AL F7 Bio.notes Gas Exchange P.1 GAS EXCHANGE
erobic respiration demands a continuous supply of oxygen into cells accompanied by the rapid removal of the carbon dioxide they produce. These two processes comprise Aexternal respiration or Gas Exchange.
I) SIZE AND LAND LIVING
Two evolutionary trends have been shaping gas exchange mechanisms in animals, namely increasing s______and increasing complexity, and the colonisation of l_____.
A) DIFFICULTIES PRESENTED BY INCREASE IN SIZE
Respiratory gases d______into and out of cells. Diffusion in water is a fairly slow process, its efficiency is determined by:
the c______g______, -- the external oxygen concentration and the rate at which oxygen is used up internally;
the a______over which diffusion takes place;
the d______over which the concentration gradient must operate (the diffusion path). a) Small Organisms
large s______a______relative to the volume, and short d______from the surface to the innermost respiring cell. in uni______organisms, coelenterates, flatworms, and other small animals whose surface is very large relative to their volume. b) Larger or More Active Animals
1) Slow-moving :
As animals increase in size their surface area to volume ratio d______so that, above a certain size, diffusion alone cannot supply the respiratory needs.
For s_____-moving animals like the earthworm, the body surface may still be large enough to supply the gas exchange requirements but the movement of the respiratory gases within the organism has to be speeded up by a trans____ system, the cir______system.
2) Larger & more active :
For l______or more a______animals, the body surface alone is NOT usually suffice to supply the respiratory needs. In such cases, large, spec______resp______sur_____ such as lungs and gills have developed.
Another development is some kind of vent______mechanisms. Such mechanisms ensure the constant renewal of the respiratory m_____ (either air or water) at the respiratory surface to maintain the c______g______.
Q. Which three development of the respiratory system are associated with an increase in size of animals ? AL F7 Bio.notes Gas Exchange P.2
B) The Colonisation of land
Organisms leaving water to live on land face the difficulty of keeping the respiratory surface mo____ in a drying terrestrial condition. Respiratory gases can only diffuse through cells in aq______solution.
Another difficulty presented by life on land is that the soft gills of aquatic animals depend (at least in part) on the surrounding water for su______, and so collapse in air.
Q. Explain with examples how land animals solve the above problems associated with gas exchange in air.
Q. Suggest four major processes that are required for living cells to acquire oxygen in Mammals.
II) GAS EXCHANGE IN MAMMALS
STRUCTURE OF THE MAMMALIAN RESPIRATORY SYSTEM
The mammalian lungs lie in the t______cavity, bounded by the r___ and a dome-shaped muscle, the d______. They are enclosed in a double layer of membranes; the p______membranes. These are separated by a thin layer of pleural f______that acts as a l______, reducing the f______when membranes to sl____ over one another during ventilation movements.
The lungs consist of two elastic sacs, each of which is sub-divided into numerous tiny sacs or a______which are lined with a film of m______. The area of the human lungs is between thirty and 90m2, that is, some 30-50 x the body surface area. AL F7 Bio.notes Gas Exchange P.3
The lungs communicate with the outside through a system of a___-tubes. The t______, leads from the ph______where its entrance, the glottis, is guarded by a flap, the epi______. The latter closes during sw______to prevent food entering the air passages.
The trachea divides into two b______which pass to each lung. The bronchi branch on entering the lungs to form a mass of smaller tubes called b______, the smallest branches of which lead to the alveoli.
The trachea, bronchi and bronchioles composed of smooth muscle lined with mu____ membrane.
Embedded in the wall of the trachea and bronchi are C-shaped rings of c______which serve to keep the air passageways open.
The nasal passages, the trachea, the bronchi and their larger branches, have a ci_____ membrane. The cilia waft particles, that become entangled in mu_____, to the back of the throat where they are s______.
B) VENTILATION
Ventilation is achieved by lowering or raising the air p______in the lungs relative to atmospheric pressure, so that air either passes into or out of the lungs.
