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Chapter 1
Understanding cells
The cell is the body’s basic building block. It’s the smallest living component of an organism.
Cell components: Cells are composed of various structures, or organelles, each with specific functions. The
organelles are contained in the cytoplasm—an aqueous mass—that’s surrounded by the cell membrane. The
largest organelle, the nucleus, controls cell activity and stores deoxyribonucleic acid (DNA), which carries
genetic material and is responsible for cellular reproduction or division.
More components
The typical animal cell is characterized by several additional elements:
• Adenosine triphosphate (ATP), the energy that fuels cellular activity, is made by the mitochondria.
• Ribosomes and the endoplasmic reticulum synthesize proteins and metabolize fat within the cell.
• The Golgi apparatus holds enzyme systems that assist in completing the cell’s metabolic functions.
• Lysosomes contain enzymes that allow cytoplasmic digestion to be completed.
Cell division and reproduction
Individual cells don’t live as long as the organism they’re a part of. They’re subject to wear and tear and
must reproduce and be replaced. Cell reproduction occurs in two stages. In the first stage, called mitosis,
the nucleus and genetic material divide. In the second stage, called cytokinesis, the cytoplasm divides,
beginning during late anaphase or telophase. At the end of cytokinesis, the cell produces two daughter
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The great divide Before division, a cell must double its mass and content. This occurs during the growth
phase, called interphase. Chromatin, the small, slender rods of the nucleus that give it its granular
appearance, begins to form. Replication and duplication of DNA occur during the four phases of mitosis:
Prophase, metaphase, anaphase, and telophase. Most cells reproduce as quickly as they die.
Meiosis, another type of cell division, occurs in the reproductive cells, the egg and sperm. Meiosis
involves two cell divisions resulting in a total of four daughter cells produced, each containing 23 single
chromosomes rather than 23 pairs.
Pathophysiologic concepts
The cell faces a number of challenges through its life. Stressors, changes in the body’s health, disease, and
other extrinsic and intrinsic factors can alter the cells’ normal functioning.
Adaptation
Cells generally continue functioning despite challenging conditions or stressors. However, severe or
prolonged stress or changes may injure or destroy cells. When cell integrity is threatened, the cell reacts
by drawing in its reserves to keep functioning, by adaptive changes or by cellular dysfunction. If cellular
reserve is insufficient, the cell dies. If enough reserve is available and the body doesn’t detect
abnormalities, the cell adapts by atrophy, hypertrophy, hyperplasia, metaplasia, or dysplasia. (See
Adaptive cell
changes.)
Atrophy
Atrophy is a reversible reduction in the size of the cell. It results from disuse, insufficient blood flow,
malnutrition, denervation, or reduced endocrine stimulation.
Hypertrophy
Hypertrophy is an increase in the size of a cell due to an increased workload. It can result from normal
physiologic conditions or abnormal pathologic conditions. Hypertrophy is primarily seen in cells that
cannot adapt to increased work by increasing their numbers through mitosis. Examples of cells that
cannot undergo mitosis but experience hypertrophy are cardiac and skeletal muscle cells. There are
three main types of hypertrophy:
-Physiologic hypertrophy occurs as a result of a healthy increase in the workload of a cell (i.e., increased
muscle bulk through exercise).
-Pathologic hypertrophy occurs in response to a disease state, for example, hypertrophy of the left
ventricle in response to longstanding hypertension .
-Compensatory hypertrophy occurs when cells grow to take over the role of other cells that have died.
For example, the loss of one kidney causes the cells of the remaining kidney to undergo hypertrophy.
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Hyperplasia
Hyperplasia, an increase in the number of cells, is caused by increased workload, or hormonal
stimulation. It can only occur in cells that undergo mitosis, such as liver, kidney, and connective tissue
cells. Hyperplasia may be:
Physiologic hyperplasia occurs monthly in uterine endometrial cells .
