Biology Textbook PDF - Gas Exchange

The exchange of gases bet,veen the individual cell and its environment takes place by diffusion. For exan1ple, in cells that are respiring aerobically there is a higher concentration of oxygen outside the cells than inside, and so there will be a continuous...

The exchange of gases bet,veen the individual cell and its environment takes place by diffusion. For exan1ple, in cells that are respiring aerobically there is a higher concentration of oxygen outside the cells than inside, and so there will be a continuous net in,vard diffusion of oxygen. In living organisms, factors that determine the rate of diffusion include: The size of the surface area available for gas exchange (the respiratory surface) - the greater this surface area, the greater the rate of diffusion. In a single cell, the respiratory surface is the whole plasma membrane. The difference in concentration (concentration gradient) - a rapidly respiring organism has a very much lower concentration of oxygen in the cells and a higher-than-normal concentration of carbon dioxide. The greater t he gradient in concentration across the respiratory surface, the greater the rate of diffusion. The length of t he diffusion path - t he shorter the diffusion path, t he greater the rate of diffusion, so the respiratory surface must be as thin as possible. rr:: T,-... -I - Energy for diffusion comes from the kinetic energy of molecules. The higher the temperature, the faster the rate of diffusion due to the increased kinetic energy of molecules. In warm-blooded animals, temperature is kept constant through homeostatic mechanisms and so the effects of temperature on diffusion do not vary. Challenges of gas exchange link The challenges of gas exchange beco111e greater as orgarusms increase in size because their surface The surface area- area-to-volume ratio decreases ,vith increasing size, and the distance from the centre of an to-volume ratio is organism to its exterior increases. The surface area of a single-celled organisn1 is large in relation covered in detail to the a1nount of cytoplasn1 it contains, so the surface of the cell is sufficient for efficient gaseous in Chapter B2.3, exchange. On the other hand, most cells in large multicellular organisms are too far from the page 262. surface of the body to receive enough oxygen by diffusion alone. In addition, animals often develop an external surface of tough or hardened skin that, while it provides protection to the body, is not suitable for gaseous exchange. These organis111s require an alternative respiratory surface. Active organisrns have an increased 111etabolic rare, and the de1nancl for oxygen in their cells is higher than in sluggish and inactive organisms. Therefore, large, active organis1ns have specialized organs fo r gaseous exchange. Properties of gas-exchange surfaces All gas-exchange surfaces need properties thar increase rhe rate of diffusion of respiratory gases across them. These properties include: permeability- to allow rhe gases across chin tissue layer - co make the shortest distance for diffusion as possible moisture - gases dissolve in the 1noisture, helping Lhem ro pass across the gas-exchange sur[ace large surface area - so Lhat large quantiries of the respiratory gases can cross at the same rime. Theme B: Form and function - Organisms  Maintenance of concentration gradients at exchange surfaces in animals Concentration Diffusion requires a concentration gradient to be n1aintained - the steeper the gradient, the faster gradient: the difference the diffusion (Figure 83.1.2). A concentration gradient needs to be n1aintained at the gas-e..' K;;t--- - high concentration of carbon dioxide.!2 "C. C 0.0 0 tow concentration of carbon dioxide blood lungs location in body Figure B3.1.2 The concentration gradient in alveoli allows carbon dioxide to diffuse from the blood How are concentration gradients maintained? There are three \vays in ,1-1hich a concentration gradient is maintained at exchange surfa1ces: a dense net\vorks of blood vessels: capillaries provide a large surface area for the diffusion of respiratory gases; blood carries the gases either in red blood ceUs (mainly O>-'Ygen) and plasma (carbon djoxide) a continuous blood no\v: this maintains the difference in concentration of molecules bet\veen the air and blood by carrying oxygen av1ay from the gas-exchange surfaces in the capillaries and carbon