The Respiratory System
External and Internal Respiration
Learning Objectives
How gases move into and out of the body (external respiration).
How gases move within the body (internal respiration).
An understanding of partial pressure.
The relationship between partial pressure and movement of gases within the body.
Introduction
Introduction
The respiratory and cardiovascular system combine to provide an efficient delivery system.
Carries oxygen (O2) to body tissues.
Removes (CO2).
This transport system involves four separate process….
Transportation
Pulmonary ventilation (breathing).
Pulmonary diffusion.
Transport of oxygen and carbon dioxide via blood.
Capillary gas exchange.
Pulmonary Ventilation
Commonly referred to as breathing.
It is the process by which air moves in and out of the lungs.
Pulmonary Ventilation
Air is typically drawn into the lungs through the nose.
The mouth can be used.
Bring air in through the nose has advantages over mouth breathing.
Air is warmed and humidified.
Dust and other particles can be filtered out.
Pulmonary Ventilation (Transport Summary)
In through nose and mouth.
Down the pharynx.
Down the larynx.
Through the trachea.
Through the bronchi.
Through the bronchioles.
Then reaches the sites of gas exchange the alveoli.
Pulmonary Ventilation
Inspiration
Inspiration
Inspiration is…
An active process.
Involving the diaphragm and the external intercostals.
The ribs and sternum are moved by the external intercostals muscles.
The ribs swing up and out.
The sternum swings up and forward.
At the same time the diaphragm contracts.
Inspiration
These actions expand all three dimensions of the thoracic cage.
In tern this expands the lungs.
This action reduces the air pressure in the lungs (intrapulmonary pressure).
This is less than the pressure outside the body.
Air rushes into the lungs to reduce the pressure difference.
Thus air is brought into the lungs during inspiration.
Exercise and Inspiration
During forced or laboured breathing – as in exercise, inspiration is further assisted by,
Other muscles such as scalenes (anterior, middle and posterior).
Sternocleidomastoid (in the neck).
Pectorals.
These help raise the ribs even more than during regular breathing.
Inspiration
The pressure changes required for adequate ventilation at rest are really quite small. For example, at
standard atmospheric pressure (760mmHg), inspiration may decrease the pressure in the lungs
(intrapulmonary pressure) by only about 3mmHg. However, during maximal respiratory effort, such as
during exhaustive exercise, the intrapulmonary pressure may decrease by 80 – 100 mmHg.
Pulmonary Ventilation
Expiration
Expiration
Expiration is…
Usually a passive process.
Involves the relaxation of the inspiratory muscles and the recoil of the tissues.
The diaphragm relaxes.
The external intercostals relax.
The elastic nature of the lung tissue causes the lungs to recoil to its resting size.
Expiration
All this activity increases the pressure in the thorax.
So air is forced out.
Thus expiration is accomplished.
Expiration and Exercise
During exercise expiration becomes a more active process.
The internal intercostals muscles can actively pull the ribs down.
This can be assisted by the latissimus dorsi and quadratus lumborum muscles.
This increases the pressure on the diaphragm, causing faster contraction, therefore accelerating return.
Review of Pulmonary Respiration
Pulmonary ventilation (breathing) is the process by which air is moved into and out of the lungs. It has
two phases: inspiration and expiration.
Inspiration is an active process through which the diaphragm and the external intercostal muscles
increase the dimensions, and thus the volume, of the thoracic cage. This decreases the pressure in the lungs
and draws air in.
Review of Pulmonary Respiration
Normal expiration is a passive process. The inspiratory muscles relax and the elastic tissue of the lungs
recoil, retuning the thoracic cage to its smaller, normal dimensions. This increases the pressure in the
lungs and forces air out.
Forced or laboured inspiration and expiration are active processes, dependant on muscle action.
Pulmonary Diffusion
Gas Exchange in the Lungs
Pulmonary Diffusion
Air was brought into the lungs during pulmonary ventilation;
Gas exchange must occur between this air and the blood.