Pathophysiologic hyperplasia can occur with excessive hormonal stimulation, which is seen in
acromegaly .
Compensatory hyperplasia occurs when cells of a tissue reproduce to make up for a previous decrease in
cells as what occurs in liver cells after surgical removal of sections of liver tissue. The compensation is
striking in its rapidity.
Metaplasia
Metaplasia is the replacement of one adult cell with another adult cell that can better endure the change or
stress. It’s usually a response to chronic inflammation or irritation. The most common example of
metaplasia is the change in the cells of the respiratory passages from ciliated columnar epithelial cells to
stratified squamous epithelial cells in response to years of cigarette smoking. Stratified epithelial cells are
better able to survive smoke damage. Unfortunately, they do not assume the vital protective role of
ciliated cells.
Dysplasia
In dysplasia, deranged cell growth of specific tissue results in abnormal size, shape, and appearance.
Although dysplastic cell changes are adaptive and potentially reversible, they can precede cancerous
changes. The most common sites of dysplasia are the respiratory tract (especially the squamous cells
present as a result of metaplasia) and the cervix. Cervical dysplasia usually results from infection of the
cells with the human papilloma virus (HPV). Dysplasia is usually rated on a scale to reflect its degree,
from minor to severe.
Cell injury A person’s state of wellness and disease is reflected in the cells. Injury to any of the cell’s
components can lead to illness. One of the first indications of cell injury is a biochemical lesion that
forms on the cell at the point of injury. This lesion changes the chemistry of metabolic reactions within
the cell. Consider, for example, a patient with HIV. The cells of the immune system may be altered,
making the patient susceptible to infection.
Draw on your reserves, adapt, or die
When cell integrity is threatened—for example, by toxins, infection, physical injury, or deficit injury—the
cell reacts in one of two ways:
• by drawing on its reserves to keep functioning
• by adapting through changes or cellular dysfunction.
If enough cellular reserve is available and the body doesn’t detect abnormalities, the cell adapts. If there
isn’t enough cellular reserve, cell death (necrosis) occurs. Necrosis is usually localized and easily
identifiable.
Toxic injury Toxic injuries may be caused by factors inside the body (called endogenous factors)
or outside the body (called exogenous factors).
Common endogenous factors include genetically determined metabolic errors, gross
malformations, and hypersensitivity reactions. Exogenous factors include alcohol, lead, carbon
monoxide, and drugs that alter cellular function. Examples of such drugs are chemotherapeutic
agents used for cancer treatment and immunosuppressive drugs that prevent rejection in organ
transplant recipients.
Infectious injury
Viral, fungal, protozoan, and bacterial agents can cause cell injury or death. These organisms
affect cell integrity, usually by interfering with cell synthesis, producing mutant cells. For
example, human immunodeficiency virus alters the cell when the virus is replicated in the cell’s
ribonucleic acid.
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Physical injury
Physical injury results from a disruption in the cell or in the relationships of the
intracellular organelles. Two major types of physical injury are thermal (electrical
or radiation) and mechanical (trauma or surgery). Causes of thermal injury include
radiation therapy for cancer, X-rays, and ultraviolet radiation. Causes of
mechanical injury include motor vehicle crashes, frostbite, and ischemia.
Deficit injury
When a deficit of water, oxygen, or nutrients occurs, or if constant temperature and
adequate waste disposal aren’t maintained, cellular synthesis can’t take place. A
lack of just one of these basic requirements can cause cell disruption or death.
Conditions of Disease or Injury
1-Hypoxia
Hypoxia is the decreased concentration of oxygen in the tissues. Cells and tissues become hypoxic when
there is inadequate intake of oxygen by the respiratory system, inadequate delivery of oxygen by the
cardiovascular system, or a lack of hemoglobin.Oxygen is required by the mitochondria for production
of ATP.
Consequences of Hypoxia
1. cell and begins to swell and burst, because NA diffuses into the cell and withdraw water.
