Download Ornithology lecture 30 - NREM/BIOL 4464 – Ornithology

NREM/ZOOL 4464 – Ornithology
Dr. Tim O’Connell
Lecture 30
29 March 2013
Last time:
•Back to avian physiology and metabolism
This time:
•avian respiratory system
Readings this week:
Gill, chapter 6
If avian red blood cells carry less oxygen per cell than do mammalian RBCs, then how do birds manage
better ability than mammals to get oxygen to their tissues?
Unlike mammalian lungs, birds have associated air sacs.
•Air sacs – generally 9 of them:
•Cervical (2)
•Interclavicular (1)
•Anterior thoracic (2)
•Posterior thoracic (2)
•Abdominal (2)
•Mammalian respiratory system
•Nares, trachea, bronchi, bronchioles, alveoli, capillary beds
•Gas exchange takes place in capillaries associated with alveoli
•In relaxed breathing, humans have a residual volume of air in the lungs that is about 50% of the volume
inhaled and exhaled with each breath.
•Oxygen taken in only during inhalation
•Avian respiratory system
•Key feature (unique to birds): unidirectional flow through lungs; never any “dead air”
•Avian lungs spongy but rigid – they don’t expand and contract like mammalian lungs
•Oxygen from “fresh air” taken in during both inhalation and exhalation.
•Structure: nares, trachea, 1° bronchi, 2° bronchi, 3° bronchi a.k.a. “parabronchi”. Parabronchi – tubes
within the avian lung that are the site of gas exchange.
•Parabronchus cross section:
•Tiny tubes carrying air from the parabronchus (“air capillaries”) exchange gases with tiny tubes carrying
blood (capillaries).
•Capillaries arranged perpendicular to the primary direction of air flow through the parabronchus, resulting
in “cross-current” gas exchange.
In the tissues surrounding the main tube of each
parabronchus, air capillaries and blood capillaries are
arranged with their fluids moving in opposite directions,
creating short segments of counter-current exchange.
Because these flows are oriented perpendicular to the
primary flow of air, however, we use the term “crosscurrent” exchange to describe it. Here’s how diffusion
gradients can be more efficient with a counter-current
flow:
With con-current flow, a strong gradient weakens such
that only 50% exchange can occur, i.e., equilibrium is
reached.
With counter-current flow, a positive diffusion gradient is maintained along the
entire length. If the path is long enough, near 100% exchange can occur.
Gas exchange in the avian lung occurs via a cross-current exchange
mechanism. It’s not as efficient as counter-current exchange, but it is a
significant advance over concurrent exchange.
Here is the mammalian gas exchange system – concurrent. Blood and air flow
toward each other at a point of diffusion, but once the concentration of oxygen
in either vessel approaches 50%, the gradient so shallow that no further net gas
exchange takes place.
•Blue = oxygen (% of maximum partial pressure)
•Red = blood vessel. Oxygen content is very low at first, but the vessel quickly
picks up oxygen. As the blood flows down the vessel it picks up oxygen from
the air tube, but the concentration of oxygen declines and the width of gradient
between the vessels decreases. Once that concentration approaches 50%,
there is no gradient in oxygen concentration and no net movement of oxygen into the blood.
Theoretically, a counter-current exchange (such as exists in the blood vessel arrangements of birds’
feet to facilitate heat transfer) could do much better than raising the oxygen content of the blood to 50%.
•Red = blood vessel. Oxygen content is very low (0%) at first, but along the entire length of its
arrangement next to the air tube, a positive diffusion gradient is maintained. As the blood vessel picks up
oxygen, it continually encounters air with a greater oxygen concentration.
Birds use cross-current exchange in their lungs.
•Red = blood vessel. At multiple points along the parabronchus, deoxygenated blood comes in close
contact with the “air capillaries” that always have a higher oxygen concentration. This is not efficient
enough to increase the oxygen content of the blood close to 100% (theoretical maximum with countercurrent system), but it is a significant improvement over the ~ 50% that could be achieved with a
concurrent system like ours.
•avian respiratory system – air sacs and unidirectional flow
•No diaphragm in birds as in mammals.
•Air brought in and out through lowering of the sternum (creating negative pressure, just like dropping
diaphragm) and in expansion and contraction of air sacs like bellows.
•Muscle activity can assist respiration in flight; many birds have rhythm to flapping that facilitates
respiration.