Pressure changes are due to changes in thoracic volume brought about by contraction and relaxation of the d______and the i______muscles..
i.e. Volume changes lead to pressure changes, which lead to flow of gases to equalize the pressure.
△V →△P → F (flow of gases)
a) Inspiration
During i______the diaphragm contracts and fl______, this enlarges the vol____ of the thoracic cavity. The latter is further enlarged by the contraction of the inter______muscles, which move the r____ upwards and outwards.
As the thoracic cavity enlarges, the elastic l______, being attached to the rib cage and diaphragm by the pl_____ membranes, also ex______. The expansion of the lungs lowers the p______inside the lungs relative to atmospheric pressure and air passes in from outside. AL F7 Bio.notes Gas Exchange P.4 b) Expiration
Quiet expiration is largely a p______process that depends more on the natural elasticity of the lungs than on muscle contraction.
As the diaphragm and the intercostal muscles r_____ and resume their initial length, the ribcage falls and ret_____ to its former position. This decreases the t______volume.
The lung tissues rec___, the lung v______decrease. Thus both the thoracic and the lung volume decrease. This compresses the alveolar air and i______the intra-pulmonary pressure above that of the a______, causing air to be expelled.
In forced expiration, two extra sets of muscle help to raise in___-pulmonary pressure, forcing air out of the lungs:
The i______intercostal muscles forcefully pull the rib cage down and the abd______muscles contract, increasing pressure within the abdomen and pushes up on the dia______.
C) GAS EXCHANGE IN THE ALVEOLUS
The alveolar wall consists of a si___ layer of cells, the ep______, 1m thick. Each alveolus is surrounded by a network of c______; these receive blood from a branch of the p______artery and convey it to a branch of the pulmonary v______.
Oxygen d______in the film of moisture that lines the alveolus, and then d______along a concentration gradient into the blood; carbon dioxide diffuses in the opposite direction. The two- cell-thick layer that separates the blood and the alveolar air offers little r______to the passage of the respiratory gases.
The concentration gradient of carbon dioxide and oxygen between the alveolar cavity and the blood is maintained by : a) Continual removal of dissolved O2 from plasma by h______(to become oxyhaemoglobin). b) Blood flow which re______oxy______blood from the alveoli, c) V______of the alveolar cavity. AL F7 Bio.notes Gas Exchange P.5
Reference reading : Diseases of the Respiratory System
The condition of the airways and the pressure difference between the lungs and atmosphere are important factors in the flow of air in and out of lungs. Many diseases affect the condition of the airways.
Asthma narrows the airways by causing an allergy-induced spasms of surrounding muscles or by clogging the airways with mucus.
Bronchitis is an inflammatory response that reduces airflow and is caused by long-term exposure to irritants such as cigarette smoke, air pollutants, or allergens.
Cystic fibrosis is a genetic defect that causes excessive mucus production that clogs the airways.
D) CONTROL OF RESPIRATION a) Respiratory centers and Breathing rhythm
The rate and depth of ventilation are controlled by the respiratory centre situated in the me____ oblongata of the brain.
The respiratory centre consists of two regions: the ins______centre which acts to increase inspiratory rate, and the ex______centre which stops inspiratory activity and promotes expiration.
The breathing rhythm is neurogenic, it is initiated through impulses by the respiratory centres on the respiratory muscles. It is basically involuntary but conscious control is also possible by the cerebrum. E.g. singing
The breathing rhythm are regulated through n______feedback involving the respiratory centres and various central and peripheral chemoreceptors, adjusting concentration of respiratory gases according to body needs. . AL F7 Bio.notes Gas Exchange P.6 b) Basic breathing rhythm:
The inspiratory centre sends out nerve impulses to the diaphragm and intercostal m _____ causing them to increase the rate at which they contract.
Inspiratory activity inflates the alveoli, and s ______receptors located here and in the bronchial tree are stimulated.
Impulses are discharged to the expiratory centre which automatically cuts off inspiratory activity. The respiratory muscles therefore relax and ex ______takes place.
