Lecture 16: Gas Exchange
Silverthorn Chapter 18
Gas exchange between alveoli and blood
1. Several factors impact the rate of gas exchange between the alveoli capillaries
A. Distance between air in ALVEOLI and fluid in CAPILLARIES for gas exchange
i. This TINY distance allows for incredibly FAST gas exchange (gasses come to equilibrium within 1 second)
ii. Diseases that affect this factor decrease the efficiency of gas exchange
a. Ex: Pulmonary edema
I. Various factors can lead to increased filtration
II. Ex: Usually, blood pressure in your pulmonary system is really low…25/8 mm Hg!
III. Increased pressure can lead to increased filtration…
B. Surface area of exchange membrane
i. Decreased in emphysema!
C. How easily the gas in question diffuses into water
i. Gases differ in their solubility
ii. CO2 is more soluble in water that O2
D. Partial pressure differences between the air and blood
i. The PARTIAL PRESSURE of a gas in AIR is proportional to its conc gradient
a.
b.
c.
d.
Conc = # of molecules/volume
PP= how hard those molecules bounce off things
In GAS, PP is proportional to CONC
However, a gas’ ability to dissolve in water (liquid) depends on “concentration gradient” AND “solubility”…so you
can’t ever use concentration gradient alone to determine the direction that gas molecules diffuse when diffusing
between liquid and gas! Use PP instead.
e. Check it out: Fig 18-2 (text)
ii. If in doubt about the direction a gas molecule will diffuse, compare partial pressures. The partial pressures of a gas in
air and water can be equal, but their conc. can be different…partial pressure ALWAYS wins.
Partial Pressures of CO2 and O2 in the circulatory system: (Fig 18-1)
Gas exchange occurs very fast between capillaries and the alveoli, coming to equilibrium within ONE SECOND.
Transport of oxygen through body
1. ≈ 2% of oxygen is dissolved in plasma
2. ≈ 98% of oxygen in blood is bound to hemoglobin
A. Hemoglobin structure: 4 protein subunits, each with
a central “heme” or iron unit
B. Hemoglobin function: The central iron forms a weak
bond with oxygen…key word: WEAK!
C. Hemoglobin’s ability to bond with O2 or let go of
O2 is related to the partial pressure of oxygen…
Oxyhemoglobin dissociation curve Fig 18.9
WHY does hemoglobin’s attraction
to oxygen CHANGE when exposed to
different concentrations of oxygen???
WHAT would the graph look like if there wasn’t a change in affinity?
Bio 7: Human Physiology
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Spring 2014: Riggs
Shifting the dissociation curve
Various factors “move” the curve to the right or left…making Hb more or less sticky at different concentrations
1. pH
2. Temperature
3. CO2 concentration
But how will we REMEMBER this??? Ask yourself this series of questions:
A. During exercise, will the cells need MORE or LESS oxygen?
B. Would you then rather have GREATER or LESSER affinity between Hb and O2?
C. Hypothesize about the DIRECTION you think the curve will move in each scenario…
Some other scenarios…what do you think?
D. Fetal hemoglobin has a higher affinity for oxygen (why??? Less available!!!)
E. Altitude?
Transporting CO2
1. 70% dissolves in blood as bicarbonate ions
A. First, CO2 gets into RBC, where an enzyme called carbonic anhydrase turns it to HCO3-
CO2 + H2O ↔ (H2CO3) ↔ H+ + HCO3B. Dynamic equilibrium: when rate of forward reaction equals rate of reverse reaction
C. Bicarbonate is the most important buffer in the body!!! (Prevents major pH swings!)
2. 23% binds to hemoglobin
3. 7% dissolves in plasma
Bio 7: Human Physiology
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Spring 2014: Riggs
External Brain 16: Gas Exchange
Study Guide Questions:
1. What is partial pressure? How is it different from concentration?
2. Given a scenario with known concentration and partial pressure gradients, predict the movement of gas molecules.
3. Be able to label PO2 and PCO2 values on a diagram that includes the pulmonary circulation, the alveoli, the systemic
circulation, and the body’s cells.
4. Predict changes in PO2 and PCO2 in various places in the body (including the locations described in previous question),
based on different scenarios. (Ex: during exercise, at altitude, when anemic...)
5. Describe the general structure of hemoglobin (to the detail described in lecture).
6. Be able to describe the oxygen-hemoglobin dissociation curve (figure 18.9). Know important markers (PO2 in resting cell,
PO2 during exercise, PO2 in alveoli).
7. Talk about the factors that can move the oxygen-hemoglobin dissociation curve (figure 18.9) TO THE RIGHT. What factors
move the oxygen-hemoglobin dissociation curve (figure 18.9) TO THE LEFT? What does this mean, exactly?
8. Given any scenario, be able to predict what the oxygen-hemoglobin dissociation curve (figure 18.9) would look like. Ex:
Exercise? Hypothermia?
9. When a patient’s blood is taken to determine her arterial oxygen content, the blood is immediately put on ice. Explain why
this is necessary, and describe how the results could be flawed if the blood is not put on ice. Your answer should relate directly
to the oxygen-hemoglobin dissociation curve!
10.Explain the 3 ways CO2 is transported in the blood.
11. What is carbonic anhydrase? What does it do? Where is it located?
12.Be able to describe the dynamic equilibrium that exists between CO2 and bicarbonate ions in the blood going by the cells in
your body that undergo cellular respiration and in the blood near the alveoli.
Bio 7: Human Physiology
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Spring 2014: Riggs