Comparative Phys. Test 1

Why are mammals unable to breathe water? Please answer this question in terms of the physical properties of water and air, ventilatory mechanics, and respiratory anatomy.

-Mammals do tidal convection (medium flows in, and out through the same pathway), which is less efficient in water due to its higher density (400x) and viscosity (50x) compared to air. Mammals would require much more energy to accomplish this tidal flow in water.

-Mammalian lungs are adapted for gas exchange in air with tidal flow, making them ineffective for extracting oxygen from water. If the alveoli were composed only of water, there would be a risk of collapse upon exhalation

-High heat capacity of water would absorb admitted heat from animal making it hard for endothermic mammals not equipped for water breathing to thermoregulate.

-Water has a lower oxygen concentration than air, mammals require lots of oxygen for their high rates of metabolism which would not be possible in water.

Why can’t a trout breathe air? Please answer this question in terms of the physical properties of water and air, ventilatory mechanics, and respiratory anatomy.

-Gills secondary lamellae require constant water flow to maintain their structure and function, and exposure to air can lead to gill collapse/ atrophy leading to inefficient gas exchange due to the limited SA of the lamellae

-In air there is far too much oxygen (30 x higher than in water) so the fish would never need to increase their ventilation rate. Too much oxygen could result in apnea and the decrease in ventilation results in CO2 build up in the animal.

Explain why larval fish do not require gills for gas exchange while adults do.

Larval fish rely on diffusion through their skin for gas exchange due to their small size and high surface area to volume ratio, which allows sufficient oxygen intake without the need for specialized respiratory structures. As they mature, their size increases and metabolic demands rise, necessitating the development of gills to efficiently extract oxygen from water.

Tuna are considered a high performance fish species while sole are typically viewed as sluggish. Compare these two species regarding the following: gill morphology, Ucrit, myocardial properties, ventilatory mode, and coronary blood supply.

-Tuna have more complex gill morphology with larger surface area due to large amounts of closely spaced more efficient (thin) lamellae, allowing for higher oxygen extraction rates. They have a very high Ucrit (critical swimming speed) indicating superior swimming endurance necessary for maintaining oxygen demands. Tuna possess a more developed and compact myocardium with high mitochondrial counts and a well developed coronary circulation supporting necessary high cardiac outputs. Tuna are obligate ram ventilators, meaning they must swim continuously to force water over their gills, reducing energy expenditure on active pumping.

-Sole have simpler gill structures (further distance thick lamellae with smaller surface area) suited for a sedentary lifestyle, have less muscular hearts, and little to no coronary circulation. Sole have a much lower Ucrit which supports their lower metabolic demands. Sole are buccal pump ventilators, actively drawing water over their gills using muscular pumping, which allows them to remain stationary on the seabed.

Define hypoxia, normoxia, and hyperoxia.

Hyperoxia: occurs during exposure to too much oxygen.
Hypoxia: especially low level of oxygen in tissues

Normoxia: “normal” or homeostatic level of oxygen in tissues

What is it about their physical and biological environments that requires the pupfish to be so hypoxia-tolerant?

The pupfish lives in Death Valley, California in nearly evaporated tiny streams that are hot and have a high salinity which both would be connected to lower concentrations of oxygen in the water.  Oxygen levels can be even lower at night when photosynthetic plants are no longer producing oxygen. Thus, the pupfish do not have a lot of extra oxygen to work with and must be well evolved to deal with hypoxic conditions and extract as much O2 as possible from their environment.

What is the primary respiratory organ of the pupfish? Describe its anatomical arrangement relative to the water and how it allows a more efficient extraction of oxygen from the water than other gas exchange organs. Feel free to supplement your answer with a diagram.