After this has occurred, the alveoli are no longer stretched and the stretch receptors no longer stimulated. Therefore the ex ______centre becomes inactive and inspiration can begin again. The whole cycle is repeated rhythmically throughout the life of the organism.
ii) Regulation of Breathing rate / Concentration of respiratory gases
The adjustment of the d______and r_____ of breathing plays an important part in the hom______control of the respiratory gases. The supply of oxygen and removal of carbon dioxide must keep pace with the varying n_____ of the animal.
As the metabolic rate increases, so the oxygen requirement and carbon dioxide production will also increase. If it contains too much oxygen, cells will be destroyed by unwanted ox______; on the other hand, accumulation of carbon dioxide will tend to lower the _____ (increase the acidity) of the body fluids, which can cause en______inhibition.
The major stimulus that controls the rate and depth of breathing is the concentration of CO2 in the blood (partial pressure of CO2). Oxygen concentration also has an effect on the breathing rate. AL F7 Bio.notes Gas Exchange P.7
However, under normal circumstances there is an abundance of oxygen available, and its influence is relatively minor.
Changes in PCO2 are detected by chemo______in the respiratory centres of the m______. These receptors actually respond to a decrease in the ____ (increased acidity) of the cerebrospinal fluid, but this accurately reflects increased arterial PCO2.
During increased activity the level of ______rises and stimulates the ______in the medulla, causing nerve i______to be sent along efferent nerves to the r______muscles.
The arrival of these impulses increases the v______rate (breathing rate x tidal volume), and so speeds up the re______of carbon dioxide.
Chemoreceptors sensitive to PCO2, are also found in the c______and a______bodies. These peripheral receptors monitor the a______PCO2 directly, but seem to play a subsidiary role to the central chemoreceptors in the medulla.
Some of the chemoreceptors in the carotid and aortic bodies respond to decreased o______concentration (PO2). In normal circumstances they appear to enhance the effects of increased PCO2 rather than initiate corrective mechanisms themselves.
Increased Increased Arterial PCO Arterial PCO2 2
Increased PCO , Increased PCO2 , Decreased pH in2 Decreased pH in cerebrospinal fluid cerebrospinal fluid
Peripheral Peripheral Central chemoreceptors chemoreceptors Central chemoreceptors chemoreceptors in medulla in carotid and aortic in medulla in carotid and aortic (mediate 70% of the bodies (mediate 30% of (mediate 70% of the bodies (mediate 30% of response) the response) response) the response)
Afferent impulses Negative feedback Respiratory centers Respiratory centers in Medulla in Medulla
Efferent impulses
Respiratory muscles Respiratory muscles
Increased ventilation Increased ventilation (more CO2 exhaled) (more CO2 exhaled)
Arterial PCO2 and pH Arterial PCO2 and pH return to normal return to normal
Negative feedback mechanism by which changes in PCO2 and pH regulate ventilation AL F7 Bio.notes Gas Exchange P.8
Reference reading:
1. Breathing oxygen
Although breathing oxygen at 2 atmospheres presents no problems for short periods of time, oxygen toxicity develops rapidly when PO2 is great than 2.5-3 atmospheres. Excessive oxygen concentration generate large amount of harmful free radicals. The result is profound central nervous system disturbances, culminating in coma and death.
2. Hyperventilation
i) Homeostatic regulation
It can results from increased PCO2 or a drop in blood pH which excite the central chemreceptors. As a result, the depth and rate of breathing are increased. The enhanced alveolar ventilation quickly removes CO2 out of the blood, raising the blood pH. Hyperventilation is usually self-limiting, ending when homeostatic blood PCO2 level are restored.
ii) Anxiety attacks
People experiencing anxiety attacks may hyperventilate involuntarily, to the point where they become dizzy and may faint. This reflects the fact that low CO2 levels in blood cause cerebral blood vessel to constrict, reducing blood supply to the brain. Such attacks may be prevented by breathing into a paper bag. Why?
iii) Voluntary Hyperventilation
This can lead to abnormally low PCO2, respiration movement may become inhibited and become slow and shallow -- Breathing cessation. Normal breathing may occur until arterial PCO2 rises again and stimulates respiration.
