Download Pulmonary System

Pulmonary System
There are many topics that are covered in the book that I did not have time to cover in
lecture (ex. Pleurisy). You will not be responsible for these topics for the exam. Focus your
reading and study time on the topics that I covered in lecture.
The pulmonary or respiratory system consists of two major components an upper respiratory
tract and a lower respiratory tract. The upper respiratory tract includes the nasal cavities,
pharynx, and larynx. The lower respiratory tract includes the trachea, bronchi, bronchioles,
alveolar ducts, and the alveoli. The primary function of the pulmonary system is the exchange
of oxygen and carbon dioxide between the blood and the environment. While some exchange of
these gases occurs in the respiratory bronchioles (smallest bronchioles), and the alveolar ducts,
the primary site of gas exchange is in the alveoli. Although the alveoli are small in volume, they
are numerous and provide a surface for gas exchange that measures about the size of a tennis
court. The walls of the alveoli are composed of a single layer of squamous epithelial cells. This
thin wall minimizes the distance across which gas exchange has to occur.
The large, thin surface area provided by the alveoli, while an advantage for gas exchange,
creates two problems. First, the thin walls of the alveoli tend to collapse as a result of surface
tension created by the water molecules that line the surface of the alveoli. Second, the large, thin
surface area and its rich blood supply expose the body to potential infection by airborne
pathogens. The strategies used to address these problems were reviewed in lecture. In addition to
squamous epithelial cells, the walls of the alveoli also include surfactant cells. The surfactant
cells secrete surfactant that disrupts the attraction between water molecules and so relieves the
surface tension on the walls of the alveoli. The surfactant cells develop at about six months after
conception, but they don’t start secreting sufficient surfactant until about the eighth month. So,
premature babies born before the eighth month have difficulty with respiratory gas exchange
because their alveoli are collapsing. This condition is called neonatal distress syndrome
(NRDS). The treatments for NRDS were described in lecture and you should know these for the
exam. A similar condition can occur in adults and is called acute respiratory distress syndrome
(ARDS). The problem of exposure to pathogens is dealt with by the presence of macrophages in
the alveoli, and by the epithelial lining of the respiratory tract that consists of pseudostratified
ciliated columnar epithelium. This type of epithelium includes goblet cells that secrete mucous
onto the surface of the epithelium. The mucous serves to catch pathogens and other foreign
particles in the inhaled air, effectively preventing them from reaching the deeper airways and
alveoli. The ciliated cells continuously push the mucous up out of the lungs and into the throat
where it is swallowed or spit out. The mucous secreted by the goblet cells is thick and requires
dilution by water. The water comes from the tissues as a result of an osmotic gradient. The
osmotic gradient is created by the goblet cells, which secrete chloride ions along with the
mucous. The transport of chloride ions is carried out by a transport protein called the cystic
fibrosus transmembrane regulator (CFTR). The disease cystic fibrosus (CF) is an autosomal
recessive disease in which the gene that codes for the CFTR protein is mutated. As a result
chloride ion transport is disrupted and the mucous secreted by the goblet cells is not thinned and
gets trapped in the airways. The trapping of the mucous causes obstructions to airflow and
increases the incidence of lung infection. The respiratory treatments for CF were discussed.
The process by which inhalation and exhalation are achieved were reviewed and you
should be prepared to summarize these mechanisms and name the muscles involved during
normal quiet breathing. The movement of air into and out of the lungs requires that the thoracic
cavity be a closed chamber, and that the lungs have one opening to the environment through the
trachea. If there is damage to the chest wall that allows air to be drawn into the thorax during
inspiration, then the negative pressure required to fill the lungs can’t be generated and the lungs
will not fill. This is what happens in a pneumothorax. In a tension pneumothorax, air pressure
inside the chest cavity builds up and causes the lung to collapse. A collapsed lung is known as
atelectasis. Air pressure may build up in the chest cavity as a result of a puncture wound to the
chest that allows air to enter the chest during inspiration, but does not allow air to escape during
expiration (yes, this is known as a sucking chest wound). A pneumothorax can also be caused by
the rupture of a blister-like formation on the lung known as a bleb. The rupture of a bleb allows
air to pass from the lung into the chest cavity. Pneumothorax due to bleb rupture is called a
spontaneous pneumothorax. The cause of blebs is unknown. Treatment for pneumothorax
involves repair of the chest wall and needle aspiration of the air in the chest cavity. After air
removal a collapsed lung will normally reinflate on its own. Bleeding into the chest cavity, called
a hemothorax, can also prevent lung inflation. Hemothorax requires stopping the source of the
bleeding and draining the blood from the chest cavity.
Obstructive lung diseases (OPDs) are characterized by difficulty in expiration. That is
that people with an OPD have trouble getting air out of their lungs. The most common OPD is
asthma. Asthma has many causes or triggers including allergic reaction, cold air, exercise,
emotional upset, etc. Asthma is an inflammatory disease involving a hypersensitivity response of
the airways with spasmodic contraction of the smooth muscle of the bronchioles. The decrease in
bronchiole diameter and excess mucous secretion increases the resistance of the airways to
airflow. The increased airway resistance decreases primarily expiratory airflow, resulting in
trapping of air in the affected part(s) of the lung. During the early stages of an asthma attack
hypoxemia (decreased oxygen levels in the arterial blood) and hypocapnia (decreased carbon
dioxide levels in the arterial blood) occur due to inadequate ventilation of the alveoli. If
treatment measures fail to resolve the attack and bronchoconstriction worsens, hypercapnia
(increased carbon dioxide levels) may develop. Hypercapnia causes the blood pH to decrease, a
condition called respiratory acidosis. Once acidosis develops the asthma attack becomes life
threatening. Treatment for asthma involves avoiding triggering factors and the use of
pharmacologic agents to relieve attacks. The pharmacologic agents and their actions were
discussed in lecture and you will be expected to know these actions.
