Respiratory System Physiology

Pulmonary ventilation

moving air into and out
of the lungs

External respiration

gas exchange between the
lungs and the blood

Gas transport

transport of oxygen and carbon
dioxide between the lungs and tissues

Internal respiration

gas exchange between
systemic blood vessels and tissues

Pulmonary
ventilation

… is the physical process of moving air into and out
of the lungs. Pulmonary ventilation relies on the fact that air
will flow from areas of high pressure to areas of low pressure!
Let’s look at the pressures involved:

Atmospheric pressure (pATM)

pressure exerted by
the weight of the air surrounding the body at Earth’s
surface (760 mm Hg at sea level). This is the pressure of
the air outside our bodies

Alveolar pressure (or intrapulmonary pressure)

pressure within the alveoli. This pressure can be equal to
atmospheric pressure (~760mmHg), but it will also rise
above and fall below atmospheric pressure

Negative respiratory pressure

is less than pATM

Positive respiratory pressure

is greater than pATM

Intrapleural pressure (or intrathoracic pressure)

pressure within the intrapleural cavity. This
pressure is always below atmospheric and alveolar
pressure (~756mmHg when the lungs are at rest). At
rest, the intrapleural pressure is 4 mmHg lower than
pATM. It is, therefore, referred to as a -4mmHg
pressure. Intrapleural pressure exists because the
lungs always want to recoil away from the thoracic
wall and the thoracic wall always wants to expand
away from the lungs; this increases the size of this
space (ever so slightly), dropping the pressure below
atmospheric pressure.

Intrapleural fluid cohesiveness

the strong cohesion of water
molecules result in wet surfaces typically sticking together. Within
the pleural cavity, this means that the wet pleural membranes are
strongly attracted to one another. (Remember: one pleural
membrane is found on the lungs and one is found on the inner
thoracic wall). This helps to keep the lungs adhered to the body
wall.

Transmural pressure gradient

the atmospheric pressure
(760mmHg) is greater than the intrapleural pressure (756mmHg).
This stretches the lungs out towards the thoracic wall.

Boyle’s law

states that pressure and volume are inversely related at any
constant temperature. If the volume of a gas were to increase,
therefore, the pressure of the gas would go down proportionately.
On the other hand, if the volume of a gas were to decrease, the
pressure of the gas would go up proportionately.

The equation for Boyle’s law is:

P1V1=P2V2

Why is Boyle’s law important?

Boyle’s law reminds us of the
intimate relationship between pressure and volume. We can
change the pressure of an environment by changing its volume.
In particular, we can change the pressure of the gas inside our
lungs by changing the volume of our lungs. In order to
understand ventilation, therefore, what we must really
understand is how we change the volume of our lung

inhalation

is the active process of bringing air into the lungs. It occurs when the
phrenic nerve stimulates the diaphragm to contract and lower & when the
intercostal nerves stimulate the external intercostals to contract and the rib cage
to rise. Together, the movement of these two muscles increases the volume of
the thoracic cavity. As the volume of the thoracic cavity expands, the lung
volume increases as well.

Exhalation

by contrast, is a passive process. It occurs when the
diaphragm and external intercostals relax, reducing the volume of the chest
(rib cage descends due to the force of gravity and elasticity of tissue and the
diaphragm resumes its natural dome shape). The lungs readily rebound to
their smaller size when the muscles are relaxed due to elastic recoil.
Because the volume in the lungs is now smaller, the pressure of the air in
the lungs goes up.

Active breathing

is voluntary breathing
or ventilation above ~30-40 breaths per minute. When our
breathing rate increases substantially, this requires the use of the
internal intercostals and accessory respiratory muscles (in the
thorax and abdomen). Engaging these additional muscles
increases the volume in the thoracic cavity during inhalation
and, therefore, increases the amount of air moving into the lungs
during inhalation. We also have muscles that increase the force
of our exhalation.

Bronchoconstriction

increases resistance in response to parasympathetic
stimulation by promoting bronchiolar smooth muscle contraction (decreases
air flow)

Bronchodilation

in response to sympathetic stimulation decreases resistance
by promoting bronchiolar smooth muscle relaxation (increases air flow)

Compliance

describes the
lungs ability to stretch. The
more stretchy the lungs, the
easier it is to inflate them.

High compliance

means the
lungs stretch easily and
ventilation can occur
normally.

Low compliance

means the
lungs are difficult to stretch
and inhalation is impaired.

Pulmonary surfactant

decreases the work of breathing by decreasing
the surface tension in the lungs. This makes the lungs easier to stretch
and inflate. The surface tension is created by water in the alveoli and is
directed toward the center of the alveolus. (Surface tension is the
attraction of liquid molecules to one another at a liquid-gas interface.)
The fluid in the lungs, therefore, is always acting to reduce the size of
alveoli. Surfactant counteracts this, disrupting the surface tension and
making it easier to inflate the lungs.

Elastic recoil

describes how readily the lungs rebound after being stretched. Elastic
recoil of the lungs is largely due to the large quantity of elastic fibers in
the pulmonary connective tissue. The more elastic recoil the lungs
have, the easier it is for the lungs to deflate during exhalation.

spirometer

Measurements of lung function can be made by a
… and shown with a spirogram.

