Unit 6 - Gas Exchange & Transport Learning Objectives

Question 1

List four factors that influence the diffusion of gases across the alveoli.

Answer 1

a. Concentration (Pressure) gradient of gasses
b. Surface area for exchange
c. Diffusion barrier permeability
d. Diffusion distance

Question 2

Explain the difference between the concentration and the partial pressure of that gas in solution.

Answer 2

Partial pressures of gasses determined by % of gas in a mixture of gases:
• E.g. O2 at sea level = 20.95% of atmospheric air, so
partial pressure of O2 is = 20.95% x 760 mmHg (air
pressure at sea level) = 159.2 mmHg (~160mmHg)
Ø Partial pressure gradients for each gas promote their
movement (diffusion) from alveoli to blood and blood to
cells and vice versa..

Concentration (Pressure) gradient of gasses - in healthy lungs = primary factor
affecting gas exchange.
- The steeper the gradient the more gas diffusion will occur
- Gradient is affected by composition of air and alveolar ventilation (see previous slides).
- Concentration of oxygen in air and in solution()plasma) is measured in mmol O2/L. Concentration depends on solubility of the gas in the fluid of the plasma. O2 has low solubility in water, and so its concentration in blood plasma leaving the lungs is lower than in alveolar air, even though they both have the same partial pressure.

Question 3

Compare and contrast the solubility of oxygen and carbon dioxide.

Answer 3

Oxygen exhibits low solubility in aqueous solutions, as a result, very little can be carried in the plasma (only ~0.3 mL O2 dissolved per 100 mL of plasma). In contrast CO2 solubility is 20X higher. For a given partial pressure, more CO2 will dissolve in water than O2.

Question 4

Describe normal physiological pressures of oxygen and carbon dioxide in the following locations: alveoli, arterial blood, resting cells, and venous blood.

Answer 4

• Arterial PO2 = 100 mmHg
• Resting venous PO2 and ISF (interstitial fluid) PO2 = 40 mmHg
• Intracellular (ICF) PO2 = < 40 mmHg (since the cell is constantly using oxygen for cellular respiration – ATP production).
• Arterial PCO2 = 40 mmHg
• Intracellular Fluid PCO2 = >46 mmHg
• Interstitial Flujd PCO2 = 46 mmHg
• Resting Venous PCO2 = 46 mmHg

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Question 5

Diagram the pressure gradients at the sites of gas exchange and show the direction of oxygen and carbon dioxide movement.

Answer 5

done

Question 6

Describe how the law of mass action applies to gas transport in the body fluids.

Answer 6

Loading and unloading of O2 onto hemoglobin obeys the law of mass action:
- Increasing the concentration of one substance involved in a reversible reaction drives that reaction
towards the opposite direction

Reaction follows the law of mass action (build up of
substrates/products will push the equation in one direction or the other)

The Hb binding rxn Hb + O2 HbO2 obeys the law of mass action

• as the [free O2] increases, more O2 binds to HB & the equation shifts to the RIGHT, producing MORE HbO2
• if the [O2] decreases, the equation shifts to the LEFT
• Hb releases O2, & the amount of oxyhemoglobin DECREASES

Question 7

Describe the structure of hemoglobin and its capacity to bind oxygen.

Answer 7

Found only in erythrocytes (RBCs)

Allows more oxygen to be transported in the blood

Adult Hb (HbA) is compsed of two a-globin and two b-globin chains, each of which is bound to an iron (Fe2+) containing heme group.

Oxygen binds reversibly to Fe2+
- Fe2+ BINDS oxygen when plasma PO2 is HIGH (in pulmonary
capillaries)
- Fe2+ RELEASES oxygen when PO2 is LOW (in systemic capillaries).

Question 8

Describe and draw the oxygen-hemoglobin dissociation curve (for normal conditions) and explain the physiological significance of the shape of this curve.

Answer 8

Curve is sigmoidal due to cooperative oxygen binding
between Hb and O2. When oxygen binds to one site on the
Hb tetramer, a conformational change occurs that increases
the oxygen binding affinity at the other sites (and vice
versa). So Hb releases/takes up O2 quickly through the
middle portions of the graph resulting a steep section
(where O2 is gained/lost easily). But in the plateau where
Hb is saturated even large changes in PO2 have no impact
on saturation (more difficult to gain/lose oxygen).

Question 9

Describe and diagram the shifts in the oxygen-hemoglobin dissociation curve that result from changes in pH, temperature, and 2,3-DPG levels.

Answer 9

Factors causing shifts to RIGHT in the O2-Hb Saturation Curve (↑ P50 )

1. ↓ pH (↑ H+)
2. ↑ PCO2
3. ↑ 2,3-diphosphoglycerate (DPG)
4. ↑ Temperature

Factors causing shifts to LEFT in the O2-Hb Saturation Curve (↓P50)

1. ↑ pH (↓ H+)
2. ↓ PCO2
3. ↓ 2,3-diphosphoglycerate (DPG)
4. ↓ Temperature

Question 10

Describe the half saturation oxygen pressure (P50) and its importance in determined shifts in the Hb-O2 equilibrium curves.

Answer 10

Half Saturation oxygen pressure P50

• The affinity of Hb (or blood) for O2 is often expressed as its P50
• = plasma PO2 at which 50% of the Hb molecules are saturated with oxygen (i.e. 50% of Hb are in the form of DeoxyHb and 50% are in the form of OxyHb).

