Chapter 48
Gas Exchange in Animals
Gas Exchange in Animals
Physical Processes of Respiratory Gas Exchange
Adaptations for Respiratory Gas Exchange
Gas Exchange in Human Lungs
Blood __________ of Respiratory Gases
Regulation of Breathing
Physical Processes of Respiratory Gas Exchange
The respiratory gases are oxygen (O2) and carbon dioxide (CO2).
Cells require O2 from the __________ to produce __________ by cellular respiration.
Cellular respiration produces CO2 as an end product, which must be lost to the environment to prevent toxic effects.
__________ is the only means to exchange these gases.
Physical Processes of Respiratory Gas Exchange
It is easier to obtain O2 from __________ than from __________.
The O2 content in __________ is about 20 times higher than in __________.
O2 diffuses 8,000 times more rapidly in __________.
Breathing air, which is less __________ and less __________ than water, requires less work for the animal.
Physical Processes of Respiratory Gas Exchange
Slow molecular diffusion of O2 is a problem because O2 must diffuse through the aqueous cytoplasm of the cell to reach the mitochondria.
Diffusion of O2 in water is so slow that even cells with low metabolism must be only 12 mm from the O2 source.
Animals that have no internal transport of O2 are either severely limited in size or have evolved bodies that are flattened or built around a central cavity.
Physical Processes of Respiratory Gas Exchange
Most water breathers have body temperatures close to the water temperatures surrounding them (they are ectothermic).
As temperatures rise, so does the animals metabolism and the need for more O2.
However, warm water holds __________ dissolved O2 than cold water.
Therefore, more energy must be expended to get a decreasing O2 supply.
Physical Processes of Respiratory Gas Exchange
The decrease in O2 as altitude increases depends on barometric pressure.
At sea level the partial pressure of O2 (PO2) is about 159 mm Hg.
At the top
of
The diffusion rate of O2 into cells under reduced PO2 is greatly reduced, and the O2 uptake is constrained.
Humans venturing to great heights must breathe O2 from pressurized containers.
Physical Processes of Respiratory Gas Exchange
CO2 diffuses out of the body at about the same rate as O2 diffuses in.
The partial pressures of the gases are not the same because the amount of CO2 in the atmosphere is low (0.03 percent).
This usually means there is a good partial pressure gradient for loss of CO2 from air-breathing animals.
For water-breathing animals, a surrounding environment high in decaying organic material (generating high levels of CO2) may be unable to support life.
Physical Processes of Respiratory Gas Exchange
Ficks law of diffusion:
Q = DA (P1 - P2/L)
Q is the rate at which a substance diffuses between two locations.
D is the diffusion coefficient.
A is the cross-sectional area over which the substance is diffusing.
P1 and P2 are the partial pressures of the gas at two locations.
L is the distance between these locations.
Physical Processes of Respiratory Gas Exchange
Animals maximize the diffusion coefficient by using air rather than water for diffusion whenever possible.
Other adaptations for maximizing respiratory gas exchange must influence the surface area for exchange (A) or the partial pressure gradient across that surface area [(P1 P2)/L].
Adaptations for Respiratory Gas Exchange
Anatomical adaptations to maximize the __________ __________ for gas diffusion (A in Ficks law) include external and internal gills and lungs.
External gills are branched and folded thin membranes on the body surface that provide a __________ diffusion area.
Internal gills are similar to external gills but are __________ from damage by their location inside body cavities.
Adaptations for Respiratory Gas Exchange
Lungs are __________ cavities for gas exchange in air breathers.
Lungs are highly divided to provide __________ surface area and are elastic to permit inflation and deflation.
Insects have a unique system for gas exchange called tracheae, which are branched tubules.
The tracheae penetrate through fine terminal branches to tissues and cells, presenting an enormous surface area for gas exchange.
Adaptations for Respiratory Gas Exchange
Driving diffusion of gases across gas exchange membranes (i.e., maximizing the partial pressure gradients(P1 P2)/L in Ficks law) is accomplished in several ways:
Thin membranes shorten the diffusion path (L).
