Mammalian Transport Systems and Gas Exchange
This focuses on the mammalian circulatory system, with specific emphasis on the transport of
oxygen and carbon dioxide, the structure of blood vessels, and the dynamics of the oxygen
dissociation curve.
Transport of Oxygen and Carbon Dioxide: Learning Outcomes
1. The Role of Red Blood Cells in Gas Transport
Red blood cells (erythrocytes) are highly specialized for the transport of oxygen and carbon
dioxide. Their biconcave shape increases the surface area to volume ratio for rapid diffusion,
while their small diameter ( 7µm ) ensures that no haemoglobin molecule is far from the cell
surface.
● Haemoglobin: This globular protein consists of four polypeptides, each containing a
haem group with an iron atom. Each haem group can combine with one oxygen
molecule ( O2 ), allowing one haemoglobin molecule to carry up to four O2 molecules
(eight oxygen atoms), forming oxyhaemoglobin.
● Carbonic Anhydrase: This enzyme, found in the cytoplasm of red blood cells,
catalyzes the reaction between carbon dioxide and water to form carbonic acid ( H2CO3).
This is the first step in the primary method of carbon dioxide transport.
● Formation of Haemoglobinic Acid: Carbonic acid dissociates into hydrogen ions ( H+ )
and hydrogencarbonate ions ( HCO3- ). Haemoglobin readily combines with these H+
ions to form haemoglobinic acid ( HHb). This process allows haemoglobin to act as a
buffer, maintaining the blood's pH close to neutral.
● Formation of Carbaminohaemoglobin: Approximately 10% of carbon dioxide is
transported by combining directly with the terminal amine groups of haemoglobin
molecules.
2. The Chloride Shift and Its Importance
As hydrogencarbonate ions are produced in the red blood cell, they diffuse out into the blood
plasma. To maintain electrical neutrality and prevent the inside of the cell from becoming too
positively charged due to accumulated H+ ions, chloride ions ( Cl- ) move from the plasma into
the red blood cell. This movement is known as the chloride shift .
3. The Role of Plasma in Carbon Dioxide Transport
Blood plasma serves as the medium for transporting carbon dioxide in two main forms:
● As Hydrogencarbonate Ions: About 85% of CO2 is carried in the plasma as HCO3-
ions after they diffuse out of the red blood cells.
● As Dissolved Gas: Approximately 5% of CO2 is carried simply as dissolved molecules
in the plasma.
,4. The Oxygen Dissociation Curve of Adult Haemoglobin
The oxygen dissociation curve is an S-shaped (sigmoid) graph that plots the percentage
saturation of haemoglobin against the partial pressure of oxygen ( pO2 ).
● Sigmoid Shape: At low pO2 , saturation is low. Once the first oxygen molecule binds to
a haem group, the haemoglobin molecule's tertiary structure changes (distorts), making
it easier for subsequent oxygen molecules to bind. This causes the curve to rise steeply
at mid-range partial pressures.
5. Importance of the Curve at the Lungs and Respiring Tissues
● In the Lungs: The pO2 is high (approx. 12 kPa). At this pressure, haemoglobin has a
high affinity for oxygen and becomes 95–97% saturated.
● In Respiring Tissues: The pO2 is low (approx. 2 kPa in active muscle). The curve
shows that at these lower pressures, haemoglobin's affinity for oxygen drops
significantly, causing it to release about three-quarters of its oxygen load to the cells for
aerobic respiration.
6. The Bohr Shift and Its Importance
The Bohr shift is the decrease in haemoglobin's affinity for oxygen in the presence of high
partial pressures of carbon dioxide ( pCO2 ).
● Mechanism: High pCO2 leads to more H+ ions and the formation of haemoglobinic
acid, which triggers the release of oxygen.
● Importance: In actively respiring tissues, where CO2 levels are high, the Bohr shift
ensures that haemoglobin releases even more oxygen than it would based on pO2 alone.
On a graph, this causes the dissociation curve to shift downwards and to the right.
Short-Answer Quiz
1. Describe how the structural lack of organelles in a red blood cell aids its primary
function.
2. How does the biconcave shape of a red blood cell facilitate gas exchange?
3. Explain the role of elastic fibers in the walls of the aorta.
4. What is the functional difference between the systemic and pulmonary
circulations?
5. Describe the mechanism of the chloride shift.
6. Why is it vital for haemoglobin to act as a buffer within the red blood cell?
7. How do the semilunar valves in veins contribute to blood flow?
8. Define "percentage saturation" in the context of haemoglobin.
9. What causes the "Bohr shift" to occur in active muscle tissue?
10. Describe the composition and formation of tissue fluid.
Quiz Answer Key
1. Red blood cells lack a nucleus, mitochondria, and endoplasmic reticulum to maximize
the internal space available for haemoglobin. This allows each cell to carry the maximum
possible amount of oxygen.
, 2. The biconcave shape increases the surface area to volume ratio of the cell. This large
surface area allows oxygen to diffuse into and out of the cell much more quickly than if
the cell were spherical.