Chronic obstructive pulmonary diseases (COPDs) are pulmonary disorders
characterized by chronic obstruction of airflow in the airways. Probably the best-known COPDs
is emphysema. Emphysema results from the breakdown of elastin fibers in the alveolar walls
and is most commonly caused by smoking. It also may be caused by an inherited deficiency of a
protein, 1-antitrypsin, which inhibits the proteolysis of elastin. The elastin fibers are responsible
for the elasticity of the lungs. Elasticity refers to the ability of the lungs to return to their
previous shape and volume after being stretched. During normal, quiet breathing the elasticity of
the lungs is responsible for exhalation of air. The loss of elasticity that occurs in emphysema
results in the trapping of air in the lungs. Because emphysema normally occurs across the entire
lung tissue, hyperinflation of the lungs occurs. The hyperinflation of the lungs produces
hyperexpansion (barrel chest) of the chest that characterizes this condition. The trapping of air
decreases gas exchange leading to dyspnea (feeling of inability to get enough air), hypoxemia
and hypercapnia. In response increase inhalation effort by recruiting accessory muscles, such as
the sternocleidomastoids and scalene muscles, during inhalation. The treatment for asthma is
similar to that for asthma, with the addition of breathing exercises to condition the chest muscles
for increased breathing effort, oxygen therapy, and in advanced stages of the disease lung
transplantation.
In Restrictive lung diseases (RLDs) the lungs have decreased compliance, that is, they
don’t stretch as easily. The primary cause of RLDs is the build up of scar tissue in the lungs due
to exposure to damaging environmental agents (like cigarette smoke, asbestos, coal dust, etc.)
The scar tissue does not stretch as easily as normal tissue; therefore, a person with an RLD has
difficulty inflating their lungs. In compensation for this a person with an RLD will breath faster
and shallower. This compensation helps to decrease hypercapnia, but not the hypoxemia. This is
because carbon dioxide is able to diffuse across the wall of the alveoli more rapidly than oxygen.
The treatment goal for RLDs is to avoid exposure to the agent causing the disease, oxygen
therapy, and reducing inflammation of the airways using corticosteroids.
NOT COVERED. WILL NOT BE ON EXAM! Pulmonary hypertension is high
blood pressure in the pulmonary arteries. There are two types of pulmonary hypertension:
primary pulmonary hypertension and secondary pulmonary hypertension. The cause of
primary pulmonary hypertension is unknown. Secondary pulmonary hypertension is caused by
conditions that are secondary to the lungs or cardiovascular system. There are three causes of
secondary pulmonary hypertension. One cause is increased filling pressure of the left ventricle
(due to decreased compliance of the ventricular wall or a stenotic bicuspid valve) that causes the
backing up of blood in the pulmonary circulation. A second cause is increased blood flow into
the pulmonary circulation, which can occur in certain heart defects (ex. interventricular septal
defect) that increase the volume of blood entering the right ventricle. A third cause is
vasoconstriction or obstruction of part of the pulmonary circulation due to an embolus,
atelectasis, emphysema, or any condition that decreases blood flow to part of the lung. Treatment
for secondary pulmonary hypertension is directed at alleviating the cause (ex. Surgically
repairing the interventricular septal defect). Once the cause of the secondary pulmonary
hypertension is relieved, the hypertension resolves. Primary pulmonary hypertension is a more
serious condition. Oxygen therapy and digitalis may be given to temporarily improve the
person’s condition. However, the increase resistance of the pulmonary circulation causes right
ventricular hypertrophy (called cor pulmonale) and eventually the right heart fails and death
occurs unless a lung transplant can be performed.
Lung cancers (a.k.a. bronchogenic carcinomas) are the leading course of cancer deaths
in the U.S. Lung cancers originate from the epithelial cells that line the respiratory tract. They
are usually the result of chronic irritation of the lung epithelium, and are most commonly caused
by cigarette smoking. The high incidence of death due to lung cancer results from the tendency
of these cancers to metastasize (spread) to other organs. Metastasis of the lung cancers is
facilitated by the fact that the lungs are highly vascularized, and they have lots of lymph vessels.
These provide routes for metastasis of the cancer. Lung cancers are generally divided into four
types on the basis of the types of cells making up the tumor. The four types are squamous cell
carcinoma, adenocarcinoma, large cell carcinoma, and small cell carcinoma. The first three
types are grouped together as non-small cell carcinomas. The characteristics of each of these
types of lung cancer (including the percentages of lung cancers that each type accounts for) were
discussed and you will be expected to know these facts for the exam. Treatment for lung cancer
depends on the type and stage of the disease. Generally, if possible, the primary tumor of the
lung is removed, and may be preceeded by chemo- and/or radiation therapy to reduce the size of
the tumor. Post-surgical chemo- and/or radiation therapy is also commonly done.