Enzymes
Enzymes are one type of complex protein structure determined by DNA. (Enzymes do not enter into chemical compounds, but speed up chemical reactions.) Every one of the thousands of different chemical reactions that occur in the human body requires a specific enzyme to speed it up or facilitate the process. DNA directs the formation of thousands of different proteins to meet the needs of enzyme production.
Another task of DNA is the formation of other proteins that make up skin, muscle, blood vessels, and all internal organs. DNA builds the body’s proteins from endless combinations of 20 amino acids. If all the coded DNA instructions found in one single cell were translated into English.
Key Concept Enzymes are complex proteins that speed up chemical reactions.
Tissues
Cells of the same type and structure form tissues, each of which has a special function. The list below identifies the four principal types of human body tissues and their basic functions:
• Epithelial tissue protects body parts and produces secretions.
• Connective tissue anchors and supports other body structures. Blood is a special type of connective tissue that brings food and oxygen to the cells and carries wastes away.
• Muscle tissue provides movement of the body.
• Nerve tissue conducts impulses to and from all parts of the body.
Epithelial Tissue
Functions. The following are the main functions of epithelial tissue:
• Covers and protects all body surfaces, cavities, and lumina (hollow portions of blood vessels or body tubes)
• Absorbs and secretes substances from the digestive tract
• Secretes substances from glands
• Provides filtration in the kidneys
• Forms highly specialized epithelial tissues in the taste buds and nose
• Transports particles contained in mucus away from the lungs
Generally, epithelial tissue is avascular (without blood vessels). To receive nourishment, the tissue must receive nutrients from underlying tissue, such as connective tissue, through a process called diffusion.
In places where epithelium is subject to much destruction, the tissue is modified to provide greater protection. For example, calluses form on the bottom of the feet or on the palms of the hands to withstand greater wear and tear. Because the outer layers of epithelial cells are constantly being worn off at the body’s surface, the underlying layers of epithelium are continually producing new cells. Therefore, epithelium is in a continuous state of regeneration.
Types. Several types of cells make up epithelial tissue. Each type of cell has a characteristic shape and may be found in single or multiple layers (Table 15-4). The term simple is used when the epithelial tissue is made up of a single layer of cells. The term stratified means several layers of cells. For example, simple squamous (scaly) cells can be found in single layers, as in the alveoli (air sacs) of the lungs, or they can be stratified, as in the mouth and esophagus.
TABLE 15-4. Types of Epithelial Tissue
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DESCRIPTION |
ILLUSTRATION |
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Simple squamous (basement membrane) tissue is found in the lungs. Simple cuboidal tissue is found in the ovaries, thyroid, sweat glands, and salivary glands. |
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Simple columnar tissue is found in the stomach and intestines and in ducts of glands. |
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Pseudostratified columnar tissue (ciliated shown) is found in the mucous membranes of respiratory passages and eustachian tubes. It also exists in nonciliated form, which is found in the ducts of some glands, such as the parotid salivary gland and the male urethra. |
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Transitional tissue lines the urinary bladder. It varies in shape, depending on whether the bladder is full or empty; when full, the cells slide and stretch. |
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Stratified squamous tissue makes up the epidermis of the skin and the lining of the mouth, pharynx, ovaries, and vagina. |
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Types of epithelial tissue not shown include stratified columnar; which is found in the epiglottis, parts of the pharynx and anal canal, and the male urethra; and simple ciliatec columnar; which is found in the lining of bronchi, the nasal cavity, the oviducts, and in the lining of the uterus.
Transitional epithelium is a type of stratified squamous epithelium. It has the ability to change shape (e.g., it enables the bladder to fill and stretch without damaging its walls).
Ciliated epithelium is a type of columnar cell epithelium containing cilia. Here, in a very effective protective mechanism, specialized cilia sweep mucus with trapped dust and bacteria away from sterile areas, such as the lungs, toward nonsterile areas, such as the trachea and the mouth. Cilia are also found in the oviducts (uterine tubes) and help to move the ovum toward the uterus.
Epithelial cells shaped like goblets have the ability to form secretions.
Connective Tissue
The different types of connective tissue vary greatly in structure and function. The main functions of connective tissue are:
• Support, bind, or connect other tissues
• Provide nutrients to all body organs and remove waste
• Store vital nutrients, such as fat or calcium
• Provide protection for organs
The classification of connective tissues varies, but generally connective tissues are organized according to each specific matrix (type of structural network). For example, the matrix of blood is plasma. The matrix of adipose connective tissue is collagen fibers and adipose (fat) cells. Bone has a matrix of tightly packed cells that are rich in calcium.