This process is known as pulmonary diffusion.
Pulmonary Diffusion
Pulmonary diffusion is the process by which gases are exchanged across the respiratory membrane in
the alveoli.
Functions of Pulmonary Diffusion
It replenishes the blood’s O2 supply.
It removes CO2 from returning venous blood.
The Process of Pulmonary Diffusion
Blood from most of the body returns through the ______ (shown in red) to pulmonary (______)
The Venae Cava and Pulmonary Side of the Heart
The Process of Pulmonary Diffusion
From the right ventricle, this blood is pumped through the pulmonary artery to the lungs, ultimately
working its way into the pulmonary capillaries.
These capillaries form a dense network around the alveolar sacs.
The vessels are small enough that the red blood cells can pass through but in single file.
The Respiratory Membrane
Gas exchange between the air and the alveoli and the blood in the pulmonary capillaries occurs across
the respiratory membrane.
The Respiratory Membrane
The Respiratory Membrane
Is composed of.
The alveolar wall,
The capillary wall, and,
Their basement membranes.
It is very thin, measuring only 0.5 to 4.0 Цm.
This membrane presents a barrier for gas exchange.
Lets now look how this gas exchange occurs.
Partial Pressure
The air we breathe is a mixture of gases.
Each exerts a pressure in proportion to its concentration in the gas mixture.
The individual pressures from each gas in a mixture is referred to as partial pressures.
Therefore as according to Dalton’s law, the total pressure of a mixture of gases equals the sum of the
partial pressures of the individual gases in the mixture.
Key Point
The total pressure of a mixture of gases equals the sum of the partial pressures of the individual gases in
that mixture.
The Air We Breathe
The air we breathe is composed of:
79.04% Nitrogen (N2).
20.93% Oxygen (O2).
0.03% Carbon Dioxide (CO2).
The standard atmospheric pressure is approximately.
760mmHg.
This is considered the total pressure of the three gases or 100%.
Partial Pressures of the Gases We Breathe
Nitrogen.
% of air = 79.04%.
79.04% of total atmospheric pressure.
(0.7904 x 760mmHg).
 = 600.7mmHg.
PN2 = 600.7mmHg.
Partial Pressures of the Gases We Breathe
Oxygen:
% of air = 20.93%.
20.93% of total atmospheric pressure.
(0.2093 x 760mmHg).
= 159.0mmHg.
PO = 159.0mmHg.
2
Partial Pressures of the Gases We Breathe
Carbon Dioxide:
% of air = 0.03%.
0.03% of total atmospheric pressure.
(0.003 x 760mmHg).
= 0.3mmHg.
PCO2 = 0.3mmHg.
Important Point
Gases in our bodies are dissolved in fluids, such as blood plasma.
According to Henry’s law,
gases dissolve in liquids in proportion to their partial pressures, depending also on their solubilities in the
specific fluids and on the temperature.
Important Point
A gas’s solubility in blood is constant, and blood temperature is relatively constant.
Thus the most critical factor for gas exchange between the alveoli and blood is:
The pressure gradient between the gases in the two areas.
Gas Exchange in the Alveoli
Differences in the partial pressure of the gases in the alveoli and the gases in the blood create a pressure
gradient across the respiratory membrane.
This forms the basis of gases exchange during pulmonary diffusion.
What would happen if the pressures were equal?
Key Point
The greater the pressure gradient across the respiratory membrane, the more rapidly oxygen will diffuse
across it.
Gas Exchange in the Alveoli
Oxygen Exchange
PO2 = 159mmHg.
This drops to 100 –105mmHg when air is inhaled and enters the alveoli.
The blood, stripped of much of its O2 enters the pulmonary capillaries with a PO2 = 40 –45mmHg.
This is about 55 mmHg less than PO2 in the alveoli.
It is this pressure gradient that drives the O2 from the alveoli into the blood.
Oxygen Exchange
Oxygen Exchange
The rate in which O2 diffuses from the alveoli into the blood is referred to as the oxygen diffusion
capacity.