Tidal volume

the normal amount of air entering or leaving
the lungs each breath cycle (~500mL)

Inspiratory reserve volume

amount of air that can be
inhaled in addition to TV (~3000mL)

Inspiratory capacity

maximum about of air that can be
inspired at the end of a normal quiet exhalation (TV + IRV)
(~3500mL

Expiratory reserve volume

additional air one can
exhale after the TV (~1000mL)

Vital capacity

represents the largest amount of air that
can be moved into or out of the lungs (TV + IRV + ERV)
(~4500mL)

Residual volume

air left in the lungs after strenuous expiration (~1200mL)

Functional residual volume

amount of air remaining in the lungs
after a tidal exhalation (RV + ERV) (~2200mL

Total lung capacity

sum of vital capacity and residual capacity
(~5700mL)

Forced expiratory volume in one second

volume of air that can be
exhaled during the first second of expiration (Normally ~80% of VC)

anatomical dead space

is the volume of air in the conducting division
(~150mL). The air in the conducting division cannot do gas exchange. If we
really want to understand how much air is doing gas exchange each minute, we
have to take into account this dead space.

Alveolar ventilation

is equal to the
amount of air inhaled minus the anatomical dead space times the ventilation
rate. This calculation gives us a good estimate of the amount of air that is
performing gas exchange each minute.

total dead space

The … is the sum of the alveolar and anatomical dead spaces.

alveolar dead space

We also have …. These are alveoli that have collapsed or
become obstructed and are, therefore, no longer capable of gas exchange.
The total dead space is the sum of the alveolar and anatomical dead spaces.

respiratory membrane

Gas exchange occurs as gases move down their pressure gradients
across the … and capillary walls. In other
words, gases move from areas of high pressure to areas of low
pressure.

Dalton’s Law

tells us that the total pressure exerted by a mixture
of gases is the sum of the pressures exerted independently by each
gas in the mixture.

partial pressure.

he pressure exerted by each individual gas,
therefore, is a part of the total pressure in the system and is called a
….

Dalton’s Law

tells us that the total pressure exerted by a
mixture of gases is the sum of the partial pressures exerted
independently by each gas in the mixture. The partial pressure of
a gas can, therefore, be calculated if we know the total pressure
of the system and each gas’s relative percentage.

Henry’s Law.

This is called ….
Henry’s law states that when a mixture of gases is in contact with a
liquid (like blood), each gas will dissolve in the liquid in
proportion to its partial pressure.

permeability

Gas exchange is affected by the… of the membrane to
the particles that are diffusing.

Surface area

for gas exchange – if there is more available
surface area for gas exchange, it will happen faster and in
greater amounts. How do we increase surface area in the
lungs? During resting conditions, many pulmonary capillaries
are closed. During exercise and stressful situations, some of
these capillaries will open. This increases the surface area
available for exchange

Thickness

of gas exchange membranes – diffusion happens
best across short distances, so thicker membranes will slow
gas exchange. The thickness of our respiratory membrane and
capillaries should be constant; however, it can increase with
disease.

Hemoglobin

iron ions at the center of its four heme units bind to oxygen
(while its protein units bind to carbon dioxide). Most of the oxygen in the blood
is bound to the hemoglobin (98%). Only a small amount is dissolved in the
plasma. The amount of oxygen bound to hemoglobin depends primarily on the
O2 concentration of the air and the binding sites available on hemoglobin.

saturation curve

The amount of oxygen bound to hemoglobin can be plotted on a
saturation curve. The… for oxygen shows that at
100 mmHg (the partial pressure of oxygen in the lungs), 98% of
hemoglobin are fully saturated after gas exchange occurs. In other
words, 98% of hemoglobin are attached to 4 oxygen molecules!!!
There is incredibly efficient loading of hemoglobin with oxygen at
the lungs under normal body conditions.

2,3-BPG

is another substance
that can cause the saturation curve to shift. … is
made in glycolysis pathways inside the red blood cells
when the red blood cell is deprived of oxygen….
binds to hemoglobin inside the red blood cell decreasing
its affinity for oxygen.

medulla oblongata

The … establishes the basic
rhythm of breathing by providing nervous output to the respiratory
muscles. It is composed of two nuclei

DRG

inspiratory center consisting of inspiratory
neurons that send out signals to respiratory muscles associated with
inhalation. When the neurons of the DRG stop firing, the respiratory muscles
relax and exhalation occurs

VRG

active during forced breathing. It contains
both inspiratory and expiratory neurons for active forced breathing. It also
contains the Pre-Botzinger complex. These neurons display pacemaker
potentials and are believed to drive the firing rate of the DRG.

pons

The … is believed to exert fine tuning influences
over the centers in the medulla oblongata.

pneumotaxic center

The
… contains nuclei that inhibit
inhalation and adjust the respiratory rate by decreasing
inhalation. This allows for exhalation.

apneustic
center

The … contains nuclei to promote inhalation. It
accomplishes this by inhibiting the pneumotaxic center
and preventing the inspiratory neurons of the DRG
from being turned off.

stretch reflex

The … relies on
receptors in the respiratory
system that respond to
stretch. Upon inflation
over 1 liter, these receptors
are stimulates and result in
inhibitory messages to the
medulla.

Chemoreceptors

in
some blood vessels
and the CSF monitor
the chemical changes
in the blood.

Central
chemoreceptors

monitor the CSF
chemicals. They
respond strongly to
hydrogen ion level
changes in the CSF.
These are far more
powerful than the
peripheral
chemoreceptors.

Peripheral
chemoreceptors

monitor the blood.
They respond to pH and
CO2 changes in the
blood. They also weakly
respond to oxygen. You
might think changing
oxygen levels in the
body would cause a big
change in respiration
rates, but that is not so.
Peripheral
chemoreceptors require
huge drops in O2 levels
in order to be stimulated
to change the respiration
rate.

So, what does drive respiratory changes?

Carbon
dioxide is the primary driver of depth and rate of
breathing. High pCO2 levels increase the depth and
rate of breath. Low pCO2 levels decrease the depth and
rate of breath.

pH

can also affect ventilation rates. As pH falls, the
ventilation rate and depth increase. As pH rises, the
ventilation rate and depth decrease.