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Lecture 9

Lecture 9 Notes 2014.pdf

10 Pages
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Department
Health Studies
Course Code
HLTB21H3
Professor
Michael Inzlicht

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1
Lecture 9: Blood Pressure Continued and Lung Mechanics I
See lecture 8 notes for blood pressure regulation.
1. Lung Anatomy
Lung mechanics describe the physiological changes in pressures, pressure differences, lung
volumes, muscle contraction and air flow that occur during a single breath. These can be thought
of as the respiratory equivalent of the cardiac cycle.
There are two lungs, a right and a left, situated in the chest (thoracic cavity) and protected by the
chest wall. They are lobular - the left lung is made up of two lobes (a large superior lobe and a
small inferior lobe) while the right lung is made up of three (the previous lobes, as well as a
middle lobe). The left lung has a greater capacity than the right lung and there are slight
differences in size.
The major respiratory muscle in humans is the diaphragm, which is a curved, thin muscle
located under the lungs. It separates the thoracic cavity from the abdominal cavity below. The
diaphragm tends to curve slightly upward on the right-hand side, due to the position of the liver
immediately beneath it. The consequence of this is that the right lung is about 5 centimeters
shorter than the left lung. The cardiac notch is a slight indentation in the left lung caused by the
position of the heart. This makes the left lung slightly narrower than the right lung. Overall,
however, the left lung has a greater capacity than the right lung. When we look at a topic called
ventilation-perfusion matching, we'll see that there is also a difference in the efficiency of gas
exchange between the bottom of the lungs and the top.
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2. The Respiratory Tract
The respiratory tract is the pathway through which gases enter and exit the lungs. It opens at the
top through the mouth and the nose and continues all the way to the alveoli. From the mouth or
the nasal cavity, the respiratory tract converges in a tube it shares with the digestive system, the
pharynx, located behind the mouth. Underneath this is the larynx (vocal cords) and then the
trachea. The trachea is surrounded by rings of cartilage that make it rigid and prevent its
collapse. This rigidity is termed airway patency (this is in contrast to the esophagus, which
collapses when not in use). A lack of airway patency often results in apnea (a cessation of
breathing), and generally occurs when the upper airway muscles surrounding the trachea are
relaxed during sleep. These muscles, when active, help to “pull on” the trachea keeping it open
(patent) and preventing collapse. Apnea is often caused by obesity. When the upper airway
muscles relax, the excess weight in the throat and neck push on the trachea and cause it to
collapse. This is called obstructive sleep apnea and accounts for approximately 90% of sleep
apnea cases. The other 10% are referred to as central sleep apnea and result from abnormalities
in the respiratory control centres in the brain.
The opening of the trachea is referred to as the glottis, and a flap of tissue called the epiglottis
acts as a barrier to prevent ingested food and drink from entering the trachea rather than the
glottis. The glottis is controlled by a laryngeal branch of the vagus nerve. The epiglottis covers
the glottis during the swallowing reflex.
The trachea branches to form two primary bronchi and they branch into secondary bronchi,
tertiary bronchi and so on - normally there are between twenty to twenty-five orders of bronchi
branching depending on the person. Sometimes these bronchi are given different names; for
example, primary bronchi going to each lobe may be referred to as lobular bronchi. All of these
orders of bronchi end in what are called terminal bronchi, which have little offshoots called
respiratory bronchioles that end in alveolar sacs. Everything from the trachea down to the
terminal bronchioles is considered to be part of the “conducting zone”, whilst the respiratory
bronchioles and the alveolar sacs (and associated alveoli) are considered the “respiratory
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zone”. In the conducting zone, there is no gas exchange: the walls of cartilage and mucus are too
thick for gases to move across. Thus, it is considered 'anatomical dead space' because it is not
involved in gas exchange. However, it does play an important role by warming and humidifying
air before it enters the lungs, and helping to conserve water which is lost during exhalation.
As we proceed from the trachea to the alveolar sacs, the amount of cartilage is reduced whilst the
amount of circular smooth muscle increases. This means that the bronchi become less rigid as
they decrease in size and branch order. The smooth muscle can constrict and dilate, and the
constriction of this muscle accounts for the feelings of tightness in the chest and difficulty
breathing during an allergic reaction. A micrograph of alveoli reveals that they are covered by
thin and delicate capillaries. The delicateness of these structures is the reason why the pulmonary
circulation is under lower pressure than the systemic circuit. There is generally about 150 ml of
air in the conducting zone. Later we'll see how the anatomical dead space leads to constraints of
increasing overall breathing by increasing breathing rate. Anatomical dead space causes breath
volume (actually alveolar ventilation volume) to decrease when breathing rate increases.
Ultimately breathing can become so fast that there is no gas exchange because air only moves in
and out of the conducting zone (more on this later).

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Description
1Lecture 9 Blood Pressure Continued and Lung Mechanics ISee lecture 8 notes for blood pressure regulation1 Lung AnatomyLung mechanics describe the physiological changes in pressures pressure differences lung volumes muscle contraction and air flow that occur during a single breath These can be thought of as the respiratory equivalent of the cardiac cycle There are two lungs a right and a left situated in the chest thoracic cavity and protected by the chest wall They are lobularthe left lung is made up of two lobes a large superior lobe and a small inferior lobe while the right lung is made up of three the previous lobes as well as a middle lobe The left lung has a greater capacity than the right lung and there are slight differences in sizeThe major respiratory muscle in humans is the diaphragm which is a curved thin muscle located under the lungs It separates the thoracic cavity from the abdominal cavity below The diaphragm tends to curve slightly upward on the righthand side due to the position of the liver immediately beneath it The consequence of this is that the right lung is about 5 centimeters shorter than the left lung The cardiac notch is a slight indentation in the left lung caused by the position of the heart This makes the left lung slightly narrower than the right lung Overall however the left lung has a greater capacity than the right lung When we look at a topic called ventilationperfusion matching well see that there is also a difference in the efficiency of gas exchange between the bottom of the lungs and the top
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