1 Lecture #11 – Animal Circulation and Gas Exchange Systems

2 Key Concepts: Circulation and gas exchange – why? Circulation – spanning diversity Hearts – the evolution of double circulation Blood circulation and capillary exchange Blood structure and function Gas exchange – spanning diversity Breathing – spanning diversity Respiratory pigments

3 Animals use O 2 and produce CO 2 All animals are aerobic  Lots of oxygen is required to support active mobility  Some animals use lots of oxygen to maintain body temperature All animals produce CO 2 as a byproduct of aerobic respiration Gasses must be exchanged  Oxygen must be acquired from the environment  Carbon dioxide must be released to the environment

4 Except……breaking news! Abstract – 6 April 2010 Background Several unicellular organisms (prokaryotes and protozoa) can live under permanently anoxic conditions. Although a few metazoans can survive temporarily in the absence of oxygen, it is believed that multi- cellular organisms cannot spend their entire life cycle without free oxygen. Deep seas include some of the most extreme ecosystems on Earth, such as the deep hypersaline anoxic basins of the Mediterranean Sea. These are permanently anoxic systems inhabited by a huge and partly unexplored microbial biodiversity. Results During the last ten years three oceanographic expeditions were conducted to search for the presence of living fauna in the sediments of the deep anoxic hypersaline L'Atalante basin (Mediterranean Sea). We report here that the sediments of the L'Atalante basin are inhabited by three species of the animal phylum Loricifera (Spinoloricus nov. sp., Rugiloricus nov. sp. and Pliciloricus nov. sp.) new to science. Using radioactive tracers, biochemical analyses, quantitative X-ray microanalysis and infrared spectroscopy, scanning and transmission electron microscopy observations on ultra-sections, we provide evidence that these organisms are metabolically active and show specific adaptations to the extreme conditions of the deep basin, such as the lack of mitochondria, and a large number of hydrogenosome-like organelles, associated with endosymbiotic prokaryotes. Conclusions This is the first evidence of a metazoan life cycle that is spent entirely in permanently anoxic sediments. Our findings allow us also to conclude that these metazoans live under anoxic conditions through an obligate anaerobic metabolism that is similar to that demonstrated so far only for unicellular eukaryotes. The discovery of these life forms opens new perspectives for the study of metazoan life in habitats lacking molecular oxygen.

5 Animals use O 2 and produce CO 2 Circulation systems move gasses (and other essential resources such as nutrients, hormones, etc) throughout the animal’s body Respiratory systems exchange gasses with the environment

6 Circulation systems have evolved over time The most primitive animals exchange gasses and circulate resources entirely by diffusion  Process is slow and cannot support 3-D large bodies Sponges, jellies and flatworms use diffusion alone

7 Critical Thinking Why isn’t diffusion adequate for exchange in a 3D large animal???

8 Critical Thinking Why isn’t diffusion adequate for exchange in a 3D large animal???

9 Critical Thinking But…..plants rely on diffusion for gas exchange…..how do they get so big???

10 Critical Thinking But…..plants rely on diffusion for gas exchange…..how do they get so big???

11 Circulation systems have evolved over time The most primitive animals exchange gasses and circulate resources entirely by diffusion  Process is slow and cannot support 3-D large bodies  Surface area / volume ratio becomes too small Sponges, jellies and flatworms use diffusion alone

12 Diagram of sponge structure Virtually every cell in a sponge is in direct contact with the water – little circulation is required

13 Diagram of jellyfish structure, and photos Jellies and flatworms have thin bodies and elaborately branched gastrovascular cavities  Again, all cells are very close to the external environment  This facilitates diffusion  Some contractions help circulate (contractile fibers in jellies, muscles in flatworms)

14 Diagram of open circulatory system in a grasshopper Circulation systems have evolved over time  Metabolic energy is used to pump hemolymph through blood vessels into the body cavity  Hemolymph is returned to vessels via ostia – pores that draw in the fluid as the heart relaxes Most invertebrates (esp. insects) have an open circulatory system

15 Diagram of a closed circulatory system, plus a diagram showing an earthworm circulatory system Circulation systems have evolved over time  Metabolic energy is used to pump blood through blood vessels  Blood is contained within the vessels  Exchange occurs by diffusion in capillary beds Closed circulatory systems separate blood from interstitial fluid