dioxide to them ventilation: with air for lungs and \vith ,var.er for gills, ventilation brings oxygen ro the gas- exchange surface and removes carbon dioxide. Gas exchange in mammals t List three characteristics of The lungs an efficient gas- ln man11nals. the lungs are the organs \vhere gas exchange occurs. Lungs provide a large, thin, n1oist exchange surface. surface area that is suitable tor gaseous exchange. Ho"'1ever, the lungs are in a protected position inside Explain how the thorax (chest), so air must be brought to the respiratory surface there: the lungs 1nust be ventilated. each influences.A.. ventilation syste1n is a pumping mechanism that moves air into and out of the lungs efficiently, diffusion. thereby maintain ing the concentrarion gradients of oxygen and carbon dioxide for diffusion. In addiiion, in ma1nmals the condiLions for diffusion at the respiratory surface are improved by: Thorax, in mammals, a blood circulation system, which r-apidly moves O>-'Ygen ro the body cells as soon as it has the upper part of the body separated from the crossed the respiratory surface, thereby maintaining the concentrar.ion gradient in the lungs abdomen; in insects, the a respiratory pigment, which increases the oxygen-carrying ability of the blood. This is the region between head and haemoglobin of I he reel blood cells, which are by far Lhe most numerous of the cells in our abdomen. blood circulation. B3. l Gas exchange  lntercostal muscles: The structure of the lung of mammals muscles between the ribs The structure of the hu1nan thorax is shown in Figure B3.1.3. The lungs are in the thorax, an airtight involved in ventilation. chamber fanned by the ribcage and its n1uscles (intercostal muscles), with a do1ned floor, the Diaphragm: sheet of t issues, largely muscle, diaphragm. The diaphragn1 is a sheet of n1uscle attached to the body ,vall at the base of the ribcage, separating thorax from separating the thorax from the abdomen. The internal surfaces of the thorax are lined by the pleural abdomen in mammals. membrane, ,vhich secretes and n1aintains pleural fluid. Pleural fluid is a lub1icating liquid derived Pleural membrane: fron1 blood plasma; it protects the lungs fro1n friction du1ing breathing moven1ents. lines lungs and thorax cavity; it secretes the pleural fluid. Trachea: windpipe Bronchus (plural, bronchi): a tube connecting the trachea with the lungs. Bronchiole: small terminal branch of a bronchus. Alveolus (plural, alveoli): air sac in the ribs (in section) lung. bronchus L=~======~ pleural membranes intercos tal muscles r--:i--- - - - pleural cavity internal -===:::::(:=tJ~ l containing pleural external fluid lung - - - - - -- position of diaphragm - - - --+- the heart abdominal - - - -- -+ cavity bronchioles Figure B3.1.3 The structure of t he human thorax From trachea to alveoli The lungs connect ,,vith the rear or the mouth by the trachea (Figure 133..1.3). The trachea then divides into two bronchi, one to each lung. \Vithin the lungs the bronchi divide into smaller bronchioles. The rinest bronchioles encl in the alveoli (air sacs). The walls or bronchi and larger Link bronchioles contain smooth muscle and are also supported by rings or tiny plates or cartilage, Adaptations of t he preventing collapse that might be triggered by a sudden reduction in pressure that occurs with cells in the alveoli are po,,verrul inspirations of air. discussed in detail in Chapter B2.3, 1-ungs are extrernely elllcient, but they cannot prevent some water loss during breathing- an issue page 266. for n1ost terrestrial organi.sms. Theme B: Form and function - Organisms  Adaptations of mammalian lungs for gas exchange Alveolar structure and gaseous exchange The lung tissue consists of the alveoli, arranged in clusters, each served by a tiny bronchiole. Bronchioles exist in a branched net\vork, spreading our \\Tith.in the lung to provide an even disoibution of alveoli. Alveoli have elastic connective tissue as an integral part of their ,valls (Figure B3 1.4). There are very many small alveoli in eacb lung - the s1naUsi.ze and large number of alveoli provide a large surface area for gas exchange. A capillary sysren1 \Vraps around the clusters of alveoli (Figure B3.1.4). The extensive capillary beds are an adaptation of the lungs for gas exchange, providing a large su1i'ace area for the diffusion of OA')'gen tro1n the alveoli. into the blood and carbon dioxide our of the blood