Sometimes competing swimmers voluntarily hyperventilate. Why would they do so and why it is an extremely dangerous practice?
iv) Effect of high altitude
When one move (on a long term basis) from a sea level region to the mountains, where air density and PO2 are lower, the body initially responds with headaches, nausea, and dizziness; even though the body begins to make adaptive adjustments called acclimatization.
As a result of decrease in PO2, chemoreceptors are stimulated leading to hyperventilation in an effort to restore blood PO2 to previous levels. Since hyperventilation also reduces PCO2 in blood, blood pH rises, kidneys help restoring blood pH by excreting alkaline urine.
Because less oxygen is available to be loaded, high-altitude conditions always results in lower blood oxygen saturation. For example at altitude of 6000m, the oxygen saturation of arterial blood is only 67% (compared to 98% at sea level). But haemoglobin unloads only 20 to 25% of its oxygen at sea level, thus even at reduced haemoglobin saturations, oxygen needs of the tissues are still met adequately under resting conditions. Additionally, at high altitude, haemoglobin’s affinity for oxygen is also reduced, and more oxygen is released to the tissues during each circulatory round. However, excessive exertion at high attitude before one is acclimatized can lead to altitude / mountain sickness and can results in death.
When blood oxygen tension declines, the kidneys intervene by accelerating the production of a hormone (erythropoietin) which stimulates bone marrow production of more RBC. This phase of acclimatization occurs slowly, providing long-term adjustment for living at high altitude.
Reference reading: Role of stretch receptors**
Recent findings suggest that the inspiratory centre and the expiratory centre are reciprocally in ______, these activities probably account for the basic breathing r______.
The stretch receptors in the lungs that initiate the ‘inflation reflex’ may have little role to play in the basic rhythm of breathing. They are thought to be more a protective mechanism to prevent excessive stretching of the lungs than a normal regulatory mechanism because the threshold of these receptors is very high. AL F7 Bio.notes Gas Exchange P.9
Q1 Lung Volumes
The volume of air that passes in and out at each breath during normal unforced breathing is called the t______volume. During quiet breathing a man exchanges about 1/2 a litre at each breath. This represents only about 1/10 the total capacity of the two lungs. The v_____ capacity of the lungs is the maximum volume of air which can be exchanged during forced breathing; in man this is usually about three litres. From these figures you can see that some air always remains in the lungs. This r______air does not stagnate as it is constantly renewed by exchange with the tidal air.
The diagram above left show the lung volumes, giving also some indication of the average size of each volume. In order to give a base line representing a completely empty lung, the traces are upside down compared with the spirometer reading.
1 From the description above name 1 to 5 on the above diagram. 2 What is the vital capacity for a) a man b) a woman? 3 What is the total capacity for a) a man b) a woman?
A and B shows two spirometer tracings given by the same person. A is given at rest, B after strenuous exercise.
4 Calculate for this person a) i) the number of breaths per minute at rest and after exercise. ii) the average volume breathed in at rest and after exercise.
b) From these tracings what is the effect of exercise on breathing? Why is the tracing moving downwards from left to right?
5 What precautions for the safety of the subject should be taken while using a spirometer to obtain tracings such as those above? AL F7 Bio.notes Gas Exchange P.10
Q.2 There is about 21 per cent oxygen in normal air, and about 0.04 per cent carbon dioxide. The composition of inspired and expired air is shown in the table below.
Percentage composition of inspired and expired air Inspired air Expired air Air in lung alveoli Oxygen 20.96 16.3 14.2 Carbon dioxide 0.04 4.0 5.5 Nitrogen and other gases 79.00 79.7 80.3
It is often thought that it is a shortage of oxygen in the air which makes us breathe faster. In fact, if the oxygen content of the air is reduced without increasing the amount of carbon dioxide, there is no change in breathing until the amount of oxygen falls from 21 per cent to about 13 per cent.
The second table shows the relationship between breathing and the amount of carbon dioxide in the air.
