Biology 2601A/B Lecture Notes - As Blood Flows, Countercurrent Exchange, Respiratory System

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16 Oct 2011
Organismal Phys Lecture 16
Gas Exchange 2: Animals Breathing Air
Concurrent Gas Exchange
Medium and blood flow in the same direction
At end of flow, end up with intermediate level of O₂ in blood
As deoxygenated blood enter the gas exchange surface, it comes into
contact with the fully oxygenated external medium
As blood flows through the gas exchange surface, the PO₂ approaches
that of the exhaled medium
Counter-current Gas Exchange
(refer to last lecture)
Medium and blood flowing in opposite ways
Get high O₂ in the blood
Always a concentration gradient between the medium and the blood
Heat transfers to incoming vessel; less heat is lost to the outside world
Less heat is required to re-heat the incoming blood
Cross-current Gas Exchange
Medium intersects blood flow at various points
Multiple capillaries are arranged at an angle to the flow of the external medium
After they exit the gas exchange surface, these capillaries join into an efferent
blood vessel
The first vessel that crosses the gas exchange surface encounters a fully
oxygenated medium, yielding a high PO₂ in the capillary, but subsequent
capillaries encounter a progressively O₂-depleted medium and thus have
somewhat lower PO₂
PO₂ leaving the gas exchange surface is generally higher than concurrent, but
lower than counter-current
The slower the flow of the medium in relation to blood flow, the lower the
blood PO₂ will be leaving
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Are undergoing tidal gas exchange (tidal volume is the normal amount
of air displaced between normal inspiration and expiration when extra
effort is not applied)
Highly vascularised; O₂ moves from inhaled air into the blood
Mammal Lungs Are Inefficient: Tidal Ventilation & Dead Space in Alveoli
Similar to concurrent
As blood leaves gas exchange area, the blood contains approximately
same O₂ as the medium
Animals that tidally ventilate are generally unable to completely
empty their respiratory cavity with each ventilatory cycle
As the animal breathes in, the incoming fresh medium mixes with that
of the medium in the respiratory cavity; therefore the PO₂ in the
respiratory cavity is lower than that of the external medium
Also need to note that there is dead space in the alveoli when breathe in and
out the tidal volume is only 500mL
o When you breathe very deeply, you can increase your lung capacity
(therefore, normally have extra volume in your lungs that you don’t
Breathing Air
Lots of O₂ available
Is not so easy to get rid of CO₂, but is not impossible
There are problems with water loss (especially if air is dry)
Use lungs for gas exchange (invaginations)
Fish can use their gut, swim bladder or mouth as an air breathing organ; the
blood will go through air breathing organ in parallel to systemic tissues
Ways to Maximize O₂ Uptake
The energetic costs of flying are 2.5-3x higher than running (very expensive)
Constraints for flying is O₂ availability
There are two groups of extant flying vertebrates (birds and bats)
Organisms can maximize O₂ uptake by
o Counter-current exchange
o Increase flow rate
o Increase SA
o Reduce diffusion distance
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