into the alveoli. Each capilla1y is connected to a branch of the pulmonary artery and is drained by a branch ol' the puln1onary vein. The puln1ona1y circulation is supplied with deoxygenared blood fro111 the right side of the heart and returns oxygenated blood to the left side of the heart to be pun1ped to the rest of the body. (e Common mistake A common misunderstanding is that the spherical shape of alveoli gives the lungs a large surface area for gas exchange. In fact, a sphere has less surface area for a given volume than any other shape. It 1s the small size and large number of alveoli that gives the large surface area. Surfactant lines the inner surface of Lhe alveoli. lL is secreted by cells in the \Vall of alveoli and reduces surface tension. Because of Lhe tiny djameter of the alveoli (about 0.25 m111) they would Lend co collapse under surface rension during expirarion, with their ,valls sticking together. The lung surfaccanLlo\vers the surface Lension, permitting the alveoli to flex easily as the pressure o( the thorax falls and rises. P. Trin fjnl Make sure you know how the long is adapted for gas exchange. The adaptions include, the (e Common presence of surfactant, a branched network of bronchioles, extensive capillary beds and a large surface area. mistake Do not refer to an Blood aniving in the lungs is lo,v in OA')'gen but high in carbon dioxide. As blood flo\'IS past the 'alveolar membrane' alveoli, gaseous exchange occurs by diffusion. Oxygen dissolves in the alveolar surface filn1 of water, because this leads to diffuses across into the blood plasma and into the red blood cells, \Vhere it chemically combines,vith confusion with cell haen1oglobin to fonn oxyhaen1oglobin. Ac the same rime, carbon dioxide diffuses fron1 the blood plasma membranes. into rhe alveoli. The term 'wall' is Table B3.1.1 The com position of air in t he lungs preferable. One of the adaptations of alveoli is Component Inspired air/% Alveolar air/% Expired air/% that alveolar walls are oxygen 20.9 14 16 one cell thick. Do not carbon dioxide 0.04 5.5 4.0 confuse 'alveolar walls' nitrogen 79 81 79 with 'cell walls'. wa ter vapour variable saturated saturated B3.1 Gas exchange  movement gaseous exchange in the alveolus ca rt ilage of air nngs capillary with blood cells alveolar blood supply to alveoli branch of membrane pulmonary ve, n branch of alveoli pulmonary artery ;::-- surface f ilm of water capillary network cart ilage alveol us ----c::::t- rings elastic connective t issue occurs around the alveoli On inspiration On expiration t he volume of the the volume of the thorax thorax increases decreases t he walls of the alveolus the alveol us and terminal alveolar wall and terminal bronchiole bronchiole revert to resting (squamous position of are stretched size. due to recoil of the alveolus epithelium) elastic fibres air is drawn in - elast ic f ibres air is expelled - terminal bronchiole bands of overlapping elastic fibres alveolus wall photomicrograph of TS alveol i, HP capillary red blood position of cells capillary wall role of elast ic fibres in alveol i and bronch ioles (endothelium) Figure B3.1.4 Gaseous exchange in t he alveoli Theme B: Form and function - Organisms (e Common How effective are mammalian lungs? Air flo"' in the lungs of 1na1nn1als is tidal: air enters and leaves by the sa1ne route. Consequently, mistake t here is a residual volu1ne of air that cannot be expelled. lncoming air mixes ,v-ith and dilutes the A common residual air, rather than replacing it. The effect of this is that air i.n tl1e alveoli contains significantly misconception is that less oi--ygen than the atn1osphere outside (see Table B3.l.l ). the gas breathed in Nevertheless, the lungs are efficient organs. Their success is due to numerous features of the alveoli fs oxygen and the that adapt rhe1n to gaseous exchange. These are listed in Table 133.1.2. gas breathed out is carbon dioxide. The air Table B3.1.2 Features of alveoli t hat adapt them t o efficient gaseous exchange breathed in and out Feature Effects and consequences contains both oxygen surface area of alveoli a huge surface area for gaseous exchange and carbon dioxide (SO m' = area of doubles tennis court) - exhaled air has a alveolar wall very thin, flattened (squamous) epithelium (S µm) means the diffusion pathway is higher concentration short of carbon dioxide capillary supply to alveoli network of capillaries around each