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Chapter 23
Respiratory System
Respiration
• Ventilation: Movement of air into and out of
lungs
• External respiration: Gas exchange between
air in lungs and blood
• Transport of oxygen and carbon dioxide in
the blood
• Internal respiration: Gas exchange between
the blood and tissues
The primary function of respiration is to deliver oxygen to
the tissues and bring back the a metabolic product,
carbon dioxide, to the lungs for exchange with oxygen.
The ratio, CO2/O2, depends upon what kinds of foods
are consumed as the energy source, and is called the
respiratory quotient (RQ).
On a mixed diet the RQ is about 0.8. For carbohydrate
1.0, for fat 0.7 and for protein 0.8.
Respiratory System Functions
• Gas exchange: Oxygen enters blood and carbon
dioxide leaves
• Regulation of blood pH: Altered by changing
blood carbon dioxide levels
• Voice production: Movement of air past vocal
folds makes sound and speech
• Olfaction: Smell occurs when airborne molecules
drawn into nasal cavity
• Protection: Against microorganisms by preventing
entry and removing them
1.
Pulmonary ventilation
The purpose of pulmonary ventilation is to
exchange gases in the alveoli.
One single respiratory cycle consists of
inspiration and expiration.
Fig 23-16 Pressure change during respiration
a.
Pressure and airflow to the lungs
Size of the thoracic cavities, where the lungs are held, is
determined by the position of the ribs and abdominal diaphragm.
The outer membrane of the lungs, visceral pleura, adheres to
parietal pleura with pleural fluid.
These membranes are kept in contact with the fluid while the lungs
expand and shrink. (Fig. 23-16).
The flows of air into the lungs and out from the lungs are
determined by changing the volume of thoracic cavity, thus creating
negative and positive pressures in the lungs relative to the outside
pressure.
In order to keep the lungs in contact with the thoracic wall, the fluid
in the pleural cavity must be maintain closed.
a.
Modes of breathing
Inhalation, while quiet breathing, involves
contractions of the diaphragm and the external
intercostals.
Exhalation is passive.
In forced breathing both inhalation and
exhalation are active.
a.
Respiration volumes and rates
The tidal volume is the amount of air moved into
or out of the lungs during on respiratory cycle.
During quiet breathing only a small fraction of
the air in the lungs is exchanged. - about 500
ml.
The total lung capacity may be divided into 4
major fractions. Fig. 23-18.
Fig 23-18 Respiratory volumes and capacities
1.
The tidal volume. Volume of air inspired or expired during
a normal inspiration or expiration - about 500 ml.
2.
Inspiratory reserve volume (IRV). The amount of air that
can be taken in over and above the tidal volume - males 3,300 ml
and females 1,900 ml.
3.
Expiratory reserve volume (ERV). The amount of air that
could be voluntarily expelled at the end of a tidal cycle - about
1,100 ml.
4.
ml.
Residual volume - After the expiratory volume - about 1,200
By combining more than two of these volumes, pulmonary
capacities are formed.
1.
Gas exchange at the respiratory membrane
Diffusive exchange of gases takes place
through respiratory membranes.
a.
Alveolar
ducts and alveoli
Study Fig. 23-10
for alveolar ducts,
alveolar sacs, and
alveoli.
The surface area of alveolar and alveoli, where gas exchanges
takes, is large and amounts up to 140 sq ft.
The alveolar epithelium consists of thin squamous cells. Fig. 2312.
Alveolar macrophages move around on the surface of the
epithelium to combat diseases.
Surfactant is released from the surfactant cells, which are
imbedded with the epithelium.
Surfactant reduces the surface tension of the epithelial cells and
keep alveoli inflated. Without the surfactant the alveoli will
collapse.
Furthermore, without surfactant, a smaller alveoli joined to a larger
alveoli will collapse and join the larger alveoli.
Fig 23-12 Alveolar organization
a.
The respiratory membrane
Exchange of gases is done from the side of
higher content to that of the lower content of
each gas through the respiratory membranes,
which consist of three layers. (Fig. 23-12b,c).
1.
The squamous epithelial cells
2.
The endothelial cells of capillaries
3.
The fused basement membranes between them.
The total thickness is about 0.1 um.
Thus, lipid soluble oxygen and carbon dioxide pass through the
membrane quickly.
Although the passage of blood going through alveolus capillaries is
fast, it is assumed that the gases in the blood coming out from the
alveolus capillaries are in equilibrium with those in the alveoli.
Pneumonia by Pneumocystis carinii commonly found in alveoli.
a. Mixed gases and partial pressures
Composition of normal air is:
20.8%
78.6%
0.5%
0.04%
Trace
Oxygen
Nitrogen
Water (This could change)
Carbon dioxide
Helium, hydrogen, etc.
The total pressure at sea level is 760 mmHg. The gravitational pull of all
the gases in the atmosphere down at the sea level amounts this much.
Since 20.8% of the air is oxygen, the contribution by oxygen as its partial
pressure is,
20.8 X 760 = 158.08 mmHg (or torr)
Thus, the partial pressure of oxygen, PO2, is 158.08 mmHg. The partial
pressures for other gasses may be calculated in a similar manner.