Breathing and the amount of carbon dioxide in inspired air:
% carbon dioxide in inspired air 0.04 0.79 1.52 2.28 3.11 5.48 6.02 Average depth of respiration cm3 673 739 794 911 1232 1854 2104 Average number of breaths per minute 14 14 15 15 15 16 27 Volume breathed per minute (normal = 100) 100 111 128 141 191 311 631 % carbon dioxide in alveolar air 5.6 5.5 5.55 5.8 5.5 6.8 6.6
1 What is the effect of an increased percentage of carbon dioxide in the inspired air on the rate and depth of breathing?
2. The increase of CO2 in inspired air is not paralleled by a similar increase in alveolar air. Suggest why.
3. When, in normal life, would you expect a similar increase in breathing to occur ?
VI) GAS EXCHANGE IN PLANTS
Plants are considerably less active than animals and so generally have a much l______oxygen requirement. In addition, plants produce oxygen during p______.
In contrast to the situation in animals, even the largest plants rely on d alone to supply their gas exchange needs. This is partly attributable to their relatively low oxygen requirements, and partly to the very extensive gas exchange surfaces provided by plant organs such as l______and r______. AL F7 Bio.notes Gas Exchange P.11
Note the extensive intercellular spaces within the leaves, and, to a lesser extent in the stem and roots, these air spaces ensure that air can circulate freely and that no cell is very far from a supply of oxygen. Each living cell inside the plant is covered with a thin film of water which allows the respiratory gases to enter and leave.
Land plants had to overcome the difficulty of keeping the respiratory surface moist. Early colonisers like mosses and liverworts are imperfectly adapted to life on land. Like their aquatic ancestors, these plants respire through the body surface and as a consequence are restricted to m______habitats.
The truly terrestrial higher plants developed a special, waterproof covering, the c______, on their aerial parts (leaves and stems) perforated by pores or s______to allow gas exchange.
In stems that become secondarily thickened and acquire an outer layer of b______for protection, the stomata are replaced by l______, areas where the c______-filled cells of the bark are l______- packed, allowing air to circulate between the outside and the living cells that lie under the bark. Lenticels appear as elongate or spherical, slightly raised areas on the bark.
The younger parts of roots and the root h______lack a waterproofing c______and so gas exchange can take place through their surface; an impermeable, protective outer layer generally develops on the older parts of the root. AL F7 Bio.notes Gas Exchange P.12
Common Misconceptions: Gas exchange and breathing AL F7 Bio.notes Gas Exchange P.13
1 During inhalation, atmospheric air rushes in through the air passages and inflates the lungs. The correct sequence of ventilation in a mammal should be:
1. Contraction of intercostal and diaphragm muscles → 2. volume of thoracic cavity increases (or the lungs expand) → 3. air pressure in the lungs decreases → 4. atmospheric air, being at a higher pressure, rushes into the lungs
2 A person is breathing out when there is a rise in lung pressure (i.e. 1.0 –3.0 seconds)
Air moves out of the lungs only when the air pressure in the lungs exceeds the atmospheric pressure and this period, as read from the graph, ranges from 2.0 to 4.0 seconds.
3 If the pleural membrane is punctured, the lungs collapse due to leaking of pleural fluid.
The two pleural membranes, with fluid between them, behave in the same way as two pieces of glass slides separated by a thin film of water. They glide over each other readily but cannot be pulled apart because of the strong surface tension of water. If the pleural membrane is punctured, the lung collapses due to the elastic recoil of the lung.
As a convention, all pressures in the respiratory system are given relative to the atmospheric pressure, which is the pressure of the air surrounding the body (760 mm Hg at sea level). Although the pressure within the lungs (the intrapulmonary pressure) rises and falls between breaths, it tends to equalise itself to the atmospheric pressures (i.e. the difference with atmospheric pressure = 0 mm Hg). The pressure within the pleural cavity, the intrapleural pressure, also fluctuates with the breathing phases, and is about 4 mm Hg less than the intrapulmonary pressure (i.e. -4 mm Hg). Hence there is pressure difference of 4 mm Hg across the alveolar wall. This pressure difference is the force that keeps the stretched lungs from collapsing (Marieb, 1992) (Fig. 15). AL F7 Bio.notes Gas Exchange P.14
When the pleural membrane is punctured, atmospheric air rushes into the pleural cavity, and the intrapleural pressure immediately changes from -4 mm Hg to 0 mm Hg (760mmHg). The negative pressure that holds the lung open is now eliminated, and the stretched lung collapses due to its elasticity. If the lung collapses, it can no longer be, inflated by inhalation, even when the diaphragm and ribs can still carry out normal breathing movements. AL F7 Bio.notes Gas Exchange P.15
