Questions Related to the Article (27 points total):
1. The article states that oxygen consumption of the crayfish declines below what it was under
normoxic conditions when the PO2 of its freshwater environment reaches about 40 mmHg
(barometric pressure = 760 mmHg). (4)
a. Assuming that air and water are in equilibrium, what is the PO2 in the air above the water?
40 mm Hg
b. What is the oxygen concentration (= oxygen content) of this water?
from Table 1.3, solubility of oxygen in water at 15 degrees is 34.1 ml oxygen per liter of
water per 760 mm Hg of PO2
from Henry's Law, volume of oxygen in 1 liter of water
= solubility * PO2 * (volume of water)
= (34.1 ml O2/liter water/760 mm Hg O2) * (40 mm Hg O2) * (1 liter water)
= 1.8 ml O2 per liter of water
c. If this experiment was done at an altitude of 2000 meters, what would be the PO2 and
oxygen concentration of the crayfish water?
PO2 at 2000m = (40 mm Hg) * (600 mm Hg / 760 mm Hg) = 31.6 mm Hg
oxygen concentration = (1.8 ml O2) * (600 mm Hg) / 760 mm Hg)
= 1.4 ml O2 per liter of water
2. Figure 1 and Table 1 reveal that crayfish oxygen consumption remains constant when the
PO2 declines from 143 mmHg (normoxic conditions) to 63 mmHg. However, heart rate
declines significantly when the PO2 falls to 63 mmHg. Based on material discussed in class,
please explain how these crayfish could maintain a constant O2 consumption when their heart
rate declines. Correct answers should not be long but should be precise and logical. (3)
The crayfish could increase their stroke volume (to maintain a constant cardiac output
despite a decreased HR). If cardiac output falls, they might nevertheless be able to
maintain oxygen consumption by increasing the percentage of oxygen extracted from the
blood.
3. Figure 2 and Table 1 show that the PO2 of crayfish arterial blood decreases when
environmental PO2 declines from 143 mmHg to 63 mmHg. By contrast, the oxygen content of
arterial blood (Ca,O2) increases when PO2 declines to 63 mmHg. Given what you know about
how O2 is carried in the blood, provide a plausible explanation that will reconcile these
seemingly contradictory observations. Complete answers will back up your explanation with
a mechanism that has foundations in the paper. You may want to come back to this question.
(4)
In a nutshell: at low PO2, the blood of these crayfish becomes more alkaline, increasing the
oxygen affinity of the hemocyanin, leading to an increased binding of oxygen and an
increased arterial oxygen content. This prevents the fall in arterial oxygen content that
would normally accompany a drop in PO2.
4. Figure 4 shows that the PCO2 of crayfish arterial blood declines when environmental PO2
declines from 143 mmHg to 63 mmHg. Based on information contained within this paper
and material discussed in lecture, by what mechanism does arterial PCO2 decline? Cite
evidence for your assertion. Clearly explain why this decrease in arterial PCO2 is associated
with an increase in pH. (3)
The low PO2 causes the crayfish to hyperventilate (Fig. 1), increasing removal of CO2 from
the blood, lowering the CO2 in the blood (Fig. 4), lowering the amount of H2CO3 formed by
carbonic anhydrase, lowering the H+ concentration, raising the pH (Table 1 and Fig. 4).
5. In your own words, what does Figure 3 illustrate? Answer this question in non-technical
terms. (2)
This was a poorly worded question, and any reasonable answer will be accepted. One
possible answer is: this graph compares the observed relationship between [HCO3- + CO32-]
and pH (data points) with the relationship predicted by measurements of the blood's
buffering capacity in vitro (dotted line).
6. The dashed line in Figure 3 is called a "buffer line". A buffer lines describes how the pH of
a sample of blood should change under "normal" conditions. What reasons do the authors
give for why measured values of pH deviate from expected values (i.e., why does the line
made by connecting the values at 140 mmHg, 63 mmHg, and 36 mmHg deviate from the
buffer line)? Please make it clear HOW what you are proposing would make the measured
values deviate from expected (e.g., answering "because the crayfish were hypoxic" doesn't
tell me HOW this would make the values deviate). (4)
Lactic acid partially counteracts the hyperventilation-induced alkalosis, explaining why the
pH doesn't rise as much as expected for a given change in [HCO3- + CO32-].
7. Figure 5 shows the oxygen binding curve for crayfish blood at pH 7.956 ± 0.018. Please
explain why the solid line is parallel to the dashed line above a PO2 of about 25 mmHg.
