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Mechanism of Breathing
UNIT B Chapter 11: Respiratory System
Inspiration: Active Phase
1.
2.
3.
4.
5.
Diaphragm contracts and
drops down
Intercostal muscles in the rib
cage contract and push up
and out
The thoracic cavity increases
in volume
Pressure in the lungs
decreases (negative air
pressure)
Air rushes into the lungs
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Section 11.2
Inhalation
UNIT B Chapter 11: Respiratory System
Expiration - passive phase
1.
2.
3.
4.
5.
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Diaphragm relaxes and moves up
Intercostal muscles in the rib cage
relax and move down and in
The thoracic cavity decreases in
volume
Pressure in the lungs increases
Air rushes out of the lungs
Section 11.2
Exhalation
Features of Thoracic Cavity
For breathing to occur, the thoracic cavity must be airtight:
1. The interior of the thoracic cavity is lined with an airtight membrane called the parietal pleura
2. Each lung is surrounded with an air-tight membrane
called the visceral pleura
3. The space between the two pleura (interpleural cavity)
contains a lubricant
4. The muscular diaphragm seals the bottom of the thoracic
cavity
UNIT B Chapter 11: Respiratory System
Section 11.2
Nervous Control of Breathing
Ventilation is controlled by a
respiratory centre in the
medulla oblongata of the brain.
Sends impulse diaphragm (which
contracts and lowers) and intercostal
muscles (rib cage move up)
air flows into alveoli
alveoli expand and stretch
stretch receptors send signal to
brain to inhibit medula oblongata
 diaphragm relaxes
 expiration occurs.
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Primary stimuli: Rising CO2 and H+ ion levels
trigger respiratory center in medulla oblongata
 nerve impulse sent along intercostal nerve to contract
intercostal muscles and
 along phrenic nerve
to contract diaphragm
 Increased rate and
depth of breathing
Secondary stimuli for Breathing:
Decreasing O2 levels
 trigger chemoreceptors in
carotid bodies of carotid arteries
and aortic bodies of aorta
 nerve impulse to
respiratory center of
medulla oblongata
UNIT B Chapter 11: Respiratory System
Section 11.3
Gas Exchanges in the Body
Respiration includes:
1. External respiration: exchange of gases in the lungs
2. Internal respiration: exchange of gases in the tissues
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Movement of CO2
Carbon dioxide molecules are transported in the blood from body
tissues to the lungs by one of three methods:
1. Dissolution directly into the blood (Partial pressure, capillaries and
alveoli are both one cell layer thick)
2. Binding to hemoglobin (carbaminohemaglobin)
3. Carried as a bicarbonate ion.
– CO2 diffuses into RBC. Carbonic Anhydrases converts it to carbonic
acid H2CO3.
UNIT B Chapter 11: Respiratory System
Section 11.3
External Respiration
• Exchange of gases between air (alveoli) blood (pulmonary
capillaries)
• Both are one cell thick
• Gas exchange is by diffusion alone
• CO2 leaves the blood, and O2 enters the blood.
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Gas Exchange: CO2
• Partial pressure of CO2 in plasma > atmospheric pressure
• So CO2 diffuses out of blood to lungs
• CO2 in plasma is carried as bicarbonate ion
• As CO2 diffuses out of plasma, carbonic anhydrase
speeds up break down carbonic acid, driving reaction to
right
UNIT B Chapter 11: Respiratory System
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Section 11.3
UNIT B Chapter 11: Respiratory System
CO2 Exits the Blood
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Section 11.3
UNIT B Chapter 11: Respiratory System
Section 11.3
Gas Exchange: O2
o Blood returning from the systemic capillaries has a lower
partial pressure of O2 (PO2) than the air in the alveoli
o O2 diffuses into plasma and then into red blood cells
o Hemoglobin takes up O2 and becomes oxyhemoglobin (HbO2)
o Hemoglobin has a higher affinity for O2 at lower temperatures
and higher pH
o Why is O2 transported via hemoglobin?
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Most of the oxygen carried in the blood is attached to
the heme portion of hemoglobin (Hb), a protein found
in red blood cells.
UNIT B Chapter 11: Respiratory System
O2 Enters the Blood
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Section 11.3
Carbon Monoxide
Difficult to detect: Colourless, odorless gas. Lighter than air.
Lowers Oxygen delivery: Binds to hemoglobin with greater
affinity than oxygen. Longer the exposure, the more sites
taken up, less oxygen getting to tissues, leading to possible
death of tissues.
Long clearing time: 4-6 hr
Produced by burning gas, wood, propane, charcoal or other fuel
UNIT B Chapter 11: Respiratory System
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Section 11.3
UNIT B Chapter 11: Respiratory System
Section 11.3
Internal Respiration:
Exchange of gases between the blood (systemic capillaries) and
tissue fluid.
• O2 leaves the blood, and CO2 enters the blood.
• Tissues have a higher temperature and lower pH, and the partial
pressure of O2 (PO2) in tissue fluid is lower than in blood
o Therefore, oxyhemoglobin gives up O2, which diffuses out of
the blood into the tissues:
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UNIT B Chapter 11: Respiratory System
Section 11.3
 CO2 diffuses into the blood from the tissues because the
PCO2 of tissue fluid is higher than in blood
– About 10% CO2 is taken up by hemoglobin =
carbaminohemoglobin (HbCO2)
o The remaining CO2 combines with water in the plasma,
forming carbonic acid (H2CO3), which dissociates into
hydrogen ions (H+) and bicarbonate ions (HCO3-).
o The increased concentration of CO2 in the blood drives
the reaction to the right
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UNIT B Chapter 11: Respiratory System
Section 11.3
o The globin portion of hemoglobin combines with excess H+ to
become reduced hemoglobin (HHb), darker in color
− Carbon monoxide (CO) poisoning occurs because it has a
greater affinity for hemoglobin than O2, stays combined for
hours
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UNIT B Chapter 11: Respiratory System
CO2 Enters the Blood
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Section 11.3
UNIT B Chapter 11: Respiratory System
Section 11.3
Figure 11.10 Internal respiration. During internal respiration in the tissues, oxygen
leaves the blood, and carbon dioxide enters the blood.
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Binding Capacity of Hemoglobin affected
by pH and Temperature
Cooler temperature (37 C) and higher pH (7.40) of lungs raises
oxygen binding capacity of hemoglobin to 98%
 optimal binding of Hb and O2 in lungs where it is cooler
and less acidic
Warmer temperature (38 C) and lower pH (7.38) of body cells
lowers the oxygen binding capacity of hemoglobin to 60%
Hb will release oxygen to tissue where its warmer and more
acidic
15-27
Saturation of Hb relative to temperature
temperature
15-28
Saturation of Hb relative to pH
Check your progress
1. Explain the role of hemoglobin.
2. Discuss why arterial blood is bright red in colour, but venous
blood is darker. This being the case, why does blood oozing
from a cut always appear to be bright red?
3. Explain why carbon monoxide poisoning can be rapidly fatal.
4. How is CO2 transported?
5. What is reduced hemoglobin?
Blood pH
The pH of the blood is normally 7.35 to 7.45 – a narrow range.
During exercise, the concentration of carbon dioxide in the
blood and respiring tissues increases. This could lower the pH
(making the blood more acidic).
To prevent this happening:
substances in blood plasma react with the excess carbon dioxide
increases in the rate and depth of breathing speed up the rate at
which carbon dioxide is removed from the bloodstream