4 When one is shot in the lung, he couldn’t breathe because the lungs would collapse.
When one is shot in one side of the two lungs, provided that the other side of the thoracic cage and pleural membranes are intact, he could still breathe with ONE side of his lung— which wouldn’t collapse because the two sides of the lungs are separately wrapped with pleural membranes. A collapsed lung, or pneumothorax, occurs when all or part of a lung collapses or caves inward. This occurs when air gets in the area between the lung and chest wall. When this happens the lung cannot fill up with air, breathing becomes hard, and the body gets less oxygen. A collapsed lung can occur spontaneously in a healthy person or in someone who has lungs compromised by trauma, asthma, bronchitis, or emphysema.
The presence of air in the pleural cavity or pneumothorax is sometimes artificially induced in a patient to allow an infected lung to rest until it recovers; the other lung can still carry out its normal function of ventilation as the pleural cavities surrounding the two lungs are completely separated. This condition can be reversed by closing the hole and drawing air out of the pleural cavity.
5 During exhalation, the lungs are compressed and air is expelled from the air sacs.
The lungs cannot be compressed completely during exhalation, most of the air in the air sacs cannot be expelled out of the body and this constitutes the residual air.
6 Exhaled air is rich in carbon dioxide.
Exhaled air is richer in carbon dioxide compared to inhaled air. Exhaled air is still richer in oxygen (16%) than carbon dioxide (4%)
7 Dead space = residual volume
The dead space, with a volume of about 150 ml, comprises the air passage in the nose, pharynx, trachea, bronchi and bronchioles, where no gas exchange can occur.
The residual volume refers to the volume of air remaining in the lungs after exhalation and is approximately 2500 ml during quiet breathing and 1200 ml after a maximal active exhalation.
8 Inhaled air fills alveoli and gas exchange takes place between the inhaled atmosphere air and the blood capillary directly. Of the 500 ml of fresh air taken into the breathing system in each breath, due to the presence of the dead space, only about 350 ml reach the alveoli for gas exchange. AL F7 Bio.notes Gas Exchange P.16
Moreover, the relatively large proportion of residual air to tidal air further reduces the efficiency of gas exchange inside the alveoli.
As the residual air is in direct contact with the alveolar membrane, the oxygen in the tidal air has to diffuse for some distance before reaching the gas exchange surface and entering into the blood (Fig. 16). This process again reduces the efficiency of the body in extracting oxygen from the tidal air. AL F7 Bio.notes Gas Exchange P.17
9 The efficiency of ventilation is higher when a person breathes rapidly and shallowly compared to when he breathes slowly and deeply
Increased depth of breathing is far more effective in elevating alveolar ventilation than an increase in breathing rate.
The table below summaries the effect of different breathing pattern on the ventilation of the alveoli (Vander et al 1994)
The amount of residual oxygen (~16%) in exhaled air explains why it is possible to use exhaled air in mouth to mouth resuscitation. This also suggests that the mammalian system of gas exchange is somewhat inefficient.
Further notes:
Although the residual air in the air sacs cannot be expelled out of the body directly, there is a continual diffusion of oxygen from the tidal air to the residual air and of carbon dioxide in the reverse direction.
Because of the existence of residual air, which prevents direct gas exchange to take place between the blood and tidal air, the process of gas exchange is NOT very efficient and the exhaled air still contains as much as 16% of oxygen and only 4% carbon dioxide.
The relatively high concentration of residual oxygen in the exhaled air is an indication of the relative inefficiency of the mammalian ventilation mechanism . Birds are metabolically more active than mammals because of the need for flight. Compare its design to that of the mammals and suggest how this is more efficient than that the mammalian system.