__________ brings in fresh air with the high PO2 and the low PCO2.
Perfusion by the circulatory system helps maintain the low PO2 and the high PCO2 on the inside of exchange surfaces.
Adaptations for Respiratory Gas Exchange
An animals gas exchange system is made up of its gas exchange surfaces and the mechanisms it uses to ventilate and perfuse those surfaces.
Adaptations for Respiratory Gas Exchange
The tracheae of insects begin at openings on the outside of the body called spiracles, which admit air.
By ever-finer branches of air tubes, O2 is delivered to air capillaries not more than a few micrometers away from cell mitochondria.
Since O2 diffuses at a higher rate in air than in water, this system ensures an abundant supply for high metabolism.
However, small diameter and total length of these dead-end airways limits body size.
Adaptations for Respiratory Gas Exchange
Some insects that stay under water for extended periods carry a bubble of air with them.
Even when the O2 lowers in the bubble, more diffuses in from the water, giving them nearly limitless time underwater.
Adaptations for Respiratory Gas Exchange
In fish, water passes into the mouth, over the gills and out the opercular flaps. This constant water flow over the gills maximizes the PO2 on the external surfaces.
Blood flow on the internal side minimizes the PO2 by sweeping O2 away rapidly.
The gills maximize the surface area for gas exchange (A).
Each gill has hundreds of subunits called gill filaments; each filament is covered with lamellae, the gas exchange surfaces.
The structure of the lamellae minimizes the path length for diffusion (L).
Adaptations for Respiratory Gas Exchange
The perfusing blood flow on the inner surface of the lamellae is __________.
Afferent (to gills) and efferent (away from gills) blood vessels ensure a countercurrent flow to maximize the PO2 gradient.
Some fish, such as sharks and tuna, swim almost constantly with mouths open to ventilate their gills.
Most fish use a two-pump mechanism activated by opening and closing the mouth to push water over the gills.
Adaptations for Respiratory Gas Exchange
Birds can sustain high activity levels much longer and at higher altitudes than mammals can.
Air flows __________ through the lungs rather than in and out via the same airway as in mammals.
Birds also have air sacs in the body that are connected to the lungs, but are not gas exchange surfaces.
In birds, the bronchi divide into parabronchi that run parallel to one another. Branching from the parabronchi are air capillaries where gas exchange occurs.
Adaptations for Respiratory Gas Exchange
The pathway of air through bird lungs was determined by experiments in which oxygen sensors where placed in different locations.
A single breath remains in the birds gas exchange system for two cycles of inhalation and exhalation.
Air passes from the trachea to posterior air sacs, through the lungs, into anterior air sacs, and back to the trachea.
The system results in a continuous flow of air with high PO2.
Adaptations for Respiratory Gas Exchange
In mammal lungs, ventilation is tidal: Air flows in and out by the same route.
At rest, the amount of air exchanged is the tidal volume.
The additional volume of air taken in by inhaling deeply is the inspiratory reserve volume.
The additional volume we can exhale is the expiratory reserve volume.
The total of these three volumes in the vital capacity.
Adaptations for Respiratory Gas Exchange
Even with forceful breathing, there is residual volume of air that keeps the lungs from collapsing. Some of this exists in what is called the anatomical dead spaceairways in which gas exchange cannot occur.
The total lung capacity is the sum of the residual volume and the vital capacity.
Adaptations for Respiratory Gas Exchange
In tidal breathing, the incoming air mixes with the stale air remaining in the lung, which severely limits the PO2 gradient.
The volume of this stale air is the sum of the residual volume and the expiratory reserve volume.
Tidal breathing also reduces gas exchange efficiency by not permitting countercurrent gas exchange between air and blood.
To offset the inefficiencies of tidal breathing, mammalian lungs have an enormous surface area and a very short path length for diffusion.
Gas Exchange in Human Lungs
The air pathway in humans consists of the following components:
An oral or nasal cavity, followed by the pharynx (an area for both food and air).
The larynx (voice box), which leads to the trachea.
The trachea branches into two bronchi (both of these have cartilage support).
The bronchi branch repeatedly into bronchioles, which terminate in the alveoli.