Question 11

Describe the factors that affect Hb-O2 affinity, including pH, temperature, 2, 3-DPG levels, carbon monoxide, and chloride ions.

Answer 11

Curve (↑ P50 )

1. ↓ pH (↑ H+)
2. ↑ PCO2
3. ↑ 2,3-diphosphoglycerate (DPG)
4. ↑ Temperature

Factors causing shifts to LEFT in the O2-Hb Saturation Curve (↓P50)

1. ↑ pH (↓ H+)
2. ↓ PCO2
3. ↓ 2,3-diphosphoglycerate (DPG)
4. ↓ Temperature

Question 12

Compare and contrast fetal hemoglobin with hemoglobin found in adults.

Answer 12

O2 transport from maternal to fetal circulation is
facilitated by expression of a different Hb isoform
in the fetus (fetal Hb, or HbF)

Fetal hemoglobin has (GAMMA) g-globin chains
instead of the beta-globin chains seen in HbA.

This change decreases the number of bonds that DPG
can form with Hb, which decreases fetal blood P50 to ~
21 mmHg which is lower than that of maternal blood
(~28 mmHg).

The lower P50 acts as a shift to the left in the placenta,
and so promotes the transfer of oxygen from the HbA
of the mother’s blood to the HbF of the fetus. Oxygen
loading of HbF increases (offloading decreases), and
so we observe a higher saturation of fetal blood with
oxygen for the same PO2.

Question 13

Describe or diagram the factors affecting total oxygen content of arterial blood.

Answer 13

The total O2 content of arterial blood depends on the amount of oxygen dissolved in plasma & bound to Hb

1. Oxygen dissolved in plasma is influenced by:
   - Comp of inspired air
   - Alveolar ventiliation
   - O2 diffusion b/t alveoli & blood
   - Adequate perfusion of alveoli
2. Oxygen bound to Hb
   - & saturation of Hb
   - Total # of binding sites

Question 14

Write the chemical reaction for the conversion of carbon dioxide to bicarbonate ions, including the enzyme that catalyzes the reaction.

Answer 14

Conversion of CO2 into HCO3
- at the systemic tissue
capillaries (and back to CO2 in the pulmonary capillaries of
the lungs) is catalyzed in RBCs by the enzyme carbonic
anhydrase (CA) – but some CA is also found on the
endothelium of the capillaries in both locations.

CO2 + H2O –> (CA) H2CO3 –> H+ + HCO3

Question 15

Explain how bicarbonate acts as a buffer.

Answer 15

CO2 + H2O –> (CA) H2CO3 –> H+ + HCO3

H+

Question 16

Map the three ways in which carbon dioxide is transported in the blood and explain the specific mechanisms in each case. Also, be sure you can map the reverse mechanisms in order to show how these transport methods interact to remove CO2 from the body.

Answer 16

1. Dissolved in the plasma (~5%, but up to 15% during strenuous exercise).
2. Bound to amino groups of plasma proteins (<1%).
   CO2 + protein-NH2 ↔ protein-NHCOO- + H+
3. Bound to amino groups on globin portion of hemoglobin = carbaminohemoglobin (~5%).
4. As bicarbonate ions (HCO3-) in the blood plasma (~90% total)

Question 17

Describe the current model for the neural control of breathing.

Answer 17

1. Respiratory centres in the medulla
   a. Ventral Respiratory Group (VRG)
   b. Dorsal Respiratory Group (DRG)
2. Pontine respiratory centres
3. Voluntary Control
4. Pulmonary stretch receptors
5. Chemical control (chemoreceptors)
   a. Peripheral chemoreceptors
   b. Central chemoreceptors

Question 18

Describe the specific mechanisms by which central and peripheral chemoreceptors monitor carbon dioxide, oxygen, and pH levels for the purpose of regulating ventilation.

Answer 18

a. Peripheral chemoreceptors
- Strongly activated when plasma PO2 drops below 60 mmHg (emergency situation)
- More often stimulated by increases in both arterial PCO2 and/or [H+]

b. Central chemoreceptors
Respond only to changes in the pH of the cerebrospinal
fluid (CSF) surrounding the medulla (this pH decreases
when there is an increase in PCO2).

Question 19

Describe protective reflexes that guard the lungs.

Answer 19

Summary of central and peripheral chemoreception and their impact on ventilation

*this pathway overrides voluntary
control and is the most powerful
pathway regulating ventilation.
For example, if you try to hold your
breath for as long as possible, PCO2
increases and is converted into H+ in
the medulla. This triggers the central
chemoreceptors, which signal to the
medullary respiratory centres to
increase ventilation rate. Firing of
inspiratory neurons will override the
voluntary control from the cerebral
cortex and you will be forced to take a
breath.

Question 20

Describe the influence higher brain centres can exert on breathing patterns.

Answer 20

For example, if you try to hold your
breath for as long as possible, PCO2
increases and is converted into H+ in
the medulla. This triggers the central
chemoreceptors, which signal to the
medullary respiratory centres to
increase ventilation rate. Firing of
inspiratory neurons will override the
voluntary control from the cerebral
cortex and you will be forced to take a
breath.