At rest approx 23 ml of O2 diffuses per minute.
During maximal exercise this can raise to 45 ml in the untrained athlete and 80 ml in the trained athlete.
Training and O2 Diffusion Capacity
Athletes with large aerobic capacities often also have greater oxygen diffusion capacities.
This is likely the combined result of:
Increased cardiac output.
Increased alveolar surface area, and,
Reduced resistance to diffusion across respiratory membranes.
Carbon Dioxide Exchange
CO2 exchange, like O2 exchange, moves along a pressure gradient.
Blood passing through the.
Alveoli has PCO2 of about 45mmHg.
The alveoli air has PCO2 = 40mmHg.
This results is a relatively small pressure gradient of about 5mmHg.
Is this a problem? How do we over come this problem?
Pulmonary Diffusion Review
Pulmonary diffusion is the process by which gases are exchanged across the respiratory membrane in the
alveoli.
Pulmonary Diffusion Review
The amount of gas exchange that occurs across a membrane primarily depends on the partial pressure of
each gas, though gas solubility and temperature are also important. Gases diffuse along a pressure gradient,
moving from an area of higher pressure to one of lower pressure. Thus oxygen enters the blood and carbon
dioxide leaves it.
Pulmonary Diffusion Review
Oxygen diffusion capacity increases as you move from rest to exercise. When your body needs more
oxygen, oxygen exchange is facilitated.
The pressure gradient for carbon dioxide exchange is less than for oxygen exchange, but carbon dioxide’s
membrane solubility is 20 times greater than that of oxygen, so carbon dioxide crosses the membrane
easily, even without a large pressure gradient.
Transport of Oxygen and Carbon Dioxide
How Gases Are Transported
Transport of Oxygen and Carbon Dioxide
Now we have considered how we bring air into our lungs via pulmonary ventilation and how gas
exchange occurs via pulmonary diffusion. Next we must consider how gases are transported in our blood
to deliver the oxygen to the tissues and to remove the carbon dioxide that the tissues produce. We will
consider separately the transport of each gas.
Oxygen Transport
O2 is transported by the blood either,
 Combined with haemoglobin (Hb) in the red blood cells (>98%) or,
Dissolved in the blood plasma (<2%).
Oxygen Transport
Only about 3 ml of O2 are dissolved in each litre of plasma.
Assuming we have a total plasma volume of 3 to 5 litres, only about 9 – 15 ml of O2 can be carried in the
dissolved state.
Oxygen Transport
This is not enough to supply even the resting body, which requires 250ml per minute.
Fortunately we have four to six billion haemoglobin containing red blood cells.
The haemoglobin allows nearly 70 times more O2 than dissolved in plasma.
Haemoglobin Saturation
Haemoglobin saturation is the amount of oxygen bound by each molecule of haemoglobin
Haemoglobin Saturation
Each molecule of haemoglobin can carry four molecules of O2.
When oxygen binds to haemoglobin, it forms OXYHAEMOGLOBIN;
Haemoglobin that is not bound to oxygen is referred to as DEOXYHAEMOGLOBIN.
Show the haemoglobin.
Haemoglobin Saturation
The binding of O2 to haemoglobin depends on the PO2 in the blood and the bonding strength, or affinity,
between haemoglobin and oxygen.
The graph on the following page shows an oxygen dissociation curve, which reveals the amount of
haemoglobin saturation at different PO2 values.
Dissociation Curve
Reveals the amount of haemoglobin saturation at different PO values.
2
Haemoglobin Saturation
A high blood PO2 results in almost complete haemoglobin saturation, which means the maximum amount
of oxygen is bound.
But as the PO2 is reduced, so is haemoglobin saturation.
Factors Affecting Haemoglobin Saturation
Blood acidity…
Blood temperature…
Factors Affecting Haemoglobin Saturation – Blood Acidity
If the blood becomes more acidic the dissociation curve shifts right.
This means that more oxygen is being uploaded from the haemoglobin at tissue level.