Gas Exchange in Human Lungs
The alveoli are thin-walled air sacs and are the sites of gas exchange.
Capillary blood vessels closely surround the alveoli, resulting in a diffusion path of less than 2 m, which is less than the diameter of a red blood cell.
Gas Exchange in Human Lungs
Two adaptations that aid the breathing process in mammals are mucus and surfactants.
Cells lining the airways produce a sticky mucus that captures dirt and microbes.
This mucus is cleared by __________ beating upward toward the trachea and pharynx, where it is swallowed.
This phenomenon has been called the mucus escalator, and it can be immobilized by smoking.
In cystic fibrosis, a faulty __________ channel leads to mucus that is thick and difficult to clear, resulting in blockage and infection.
Gas Exchange in Human Lungs
A __________ is a chemical substance that reduces the surface tension of a liquid.
The aqueous lining of the lung has surface tension that must be overcome to permit inflation.
Cells in the alveoli produce surfactant molecules when they are stretched.
Premature babies may develop respiratory stress syndrome if they are born before cells in the alveoli are producing surfactant.
Gas Exchange in Human Lungs
The human lungs are suspended in the thoracic cavity in separate, closed pleural cavities.
The thoracic cavity is bounded by the shoulder girdle, rib cage, and the diaphragm muscle.
Breathing involves changes in volume of the thoracic cavity.
An increase in its volume creates negative pressure (suction) inside the pleural cavity.
Between breaths, there is a slight negative pressure inside the pleural cavity keeping the alveoli partially inflated.
Gas Exchange in Human Lungs
With inhalation, the diaphragm muscle contracts downward to create suction, and air flows into the lung.
When the diaphragm relaxes, it pushes upward, and exhalation occurs.
In the rib cage, intercostal muscles lift the ribs up and down to increase thoracic cavity volume.
External intercostal muscles expand the thoracic cavity and increase the volume of air inhaled.
Internal intercostal muscles decrease the volume of the thoracic cavity and increase the amount of air exhaled.
Blood Transport of Respiratory Gases
Ventilation and perfusion work together. Ventilation delivers O2 to the environmental side of the exchange surface; perfusion delivers CO2 to the exchange surface, where it diffuses out and is swept away by ventilation.
As O2 diffuses from the alveoli into the blood, it is swept away and delivered to the cells and tissues of the body.
Most O2 is carried by the oxygen-binding pigment, hemoglobin, in red blood cells.
__________ has 60 times the capacity of plasma to transport O2.
Blood Transport of Respiratory Gases
Hemoglobin is a protein consisting of four polypeptide subunits, each with a heme (iron-containing) group.
Each heme group can reversibly bind a molecule of O2.
As O2 diffuses into the blood, it binds to hemoglobin, increasing the PO2 gradient and driving diffusion of O2 into red blood cells.
Blood Transport of Respiratory Gases
The ability of hemoglobin to bind and release O2 depends on the PO2 of its environment.
When the PO2 of blood plasma is high, as in the lung capillaries, each hemoglobin complex can carry four molecules of O2.
As the red blood cell circulates to the body, the PO2 values drop, and the hemoglobin releases some of the O2 it is carrying.
Blood Transport of Respiratory Gases
The relationship between saturation of the hemoglobin polypeptides and PO2 values follows a sigmoid curve.
O2 binding by hemoglobin is influenced by positive cooperativity; that is, binding the first molecule makes the second binding easier, and so on.
However, it takes a relatively greater PO2 to bind the fourth molecule and achieve 100 percent saturation.
Blood Transport of Respiratory Gases
Carbon monoxide (CO) binds to hemoglobin with a much higher affinity than does O2.
CO is a deadly poison, as it destroys the ability of hemoglobin to transport O2.
Blood Transport of Respiratory Gases
When blood circulates through the body, it releases, on average, only about one in four of the molecules of O2 it carries.
The hemoglobin keeps 75 percent of its O2 in reserve for __________ demands.
If a tissue is starved for O2 and its local PO2 falls below 40 mm Hg, the hemoglobin will release the reserved O2 to the starved tissue.