alveolus (supplied with deoxygenated blood from the pulmonary artery and draining into pulmonary veins) maintains the than inhaled air, and concentration gradient of 0 2 and CO2 inhaled air has a surface film of moisture 0 2 dissolves in water lining the alveoli; 0 2 diffuses into the blood in solution higher concentration surfactant producing a detergent secreted by cells in the walls of alveoli; it reduces surface tension and of oxygen. stops alveoli walls from sticking together, keeping alveoli open (e Common mistake Be careful with word choice. It is not the alveolus that is one cell thick, but the alveolar wall. ATL 83.1A The production of adequate amounts of surfactant in the foetus, frorn about five months of pregnancy 1n humans, marks the beginning of the possibility of independent life. Premature babies born before this stage are unable to inflate their lungs and breathe; those born after it can :!. Outline how do so and, with intensive care, can survive. the structure of Find out about the effects of premature birth on babies. How are these effects treated? Use your alveoli adapts them biological knowledge to produce an information leaflet (of the type found in hospitals and for efficient gas doctors' surgeri es), w hich explains to the general public the problems caused by premature birth exchange. for babies and how these can be treated. Ventilation of the lungs Air is drav,rn into the alveoli v,d1en the air p ressure in the lungs is lower than arrnospheric pressure. Air is then forced our when pressure is higher in the lungs than atmospheric p ressure. Since the thorax is an airtight cha1nber, pressure changes i.n the lungs occur ,vhen the volume of the thorax changes. How the volume of the thorax is changed during breathing is illus trated in Figure B3.l.5 and s u1nmarized in Table B3.l.3. Tnn jro! Make sure you understand the role of the diaphragm, intercostal muscles, abdominal muscles and ribs in ventilation. Figure B3.l5 and Table B3.1.3 describe the roles in detail. B3.1 Gas exchange inspiration: expiration: external external intercostal ribs moved intercostal ribs moved _muscles contract } upwards and _muscles relax } downwards and internal fntercostal outwards, and the internal fntercostal inwards, and the n:iuscles relax diaphragm down muscles contract diaphragm up diaphragm muscles diaphragm muscles contract relax air out t ---- - trachea backbone - -+,L: (ribs articulate \ ""~- - pleural with the thoracic fluid vertebrae) sternum \ll.----(most ribs are attached lung tissue -lc4, 80 at high p02 16 " 0 :, X - oxygen binds....,"'" 0.r. :, 14 -~ 70 haem group C 60 12...-· ·- 0 -- :, (non-protein).rJ 0 n en 50 10 3w 0 E -0 40 8 QI..."' -"'.r. 0 C 0 30 6 0 0 "3w Cooperative ·. :; 20 4 binding: process E....0.... :, 10 low affinity at high p02 2 - C" where the addition of a "'"' - oxygen released 0 0 substance to the subunit a combination of protein and non-protein 0 0 Q. of a macromolecule means that haemoglobin is a conjugated 0 2 4 6 8 10 12 14 protein partial pressure of oxygen p02 / kPa increases the affi nity of a neighbouring subunit for Figure B3.1.16 The structure of haemoglobin and its affinity for oxygen the same substance. The shape of haemoglobin is altered as each oxygen molecule binds, making each successive binding of oxygen easier. This is kno~vn as cooperative binding, As a result, haemoglobin has a higher affinity for oxygen in oxygen-rich areas (such as the air spaces in lung alveoli), promoting oxygen loading, and has a lo\ver affin ity [or oxygen in oxygen-depleted areas (such as the 1nuscles), The f unction of promoLlng oxygen unload ing (see Figure B3 1.16) haemoglobin Notice that the oxygen dissociation curve is S-shaped (sign1oid curve). Tbis tells us chat, in the depends on complex haemoglobin molecule, the first oxygen 1nolecule attaches ,,vitb di(ficuky but, once it has. whet her it is the second combines more easily, and so on until all four are attached and the n1olecule is saturated. exposed to oxygen- rich or oxygen- In oilier,vorcL,;, the amount of oxygen held by haen1oglobin depends on the partial pressure of oxygen. depleted areas. In the former it ti I picks up oxygen When haemoglobin saturation with oxygen Is plotted as a function of the partial pressure of and, in the latter, it oxygen, a stgmoidal (or '5-shaped') curve is seen. This indicates that the more oxygen is bound to unloads it. haemoglobin, the easter it