16 Open vs. Closed…both systems are common Open systems…. Use less metabolic energy to run Use less metabolic energy to build Can function as a hydrostatic skeleton Most invertebrates (except earthworms and larger mollusks) have open systems Closed systems…. Maintain higher pressure Are more effective at transport Supply more oxygen to support larger and more active animals All vertebrates have closed systems

17 All vertebrates have a closed circulatory system Chambered heart pumps blood  Atria receive blood  Ventricles pump blood Vessels contain the blood  Veins carry blood to atria  Arteries carry blood from ventricles Capillary beds facilitate exchange  Capillary beds separate arteries from veins  Highly branched and very tiny  Infiltrate all tissues in the body We’ll go over these step by step

18 Diagram of a chambered heart Chambered heart pumps blood Atria receive blood Ventricles pump blood One-way valves direct blood flow

19 Critical Thinking Atria receive blood; ventricles pump Given that function, what structure would you predict???

20 Critical Thinking Atria receive blood; ventricles pump Given that function, what structure would you predict???

21 Diagram of a chambered heart Chambered heart pumps blood Atria receive blood  Soft walled, flexible Ventricles pump blood  Thick, muscular walls One-way valves direct blood flow

22 Diagram showing artery, vein and capillary bed Vessels contain the blood Arteries carry blood from ventricles  Always under pressure Veins carry blood to atria  One-way valves prevent back flow  Body movements increase circulation  Pressure is always low

23 Diagram of blood circulation pattern in humans Note that blood vessel names reflect the direction of flow, NOT the amount of oxygen in the blood Arteries carry blood AWAY from the heart  Arterial blood is always under pressure  It is NOT always oxygenated Veins carry blood TO the heart

24 Diagram showing artery, vein and capillary bed Capillary beds facilitate exchange Capillary beds separate arteries from veins Highly branched and very tiny Infiltrate all tissues in the body More later

25 All vertebrates have a closed circulatory system – REVIEW Chambered heart pumps blood  Atria receive blood  Ventricles pump blood Vessels contain the blood  Veins carry blood to atria  Arteries carry blood from ventricles Capillary beds facilitate exchange  Capillary beds separate arteries from veins  Highly branched and very tiny  Infiltrate all tissues in the body

26 Diagram showing progression from a 1- chambered heart to a 4-chambered heart. This diagram is used in the next 12 slides. Evolution of double circulation – not all animals have a 4-chambered heart

27 Fishes have a 2-chambered heart One atrium, one ventricle A single pump of the heart circulates blood through 2 capillary beds in a single circuit  Blood pressure drops as blood enters the capillaries (increase in cross-sectional area of vessels)  Blood flow to systemic capillaries and back to the heart is very slow  Flow is increased by swimming movements

28 Two circuits increases the efficiency of gas exchange = double circulation One circuit goes to exchange surface One circuit goes to body systems Both under high pressure – increases flow rate

29 Amphibians have a 3-chambered heart Two atria, one ventricle Ventricle pumps to 2 circuits  One circuit goes to lungs and skin to release CO 2 and acquire O 2  The other circulates through body tissues Oxygen rich and oxygen poor blood mix in the ventricle  A ridge helps to direct flow Second pump increases the speed of O 2 delivery to the body

30 Most reptiles also have a 3-chambered heart A partial septum further separates the blood flow and decreases mixing  Crocodilians have a complete septum Point of interest: reptiles have two arteries that lead to the systemic circuits  Arterial valves help direct blood flow away from pulmonary circuit when animal is submerged

31 Critical Thinking What is a disadvantage of a 3 chambered heart???

32 Critical Thinking What is a disadvantage of a 3 chambered heart???

33 Mammals and birds have 4-chambered hearts Two atria and two ventricles Oxygen rich blood is completely separated from oxygen poor blood  No mixing  much more efficient gas transport  Efficient gas transport is essential for both movement and support of endothermy  Endotherms use 10-30x more energy to maintain body temperatures

34 Mammals and birds have 4-chambered hearts Mammals and birds are NOT monophyletic What does this mean???

35 Phylogenetic tree showing the diversification of vertebrates Mammals and birds have 4-chambered hearts Mammals and birds are NOT monophyletic

36 Mammals and birds have 4-chambered hearts Mammals and birds are NOT monophyletic Four-chambered hearts evolved independently What’s this called???