Gills are the primary respiratory organ of the pupfish, the unidirectional flow of water and the countercurrent flow of water and blood within the gills allow for the most efficient extraction of oxygen from water. Countercurrent gas exchange maintains a constant oxygen partial pressure gradient ensuring that even as water loses oxygen, it remains higher than the oxygen content of blood in adjacent capillaries resulting in more oxygen being efficiently extracted across the entire gill surface.

Under normal conditions, how does the arterial Pco2 of the pupfish compare to a dog? What, specifically, accounts for this difference?

The arterial Pco2 of the pupfish is typically much lower than that of a dog due to their different respiratory systems. CO2 is highly soluble in water, so as water flows over the pupfish’s gills, CO2 rapidly diffuses out of the blood and into the environment decreasing the total amount of CO2 in the blood. Dogs (and other mammals’) ventilation is primarily adapted to remove CO2 from their body, CO2 builds up in the body at the end of each breath and must be expelled during exhalation. This difference in arterial pCO2 is specifically due to the different respiratory mechanisms and high ventilation rates of the pupfish.

	Solution A	Solution B
Oxygen concentration (mg/ml)	100	50
Po2 (mmHg)	50	100

All else being equal, which solution is more likely to have a higher osmolarity?

All else being equal, which solution is more likely to have a warmer temperature?

Which solution is most like blood?

Which solution has the higher oxygen solubility?

If solutions A and B were separated by a gas permeable membrane (like gill epithelium), what direction will oxygen diffuse?

Solution A has higher osmolarity

Solution B is warmer

Solution A is most like blood

Solution A has higher oxygen solubility

Oxygen would diffuse from solution B to solution A

Describe the basic model of how a fish ventilates its gills. (Summers and Ferry-Graham)

Continuous flow model with two cyclical pumps allowing for continuous unidirectional flow over gills. Buccal and opercular pumps are always in phase. The buccal cavity is always positive and the opercular cavity behind the gills is always negative in relation to each other.

1. The buccal cavity is depressed creating negative pressure that pulls water into the mouth.
2. The opercular cavity expands, but it is delayed to create a negative pressure behind the gills. (delay ensures movement of water towards gills)
3. Water moves across gills from the buccal cavity. The flow of water across lamellae is countercurrent to the flow of blood.
4. Mouth closes and the buccal cavity compresses increasing pressure in that cavity pushing water toward opercular cavity.

5. Opercular cavity contacts, deoxygenated water is forced out through opercular slit as opercular pressure is higher than in the environment.

What were the purposes of their experiments and what about the status quo led them to initiate their study in the first place? (Summers and Ferry-Graham)

Previous studies show that the pumps do not always work in perfect phase showing that there is sometimes backflow of water across the gills meaning that for some fish a portion of the respiratory cycle is co-current rather than counter-current. The researchers wanted to determine if asynchrony in pressure leads to flow asynchrony in order to challenge the continuous flow model.

Describe the methods that they used to investigate their problem. (Summers and Ferry-Graham)

1) Implanted sonomicrometers at 8 sites around the gills, spiracle and mouth to measure displacement (movement) throughout the ventilatory cycle.

2) Implanted pressure transducers and endoscopy probes into chambers in the mouth and gills to measure pressure and visualize flow movements.

What finding challenged the accepted model of gill ventilation in fishes? What are the implications for gas exchange? (Summers and Ferry-Graham)

Found that flow is not always unidirectional and thus is not always countercurrent which challenges the continuous flow model. Discovered that water flows through gills using 5 different respiratory modes, mode 2 involves water flowing in through the mouth and the spiracle and out through the gill slits. Tracking of the pressure gradients throughout mode 2 found that there is a negative pressure differential at a certain point that indicates the reversal of flow across the gills meaning water is moving co-current to blood at this time. Gas exchange through mode 2 is much less efficient due to this time of co-current rather than the much more effective counter-current exchange.

What, specifically, did Moalli and Burggren challenge about the existing model of gas exchange in frogs?