Blood. Blood is usually classified as a form of connective tissue because it has a matrix of specialized cells called plasma, the liquid portion of blood. The major formed elements of plasma are the three types of blood cells: erythrocytes (red blood cells or RBCs), leukocytes (white blood cells or WBCs), and thrombocytes (platelets). RBCs carry the oxygen and carbon dioxide needed for cellular respiration, including the formation of energy and waste removal. WBCs destroy pathogens and develop immunity to some diseases. Platelets are cell fragments that play a major role in the blood-clotting process.
Soft Connective Tissue. The three types of soft connective tissue are areolar (loosely structured), fibrous (densely structured), and adipose (fatty).
Areolar tissue (loose connective tissue) is the most abundant connective tissue in the body. It resembles a packing material that holds things in place. It provides support for body parts and allows for some stretching in all directions. It is highly vascular, meaning that it contains numerous blood vessels; therefore, areolar tissue plays an important role in cell nutrition. It is perfectly suited for the diffusion of nutrients and waste materials across cell membranes.Areolar tissues are located where the body can intercept pathogens before they enter the bloodstream, such as just underneath the skin and beneath the epithelial tissue that lines the digestive and respiratory tracts.
Fibrous connective tissue is found where a need for flexible strength exists, such as in the dermis layer of the skin and in ligaments and tendons. (Ligaments connect bone to bone; tendons connect muscle to bone.) The blood supply to fibrous connective tissue is poor, which is the reason it heals relatively slowly.
Elastic connective tissue can stretch to one and one-half times its original size. As with a rubber band, it snaps back to its original size. It is found in areas that are stretched on a regular basis, such as the large arteries, the larynx (voice box), the alveoli (air sacs), and the external ear.
Adipose tissue is fatty. It stores fats (lipids) as a food reserve and is found throughout the body. It serves as a padding to protect various body structures, such as the eyeballs and kidneys. It insulates against heat loss. Researchers have noted that most of our body’s adipose tissue is formed prenatally (before birth) and during the first year of life. Although dieting and exercise may eliminate stored fat within the adipose tissue, the adipose tissue itself remains, waiting to be restocked with new energy stores (i.e., new fat).
Special Considerations : LIFESPAN
Adipose Tissue
Theorists have suggested that increased amounts of fats eaten during the child’s first year of life will lead to an increase in adipose tissue, which may predispose a person to obesity in later years. This idea has not been proved, but many authorities hold it in high regard.
Hard Connective Tissue. Hard connective tissue includes bone (the hardest connective tissue) and cartilage. Bone gives the entire body structure, support, mobility, and protection. Being well supplied with blood vessels (vascular), bone is also the site of numerous metabolic activities, such as the storage of calcium. Some bones contain red bone marrow, which produces RBCs.
Cartilage is tough, elastic tissue found between segments of the spinal cord (the vertebrae) and between the ends of the long bones. Cartilage gives shape to the nose, larynx, and external ear. Between bones, cartilage serves as a shock absorber and reduces friction between moving parts. (Formation of most bones first begins in the fetus with a type of cartilage that later converts into bone through a process known as ossification.) Cartilage is poorly supplied with blood vessels; therefore, injured cartilaginous tissue heals slowly, if at all.
Muscle Tissue
Muscle tissues contain unique fibers that can contract (shorten) and relax, bringing about movement. Chemicals sent as stimuli to the muscles from the nervous system supply the stimulus to contract.
Muscle tissue may be classified in several ways. It may be classified according to the following:
• Function—skeletal, smooth, and cardiac
• Appearance—striated (striped) and nonstriated
• What controls its action—voluntary or involuntary (Chapter 18 describes muscle tissue and its function in more detail.)
Nerve Tissue
Nerve tissue is composed of neurons and neuroglia. Neurons are the actual working nerve cells that respond to stimuli. There are several types of neurons, but the two main types are the sensory (afferent—toward the brain) nerves and the motor (efferent—away from the brain) nerves. They send impulses to, and receive impulses from, all parts of the body (see Chapter 19).
NCLEX Alert An understanding of the structure, composition, and functions of various tissues found in the human body will help you choose the correct clinical nursing actions. The NCLEX requires that you are able to differentiate between basic normal and abnormal anatomy and physiology
Membranes
Membranes are sheets of epithelial or connective tissue that act together to cover surfaces, line surfaces, or separate organs or lobes (such as in the lungs). Some membranes produce secretions.
Epithelial membranes are subdivided into mucous and serous membranes. Connective tissue membranes are subdivided into skeletal and fascial membranes.
Epithelial Membranes. Mucous membranes are also known as mucosa. They line body cavities that open to the outside of the body. The mucus secreted by these membranes lubricates and protects against bacterial invasion and other foreign particles.