3. Elastic fibers allow the aorta to stretch when high-pressure blood surges out of the
ventricles and then recoil when the heart relaxes. This recoiling action helps even out the
blood flow and maintains pressure between heartbeats.
4. The systemic circulation pumps oxygenated blood from the left side of the heart to the
rest of the body (except the lungs) and returns deoxygenated blood. The pulmonary
circulation pumps deoxygenated blood from the right side to the lungs and returns
oxygenated blood to the left side.
5. When hydrogencarbonate ions (produced from CO2 ) diffuse out of the red blood cell
into the plasma, they leave behind a positive charge. Negatively charged chloride ions
move from the plasma into the cell to balance this charge and maintain electrical
neutrality.
6. When CO2 dissolves, it produces hydrogen ions which would make the blood very acidic
if left in solution. Haemoglobin binds to these ions to form haemoglobinic acid,
maintaining the pH near neutral so cellular processes can function normally.
7. Semilunar valves prevent the backflow of blood in the low-pressure venous system.
When surrounding skeletal muscles contract, they squeeze the veins, pushing blood
through the valves toward the heart; the valves then close to prevent gravity from pulling
the blood back down.
8. Percentage saturation is a measure of the degree to which haemoglobin is combined
with oxygen. It is calculated as a percentage of the maximum amount of oxygen the
haemoglobin can carry when all its haem groups are occupied.
9. High concentrations of carbon dioxide from respiring cells diffuse into the blood, leading
to an increase in hydrogen ions as carbonic acid dissociates. These ions react with
haemoglobin, causing it to change shape and release its oxygen more readily.
10. Tissue fluid is formed when blood plasma leaks through gaps in the capillary walls due to
high hydrostatic pressure at the arterial end. It is similar to plasma but contains far fewer
proteins, as most protein molecules are too large to pass through the endothelium.
Essay Questions
1. Comparative Anatomy of Vessels: Compare and contrast the structures of arteries,
veins, and capillaries. Discuss how the specific tissue layers (endothelium, smooth
muscle, and elastic fibers) in each vessel relate to the pressure and volume of blood
they carry.
2. Dynamics of Oxygen Transport: Explain the S-shaped nature of the oxygen
dissociation curve. How do the properties of the haemoglobin molecule allow for efficient
oxygen uptake in the lungs and effective release in respiring tissues?
3. Carbon Dioxide Homeostasis: Describe the three ways carbon dioxide is transported
in the mammalian blood. Include a detailed explanation of the chemical reactions
involving carbonic anhydrase and the resulting chloride shift.
This focuses on the mammalian circulatory system, with specific emphasis on the transport of
oxygen and carbon dioxide, the structure of blood vessels, and the dynamics of the oxygen
dissociation curve.
Transport of Oxygen and Carbon Dioxide: Learning Outcomes
1. The Role of Red Blood Cells in Gas Transport
Red blood cells (erythrocytes) are highly specialized for the transport of oxygen and carbon
dioxide. Their biconcave shape increases the surface area to volume ratio for rapid diffusion,
while their small diameter ( 7µm ) ensures that no haemoglobin molecule is far from the cell
surface.
● Haemoglobin: This globular protein consists of four polypeptides, each containing a
haem group with an iron atom. Each haem group can combine with one oxygen
molecule ( O2 ), allowing one haemoglobin molecule to carry up to four O2 molecules
(eight oxygen atoms), forming oxyhaemoglobin.
● Carbonic Anhydrase: This enzyme, found in the cytoplasm of red blood cells,
catalyzes the reaction between carbon dioxide and water to form carbonic acid ( H2CO3).
This is the first step in the primary method of carbon dioxide transport.
● Formation of Haemoglobinic Acid: Carbonic acid dissociates into hydrogen ions ( H+ )
and hydrogencarbonate ions ( HCO3- ). Haemoglobin readily combines with these H+
ions to form haemoglobinic acid ( HHb). This process allows haemoglobin to act as a
buffer, maintaining the blood's pH close to neutral.
● Formation of Carbaminohaemoglobin: Approximately 10% of carbon dioxide is
transported by combining directly with the terminal amine groups of haemoglobin
molecules.
2. The Chloride Shift and Its Importance
As hydrogencarbonate ions are produced in the red blood cell, they diffuse out into the blood
plasma. To maintain electrical neutrality and prevent the inside of the cell from becoming too
positively charged due to accumulated H+ ions, chloride ions ( Cl- ) move from the plasma into
the red blood cell. This movement is known as the chloride shift .
3. The Role of Plasma in Carbon Dioxide Transport
Blood plasma serves as the medium for transporting carbon dioxide in two main forms:
● As Hydrogencarbonate Ions: About 85% of CO2 is carried in the plasma as HCO3-
ions after they diffuse out of the red blood cells.
● As Dissolved Gas: Approximately 5% of CO2 is carried simply as dissolved molecules
in the plasma.
,4. The Oxygen Dissociation Curve of Adult Haemoglobin
The oxygen dissociation curve is an S-shaped (sigmoid) graph that plots the percentage
saturation of haemoglobin against the partial pressure of oxygen ( pO2 ).