37 Mammals and birds have 4-chambered hearts Mammals and birds are NOT monophyletic Four-chambered hearts evolved independently

38 Review: evolution of double circulation

39 Blood Circulation Blood vessels are organs  Outer layer is elastic connective tissue  Middle layer is smooth muscle and elastic fibers  Inner layer is endothelial tissue Arteries have thicker walls Capillaries have only an endothelium and basement membrane

40 Critical Thinking Arteries have thicker walls than veins Capillaries have only an endothelium and basement membrane What is the functional significance of this structural difference???

41 Critical Thinking Arteries have thicker walls than veins Capillaries have only an endothelium and basement membrane What is the functional significance of this structural difference???

42 Diagram showing artery, vein and capillary bed Form reflects function… Arteries are under more pressure than veins Capillaries are the exchange surface

43 Graph showing relationships between blood pressure, blood velocity, and the cross- sectional area of different kinds of blood vessels – arteries to capillaries to veins. This same graph is on the next 3 slides. Blood pressure and velocity drop as blood moves through capillaries

44 Total cross- sectional area in capillary beds is much higher than in arteries or veins; slows flow

45 Velocity increases as blood passes into veins (smaller cross- sectional area); pressure remains dissipated

46 One-way valves and body movements force blood back to right heart atrium

47 Critical Thinking What makes rivers curl on the Coastal Plain???

48 Critical Thinking What makes rivers curl on the Coastal Plain???

49 Emphasize the difference between velocity and pressure!!! Velocity increases in the venous system; pressure does NOT

50 Capillary Exchange Gas exchange and other transfers occur in the capillary beds Muscle contractions determine which beds are “open”  Brain, heart, kidneys and liver are generally always fully open  Digestive system capillaries open after a meal  Skeletal muscle capillaries open during exercise  etc…

51 Diagram showing sphincter muscle control over capillary flow. Micrograph of a capillary bed. Bed fully open Bed closed, through- flow only Note scale – capillaries are very tiny!!

52 Capillary Transport Processes: Endocytosis  exocytosis across membrane Diffusion based on electrochemical gradients Bulk flow between endothelial cells  Water potential gradient forces solution out at arterial end  Reduction in pressure draws most (85%) fluid back in at venous end  Remaining fluid is absorbed into lymph, returned at shoulder ducts

53 Capillary Transport Processes: Endocytosis  exocytosis across membrane Diffusion based on concentration gradients Bulk flow between endothelial cells  Water potential gradient forces solution out at arterial end  Reduction in pressure draws most (85%) fluid back in at venous end  Remaining fluid is absorbed into lymph, returned at shoulder ducts

54 Bulk Flow in Capillary Beds Remember water potential: Ψ = P – s Remember that in bulk flow P is dominant  No membrane  Plus, in the capillaries, s is ~stable (blood proteins too big to pass) P changes due to the interaction between arterial pressure and the increase in cross- sectional area

55 Diagram showing osmotic changes across a capillary bed Bulk Flow in Capillary Beds Remember: Ψ = P – s

56 Capillary Transport Processes: Endocytosis  exocytosis across membrane Diffusion based on concentration gradients Bulk flow between endothelial cells  Water potential gradient forces solution out at arterial end  Reduction in pressure draws most (85%) fluid back in at venous end  Remaining fluid is absorbed into lymph, returned at shoulder ducts

57 Blood structure and function Blood is ~55% plasma and ~45% cellular elements  Plasma is ~90% water  Cellular elements include red blood cells, white blood cells and platelets

58 Chart listing all blood components – both liquid and cellular Blood Components

59 Plasma Solutes – 10% of plasma volume Solutes  Inorganic salts that maintain osmotic balance, buffer pH to 7.4, contribute to nerve and muscle function  Concentration is maintained by kidneys Proteins  Also help maintain osmotic balance and pH  Escort lipids (remember, lipids are insoluble in water)  Defend against pathogens (antibodies)  Assist with blood clotting Materials being transported  Nutrients  Hormones  Respiratory gasses  Waste products from metabolism

60 Cellular Elements Red blood cells, white blood cells and platelets  Red blood cells carry O 2 and some CO 2  White blood cells defend against pathogens  Platelets promote clotting

61 Red Blood Cells Most numerous of all blood cells 5-6 million per mm 3 of blood! 25 trillion in the human body Biconcave shape No nucleus, no mitochondria  They don’t use up any of the oxygen they carry! 250 million molecules of hemoglobin per cell  Each hemoglobin can carry 4 oxygen molecules  More on hemoglobin later…