is for more oxygen to bind, until all binding sites are saturated. Theme B: Form and function - Organisms  What is the significance of the oxygen dissociation curve in the working body? ln the body che an1ounc of oxygen held by hae1noglobin depends on che partial pressure. ln che lungs, air is saturated with water vapour, so the partial pressure of the component gases is different Iron, char oucsi.cle, in dry air (Table B3.l."f). Table B3.1.4 Partial pressures of the components of air in the alveoli Component gase s Composition /% Partial pressure/ kPa nitrogen 75.5 76.4 oxygen 13.1 13.3 carbon dioxide 5.2 5.3 water vapour 6.2 6.3 Fro1n che oxygen dissociation curve, \Ve can see chat the hae1noglobi.n in red blood cells in che capillaries around the alveoli in the lungs ,-viii be about 95% sacuracecl Ho,vever, in respiring tissues, the oxygen partial pressure is much lo\ver clue to aerobic respiration there. Ln lact, the oxygen partial pressure in actively respiring tissues may be 0.0-4.0 kPa. l\t these partial pressures, o>ryhaen1oglobin breaks down, releasing oxygen in solution chat rapidly diffuses into the surrounding tissues. Clearly, the chen1is try of haemoglobin n1akes it an elEic.i.ent ,~ray or transporting O>--'Jgen, given the partial pressure o[ o>--1,gen in respiring tissue con1pared \vith chat in the lungs. Adaptations of foetal and adult haemoglobin for the transport of oxygen The [oetus obtains oxygen fron1 its mother's blood through the placenta, foetal haemoglobin ,vhere the n1acernal and foetal c.irculations come very close together but do not mix. The haemoglobin of the adult mammal and the hae111oglobin in 100 ---~,.,.,.:::::::::::====-- foetal the foetal circulation differ slightly in thei.r molecular composition. Foetal haemoglobin - 80 haemoglobin has a higher affinity for o>--7gen (Figure B3.Ll7). This 1neans C: C: 0 OJ ·-... o, >, 60 >--- adul1haemoglobin cbac the hae1noglobin present in the circularion of the foetus combines with....S )( :, 0 40 oxygen n1ore readily than n1acernal hae1noglobin does at rhe sa1ne partial.§ pressure. Because foe.cal haemoglobin has a higher aOinicy for OX}1gen, the 20 dissociation curve is shifred co the left. lr is obvious ,vhy it is advantageous for foetal haemoglobin co have this property, given that the only access co an o..j,--'Jgen when partial pressu re of oxygen p02 / kPa adult haemoglobin is unloading it (i.e. in the placenta). lf [oecal haemoglobin Figure 8 3 _1_17 The oxygen dissociatTon curve of bad a lower affinity than 1naternal haemoglobin, O>--')'gen ,vould pass from the foetal haemoglob1n compared to adult haemoglobin foerus to the mother. Follo,ving birth, foetal haemoglobin is almost con1pletely replaced by adult baemoglobin. Allosteric protein: a We have already seen rhat the binding of oxygen 10 one of the subunits affects the interaction of protein that can exist in oxygen ,virh Ihe or her subunits (cooperative b inding). This is because haemoglohin is an allosteric multiple conformations protein. The binding of oxygen to one h aemoglobin subuni1 causes confo1marional changes that are (shapes) depending on the binding of a molecule relayed to rhe oLher suhunits, making them more able 10 bind oxygen by raising rheir affinity ror (at a site other than the rhis rnolecule. catalyt1c site) Carbon dioxide is an allosteric effector of haemoglobin. It attaches to haemoglobin, making it n1ore difficult for oxygen to bind, lowering the affinity of haemoglobin lor oxygen. In foetal red blood cells, carbon dioxide has a lo,ver allosteric effect, ,vhich leads to a higher a[flniry ol oxygen for loeral hae1noglobi n. B3.1 Gas exchange  100 the oxygen dissociation curve - ~ I Bohr shift of haemoglobin at the CO2 0 ' "ii" 90 concentration of t he blood in._., C: transit around t he body, l' The blood circulation also transports carbon dioxide fron1 respiting tissues, ,..,here it is at relatively high partial pressures, to Cl 80 i.e. about 5.3 kPa CO I 2 I 70 ' the lungs. In respiring cells, the concentration of carbon dioxide.., the oxygen dissociation curve j is approximately 9. 3 kPa, ,~ hereas in the lungs, as we have seen in 1 60 of haemoglobin at the CO2 ·- C:..c 55 concentration around respiring! Table B3. l.4, it is 5.3 kPa. The effects of these partial pressures o[..2 Cl 50 cells, 1.e. about 9.3 kPa CO2 l carbon dioxide on the oxygen dissociation curve of haemoglobin is 0 l E QJ 40 as the CO2 concentration 'I noticeable (Figure B3.l.18). -"'..r:. 