● Sigmoid Shape: At low pO2 , saturation is low. Once the first oxygen molecule binds to
a haem group, the haemoglobin molecule's tertiary structure changes (distorts), making
it easier for subsequent oxygen molecules to bind. This causes the curve to rise steeply
at mid-range partial pressures.
5. Importance of the Curve at the Lungs and Respiring Tissues
● In the Lungs: The pO2 is high (approx. 12 kPa). At this pressure, haemoglobin has a
high affinity for oxygen and becomes 95–97% saturated.
● In Respiring Tissues: The pO2 is low (approx. 2 kPa in active muscle). The curve
shows that at these lower pressures, haemoglobin's affinity for oxygen drops
significantly, causing it to release about three-quarters of its oxygen load to the cells for
aerobic respiration.
6. The Bohr Shift and Its Importance
The Bohr shift is the decrease in haemoglobin's affinity for oxygen in the presence of high
partial pressures of carbon dioxide ( pCO2 ).
● Mechanism: High pCO2 leads to more H+ ions and the formation of haemoglobinic
acid, which triggers the release of oxygen.
● Importance: In actively respiring tissues, where CO2 levels are high, the Bohr shift
ensures that haemoglobin releases even more oxygen than it would based on pO2 alone.
On a graph, this causes the dissociation curve to shift downwards and to the right.
Short-Answer Quiz
1. Describe how the structural lack of organelles in a red blood cell aids its primary
function.
2. How does the biconcave shape of a red blood cell facilitate gas exchange?
3. Explain the role of elastic fibers in the walls of the aorta.
4. What is the functional difference between the systemic and pulmonary
circulations?
5. Describe the mechanism of the chloride shift.
6. Why is it vital for haemoglobin to act as a buffer within the red blood cell?
7. How do the semilunar valves in veins contribute to blood flow?
8. Define "percentage saturation" in the context of haemoglobin.
9. What causes the "Bohr shift" to occur in active muscle tissue?
10. Describe the composition and formation of tissue fluid.
Quiz Answer Key
1. Red blood cells lack a nucleus, mitochondria, and endoplasmic reticulum to maximize
the internal space available for haemoglobin. This allows each cell to carry the maximum
possible amount of oxygen.
, 2. The biconcave shape increases the surface area to volume ratio of the cell. This large
surface area allows oxygen to diffuse into and out of the cell much more quickly than if
the cell were spherical.
3. Elastic fibers allow the aorta to stretch when high-pressure blood surges out of the
ventricles and then recoil when the heart relaxes. This recoiling action helps even out the
blood flow and maintains pressure between heartbeats.
4. The systemic circulation pumps oxygenated blood from the left side of the heart to the
rest of the body (except the lungs) and returns deoxygenated blood. The pulmonary
circulation pumps deoxygenated blood from the right side to the lungs and returns
oxygenated blood to the left side.
5. When hydrogencarbonate ions (produced from CO2 ) diffuse out of the red blood cell
into the plasma, they leave behind a positive charge. Negatively charged chloride ions
move from the plasma into the cell to balance this charge and maintain electrical
neutrality.
6. When CO2 dissolves, it produces hydrogen ions which would make the blood very acidic
if left in solution. Haemoglobin binds to these ions to form haemoglobinic acid,
maintaining the pH near neutral so cellular processes can function normally.
7. Semilunar valves prevent the backflow of blood in the low-pressure venous system.
When surrounding skeletal muscles contract, they squeeze the veins, pushing blood
through the valves toward the heart; the valves then close to prevent gravity from pulling
the blood back down.
8. Percentage saturation is a measure of the degree to which haemoglobin is combined
with oxygen. It is calculated as a percentage of the maximum amount of oxygen the
haemoglobin can carry when all its haem groups are occupied.
9. High concentrations of carbon dioxide from respiring cells diffuse into the blood, leading
to an increase in hydrogen ions as carbonic acid dissociates. These ions react with
haemoglobin, causing it to change shape and release its oxygen more readily.
10. Tissue fluid is formed when blood plasma leaks through gaps in the capillary walls due to
high hydrostatic pressure at the arterial end. It is similar to plasma but contains far fewer
proteins, as most protein molecules are too large to pass through the endothelium.
Essay Questions
1. Comparative Anatomy of Vessels: Compare and contrast the structures of arteries,
veins, and capillaries. Discuss how the specific tissue layers (endothelium, smooth
muscle, and elastic fibers) in each vessel relate to the pressure and volume of blood
they carry.
2. Dynamics of Oxygen Transport: Explain the S-shaped nature of the oxygen
dissociation curve. How do the properties of the haemoglobin molecule allow for efficient
oxygen uptake in the lungs and effective release in respiring tissues?
3. Carbon Dioxide Homeostasis: Describe the three ways carbon dioxide is transported
in the mammalian blood. Include a detailed explanation of the chemical reactions
involving carbonic anhydrase and the resulting chloride shift.