62 Critical Thinking Tiny size and biconcave shape do what???

63 Critical Thinking Tiny size and biconcave shape do what???

64 White Blood Cells All function in defense against pathogens We will cover extensively in the chapter on immune systems

65 Platelets Small fragments of cells Formed in bone marrow Function in blood clotting at wound sites

66 Diagram showing the clotting process The Clotting Process

67 Diagram showing blood cell production from stem cells in bone marrow Blood Cell Production Blood cells are constantly digested by the liver and spleen  Components are re- used Pluripotent stem cells produce all blood cells Feedback loops that sense tissue oxygen levels control red blood cell production Fig 42.16, 7 th ed

68 Key Concepts: Circulation and gas exchange – why? Circulation – spanning diversity Hearts – the evolution of double circulation Blood circulation and capillary exchange Blood structure and function Gas exchange – spanning diversity Breathing – spanning diversity Respiratory pigments

Hands On Dissect out the circulatory system of your rat Start by clearing the tissues around the heart Be especially careful at the anterior end of the heart – this is where the major blood vessels emerge Trace the aorta, the vena cava, and as many additional vessels as possible – use your manual and lab handout for direction! 69

Hands On Feel and describe the texture of the atria vs. the ventricles Take cross sections of the heart through both the atria and the ventricles Examine under the dissecting microscope Do the same with aorta and vena cava Try for a thin enough section to look at under the compound microscope too 70

71 Gas Exchange Gas Exchange ≠ Respiration ≠ Breathing  Gas exchange = delivery of O 2 ; removal of CO 2  Respiration = the metabolic process that occurs in mitochondria and produces ATP  Breathing = ventilation to supply the exchange surface with O 2 and allow exhalation of CO 2

72 Diagram showing indirect links between external environment, respiratory system, circulatory system and tissues.

73 Gas Exchange Occurs at the Respiratory Surface Respiratory medium = the source of the O 2  Air for terrestrial animals – air is 21% O 2 by volume  Water for aquatic animals – dissolved O 2 varies base on environmental conditions, especially salinity and temperature; always lower than in air

74 Gas Exchange Occurs at the Respiratory Surface Respiratory surface = the site of gas exchange  Gasses move by diffusion across membranes  Gasses are always dissolved in the interstitial fluid Surface area is important!

75 Evolution of Gas Exchange Surfaces Skin  Must remain moist – limits environments  Must maintain functional SA / V ratio – limits 3D size Gills  Large SA suspended in water Tracheal systems  Large SA spread diffusely throughout body Lungs  Large SA contained within small space

76 Skin Limits Sponges, jellies and flatworms rely on the skin as their only respiratory surface

77 Evolution of Gas Exchange Surfaces Skin  Must remain moist – limits environments  Must maintain functional SA / V ratio – limits 3D size Gills  Large SA suspended in water Tracheal systems  Large SA spread diffusely throughout body Lungs  Large SA contained within small space

78 Diagrams and photos of gills in different animals. Invertebrate Gills Dissolved oxygen is limited Behaviors and structures increase water flow past gills to maximize gas exchange Fig 42.20, 7 th ed

79 Diagram of countercurrent exchange in fish gills Countercurrent Exchange in Fish Gills Direction of blood flow allows for maximum gas exchange – maintains high gradient Fig 42.21, 7 th ed

80 Figure showing countercurrent vs co-current flow effects on diffusion How countercurrent flow maximizes diffusion

81 Evolution of Gas Exchange Surfaces Skin  Must remain moist – limits environments  Must maintain functional SA / V ratio – limits 3D size Gills  Large SA suspended in water Tracheal systems  Large SA spread diffusely throughout body Lungs  Large SA contained within small space

82 Diagram and micrograph of insect tracheal system. Tracheal Systems in Insects Air tubes diffusely penetrate entire body Small openings to the outside limit evaporation Open circulatory system does not transport gasses from the exchange surface Body movements ventilate

83 Tracheal Systems in Insects Rings of chitin Look familiar???

84 Critical Thinking Name 2 other structures that are held open by rings

85 Critical Thinking

86 Evolution of Gas Exchange Surfaces Skin  Must remain moist – limits environments  Must maintain functional SA / V ratio – limits 3D size Gills  Large SA suspended in water Tracheal systems  Large SA spread diffusely throughout body Lungs  Large SA contained within small space