Correct answers need not be long but should be precise. (3)
The dashed line represents oxygen dissolved in blood. The solid line is parallel to the
dashed one above a PO2 of ~25 mm Hg because, at this PO2, the hemocyanin is essentially
saturated with oxygen. Further increases in PO2 will not increase the amount of oxygen
bound to hemocyanin; they'll only increase the amount of oxygen dissolved in the water.
8. Given what you know about factors that determine the O2 affinity of blood (remember what
we did in lab?), please circle the statement below that is most likely to be correct for the
animals used in this study. (4)
a) Hemocyanin from normoxic crayfish should have a HIGHER oxygen affinity that blood
from hypoxic crayfish.
b) Hemocyanin from normoxic crayfish should have a LOWER oxygen affinity that blood
from hypoxic crayfish.
c) Hemocyanin from normoxic and hypoxic crayfish should have the SAME oxygen
affinity.
Please clearly explain how you came to this conclusion, citing evidence from material
presented in lab, lecture, and the paper.
B is correct. Acidity decreases the oxygen affinity of hemocyanin, and the crayfish had a
lower pH under normoxic conditions than under hypoxic ones (Table 1 and Fig. 4).
Questions about avian lungs (14 points total):
9. In lecture we discussed the structure and function of tidal lungs and gills. In particular, we
discussed the pathways of medium flow and blood flow and how the pattern of flow
influences oxygen extraction efficiency.
a) Compare and contrast the structure of mammalian tidal lungs and avian lungs. Complete
answers will address both the conduction and gas-exchange regions of the lungs. (2)
Both mammals and birds have a large trachea that divides into two bronchi, each of
which supplies one lung with air. However, in mammals the bronchi branch into
smaller and smaller conduction tubes, whereas in birds the bronchi themselves are the
primary conduits between the trachea, the air sacs, and the lungs. The gas-exchange
region of mammalian lungs consists of alveoli (described by Knut as "saclike") with
single openings through which air flows in and out, whereas gas exchange in bird lungs
is carried out by very thin tubes, parabronchi, that are open at both ends (Fig. 1.23).
b) Please describe how air and blood flow through avian lungs. Complete answers will
address (i) how air is drawn through the avian lung, and (ii) the relative directions of air
and blood flow. (2)
(i) Inflation and deflation of the air sacs creates the pressure gradients that drive the air
through the avian lungs. Air goes in the bronchus back to the caudal air sacs, then
through the bronchus to the gas-exchange region, then from the bronchus to the
cranial air sacs, etc. (Fig. 1.26).
(ii) As shown in Fig. 1.28, the air and blood flow "cross-currently" to each other. This
is similar to countercurrent flow except that the blood flows both opposite to and
perpendicular to the air.
c) Please explain the mechanism of oxygen extraction in avian lungs. Feel free to draw
well-labeled figures to include in your answer. (2)
The avian mechanism is similar to the mechanism for fish gills (shown in Fig. 1.8).
Among the key features: (a) a PO2 gradient is maintained along the entire length of the
gas-exchange region; (b) countercurrent/cross-current flow allows blood exiting the
lungs to have a higher PO2 than air exiting the lungs.
d) Why is the percentage of oxygen in the caudal air sacs less than 20.9%? (2)
The fresh air from the environment (which is 20.9% oxygen) must mix with some
"used" air that has been through the lungs (and that is low in oxygen) but hasn't been
exhaled yet.
e) Is gas exchange in avian lungs more similar to that of fish gills or tidal lungs? Please
explain your reasoning. (3)
The most straightforward argument would be that bird lungs are more like fish gills
because of the unidirectional flow of fluid through the gas-exchange region and the
quasi-countercurrent exchange mechanism.
f) Based on the structure and function of avian lungs, tidal lungs, and fish gills, what should
be the correct order of oxygen extraction efficiency. Please circle the correct answer.
i. (highest) fish gills > avian lungs > tidal lungs (lowest)
ii. (highest) fish gills > tidal lungs > avian lungs (lowest)
iii. (highest) avian lungs > fish gills > tidal lungs (lowest)
Please explain how you came to this conclusion. (3)
(i) is correct. The bird lung is not a true countercurrent system; it's a cross-current
system in which blood flowing through different parts of the lung leave the lung with
different levels of oxygenation. Therefore bird lungs are not as efficient as fish gills.
However, they are still better than tidal lungs, in which exiting blood can never have a
PO2 higher than exiting air.