Serous membranes are also known as serosa. They line body cavities that do not open to the exterior. These membranes secrete serous fluid, which is thinner than mucus. Serous fluid prevents friction when organs are in contact with one another. Serous membranes are divided into two layers: parietal and visceral. The parietal layer is the portion of the membrane attached to the wall of the body cavity; the visceral layer covers internal organs.
Connective Tissue Membranes. Skeletal membranes are connective tissue membranes that cover bones and cartilage. They act chiefly to support body structures. Synovial membrane is a type of skeletal membrane that lines freely moveable joint cavities and secretes synovial fluid. (Synovial fluid is a lubricant that provides for the smoother motion of bone, reducing friction between the moving parts.)
Fascial membranes are sheets of tissues that hold organs in place. The superficial fascia is a layer that connects the skin to underlying structures. Deep fascia binds muscles to tendons in order to anchor bones, and separates muscles into functional groups.
TABLE 15-5. Major Body Systems and Their Components
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SYSTEM |
COMPONENTS |
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Integumentary |
Skin and its appendages (hair, nails, and sweat and oil glands) |
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Musculoskeletal |
Bones, joints, and muscles that enable the body to move and give it shape |
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Nervous |
Brain, spinal cord, and nerves that carry impulses and interpret them |
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Endocrine |
Organs (thyroid, pituitary, adrenal, pancreas, ovary, testis) that produce hormones that regulate body functions |
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Sensory |
Organs (eyes, ears, tongue, nose, skin) that supply the body with information |
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Cardiovascular |
The heart, along with blood vessels and tissues that transport blood to and from all parts of the body |
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Hematologic and lymphatic |
Blood and its components (including erythrocytes, leukocytes, platelets, and plasma) that carry oxygen, nutrients, and wastes to and from all parts of the body |
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Immunologic |
Specific blood cells and lymphatic organs that help to prevent disease |
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Respiratory |
Lungs and passages leading to the lungs that take part in oxygen and carbon dioxide exchange |
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Digestive |
Organs (mouth, esophagus, stomach, intestines, liver; gallbladder; pancreas) involved in taking in and converting food to substances the body can use |
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Urinary |
Organs (kidneys, ureter, bladder, and urethra) that rid the body of waste and water |
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Male reproductive |
External sex organs and all related internal structures (penis, testes, vas deferens, epididymis) |
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Female reproductive |
External sex organs and all related internal structures (vagina, oviducts, ovaries, uterus, vulva) |
Organs and Systems
An organ is a group of different types of tissues that form in a specific manner to perform a definite function. For example, the heart is a combination of muscle, nerve, connective (blood), and epithelial tissues. Organs do not work independently but are associated with other organs. Organs have many functions.
Groups of organs are called systems. Each organ contributes its share to the function of the whole. Systems do specialized work in the body, but all systems depend on one another. An understanding of the structure and function of body systems is the basis for all nursing. See Table 15-5 for specific body systems and the components of each system.
It is important to remember that the organs and systems of the body are interdependent. That is, they depend on each other for proper functioning. In general, a malfunction of one system of the body often causes malfunctions in other systems as well. Disorders of the various body systems in adults are described and discussed in Unit 12. Disorders in children are discussed in Unit 11.
Key Concept The body operates as an integrated whole. Ihe optimum functioning of one body system usually depends on the functioning of other systems.
KEY POINTS
• The human body is made up of solids, liquids, and gases that function independently, but are interrelated.
• The study of the human body can be subdivided into the studies of anatomy (structure), physiology (function), and pathophysiology (disorders of body structure and/or function).
• Medicine has developed a sophisticated system of describing anatomy and physiology called medical terminology. To assist the learner, much of this terminology can be broken down into prefixes, suffixes, and root words.
• The body is described in terms of superior, inferior, dorsal, and ventral directions. It is also described in terms of transverse, frontal, and sagittal planes, and specific cavities containing viscera.
• An individual’s physical and mental functioning is kept in equilibrium through dynamic interactions between anatomy and physiology. This is known as homeostasis.
• Substances are capable of undergoing physical changes in outward appearance or chemical changes with the transfer of energy and change in structure.
• The body can be described in terms of a single cell, which collaborates with similar cells in groups called tissues.
• Each cell is composed of many complex structures. Each structure has a specific duty that relates to the body as a whole.
• Cells have similar abilities, but have developed specialized functions. Some special abilities include metabolism, contractility, conductivity, and irritability.
• Genes, the controllers of heredity, are found on chromosomes. Human cells have 46 chromosomes.
• Body cells replicate (reproduce) through mitosis. Sex cells (eggs and sperm) replicate through meiosis.
• The body is made up of four basic kinds of tissue: epithelial, connective, muscle, and nerve.
• The body is organized according to systems (groups of organs) that work together to perform certain functions that contribute to the overall workings of the body.