2. production of lactic acid, decreased ph causes damage to the nuclear structures.
The effects of hypoxia are reversible if oxygen is returned within a certain period of time, the amount of
which varies and depends on the tissue.
Signs & Symptoms:
- Increased heart rate.
- Increased respiratory rate.
- Muscle weakness.
- Decreased level of consciousness.
Complications
- Altered consciousness progressing to coma and death if prolonged cerebral (brain) hypoxia occurs.
-Organ failure, including adult respiratory distress syndrome, cardiac failure, or kidney failure, may occur
if hypoxia is prolonged.
Treatment: Increase oxygen in inspired air through a mask or mechanical ventilation.
2-Temperature Extremes
- Exposure to very high temperatures can cause burn injuries, which directly kill cells or indirectly by
causing coagulation of blood vessels or the breakdown of cell membranes .
- Exposure to very cold temperatures injures cells in two ways: vasoconstriction which decreases deliver
nutrients and oxygen to the extremities and the formation of ice crystals in the cells. These crystals
directly damage the cells and can lead to cell lysis (bursting). Prolonged exposure to the cold can lead to
hypothermia.
Signs & symptoms:
- Numbness or tingling of the skin or extremities.
- Pale or blue skin that is cool to the touch.
- Shivering early on, then lack of shivering as condition worsens.
- Decreased level of consciousness, drowsiness, and confusion.
Complications
- Blood clotting, characterized by pain and a decrease in pulse downstream from the clot. If blood flow is
inadequate for an extended time, gangrene may result.
- Frostbite.
- Ventricular dysrhythmia.
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3-Radiation Injury
Radiation energy may be higher or low energy. High-energy radiation (including ultraviolet radiation) is
called ionizing radiation. Low-energy radiation is called non-ionizing radiation. Ionizing radiation may
injure or kill cells directly by destroying the cell membrane and causing intracellular swelling and cell
lysis. As cells are killed or injured, the inflammatory response is stimulated, causing capillary leakiness,
interstitial edema, white blood cell accumulation, and tissue scarring. It may also lead to mistakes in
DNA replication or transcription which may cause programmed cell death or subsequent cancer .
Cells most susceptible to damage by ionizing radiation are cells that undergo frequent divisions,
including cells of the gastrointestinal (GI) tract, the integument (skin and hair), and the blood-forming
cells of the bone marrow. Ionizing radiation is emitted by the sun, in x-rays, and in nuclear weapons.
Non-ionizing radiation includes microwave and ultrasound radiation. The energy of this radiation is too
low to break DNA bonds or damage the cell membrane.
Clinical Manifestations of Ionizing Radiation
- Skin redness or breakdown.
- With high doses, vomiting and nausea caused by GI damage.
- Anemia if the bone marrow is destroyed.
- Cancer may develop years after the exposure .
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Pediatric Consideration
Fetal cells rapidly undergo cellular division and are highly susceptible to the damaging effects of
ionizing radiation. Infants and young children also experience periods of rapid cellular growth and are
at risk of genetic damage from ionizing radiation. Studies suggest that there are no apparent health risks
to fetuses exposed to non-ionizing radiation .
4-Injury Caused by Microorganisms Cells of the body may be destroyed directly by the
microorganism or by a toxin released from the microorganism, or may be indirectly injured as a result
of the immune and inflammatory reactions .
Clinical Manifestations Infection by bacteria and viruses, often results in:
- Regional lymph node enlargement
- Fever (usually low-grade with a viral infection)
- Body aches
- Skin rash or eruption, especially with viral infections
- Site-specific responses, such as pharyngitis, cough, otitis media
Treatment
- Bacteria and mycoplasmas are treated with antibiotics, preferably after culturing the infection to
determine what the infecting microorganism is and to what antibiotic it is sensitive.
- Certain viral infections may be treated with antiviral agents. Other viral infections usually are left to
resolve on their own, with care taken that a subsequent bacterial infection does not infect the original
site or elsewhere.