Challenged the assumption that cutaneous gas exchange is passive and not regulated, wanted to see if frogs actively regulated cutaneous gas exchange via dilations of blood vessels on the skin surface

Describe the experimental approach that they used to investigate their problem. (Moalli and Burggren)

-measured oxygen consumption and CO2 production across the skin and from the lungs by submerging frogs in water with a dome for air access, measured O2 and CO2 differences before and after respiration, also measured pH of water (which was diluted with NaOH) to determine how much CO2 was added cutaneously

-injected radioactive microspheres under 3 conditions: after air exposure, at the end of air exposure, and an hour after being returned to water. Different tissues were sampled and the radioactivity in these samples told them how the microspheres dispersed during these different conditions

-visual quantification of capillary recruitment in foot webbing using phenoxybenzamine which alters receptors that regulate blood flow through blood vessels. thought that if blood flow at the skin of a frog was under nervous control the drug should affect blood flow through the thin webbing of a frogs foot

What happened to oxygen consumption and carbon dioxide production when the frogs were moved from water to air, and then back to water?  What physiological mechanisms accounted for these changes? (Moalli and Burggren)

-Frogs in water typically have a respiratory exchange ratio of around 1.

Frogs from water to air

-Respiratory exchange ratio starts to fall (less than 1) due to a reduction in CO2 elimination relative to consumed oxygen.

-high O2 consumption due to oxygen availability in air

Frogs from air to water

- CO2 elimination spikes while oxygen consumption stays the same (or likely decreases), resulting in a respiratory exchange ratio higher than 1.

-CO2 elimination is high and CO2 is highly soluble in water

Frogs have very thin and delicate skin that allows it to function as such a good gas exchange organ but also makes them susceptible to water loss. When in air, frogs must regulate their cutaneous gas exchange by redirecting blood flow away from the skin to both decrease CO2 elimination and minimize water loss.

How did their experiments change the model that described cutaneous gas exchange in amphibians at that time? (Moalli and Burggren)

The old model assumes the skin is a respiratory organ with fixed properties. The new model assumes that the skin is a respiratory organ without fixed properties (thus is actively regulated).

 

Why do you think the frog relies on its skin to excrete carbon dioxide?

-lungs evolved in early tetrapods to provide accessory oxygen to the animal and thus were not well adapted for CO2 elimination as these animals were still relying on their gills for that

-Dermal armor started to be lost as the early amphibians started to spend more time on land and thus they were able to rely more on cutaneous CO2 removal

- spending more time on land also meant loss of the gills- great because accessory lung has so much oxygen in the air to meet metabolic requirements, but the lung is still bad at CO2 elimination thus relying on their skin for that

How do the mechanics of lung ventilation differ in frogs compared to other tetrapods? (Brainerd and Owerkowicz and Owerkowicz et al)

-frogs rely on buccal pumping to ventilate their lungs (constrains head shape)

-other tetrapods rely on the costal ventilation pump to ventilate their lungs (mammals, crocs, and birds lost the buccal pump but some lizards and snakes retained both the costal ventilation and buccal pump)

What is costal breathing, which amniotes use it, and which amniotes do not?  For those that do not, how do they ventilate their lungs?

 

-Costal ventilation involves the use of the rib cage and rib muscles to expand and contract the lungs.

-Mammals, birds, most reptiles use costal breathing

- turtles and varanid lizards can not do costal ventilation. Turtles rely on buccal pumping and specialized air holding gaps to ventilate their lungs. varanid lizards rely on gular pumping.

What are the two main anatomical constraints in the evolution of respiratory pumps in amniotes?

-head and tongue shape was constrained by buccal pump. flat broad heads with large buccal cavities and less mobile tongues were required to achieve maximal oxygen uptake using buccal pumping. head and tongue shape constraint meant less developed feeding strategies.

-locomotor endurance constrained by costal aspiration pump (axial constraint). early amniotes had a very slow walk that minimized side-to-side movement to reduce the interference of locomotion on costal aspiration.

