occurs by repair of injured tissue. Note that the formation of a scab in some wounds is a normal event in the process of wound healing.

As the edges of epithelium grow back together, the scab eventually disappears. A scar, however, never completely disappears. It may fade over a period of years, but the scar will always be there. Scar tissue is essentially the body's method of sealing the edges of a wound. 

The scar itself is a dense mat of fibrous material that appears only after normal tissue is damaged (a normal response to injury). Scars aren't limited to the skin and can be found anywhere in the body. Since a heart attack is the death of part of the heart muscle, this dead muscle is eventually replaced by scar tissue.

Cells and components of the immune system are responsible for the overall process of wound healing. 

The speed and effectiveness of wound healing depends on many factors. Simple wounds heal faster and better than complex, deep, or dirty wounds. Healthy individuals heal wounds better than those individuals who have chronic diseases.

The following factors can effect wound healing:

• Poor circulation to the wound area means that the wound isn't receiving adequate oxygen for the process of healing.
• Infection in the wound will prevent the cells of the immune system from functioning normally.
• Several drugs can interfere with the immune system.
• Lack of sufficient protein or vitamins in the diet will deny the body the necessary building blocks for wound repair.

 

Key Points

• The structures of the human body consist of cells, tissues, organs, and systems. Cells form tissue, tissue builds organs, organs make up body systems, and body systems are the essential components of the human body.
• There are three main cell parts: the plasma membrane, the cytoplasm, and the nucleus. 
• A tissue is a body of cells organized to perform a certain function. There are four tissue types: epithelial, connective, muscle, and nerve.
• The two major classifications of membranes are epithelial and connective.
• Two layers of connective tissue form to create connective membranes.