See overhead.
Factors Affecting Haemoglobin Saturation – Blood Acidity
Look at the overhead.
The rightward shift of the curve is due to a decline in pH. This is referred to as the BOHR effect.
So why is this important to us?????
Factors Affecting Haemoglobin Saturation – Blood Acidity
The pH in the lungs is generally high.
What does this mean?
So haemoglobin passing through the lungs has a strong affinity for oxygen, encouraging high saturation.
At the tissue level, however the pH is lower, causing oxygen to dissociate from haemoglobin, thereby
supplying oxygen to the tissues.
Factors Affecting Haemoglobin Saturation – Blood Acidity
With exercise, the ability to upload oxygen to the muscles increases as the muscle ph decreases.
Factors Affecting Haemoglobin Saturation – Blood Temperature
Look at overhead.
Increased blood temperature shifts the dissociation curve to the right, indicating that oxygen is uploaded
more efficiently.
Factors Affecting Haemoglobin Saturation – Blood Temperature
Because of this, the haemoglobin will upload more oxygen when blood circulates through the
metabolically heated active muscles.
In the lungs, where the blood might be a bit cooler, haemoglobin’s affinity for oxygen is increased. This
encourages oxygen binding.
Key Point
Increased temperature and hydrogen ion (H+) (pH) concentration in exercising muscle affect the oxygen
dissociation curve, allowing more oxygen to be uploaded to supply the active muscles.
Carbon Dioxide Transport
Carbon dioxide also relies on the blood fro transportation. Once carbon dioxide is released from the
cells, it is carried in the blood primarily in three ways…
Dissolved in plasma,
As bicarbonate ions resulting from the dissociation of carbonic acid,
Bound to haemoglobin.
Dissolved Carbon Dioxide
Part of the carbon dioxide released from the tissues is dissolved in plasma. But only a small amount,
typically just 7 – 10%, is transported this way.
This dissolved carbon dioxide comes out of solution where the PCO 2 is low, such as in the lungs.
There it diffuses out of the capillaries into the alveoli to be exhaled.
Bicarbonate Ions
The majority of carbon dioxide ions is carried in the form of bicarbonate ion.
60 - 70% of all carbon dioxide in the blood.
The following bit is quite heavy just listen hard.
Bicarbonate Ions
Carbon Dioxide and water molecules combine to form carbonic acid (H2CO3).
This acid is unstable and quickly dissociates, freeing a hydrogen ion (H +) and forming a bicarbonate ion
(HCO3-):
CO2 + H2O
H2CO3
CO2 + H2O
Bicarbonate Ions
The H+ subsequently binds to haemoglobin and this binding triggers the BOHR effect (mentioned
earlier).
This shifts the oxygen-haemoglobin dissociation curve to the right.
Thus formation of bicarbonate ion enhances oxygen uploading.
Bicarbonate Ions
This also plays a buffering as the H+ is neutralised therefore preventing any acidification of the blood.
When blood enters the lungs, where the PCO2 is lower, the H+ and bicarbonate ions rejoin to form
carbonic acid, which then splits into carbon dioxide and water.
In other words the carbon dioxide is re-formed and can enter the alveoli and then be exhaled.
Key Point
The majority of carbon dioxide produced by the active muscles is transported back to the lungs in the
form of bicarbonate ions.
Carbaminohaemoglobin
CO2 transport also can occur when the gas binds with haemoglobin, forming a compound called
Carbaminohaemoglobin.
It is named so because CO2binds with the amino acids in the globin part of the haemoglobin, rather than
the haeme group oxygen does.
In Review
Oxygen is transported in the blood primarily bound to haemoglobin though a small amount is dissolved
in blood plasma.
Haemoglobin oxygen saturation decreases.
When PO2 decreases.
When pH decreases.
When temperature increases.
In Review
Each of these conditions can reflect increased local oxygen demand. They increase oxygen
uploading in the needy area.