Cell degeneration
A type of nonlethal cell damage known as degeneration generally occurs in the cytoplasm of the cell, while
the nucleus remains unaffected. Degeneration usually affects organs with metabolically active cells, such
as the liver, heart, and kidneys, and is caused by these problems:
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• increased water in the cell or cellular swelling
• fatty infiltrates
• atrophy
• autophagocytosis, during which the cell absorbs some of its own parts
• pigmentation changes
When changes within cells are identified, degeneration may be slowed or cell death prevented through
prompt treatment. When a disease is diagnosed before the patient complains of any symptoms, it’s termed
subclinical identification. Unfortunately, many cell changes remain unidentifiable even under a
microscope, making early detection impossible.
Cell aging During the normal process of cell aging, cells lose structure and function. Lost cell structure
may cause a decrease in size or wasting away, a process called atrophy. Two characteristics of lost cell
function are:
• hypertrophy, an abnormal thickening or increase in bulk
• hyperplasia, an increase in the number of cells. Cell aging may slow down or speed up, depending on the
number and extent of injuries and the amount of wear and tear on the cell
Warning: This cell will self-destruct
Signs of aging occur in all body systems. Examples of the effects of cell aging include
decreases in elasticity of blood vessels, bowel motility, muscle mass, and subcutaneous fat. The cell aging
process limits the human life span. One theory says that cell aging is an inherent self destructive
mechanism that increases with a person’s age. Cell death may be caused by internal (intrinsic) factors that
limit the cells’ life span or external (extrinsic) factors that contribute to cell damage and aging. (See In’s
and out’s of cell aging.)
Cell Death; There are two main categories of cell death:
1-necrotic cell death, characterized by cell swelling and rupture of internal organelles.Common causes of
necrotic cell death include prolonged hypoxia and infection .
2- apoptosis, is not characterized by swelling or inflammation, but rather the dying cell shrinks on itself
and then is engulfed by neighboring cells. Programmed cell death begins during embryogenesis and
continues throughout the lifetime of an organism. Viral infection of a cell will often turn on apoptosis,
ultimately leading to the death of the virus and the host cell. Deficiencies in apoptosis have been
implicated in the development of cancer .
Results of Cell Death
Dead cells are removed from the area or isolated from the rest of the tissue by immune cells in the
process of phagocytosis. If mitosis is possible and the area of necrosis is not too large, new cells of the
same type fill in the empty space. Scar tissue will form in the vacated space if cell division is impossible
or if the area of necrosis is extensive.
Gangrene refers to the death of a large mass of cells. Gangrene may be classified as dry or wet.
Dry gangrene spreads slowly with few symptoms and is frequently seen in the extremities, often
as a result of prolonged hypoxia.
Wet gangrene is a rapidly spreading area of dead tissue, often of internal organs, and is associated
with bacterial invasion of the dead tissue. It exudes a strong odor and is usually accompanied by
systemic manifestations.
Gas gangrene is a special type of gangrene that occurs in response to an infection of the tissue by a type
of anaerobic bacteria called clostridium. It is seen most often after significant trauma. Gas gangrene
rapidly spreads to neighboring tissue as the bacteria release deadly toxins that kill neighboring cells.
This type of gangrene may prove fatal.
Wound Repair
Destroyed or injured tissues must be repaired by regeneration of the cells or the formation of scar tissue.
The goal is to fill in the areas of damage. Tissues that heal cleanly and quickly are said to heal by
primary intention. Large wounds that heal slowly and with a great deal of scar tissue heal by secondary
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intention. Several factors can delay healing, such as: malnutrition, systemic disease, poorly functioning
immune system or if there is reduced blood flow to the injured tissue or if an infection develops.