Why has the loss of lungs in plethodontid salamanders been a successful evolutionary strategy?

Lunglessness (via relying solely on cutaneous gas exchange) freed the hyobranchial apparatus to evolve complex feeding movements in lungless salamanders. These salamanders have a very fast and extendable tongue that is supported by specialized muscles and cartilage in their throat.

Describe, in detail, the conflict between locomotion and ventilation in lizards.  Specifically, how have varanid lizards resolved this conflict?

-when a lizard runs, it shows lateral flexion which utilizes the bodies core muscles AKA the ones they need to ventilate their lungs. Each time that the animal flexes in one direction the lung is compressed on one side and expanded on the other. The compressed lung cannot pull in air and the expanded lung cannot effectively push air out meaning that lizards are not able to breathe effectively at high speeds.

-Varanid lizards resolved the axial constraint by evolving gular pumping (throat muscles force air into lungs and bypass costal aspiration breathing) to ventilate their lungs during locomotion.

In Owerkowicz et al, what were the conclusions? Describe the experimental design that led them to their conclusion.

-masks were placed on lizards and small tubes were placed in the animal’s throat and lung to measure pressure changes. found that after each gular pump pressure increases in the lung allowing them to conclude that gular pumping does pressurize the lungs.

-tied gular pouch and tracked ventilation rates while the lizard ran on a treadmill. found that limited gular pumping in varanid lizards decreased ventilation rates and oxygen consumption and therefore locomotion.

concluded that varanid lizards are able to maintain effective ventilation during locomotion due to the role of the gular pump.

What is the arrangement of air flow relative to blood flow called in the bird lung? How does the Po₂ of the arterial blood compare to the Po₂ of the expired air?  How does this compare to reptilian and mammalian lungs?

 

- the arrangement of air relative to blood flow in the bird is referred to as cross-current

-pO2 of the arterial blood is higher than that of expired air for birds

-mammals and reptiles arterial blood pO2 is just slightly above the spent air pO2

-birds have higher arterial po2 than mammals, enabling them to have better performance in high altitude with lower oxygen concentrations because they can squeeze all of the oxygen out of each breath of air.

What are two other differences between bird and non-crocodilian reptile lungs?

-bird lungs are isovolumic (not changing volumes) while reptile lungs do change volumes. birds have variable volume air sacs that ventilate the lungs rather than the lung changing volume.

-Reptiles have tidal gas exchange while birds have unidirectional gas exchange.

What is the purpose of the study of Bretz and Schmidt-Nielsen?

To better understand the pattern of air flow through the avian respiratory system. Wanted to see where and when air flow was occurring.

Describe the design of their experiment.(Bretz and Schmidt-Nielsen)

-mallard ducks had ports implanted into the trachea and gas sampling tubes into different air sacs

-argon gas was used as a tracer gas to track the movement of gas through the system

-ducks were harnessed and breathed in the argon gas, partial pressure of argon gas was measured by tubes in air sacs allowing them to track its movement through multiple ventilatory cycles

What was the most important finding of the study by Bretz and Schmidt-Nielsen?

found that it takes two ventilatory cycles to move a bolus of air through an avian respiratory system. found that the posterior air sacs contained the argon during the first cycle and that the anterior air sacs contained the argon during the second cycle and after the third the argon was (essentially) gone. leftover argon after 3rd round can be associated with the effective cross-current gas exchange that birds use to squeeze out every bit of oxygen from the air.

Describe how air flows in the avian respiratory system during a single ventilatory cycle.

-first inspiration: inhaled air goes into posterior air sacs, and expansion of both sets of air sacs creates a negative pressure that pulls air in from the environment into the posterior air sacs while the anterior air sacs fill with stale gas that has already passed the gas exchange organ

-first exhalation: posterior and anterior air sacs are compressed increasing their pressure and forcing fresh air into the parabronchi where gas exchange occurs

-second inspiration: stale air (now depleted of O2) moves into anterior air sacs while a new breath of air has entered the posterior air sacs. remember that this movement occurs due to both air sacs expanding.