Achieve a one-way flow though the lungs in birds:
No residual volume; all old (stale) air leaves with each breath.
One-way flow is accomplished by the use of air sacs as illustrated below.
During inspiration, the air sacs fill. During expiration, they empty.
10 Water vapour is the main component of exhaled air
Exhaled air might be saturated with water vapour ---- but that only makes up 6% of the exhaled air. (compared to usually less than 1 % in the inhaled air) AL F7 Bio.notes Gas Exchange P.18
11 During vigorous exercise, exhaled air contains a higher conc. of carbon dioxide and a lower concentration of oxygen than that produced at rest.
During vigorous exercise, ventilation can increase from the resting level of about 8 litres per minute to well over 90 litres per minute through an increase in rate and depth of breathing. The increase in ventilation is so remarkable that, except in the most exhaustive type of exercise, CO2 is exhaled as fast as it is produced by the active tissues. So the concentration of CO2 is relatively constant regardless of activity.
Total amount of CO2 produced =
Ventilation rate X CO2 concentration in arterial blood / CO2 concentration in exhaled air during exercise
As the total amount of CO2 produced increases rapidly during exercise, it is removed immediately by a corresponding increase in the rate and depth of breathing which leads to a much greater ventilation rate. Thus the concentration of CO2 in the arterial blood or exhaled air remains rather constant (Vander et. al., 1994.)
Fig 17 The effects of exercise on ventilation rate, and conc. of oxygen and carbon dioxide in blood
Despite the extremely rapid production of CO2 during exercise and the simultaneous consumption of. O2, ventilation increases so greatly that this prevents the blood concentration of these gases from changing significantly from normal.
There is even a drop in blood CO2 level during maximal activity. Therefore, most physiologists believe that the increase in ventilation during exercise is NOT caused by changes in the level of either or both of these two respiratory gases.
12 During exercise, the production of a large amount of CO2 causes a build-up of CO2 in the blood that stimulates the respiratory centre.
It has been established that the respiratory centre in the medulla oblongata is stimulated by an increase in CO2 level in blood, leading to an increase in the rate and depth of breathing. However, as shown in Fig. 17, there is no increase in CO2 concentration in the arterial blood during vigorous exercise. Therefore, the increase in ventilation rate during exercise cannot be mediated through the stimulation of the respiratory centre by a higher CO2 AL F7 Bio.notes Gas Exchange P.19
level in blood. It has to be operated through a different mechanism. There are empirical evidences that establish a neural mechanism for the increase in ventilation rate during vigorous exercise.
1. When the venous levels of C02, 02 and pH of a person are artificially maintained at resting levels, muscular activity results in a marked increase in ventilation. [What can you imply from this observation?] This result suggests that the increase in ventilation is caused by impulses reaching the respiratory centre from the stretch receptors (proprioceptors) in contracting skeletal muscles. Passive ‘pumping’ of the limbs to excite these stretch receptors also increases the ventilation rate.
2. When the cerebral cortex sends impulses to the contracting muscles, it is also sending parallel signals to the respiratory centre at the same time to increase the rate and depth of breathing. This is similar to the role of the cerebral cortex in increasing the cardiac output during exercise. Consequently, the respiratory and circulatory adjustments to exercise are well integrated through these higher levels of the nervous system.
If these fail to increase ventilation adequately, only then do CO2 and hydrogen ions begin to accumulate in the body fluids and O2 level begins to drop. These changes then excite the respiratory centre or chemoreceptors in the circulatory system to trigger off a second mechanism to increase ventilation.
13 The vital capacity of a person increases during exercise. Vital capacity is defined as the maximum amount of air that can be exhaled after the deepest inhalation. By definition, this value should be the same irrespective of whether a person is at rest or during exercise.
This value, however, can be raised by physical training. Training that leads to the development of stronger respiratory muscles, e.g. external and internal intercostal muscles, diaphragm and abdominal muscles, will result in a higher vital capacity.
The end