PHGY 210 Study Guide - Final Guide: Respiratory Minute Volume, Carotid Body, Vasodilation
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Physiology 210: Respiration
Structure of the Lungs and Chest Wall
A. Function of Respiration
The primary function of respiration is gas exchange.
In mammals, gas exchange occurs in the lungs.
During inspiration, air rich in O2 is inhaled in the lungs. During expiration, CO2 produced during the oxidative
processes of the body is exhaled from the lungs.
Both gases are transported by the blood. Therefore, both the cardiovascular system and the respiratory system
are involved with supplying body cells with O2 and eliminating their waste product, CO2.
B. The Respiratory Tract
Air flows through a series of air passages that connect the lungs
to the nose and mouth.
Inhaled air passes over a complex series of surfaces when it goes
through the nose: the nasal septum and the nasal turbinates.
These surfaces clean the air of big dust particles.
From the nose, warmed and moistened air flows through the
common passages for air and food, the pharynx, and then
continues through the larynx.
Air finally reaches the periphery of the lungs via the trachea and
bronchi (figure 1).
The lungs and the airways share the chest cavity with the heart,
the great vessels, and the esophagus.
The airways consist of a series of tubes that branch and become
narrower, shorter and more numerous as they penetrate into the
The trachea divides into 2 main bronchi, each of which divides into
lobar and segmental bronchi.
The right main bronchus has 3 lobar bronchi (the right lung has 3
lobes), while the left main bronchus divides into only 2 bronchi (the
left lung has only 2 lobes).
The segmental bronchi divide further into smaller branches.
The smallest airways without alveoli are the terminal bronchioles
Brain tells respiratory muscles to contract which leads to the
inflation of the lungs which occurs due to the pleural space
Pleura and pleural surfaces
o Pleura: Thin cellular sheet attached to the thoracic cage interior
(parietal pleura) and, folding back upon itself, attached to the
lung surface (visceral pleura; forms two enclosed pleural sacs in
o A way to visualize the apposition of the two pleural surfaces is to put a drop of water between two glass
microscope slides. The two slides can easily slide over
each other but are very difficult to pull apart.
o The pressure in the pleural space is negative
o Pneumothorax is the uncoupling of the chest and the
Air goes from the atmosphere into the pleural space
Lungs collapse and the rib cage springs out
C. Conducting and Respiratory Zones
The airways are divided into 2 zones: the conducting zone
(made up of the conducting airways) and the respiratory zone
The conducting airways consist of the airways from the mouth and nose openings, all the way down to the
These airways conduct air from the atmosphere to the respiratory part of the lungs.
The conductive airways do not contribute to gas exchange, and are thus said to compose the anatomical dead
The respiratory part of the lungs (the respiratory zone) begins where the terminal bronchioles divide into
respiratory bronchioles, which have some alveoli opening into their lumen (figure 3).
Beyond the respiratory bronchioles are the alveolar ducts lined with alveoli. The alveolated region of the lungs is
the site of gas exchange, and is called the respiratory zone.
Because of such abundant branching of the airways, the respiratory zone makes up most of the lungs. The
smallest physiological unit of the lungs (distal to the terminal bronchioles) is the acinus (figure 3).
Smooth muscle around airways cause trouble, e.g. asthma
D. Functions of the Conducting Airways
The conducting airways have 4 main functions:
1. Defense against bacterial infection and foreign particles: the epithelial lining of the bronchi has hair-like
projections called cilia which move mucus upwards to get rid of it. The epithelial glands secrete a thick
substance, mucous, which lines the respiratory passages as far down as the bronchioles. Foreign particles
stick to the mucous and the cilia constantly sweep the mucous up into the pharynx. This is called the
mucociliary defense system.
Cilia get paralyzed by nicotine and mucus builds up
2. Warm and moisten inhaled air.
3. Sound and speech are produced by the movement of air passing over the vocal cords.
4. Regulation of air flow: smooth muscle around the airways may contract or relax to alter resistance to air
E. Function of the Respiratory Zone
The respiratory zone is the site of gas exchange between the air in the alveoli and the blood in the pulmonary
capillaries. There are roughly 300 million alveoli in the human lungs, and each alveolus may be associated with
as many as 1000 capillaries.
F. Blood Supply
The lungs have two circulations: the pulmonary circulation (figures 4 & 5), bringing mixed venous blood (blood
that comes from different body organs with different metabolic activities) to the lungs (figure 4), allowing for
the blood to get oxygenated, and then back to the left heart, and the bronchial circulation (figure 5), supplying
oxygenated blood from the systemic circulation to the tracheobronchial tree (this circulation allows for the
airways to get oxygenated).
There are two circulations:
1. Pulmonary circulation
Blood to the pulmonary capillaries is supplied via the pulmonary artery, which originates from the right
ventricle (figures 4 and 5). Branches of the pulmonary artery run with the airways. When the alveoli are
reached, arterioles divide into a capillary bed. The pulmonary arteries supply with blood all capillaries
within the alveolar walls, which constitute the respiratory surface of the lungs where the gas exchange
takes place. Oxygenated blood (from the alveolar capillaries) leaves the lungs and returns to the left
heart via the pulmonary veins (figures 4 and 5).
2. Bronchial circulation
Bronchial arteries from the aorta supply the airway walls (figure 4). The bronchial circulation is part of
the systemic circulation.
G. Alveolar Cell Types
There are three alveolar cell types:
1. Epithelial type I and II cells
Alveoli are lined by epithelial type I and II cells. Together, all the alveolar epithelial cells form a complete
epithelial layer sealed by tight junctions. Little is known about the specific metabolic activities of type I
cells. Type II cells produce pulmonary surfactant, a substance that decreases the surface tension of the
alveoli (see sections VII- F & G).
2. Endothelial cells
Endothelial cells constitute the walls of the pulmonary capillaries. These cells may be as thin as 0.1
3. Alveolar macrophages
These remove foreign particles that may have escaped the mucociliary defense system of the airways
and found their way into the alveoli.
H. Respiratory Muscles
The lung tissue is elastic, but it is unable to expand or contract by itself. Air has to be sucked into the lungs (see
figure 40). This function is powered by the respiratory muscles of the chest wall. There are 2 types of respiratory
muscles: inspiratory and expiratory (figure 7).
The main inspiratory muscle is the diaphragm. It is
innervated by the phrenic nerves from cervical
segments 3, 4 and 5.
Contraction of the diaphragm causes its dome to
descend and the chest to expand longitudinally.
At the same time, its contraction elevates the lower
ribs because of the vertically oriented attachments of
the diaphragm to the costal margins.
Contraction of the external intercostal muscles also
raises the ribs during inspiration. As the ribs are
elevated, the anterior-posterior and transverse
dimensions of the chest enlarge (figure 7).
In addition to the diaphragm, the external intercostal
muscles, and the parasternal intercartilaginous
In contrast to inspiration, expiration is passive during
quiet breathing as a result of the recoil of the lungs
and the chest wall. It becomes active at higher levels
of ventilation (exercise), or in pathological states when
expiratory resistance increases and the movement of
airflow out of the lungs is impeded.
Muscles involved in active expiration include the
internal intercostal muscles and the abdominal
muscles. Contractions of these muscles compress the
abdominal content, depress the lower ribs, and pull
down the anterior part of the lower chest (Figure 7).
In effect, they force the diaphragm upwards. They are
essential for several body functions: e.g. coughing,
singing, talking, vomiting.
Forced maximal contraction of the expiratory muscles