-second exhalation: air sacs contract pushing stale air out of the lung and anterior air sacs and back into the environment

Above is 2 vent. cycles to move entire bolus of air. Below is single vent cycle, air only gets through parabronchi at this point.

1.      Inhalation: both sets of air sacs expand increasing internal negative pressure and allowing air to enter the trachea and pass the syrinx

2.      Air travels through the primary bronchi through the intrapulmonary bronchus and into the posterior air sacs

3.      Negative pressure in anterior air sacs pulls stale air through the parabronchi

6.      Expiration: elastic recoil of the chest compresses sacs, increasing positive pressure

7.      The posterior air sac shrinks and pushes air into the parabronchi (for gas exchange)

8.      Air from the now shrinking anterior air sac is pushed into the trachea and out of the bird

What conventional wisdom are they challenging? (Farmer and Sanders)

-Conventional wisdom says that birds are “aliens” with respiratory systems whose lungs do not change volume and who experience unidirectional flow and because alligators’ lungs change volumes and do not possess air sacks they are probably like traditional lizards.

-challenging that alligators do not have unidirectional flow and that air sacs are a necessary anatomical structure to facilitate unidirectional flow

What hypothesis are they testing in their study? (Farmer and Sanders)

Farmer and Sanders hypothesized that airflow in alligator lungs is unidirectional.

What did their experiments show? Do they make sense? Why? (Farmer and Sanders)

Found that although the ventilation in the trachea is tidal, the airflow through both the dorsal and the ventral passages is unidirectional. The findings make sense as it finally connects the evolutionary history between birds and crocodiles and show how unidirectional air flow did not just appear in evolution out of nowhere- BUT doesn’t make sense as to why/how alligators kept unidirectional flow but not air sacs.

What is the evolutionary significance of their findings? (Farmer and Sanders)

-It is almost undoubtable that dinosaurs had unidirectional airflow in their lungs.

-“Hepatic piston mechanism of breathing, which crocodilians have but birds lack, does not preclude the evolution of unidirectional flow”

-presence of air sacs and bird-like light airy bones can not be used as identification of unidirectional flow in fossil taxa.

Why are the findings considered controversial?(Farmer and Sanders)

It is difficult to determine if dinosaurs had rigid lungs because soft tissue doesn’t fossilize well. Air sacs, or more spaces that would’ve accommodated air sacs were definitely present in some birdlike dinosaurs, but the fossil record doesn’t allow us to fully understand the rigidity of a dinosaur lung that is very important to understand how they fit into the evolutionary process.

What factors determine vascular resistance in the cardiovascular system? Which factor is most important?

-viscosity of blood, length of tube that flow is occurring through (veins, arteries, etc.), and radius of such tube all determine vascular resistance in the cardiovascular system.

-Radius is the most important determinant of vascular resistance as resistance is inversely proportional to the fourth power of the vessel radius. Halving the radius of a vessel increases the resistance to flow through the vessel by a factor of 16. Thus, a smaller vessel (capillaries) will have a slower flower than a larger vessel (arteries).

What factors determine whether or not blood flows from one region of the cardiovascular system to another?

- flow=change in pressure/ resistance

- blood is flowing following the pressure gradient (high to low)

- high resistance vessels impede blood flow

-cardiac output= heart rate x stroke volume, higher cardiac output increases overall blood flow

- gravitational forces are very relevant for blood flow in air breathers (blood pooling in feet)

How does blood pressure and velocity change as it travels through the cardiovascular system? What specifically accounts for these changes?

-pressure and velocity are highest after the heart ejects blood into the arteries

-blood loses energy through heat as it moves against vessel walls

-blood pressure is very low in capillaries and the lowest in the veins (only slightly above atm, keeping blood flowing to heart)

-blood velocity is the lowest in the capillaries due to branching and high cross-sectional area allowing for efficient gas exchange to occur

What are three important differences between the cardiovascular system of fishes and that of tetrapods?