35 increases, more 0 2 1s I 'I 0 30 ,._.,____ released from haemoglobin An increase in carbon dioxide concentration shifts the oxygen C: (Bohr effect) I 0.., 20 ! dissociation curve to the r ight (see Figure 83.1.18). Where the "' I carbon dioxide concentration is high (in the actively respiring.,. 10 15 '..,:, l cells), oxygen is released fron, oxyhaen1oglobi.n even n1ore readily. 0 -1'-----.--r-+---r----,r-----r--r---.--+- Carbon dioxide lowers the pH, caused by protons from the o 22.7 f 4 6 8 10 12 partial pressure of dissociation or carbonic acid, ,vhich causes hae111oglobin to release pOi ln oxygen p02/kPa respiring p0 21n its oxygen. This very useful outco111e for living tissues is kno\vn as cells lungs the Bohr effect. Figure B3.1.18 How carbon dioxide favours release of oxygen in resp1ring tissues An increase in carbon dioxide causes increased dissociation of oxygen. This benefits cells \vith increased metabolism, such as respiring tissues, in the follo~ring ways: greater a.mounts of carbon dioxide (a product of cell respiration) are released haen,oglobin releases its oxygen (required for aerobic cell respiration) at regions of greatest respiratory need. ATL B3.1C Bohr effect: the Marine mammals, such as whales and seals, must hold their breath for long periods as they dive decrease in the oxygen beneath the surface of water to hunt for food. What would you predict about the concentrations affinity of haemoglobin of haemoglobin in their blood compared to land mammals? in response to decreased Research the levels of haemoglobin in a marine mammal of your choice. How do they compare to blood pH, resulting from increased carbon dioxide those in land mammals? This article discusses some of the issues: www,nature.comlarticles/news.2007.385 concentration in the blood. 6 De duce t he change in percentage saturation of haemoglobin if the oxygen part ial pressure drops from 4.0kPa to 2.7 kPa when t he partial pressure of CO 2 is 5.3 kPa (Figure 83.1.18). LINKING QUESTIONS 1 Hovv do multicellular organisms solve the problem of access to materials for all their cells? What is the relationship between gas exchange and metabolic processes in cells? Theme B: Form and function - Organisms  ' Guiding questions ' What adaptations faci litate transport of fluids in animals and plants? What are the differences and similarities between transport in animals and plants? This chapter covers the fo llowing syllabus content: B3.2.1 Adaptations of capillaries for exchange of materials between blood and the internal or external environment B3.2.2 Structure of arteries and veins B3.2.3 Adaptations of arteries for the transport of blood away from the heart B3.2.4 Measurement of pulse rates B3.2.5 Adaptations of veins for the return of blood to the heart B3.2.6 Causes and consequences of occlusion of the coronary arteries B3.2.7 Transport of water from roots to leaves during transpiration B3.2.8 Adaptations of xylem vessels for transport of water B3.2.9 Distribution of tissues in a transverse section of the stem of a dicotyledonous plant B3.2.10 Distribution of tissues in a transverse section of the root of a dicotyledonous plant B3.2.11 Release and reuptake of tissue f luid in capillaries (HL only) B3.2.12 Exchange of substances between tissue fluid and cells in tissues (HL only) B3.2.13 Drainage of excess tissue fluid into lymph ducts (HL only) B3.2.14 Differences between the single circulation of bony fish and the double circulation of mammals (HL only) B3.2.15 Adaptations of the mammalian heart for delivering pressurized blood to the arteries (HL only) B3.2.16 Stages in the cardiac cycle (HL only) B3.2.17 Generation of root pressure in xylem vessels by active transport of mineral ions (HL only) B3.2.18 Adaptations of phloem sieve tubes and companion cells for translocation of sap (HL only) An introduction to the blood system Living cells require a supply of water and nutrients, such as glucose and amino acids, and they need oxygen. The waste products of cellular metabolisn1 1nust be removed. In single-celled organis1ns and Mass flow: the very small organisn1s, internal di.stances are small, so 111ovements of nuuients can occur efficiently by movement of fluids down diffusion. In larger organisn1s, an internal transport system is required to 111eet the needs of the cells. a pressure gradient. Internal transport systen1s at work are CA