87 Lungs in Spiders, Terrestrial Snails and Vertebrates Large surface area restricted to small part of the body Single, small opening limits evaporation Connected to all cells and tissues via a circulatory system  Dense capillary beds lie directly adjacent to respiratory epithelium In some animals, the skin supplements gas exchange (amphibians)

88 Mammalian Lungs Highly branched system of tubes – trachea, bronchi, and bronchioles Each ends in a cluster of “bubbles” – the alveoli  Alveoli are surrounded by capillaries  This is the actual site of gas exchange  Huge surface area (100m 2 in humans) Rings of cartilage keep the trachea open Epiglottis directs food to esophagus

89 Figure and micrograph of lung and alveolus structure.

90 Mammalian Lungs Highly branched system of tubes – trachea, bronchi, and bronchioles Each ends in a cluster of “bubbles” – the alveoli  Alveoli are surrounded by capillaries  This is the actual site of gas exchange  Huge surface area (100m 2 in humans) Rings of cartilage keep the trachea open Epiglottis directs food to esophagus

91 Figure of vascularized alveolus

92 Mammalian Lungs Highly branched system of tubes – trachea, bronchi, and bronchioles Each ends in a cluster of “bubbles” – the alveoli  Alveoli are surrounded by capillaries  This is the actual site of gas exchange  Huge surface area (100m 2 in humans) Rings of cartilage keep the trachea open Epiglottis directs food to esophagus

93 Breathing Ventilates Lungs Positive pressure breathing – amphibians  Air is forced into trachea under pressure  Mouth and nose close, muscle contractions force air into lungs  Relaxation of muscles and elastic recoil of lungs force exhalation

94 Breathing Ventilates Lungs Positive pressure breathing – amphibians  Air is forced into trachea under pressure  Mouth and nose close, muscle contractions force air into lungs  Relaxation of muscles and elastic recoil of lungs force exhalation Negative pressure breathing – mammals  Air is sucked into trachea under suction Circuit flow breathing – birds  Air flows through entire circuit with every breath

95 Diagram of negative pressure breathing Negative Pressure Breathing

96 Breathing Ventilates Lungs Positive pressure breathing – amphibians  Air is forced into trachea under pressure  Mouth and nose close, muscle contractions force air into lungs  Relaxation of muscles and elastic recoil of lungs forces exhalation Negative pressure breathing – mammals  Air is sucked into trachea under suction Circuit flow breathing – birds  Air flows through entire circuit with every breath

97 Diagram of circuit flow breathing in birds Flow Through Breathing No residual air left in lungs Every breath brings fresh O 2 past the exchange surface Higher lung O 2 concentration than in mammals

98 Critical Thinking What is the functional advantage of flow- through breathing for birds???

99 Critical Thinking What is the functional advantage of flow- through breathing for birds???

100 Respiratory pigments – tying the two systems together Respiratory pigments are proteins that reversibly bind O 2 and CO 2 Circulatory systems transport the pigments to sites of gas exchange O 2 and CO 2 molecules bind or are released depending on gradients of partial pressure

101 Partial Pressure Gradients Drive Gas Transport Atmospheric pressure at sea level is equivalent to the pressure exerted by a column of mercury 760 mm high = 760 mm Hg  This represents the total pressure that the atmosphere exerts on the surface of the earth Partial pressure is the percentage of total atmospheric pressure that can be assigned to each component of the atmosphere

102 Atmospheric pressure at sea level is equivalent to the pressure exerted by a column of mercury 760 mm high = 760 mm Hg (29.92” of mercury)

103 Partial Pressure Gradients Drive Gas Transport Atmospheric pressure at sea level is equivalent to the pressure exerted by a column of mercury 760 mm high = 760 mm Hg  This represents the total pressure that the atmosphere exerts on the surface of the earth Partial pressure is the percentage of total atmospheric pressure that can be assigned to each component of the atmosphere

104 Partial Pressure Gradients Drive Gas Transport Each gas contributes to total atmospheric pressure in proportion to its volume % in the atmosphere  Each gas contributes a part of total pressure  That part = the partial pressure for that gas The atmosphere is 21% O 2 and 0.03% CO 2  Partial pressure of O 2 is 0.21x760 = 160 mm Hg  Partial pressure of CO 2 is x760 = 0.23 mm Hg