Homeostasis The body is constantly striving to maintain a dynamic, steady state of internal balance called
homeostasis. Every cell in the body is involved in maintaining homeostasis, both on the cellular level and
as part of an organism. Any change or damage at the cellular level can affect the entire body. When an
external stressor disrupts homeostasis, illness may occur. A few examples of external stressors include
injury, lack of nutrients, and invasion by parasites or other organisms. Throughout the course of a
person’s life, many external stressors affect the body’s internal equilibrium.
Maintaining the balance Three structures in the brain are responsible for maintaining
homeostasis: the medulla oblongata (part of the brain stem that’s associated with vital
functions, such as respiration and circulation), the pituitary gland (which regulates the
function of other glands) the reticular formation (a group of nerve cells or nuclei that form a
large network of connected tissues that help control vital reflexes, such as cardiovascular
function and respiration).
Feedback mechanisms
Homeostasis is maintained by self-regulating feedback mechanisms. There are two types of
feedback mechanisms:
 a negative feedback mechanism, works to restore homeostasis by correcting a deficit within
the system
 a positive feedback mechanism, occurs when hormone secretion triggers additional hormone
secretion. This indicates trend away from homeostasis.
Accentuate the negative
For negative feedback mechanisms to be effective, they must sense a change in the body—such
as a high blood glucose level— and attempt to return body functions to normal. In the case of
a high blood glucose level, the effector mechanism triggers increased insulin production by
the pancreas, returning the blood glucose level to normal and restoring homeostasis. (See
Negative feedback, positive result).
The positive feedback mechanism is far from positive. It takes the original response and exaggerates it. It’s
said to be positive because the change that occurs proceeds in the same direction as the initial disturbance,
causing a further deviation from homeostasis. A positive feedback mechanism is responsible for
intensifying labor contractions during childbirth.
Disease and illness
Although disease and illness aren’t synonymous, they’re commonly used interchangeably. Disease occurs
when homeostasis isn’t maintained. Illness occurs when a person is no longer in a state of normal health.
It’s highly individualized and personal
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Genetic factors plus
Genetic factors (such as a tendency toward obesity), unhealthy behaviors (such as
smoking), stress, and even the patient’s perception of the disease (such as acceptance
or denial) influence the course and outcome of a disease. Diseases are dynamic and
may manifest in various ways, depending on the patient and his environment.
Cause One aspect of disease is its cause (the fancy term is etiology). The cause of disease may be intrinsic
or extrinsic. Diseases with no known cause are called idiopathic.
Intrinsic or extrinsic
The cause of disease is intrinsic when the disease occurs because of a malfunction or change within the
body. Intrinsic factors include inherited traits, the patient’s age, and the patient’s gender. Extrinsic causes
of disease come from outside the body. Examples of extrinsic causes include infectious agents,
mechanical trauma, smoking, chemical exposure, nutritional problems, drug use, temperature extremes,
radiation exposure, and psychological stress.
Development
A disease’s development is called its pathogenesis. Unless identified and successfully treated, most
diseases progress according to a typical pattern of symptoms. Some diseases are self-limiting or resolve
quickly with limited or no intervention; others are chronic and never resolve. Patients with chronic
diseases may undergo periods of remission and exacerbation. During remission, the patient’s symptoms
lessen in severity or disappear. During exacerbation, the patient experiences an aggravation of symptoms
or an increase in the severity of the disease.
Disease stages
Typically, diseases progress through these stages:
• Exposure or injury. Target tissue is exposed to a causative agent or is injured.
• Latent or incubation period. No signs or symptoms are evident.
• Prodromal period. Signs and symptoms are usually mild and nonspecific.
• Acute phase. The disease reaches its full intensity and complications commonly arise. If the patient can
still function in a preillness fashion during this phase, it’s called the subclinical acute phase.
• Remission. This second latent phase occurs in some diseases
and is commonly followed by another acute phase.
• Convalescence. In this stage of rehabilitation, the patient progresses
toward recovery after the termination of a disease.
• Recovery. In this stage, the patient regains health or normal
functioning. No signs or symptoms of the disease occur.
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