EMBRYOLOGY, ANATOMY AND PHYSIOLOGY
I. EMBRYOLOGY
II. ANATOMY
III. PHYSIOLOGY
QUESTIONS TO CONSIDER AS LEARNING OBJECTIVES:
- The first division of the bronchial tree is known by this name.
- Water may leak from capillary walls under normal and pathologic
conditions but this system is designed to prevent fluid accumulation in
the lung.
- These alveolar lining cells produce surfactant under normal conditions,
multiply after injury to re-populate the alveolar lining, and are more
resistent to injury then their pavement-like relatives within the alveolus.
- These cells have been fondly referred to as "riders on the
escalator".
- These structures allow survival of the lung after pulmonary embolus
in a healthy individual.
- These three functions are carried out by the upper respiratory tract
and are critical in preparing raw air for the lung.
- This lipid prevents alveolar collapse at small diameters.
- Inspired particles of this size and smaller may be carried into
the alveoli.
- The relationship between alveolar diameter, surface tension, and
pressure is defined by this law which was originally conceived to relate
the pressure in a gas bubble to its radius.
- This anatomical division of the conducting airways is the first
level without cartilage.
- Macrophages serve these two essential functions in the lung.
- These cells produce a thin watery lubricating fluid and begin to
appear in the epithelium of the bronchiole, replacing goblet cells.
- These three lung components are derived from the mesenchyme that
invests the developing airways.
- What are the more frequent congenital anomalies that affect the
lung?
- Define atelectasis.
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I. EMBRYOLOGY
- The lung begins as a small anterior bud off of the foregut.
- The bud elongates and branches into two, each of these two new buds
will become the mainstem bronchus of each lung. This first division of
the bronchial tree is called the carina.
- Each new mainstem bronchus continues to grow in length while branching
dichotomously until both lungs are formed. These branched tubes will serve
as the conducting airways of the adult.
- A mass of primitive cells known as mesenchyme accompanies the growing
and branching foregut-derived tube. The mesenchyme differentiates into
the muscle, nerves and blood vessels of the lung as development proceeds.
II. ANATOMY
- Alveoli are the functional units of gas exchange. They arise as
clusters at the ends of the terminal branches of the foregut tube. These
clusters of alveoli are known as an alveolar sac. A good analogy are the
flowers that arise at the tips of tree branches in spring.
- Each adult lung is divided into lobes separated from one another
by fissures.
- The right lung has three lobes: upper, middle and lower.
- The left lung has only two lobes: upper and lower. The "middle
lobe" on the left is vestigial and is represented by a small tag-like
structure called the lingula.
- The trachea and bronchi of the lungs are easily recognized in microscopic
sections by the presence of cartilage in their outer wall.
- In the trachea and mainstem bronchi, the cartilage is present as
"C" shaped rings open at the back.
- Once within the lung tissue, the cartilage of the bronchi changes
to pavement-like plates.
- The cartilage acts like an exoskeleton and helps keep the large
airways from collapsing when we breathe out.
- All of the conducting airways have smooth muscle in their walls.
The muscle can make the airway constrict so the lung can adjust airflow.
- The conducting airways are lined by a protective epithelium that
begins at the vocal cords.
- This "respiratory" epithelium is ciliated, columnar, and
pseudostratified. Mucous secreting goblet cells are dispersed throughout
the columnar cells. Mucous and cilia are an important defense mechanism
for the major conducting airways.
- This epithelium changes character deeper in the lung as the airway
diameter is measured in millimeters.
- New cells, called clara cells, replace goblet cells. They secrete
a thin watery fluid instead of mucous.
- Also, here the epithelium starts to become cuboidal rather than
columnar.
- The anatomical divisions of the conducting airways are: trachea,
bronchi, bronchioles, terminal bronchioles, respiratory bronchioles, and
alveolar ducts. All of the alveoli associated with one respiratory bronchiole
are considered an anatomical unit known as the acinus.
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III. PHYSIOLOGY
- Macrophages are the most important defense cell of the lung. Macrophages
engulf particulate material and are key coordinators of the immune response.
Macrophages migrate over all epithelial surfaces of the lung, even in the
alveoli. With the cilia of the larger airways, they form the mucociliary
escalator for disposal of entrapped inhaled material.
- The alveoli are lined by an extremely flat, specialized epithelial
cell called a Type I cell. For every 9 Type I cells there is 1 Type II
cell, a cell that produces a lipid detergent molecule called surfactant.
Surfactant helps keep the alveoli from collapsing at low lumenal pressures
(as at the end of exhalation).
- Collapse of alveoli (and therefore lung tissue) is known as atelectasis.
This phenomenon is explained by LaPlace's Law, i.e. the pressure in a gas
bubble is related to its radius and surface tension. If all of the alveoli
collapsed each time we breathed out, it would be nearly impossible to re-inflate
them on breathing in. Type II cells are the "reproductive" cells
of the alveolus, as they are responsible for repopulating the alveoli with
Type I cells after these are injured (i.e. Type II cells turn into Type
I cells after they proliferate).
- The lungs have a dual vascular supply.
- Venous blood comes out of the right ventricle and enters the lung
via the pulmonary arteries.
- Oxygenated blood from the aorta comes to the lung via the bronchial
arteries. The bronchial arteries have collateral anastomoses with the pulmonary
arteries, but normally these collaterals are closed.
- The alveoli have a "thin side" and a "thick side".
- The thin side is where most gas exchange occurs.
- The thick side has the interstitium, where a few macrophages, lymphocytes
and fibroblast-like cells reside.
- Inhaled particles are eliminated at various levels of the respiratory
tract.
- Large particles (more than 10 microns) are deposited in the nose
and nasopharynx.
- Progressively smaller particles are swept into the lung and stick
to the mucous layer on the epithelium of the large airways.
- Very small particles (less than 3 microns) may enter the alveoli
where macrophages engulf them.
- Lymphatic vessels are not present at the level of the alveoli, but
begin their drainage around the respiratory bronchioles at the "base"
of the acinus. Nevertheless, the lungs lymphatic system is very efficient
at extracting extravascular water from the interstitium.
- The three most important functions of the upper respiratory tract
are: warming, moisturizing and cleaning.
- Two more frequent congenital anomalies are (1) congenital cysts
and (2) broncho-pulmonary sequestration. Complications of both anomalies
includes infection.
- Atelectasis is the incomplete expansion of the lungs or the collapse
of previously inflated lungs producing areas of airless pulmonary parenchyma.
There are 3 types of atelectasis: obstructive, compressive and patchy.
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Outline
Go Back to Pulmonary
[ Anatomy, Embryology and Physiology
of the Lung | Edema, Embolism, Infarction, and
Pulmonary Hypertension | Lung Infections
| Adult Respiratory Distress Syndrome (ARDS) and
Chronic Interstitial Lung Disease | Airway Diseases
COPD | Lung Disease Caused by Inhaled Dust
| Lung Cancer ]
Questions?
Comments? Send a message to the CATS guru: jkessler@salus.uvm.edu