105 Partial Pressure Gradients Drive Gas Transport Each gas contributes to total atmospheric pressure in proportion to its volume % in the atmosphere  Each gas contributes a part of total pressure  That part = the partial pressure for that gas The atmosphere is 21% O 2 and 0.03% CO 2  Partial pressure of O 2 is 0.21x760 = 160 mm Hg  Partial pressure of CO 2 is x760 = 0.23 mm Hg

106 Partial Pressure Gradients Drive Gas Transport Atmospheric gasses dissolve into water in proportion to their partial pressure and solubility in water  Dynamic equilibriums can eventually develop such that the PP in solution is the same as the PP in the atmosphere  This occurs in the fluid lining the alveoli

107 Critical Thinking If a dynamic equilibrium exists in the alveoli, will the partial pressures be the same as in the outside atmosphere???

108 Critical Thinking If a dynamic equilibrium exists in the alveoli, will the partial pressures be the same as in the outside atmosphere???

109 Diagram showing partial pressures of gasses in various parts of the body. This diagram is used in the next 3 slides. Inhaled air PP’s = atmospheric PP’s Alveolar PP’s reflect mixing of inhaled and exhaled air  Lower PP of O 2 and higher PP of CO 2 than in atmosphere

110 O 2 and CO 2 diffuse based on gradients of partial pressure  Blood PP’s reflect supply and usage  Blood leaves the lungs with high PP of O 2  Body tissues have lower PP of O 2 because of mitochondrial usage  O 2 moves from blood to tissues

111 Same principles with CO 2  Blood leaves the lungs with low PP of CO 2  Body tissues have higher PP of CO 2 because of mitochondrial production  CO 2 moves from tissues to blood

112 When blood reaches the lungs the gradients favor diffusion of O 2 into the blood and CO 2 into the alveoli

113 Diagram of hemoglobin structure and how it changes with oxygen loading. This diagram is used in the next 3 slides. Oxygen Transport Oxygen is not very soluble in water (blood) Oxygen transport and delivery are enhanced by binding of O 2 to respiratory pigments Fig 42.28, 7 th ed

114 Oxygen Transport Increase is 2 orders of magnitude! Almost 50 times more O 2 can be carried this way, as opposed to simply dissolved in the blood

115 Oxygen Transport Most vertebrates and some inverts use hemoglobin for O 2 transport Iron (in heme group) is the binding element

116 Oxygen Transport Four heme groups per hemoglobin, each with one iron atom Binding is reversible and cooperative

117 Critical Thinking Binding is reversible and cooperative What does that mean???

118 Critical Thinking Binding is reversible and cooperative What does that mean???

119 Oxygen Transport Reverse occurs during unloading Release of one O 2 induces shape change that speeds up the release of the next 3

120 Graph showing how hemoglobin oxygen saturation changes with activity. Oxygen Transport More active metabolism (ie: during muscle use) increases unloading Note steepness of curve  O 2 is unloaded quickly when metabolic use increases

121 Graph showing the Bohr Shift Oxygen Transport – the Bohr Shift More active metabolism also increases the release of CO 2  Converts to carbonic acid, acidifying blood  pH change stimulates release of additional O 2 Fig 42.29, 7 th ed

122 Figure showing how carbon dioxide is transported from tissues to lungs. This figure is used in the next 3 slides. Carbon Dioxide Transport Red blood cells also assist in CO 2 transport  7% of CO 2 is transported dissolved in plasma  23% is bound to amino groups of hemoglobin in the RBC’s  70% is converted to bicarbonate ions inside the RBC’s

123 Carbon Dioxide Transport CO 2 in RBC’s reacts with water to form carbonic acid (H 2 CO 3 ) H 2 CO 3 dissociates to bicarbonate (HCO 3 - ) and H +

124 Carbon Dioxide Transport Most H + binds to hemoglobin  This limits blood acidification HCO 3 - diffuses back into plasma for transport

125 Carbon Dioxide Transport Reverse occurs when blood reaches the lungs  Conversion back to CO 2 is driven by diffusion gradients as CO 2 moves into the lungs

126 REVIEW – Key Concepts: Circulation and gas exchange – why? Circulation – spanning diversity Hearts – the evolution of double circulation Blood circulation and capillary exchange Blood structure and function Gas exchange – spanning diversity Breathing – spanning diversity Respiratory pigments

Hands On Dissect out the respiratory system of your rat Trace the trachea into the lungs Examine trachea and lungs under the dissecting microscope Try for thin enough sections to also examine with the compound microscope 127