Respiration, Breathing
Mechanics and Lung Function
Learning Objectives
• Know the basic anatomy of the pulmonary airways and
circulation, and the pleural space.
• Understand the basic autonomic and local control of
airways.
• Know the mechanics and pressure changes of inspiration
and expiration.
• Understand pulmonary elastic recoil and compliance.
• Understand alveolar surface tension and pulmonary
surfactants.
• Understand the basic pulmonary volumes and
capacitances.
• Know what the rate of alveolar ventilation is and how to
calculate it.
Lung Anatomy
Lung Anatomy - Airways
Airways
• Cartilage rings keep the trachea
open. These rings are less
extensive in the bronchi and are
gone in the bronchioles.
• Smooth muscle occupies areas that
are not cartilage in the trachea and
bronchi. Bronchioles are almost
entirely smooth muscle, except the
respiratory bronchioles.
• Many obstructive diseases of the
lungs result from narrowing of the
smaller bronchi and larger
bronchioles.
Airway Resistance
• Most of the resistance is in
the larger bronchioles and
bronchi near the trachea
(due to small numbers of
these airways).
• In disease, the resistance of
bronchioles is a bigger
concern, because these are
easily occluded by muscle
contractions in their walls,
edema, or mucus collecting.
Autonomic Control of Airways
• Sympathetic – norepinephrine and
epinephrine relax the trachea and bronchi.
Direct control of bronchioles by sympathetic
fibers is weak. Because of beta-adrenergic
receptors, epinephrine has a greater effect.
• Parasympathetic - acetylcholine causes mildto-moderate bronchiolar constriction.
Local Control
• Histamine causes bronchiolar constriction and
is released during an allergic reaction.
• What do you thing the effects are of the
typical vasodilators, such as blood levels of O2
and CO2? What do these substances do in the
lungs during exercise?
Lung Anatomy - Circulation
Note: Pulmonary arteries carry deoxygenated blood and pulmonary veins carry
oxygenated blood
Pulmonary Vessels
• Pulmonary arteries, compared to systemic
arteries, are thin, short, and have larger
diameters. This gives them a high compliance
and can accommodate the stroke volume of the
right ventricle.
• Pulmonary veins are short and immediately
empty their effluent into the left atrium.
• Bronchial vessels supply the blood needed to
maintain the lungs (oxygenated) and originate
from the systemic circulation. It empties into the
pulmonary veins.
Pulmonary Blood Pressure
• As you might expect
from short, highly
compliant vessels, the
pressures in the
pulmonary system are
low, compared to the
pressures in the
systemic vasculature.
Pleural Sacs
• The intrapleural fluid normally
consists of a very thin layer.
• The pleural pressure is slightly
negative. Thus, the lungs are
held to the thoracic cavity, but
in a fluid so that they can slide
freely as the chest expands
and contracts.
• Imagine the water between
glass microscope slides. The
glass slides are lubricated, but
hart to pull apart.
Contraction and Expansion of the
Thoracic Cage
Lungs are contracted or expanded in 2 ways:
1. Downward and upward movement of diaphragm (major force during normal, quiet breathing.
2. Elevation and depression of the ribs, using abdominal and rib cage (intercostal) muscles.
Contraction and Expansion of the
Thoracic Cage
Expiration – Diaphragm relaxes and moves up. Rib cage is pulled downward and the chest wall
and abdominal structures compress the lungs.
Inspiration – Diaphragm contracts and moves down. Rib cage is pulled upward and expanded.
Pressures Causing Ventilation
• As with blood, air moves by bulk flow from high
pressure to low pressure. What equation?
• Two pressures important in ventilation are:
- Alveolar pressure – the pressure difference between the inside of
the alveoli and the atm (atm P is the zero reference).
-Transpulmonary pressure – the pressure difference between the
inside of the alveoli and the pleural pressure.