3) Haemoglobin is usually about 98% saturated with oxygen. This reflects a much higher oxygen content
than our body requires, so the blood’s oxygen-carrying capacity seldom limits performance.
In Review
4) Carbon dioxide is transported in the blood primarily as bicarbonate ion. This prevents the formation of
carbonic acid, which can cause H+ to accumulate, decreasing the pH. Smaller amounts of carbon dioxide
are carried either dissolved in the plasma or bound to haemoglobin
Gas Exchange at the Muscles
Gas Exchange at the Muscles
Now we have considered how our respiratory and cardio-vascular system brings air into our lungs,
exchange oxygen and carbon dioxide in the alveoli, and transport oxygen to the muscles (and CO2 away
from them).
All that remains is for us to consider the delivery of oxygen to the muscles from the capillary blood.
This gas exchange between the tissue and the blood in the capillaries is our fourth and final step in gas
transportation – internal respiration.
The Arterial-venous Oxygen Difference
At rest, the oxygen content of arterial blood is about 20ml of oxygen per 100 ml of blood.
See over head.
The value drops to 15 or 16ml of oxygen per 100ml as the blood passes through the capillaries into the
venous system.
This difference in oxygen content between arterial and venous blood is referred to as the arterial-venous
oxygen difference (a-VO2diff).
The Arterial-venous Oxygen Difference
It reflects the 4-5 ml of oxygen per 100 ml of blood taken up by the tissues.
The amount of oxygen taken up is proportional to its use for oxidative energy production. Thus as the
rate of oxygen use increases, the a-vO2 diff also increases.
The Arterial-venous Oxygen Difference
E.g. during intense exercise (figure see OHP) the a-vO2 diff in contracting muscles can increase to 15 to
16 ml per 100ml of blood. During such an effort, the blood unloads more oxygen to the active muscles
because the PO2 in the muscles is drastically lower than in arterial blood.
Key Point
The a-vO2 diff increases from a resting value of about 4 to 5 ml per 100 ml of blood up to values of 15 to
16 ml per 100 ml of blood during exercise. This increase reflects an increase extraction of oxygen from
arterial blood by active muscle, thus decreasing the oxygen content of the venous blood.
Factors Influencing Oxygen Delivery and Uptake.
The rates of oxygen delivery and uptake depend on the three major variables.
The oxygen content of blood.
The amount of blood flow.
The local conditions.
As we begin to exercise, each of these variables must be adjusted to ensure increased oxygen delivery to
our active muscles.
Factors Influencing Oxygen Delivery and Uptake.
We have discussed in class that under normal circumstances haemoglobin is 98% saturated with oxygen.
Any reduction in the blood’s normal oxygen carrying capacity would hinder oxygen delivery and reduce
cellular uptake of oxygen.
Factors Influencing Oxygen Delivery and Uptake.
Exercise causes increased blood flow through the muscles. As more blood carries oxygen through the
muscles, less oxygen must be removed from each 100 ml of blood (assuming the demand remains
unchanged).
Thus increasing blood flow improves oxygen delivery and uptake.
Factors Influencing Oxygen Delivery and Uptake.
Many local changes in the muscle during exercise affect oxygen delivery and uptake.
Muscle activity increases muscle acidity because of lactate production.
Muscle temperature and carbon dioxide concentration both increase because of increased metabolism.
All of these increase oxygen uploading from haemoglobin molecule, facilitating oxygen delivery and
uptake by the muscles.
Factors Influencing Oxygen Delivery and Uptake.
During maximal exercise,however, when we push our bodies to the limit, changes in any of these areas
can impair oxygen delivery and restrict out abilities to meet oxidative demands.
Carbon Dioxide Removal
Carbon dioxide exits the cells by simple diffusion in response to the partial pressure gradient between the
tissue and the capillary blood.
E.g. Muscles generate carbon dioxide through oxidative metabolism, so the PCO 2 in muscles would be
relatively high compared to that in the capillary blood. Consequently, CO 2 diffuses out of the muscles and
into the blood to be transported to the lungs.