-fish hearts are only receiving/ pumping deoxygenated blood to the gills while tetrapod hearts are pumping both oxygenated and deoxygenated blood

-fish hearts only contain one atria and one vetricle while tetrapod hearts have fully divided (two) atrias and some have a fully devided ventricle as well

-fish heart only has one pump (tetrapod’s have 2) fish must deal with the delicate tradeoff between efficient gas exchange and protecting the fine gill capillaries. tetrapods extra pump allows for efficent gas exchange without harming capillaries with high pressure.

How do systemic blood pressures in non-crocodilian reptiles compare to those in mammals and birds? How about pulmonary blood pressures? Explain what accounts for the similarities and differences.

-Mammals and birds have higher systemic blood pressure than pulmonary blood pressures, while non-crocodilian reptiles (like turtles) have pulmonary blood pressures that are about equal to their systemic blood pressure. Mammals and birds also have higher systemic blood pressure in general than a reptile, but all three have similar pulmonary pressures.

-These differences can be accounted for by the lack of anatomical separation between the pulmonary and systemic circuits in the heart of non-crocodilian reptiles (don’t have a fully divided ventricle). Similarly to fish gills in that if the systemic pressure was too high, the capillaries would be blown out. Intraventricular septum and uncoupling of pressures between lung and systemic circulation allows for mammal and bird circulation patterns.

What directly determines the amount of right-to-left cardiac shunting in reptiles (all reptiles this time)

-ratio of pulmonary to systemic resistance

Stimulation of the vagus nerve (via the parasympathetic NS) causes the blood vessels in the lungs to constrict leading to higher resistance in pulmonary circulation which leads to a right to left shunt

How does cardiac shunting in turtles, lizards, and snakes differ from cardiac shunting in alligators?

-Crocodilians and alligators rely on the maintenance of an extra aortic arch that is not occurring within the ventricle itself due to the absence of an intraventricular septum. (Crocs only do R-L shunting)

-In other reptiles, when they are breathing they switch to left to right shunting which switches the path of arterial blood to where it is being added to venous blood, creating this recycling system where blood has multiple chances to pass through the lungs and pick up oxygen/ eliminate CO2.

According to Hicks and Wang, what are three possible benefits from having the ability to right to left shunt? Please explain how each works.

-Promotes oxygen uptake- R-L shunting maintains the partial pressure of oxygen in the range of the oxy-hemoglobin dissociation curve that facilitates loading and unloading of oxygen onto hemoglobin. (if pO2 is too high it will never dissociate from hemoglobin and thus is useless to the tissues) R-L shunt regulation keeps the pO2 at the “sweet spot”.

 

-Direct oxygenation of myocardium- Blood comes in from the lung at such a high pO2 that it is too high on the oxy-hemoglobin dissociation curve to effectively unload oxygen, R-L shunting adds a small amount of deoxygenated venous blood that brings down the pO2 and facilitates oxygen unloading directly into the tissue of the heart.

 

-Reduces plasma filtration in lung – Capillaries have a constant fluid leak that is dependent on the pressure in the capillaries. Reptiles having the ability to only expose the lung to the high pressure that would cause fluid leak during ventilation (L-R shunting) and are able to clear any excess leaked fluid out during the apnea periods (R-L shunting)

  Under what circumstances is a right to left shunt observed in reptiles? When is a left to right shunt observed?

R-L shunting is prioritized when metabolism is low and the body is in a state of rest (parasympathetic NS is active) and is either slowed or switched to L-R shunting in times of high metabolic rates with “flight or fight” (sympathetic NS is active) enabled. This reduction or total switch allows for more blood to be sent to the lungs to be oxygenated in times of high oxygen demand.