Also, the sum of all partial pressures will be 760 mmHg. If there is
moisture in the air, the percentage of other gases will be decreased by
the amount of pressure by water vapor, but the total pressure will not
change.
Thus, if the partial pressure of water is 40 torr, the total pressure of the
rest of gases will have pressure of only 720 torr (760 - 40 = 40).
a.
Alveolar versus atmospheric air
As the air gets into the respiratory system, the
composition of gases will change. The air will
get warmer and moist. More carbon dioxide
content will increase. See Fig. 23-20 for the
level of gases in different parts of the
respiratory system.
Fig. 23-20 Gas exchange
Partial pressures within the circulatory
system
Study Fig. 23-20 for the partial pressures of
oxygen and carbon dioxide in the circulatory
system of a resting human.
Gas pick up and delivery
Solubility of both oxygen and carbon dioxide are
relatively small in plasma. However, the total
contents of these two gases in plasma are
significantly increased physiologically.
Changes in Partial Pressures
PARTIAL PRESSURE OF GASES
Gas Air
Alveolar
N2
597
569
O2
159
104
H2O
3.7
47
CO2
0.3
40
Tissue, Art.
95
47
40
Tissue, Vein
40
47
45
1.
Oxygen transport
98.5% of oxygen in arterial blood are bound to
hemoglobin and only 1.5% are in the form of
dissolved oxygen.
Hemoglobin binds oxygen reversibly up to 4
molecules of oxygen.
Hb + 4O2 = Hb(O2)4
Oxygen-RBC
Dissociation Curve at Rest
Oxygen-RBC
Dissociation Curve during Exercise
Shifting the Curve
Since the partial pressure of oxygen in interstitial fluid is
about 40 torr, at this pressure, the red blood cells are
about 75% oxygen saturated - thus 25% of oxygen in
the red blood cells are unloaded.
The amounts of unloaded oxygen may increase in
response to the increased tissue oxygen demand - i. e.
more carbon dioxide.
At rest, about 5 ml of oxygen is transported to the
tissues in each 100 ml of blood at cardiac output of
5000 ml/min, thus 250 ml of oxygen per minute.
Under exercise, this may be increased up to 15 times.
Hemoglobin and RBC
What makes the difference in oxygen affinity between
hemoglobin in solution and in red blood cells?
The oxygen dissociation curves of hemoglobin in solution
and that in cells are compared below.
100%
Hb
DPG
RBC
Saturation
0%
40
mmHg
100
mmHg
The oxygen dissociation curve of purified hemoglobin is shifted
towards left of that of red blood cells.
Since Hb in solution saturates with oxygen at the lower partial
pressure of oxygen than that of red blood cells, Hb has a higher
affinity to oxygen than RBC.
The difference comes from the presence of 2,3
diphosphoglycerate, that interacts with Hb, in red blood cells.
In fact, the level of 2,3-DPG regulates the oxygen affinity of red
blood cells. In the higher altitude, its level goes up so that red
blood cells can pick up oxygen more.
1.
Carbon dioxide transport
Carbon dioxide is a waste product of aerobic metabolism. It may exist in blood
in three forms.
1.
As freely dissolved gas in plasma. 7%
2.
Bound to the amino group of hemoglobin and forms carbamino
hemoglobin. It does not bind to the heme where oxygen binds. 23%
The oxygen affinity of hemoglobin is slightly reduced, thus CO2 bound
hemoglobin releases more oxygen.
Other proteins can form carboamino-proteins, but the amount is small.
3.
CO2 enters RBC where carbonic anhydrase (CA) facilitates reaction of
CO2 with water to form carbonic acid. Carbonic acid dissociates further to
bicarbonate and proton. 70%
CA
CO2 + H2O === H2CO3 ===== H+ + HCO3-
The released proton is likely to be picked up by hemoglobin and the molecule
reduces its oxygen affinity further, thus releases more oxygen.
Bicarbonate ions made in RBC will leave the cell in exchange with chloride
ion (chloride shift).
The reverse reactions of 2 and 3 take place in the alveolar capillaries to
restore oxygen.
Study Fig. 23-24 for pick up and release of carbon dioxide by RBC.
Outside RBC, CO2 may undergo similar reaction with CA on the surface of
capillary cell walls. In this case, the released proton remains in blood
plasma, thus blood pH goes down. Without much ventilation
(hyhpoventilation) blood pH goes down. While by hyperventilation, blood pH
goes up.
Fig. 23-25 Summary of gas transport
1.
Control of respiration
a.
The respiratory centers of the brain
The respiratory center has three pairs of nuclei in the
reticular formation of the pons and medulla oblongata.
They regulate the respiratory muscles and control the
respiratory rate and the depth of breathing. (Fig. 23.27)
The respiratory rhythmicity center determines the basic
pace.
Fig. 23-27
Respiratory
centers and
reflex
Modification of Ventilation
• Chemical control
• Cerebral and limbic
system
– Respiration can be
voluntarily controlled
and modified by
emotions
– Carbon dioxide is major
regulator
• Increase or decrease in pH
can stimulate chemosensitive area, causing a
greater rate and depth of
respiration
– Oxygen levels in blood
affect respiration when a
50% or greater decrease
from normal levels exists
Modifying Respiration