•avian respiratory system – air sacs and unidirectional flow
•Inhalation 1: “fresh” air moves to abdominal (posterior) air sacs
•Exhalation 1: fresh air moves from abdominal air sacs through lungs
•Inhalation 2: “stale” air now moves from lungs to anterior air sacs – is replaced in lungs by new fresh air
waiting in abdominal air sacs.
•Exhalation 2: stale air (the original air brought in from inhalation 1) now expelled from anterior air sacs
through trachea and nares.
For cool animation of unidirectional flow and air sac involvement in respiration, check out:
http://people.eku.edu/ritchisong/birdrespiration.html
Avian physiological extremes:
1. Getting high – Bar-headed Goose
•This species winters in southern Asia and breeds in the steppes of central Asia.
•The Indian population at least migrates along a direct route – over the Himalaya.
•This species has been seen migrating over Mt. Everest. For perspective:
Cruising altitudes:
Small planes – 10,000’
Jetliners – 30,000 – 35,000’
Migrating songbirds – 4000’
Migrating waterfowl – 7000’
Migrating eagles – 10,000’
Bar-headed Goose in Himalayan migration – 29,500’
•World record holder – Ruppel’s Griffon Vulture seen at 37,000’, but the Bar-heaed Goose routinely
crosses Himalayan passes > 29,000’ – and it’s flapping the whole time. How?
On top of Mt. Everest . . .
•The windiest place on Earth. From Oct.–Mar., 3 of every 4 days will experience hurricane force winds
(74 mph). Winds routinely top out above 156 mph, the threshold for Category 5 hurricane.
•“Frikkin’ freezing Mr. Bigglesworth.” Even May temps average -13 F. -100 F has apparently been
recorded.
•Low Oxygen – about 33% partial pressure of Oxygen compared to sea level.
•How Bar-heads make it over (generally in one day, and near the middle of a 1000-mile flight):
•Larger lungs than other birds
•Take in greater volume of air with each inhalation
•Red blood cells contain a form of hemoglobin with extra Oxygen-binding affinity
•Flight muscles richer in myoglobin than other species
2. Going down – Emperor Penguin
•Emperor Penguins have been recorded at 1,752m deep.
•Confirmed holding breath for 15.8 minutes.
•How?
Below – decrease in O2 and increase in CO2 in blood after 15 minutes.
•Avian diving reflex (very similar to mammalian, which is better studied)
•Oxygen is used up and Carbon Dioxide accumulates the longer the body is deprived of Oxygen.
•The diving reflex in birds and mammals is similar, and kicks in when the nervous system is stimulated by
submersion of the face in cold water.
•The goal? Minimize oxygen use to the greatest extent possible. How achieved?
•Storage of Oxygen in tissues other than lungs – e.g., myoglobin in muscles.
•Penguins can do some air sac storage
Comparison of O2 stored in tissues in humans and select marine mammals:
How to use less oxygen? Pump less!
•Bradycardia – slowing heart rate.
Right – change in heart rate in a diving duck during
and after a dive.
•Peripheral vasoconstriction – reducing blood flow to
periphery to keep vital organs supplied with Oxygen
first.
•On deeper dives, blood flow is further restricted from
the internal organs and the brain takes priority as the
only thing getting Oxygen.
Example of reduction in blood flow to different organ
systems in Weddell Seal. On really deep dives.
Blood flow is only maintained (it’s actually slightly
increased) to the brain.
•3. Too cold and too small – Rufous Hummingbird
•Northernmost breeding hummingbird in North America
•Many really cold nights even in summer in northern Rockies – up to 12,000’
•Normal wingbeat frequency, 52–62/sec.
•How does this little dynamo keep from using up all its stored energy accumulated during the day?
•It hibernates!
•Several hummingbirds that occur in cold environments (e.g., the Andes) enter into daily torpor to reduce
heat loss to the external environment.
•Heart rate slows quickly and body temperature is allowed to decrease close to ambient temperature to
reduce heat loss and energy consumption.
Last time:
•physiological extremes
This time:
•thermoregulation strategies
Readings this week: Gill Ch. 6
Birds deal with extremes of temperature in multiple ways. We’ve already learned a bit about the countercurrent heat exchanger in legs of certain water birds.
Counter-Current Exchanger: Network of arteries and veins arranged in close proximity with opposite
directional flow to promote favorable gradient for diffusion over a maximum distance.
Right: Cross-section of counter-current heat exchange tissue in a
tuna. Thick walls – arteries, thin walls are veins.
Note that some birds can also shunt blood flow (vasoconstriction)
away from the toes to further reduce the amount that blood cools at the extremities.