• Air flows into the alveoli when the atm pressure
is greater than the alveolar pressure. Changes
pressure are achieved by changing the volume.
Pleural Pressure Changes
• During inspiration, expansion of the chest
decreases the pleural pressure (by decreasing
the volume).
• This changes the transpulmonary pressure
(alveolar pressure – pleural pressure).
• The alveolar pressure then decreases below
atm pressure and air flows into the lungs
(images on next slide).
Pressure Changes During Normal
Breathing
Elastic Recoil
• Elastic recoil is the tendency of an elastic
structure to oppose stretching.
• The lungs naturally have a tendency to
collapse because of elastic recoil. They are
held open by the negative pleural pressure
(established by lymphatic pumping of fluid).
• The chest wall naturally expands, but is also
held by the negative pleural pressure.
Punctured Lung
• What would happen if the visceral pleura was
punctured, but the lung was not damaged?
• Air would rush into the intrapleural space and the
lung would collapse – a pneumothorax.
Lung Compliance
• Compliance is the magnitude of the change in
volume produced by a given change in
pressure.
• The greater the lung compliance, the easier it
is to expand the lung at any given change in
transpulmonary pressure.
Lung Compliance Diagram
Note the elasticity of the air-filled lung.
Elastic Forces of the Lungs
• Characteristics of the compliance diagrams are
determined by 2 elastic forces:
- Elastic properties of the lung tissue itself (fairly
straightforward).
- Elastic forces caused by the surface tension of the fluid
that lines the inside walls of the alveoli.
• Looking at Fig 37-4, one can see that surface
tension forces in the alveoli represent ~ 2/3 of
the elastic forces tending to cause the lung to
collapse.
Surface Tension
• H2O molecules at the interface with air are
attracted to each other. This causes the H2O
at the surface to contract (collapsing the
alveoli).
• The pressure caused by surface tension can be
calculated from
Surfactants
• Surfactants in water decrease the surface tension
and are secreted by type II alveolar epithelial
cells.
• The body’s surfactant is a complex mixture of
phospholipids, proteins, and ions.
• Surfactant is partially dissolved in the water,
whereas the rest spreads over the water surface.
• Surfactants can decrease the surface tension by
up to 10-fold.
Surfactants in Neonates
• Premature babies often have alveoli with
diameters less than ¼ that of an adult (note effect
of pressure equation).
• Moreover, surfactant is not secreted into the
alveoli until between the 6th and 7th month of
gestation.
• Thus, the lungs of premature babies are more
likely to collapse.
• The condition respiratory distress syndrome of
the newborn is a concern in premature babies.
Pulmonary Volumes
• Tidal Volume – normal volume
of air expired or inspired each
normal breath.
• Inspiratory Reserve – the extra
volume aboe the tidal volume.
• Expiratory Reserve – The
maximum volume of air that
can be expired.
• Residual Volume – The volume
of air remaining in the lungs
after maximum expiration.
Pulmonary Capacities
• Inspiratory capacity is the tidal
volume + the inspiratory
reserve.
• Functional residual capacity is
the expiratory reserve +
residual volume.
• Vital capacity is the inspiratory
reserve + tidal volume +
expiratory reserve.
• Total lung capacity is the vital
capacity + residual volume.
Minute Respiratory Volume
• This is the total amount of new air moved into
the respiratory passages each minute.
• It equals the tidal volume x respiratory
rate/min.
• Normally, this is ~ 500 ml x 12 breaths/min ~
6L/min.
Alveolar Ventilation
• The key area to bring new air in is to the
alveoli, where gas exchange occurs with the
pulmonary blood.
• Air that does not reach the gas exchange areas
is called “dead space air”.
• The normal amount of dead space air in a
young adult male ~ 150 ml.
Alveolar Ventilation
• Alveolar ventilation per minute is determined as
follows:
• Where,
VA = vol of alveolar ventilation/min
Freq = rate of respiration/min
VT = Tidal vol
VO = dead space vol