Blood Transport of Respiratory Gases
Myoglobin in muscle cells is an oxygen-binding molecule that can take up one molecule of O2.
It has a higher affinity for O2 than hemoglobin does and provides an oxygen reserve for high metabolic demand or when blood flow is interrupted.
Diving mammals have high concentrations of myoglobin in their muscles, allowing them to stay under water for long periods.
Even in nondiving animals, muscles called on for extended periods of work frequently have more myoglobin than muscles used for short, intermittent periods.
Blood Transport of Respiratory Gases
Hemoglobin of human adults consists of two kinds of polypeptides: -globin and -globin.
Before birth, human fetuses have two -globin chains and two -globin chains which results in a greater __________ for O2.
This difference in O2 affinity facilitates transfer of O2 from the mothers blood to fetal blood in the placenta.
Mammals such as llamas that live at high altitudes have hemoglobin that becomes saturated with O2 at lower PO2 values than that of other animals.
Blood Transport of Respiratory Gases
The influence of pH on the function of hemoglobin is known as the Bohr effect.
This effect occurs when the pH of the blood falls and the H+ ions bind to hemoglobin and decrease its affinity for O2.
The oxygen-binding curve shifts to the right.
The hemoglobin will then release more O2 to the tissues where pH is low.
Blood Transport of Respiratory Gases
Another regulator of hemoglobin function is 2,3 bisphosphoglyceric acid (BPG).
In red blood cells BPG combines with deoxygenated hemoglobin and causes it to have a lower affinity for O2.
The result is that the hemoglobin releases more of its bound O2 to tissues than usual.
If a person goes to a high altitude or starts exercising, the level of BPG goes up, and hemoglobin releases more O2 where it is needed.
Blood Transport of Respiratory Gases
CO2 is highly soluble, moving easily through cell membranes into the blood, where the partial pressure of CO2 is lower.
Most CO2 is transported as bicarbonate ion (HCO3).
Capillary cells and red blood cells produce carbonic anhydrase, which speeds conversion of CO2 to H2CO3.
The H2CO3 dissociates, and bicarbonate ions enter the plasma in exchange for chloride ions (Cl).
This conversion reduces the partial pressure of CO2, promoting the diffusion of CO2 out of the tissue cells.
Blood Transport of Respiratory Gases
Some CO2 also combines with hemoglobin.
In the lung, the CO2 and bicarbonate reactions are reversed, also catalyzed by carbonic anhydrase.
CO2 diffuses from blood plasma into the alveolar air and is exhaled.
As the PCO2 in the blood falls, more bicarbonate is converted into CO2.
Regulation of Breathing
Breathing is controlled by the autonomic nervous system.
The brain stem generates and controls the breathing rhythm.
Groups of neurons within the medulla increase their firing rate just prior to inhalation.
With increased firing, the diaphragm contracts and inhalation occurs.
When the firing stops, the diaphragm relaxes, and exhalation occurs.
Exhalation is actually a passive elastic recoil of lung tissue.
Regulation of Breathing
When breathing demands are high, as during exercise, the motor neurons for the intercostal muscles are fired to increase inhalation and exhalation volumes.
Brain areas above the medulla modify breathing to allow speech, eating, coughing, and emotional states.
Regulation of Breathing
Experiments in which subjects breathed air with varying PO2 and PCO2 concentration led to the conclusion that in mammals, CO2 sensitivity is very high, but O2 sensitivity is remarkably low.
Other experiments showed that __________ of the blood is the main feedback information for breathing rate.
Regulation of Breathing
CO2 sensors are located on the medulla surface near the neurons that generate the breathing rhythm.
O2 sensors are in tissue nodes on the aorta and carotid arteries called carotid and aortic bodies.
If PO2 of blood drops, or if blood pressure drops, chemoreceptors in the bodies send nerve impulses to the brain breathing center.
Animation 48.1 Airflow in Birds
Animation 48.2 Airflow in Mammals
Video 48.1 Endoscopic view of trachea, bronchi, and bronchioles
Video 48.2 Involvement of blood vessels in gas exchange