50.0 Introduction

1. Heterotrophs Obtain Energy by Cellular Respiration
1. Process Consumes Oxygen and Generate Carbon Dioxide and Water
2. Respiration: Exchange of Oxygen and Carbon Dioxide at Organism Level fig 50.1
1. Includes mechanisms of breathing
2. Includes exchange of gases between cardiovascular system and body

50.1 Respiration involves the diffusion of gases

1. Fick's Law of Diffusion
1. Respiratory Gases Diffuse Across Plasma Membranes
1. Plasma membranes must be surrounded by water
1. External environment is aqueous
2. True even in terrestrial animals
1. Oxygen dissolves in thin layer of fluid covering respiratory surfaces
2. Includes surface of alveoli in lungs

2. Diffusion of oxygen into the epithelial aqueous layer is passive
3. Driven by the difference in oxygen concentration two sides of membrane
4. Mathematical relationship called Fick's law of diffusion
1. R = D x A (”p / d)
2. R = rate of diffusion
3. D = diffusion constant
4. A = area over which diffusion takes place
5. p = difference in partial pressures on each side
6. d = distance across which diffusion takes place

5. Evolutionary changes optimize R by favoring certain parameters fig 50.2
1. Increase surface area
2. Decrease distance d
3. Increase concentration difference ”p

6. Evolution has involved changes in all three factors

2. How Animals Maximize the Rate of Diffusion
1. Coping with Different Conditions
1. Relying on simple diffusion
1. Oxygen diffuses too slowly to be efficient over more than 0.5 mm
2. Severely limits size of organisms
3. Protists are small enough to utilize simple diffusion fig 50.2a

2. Creating a water current
1. Most primitive invertebrate phyla possess no special respiratory organs
2. Can obtain oxygen via diffusion by increasing p in Fick's equation
1. Increase difference in O₂ concentration by creating a water current
2. Constantly replace water over diffusion surface
3. p does not decrease as diffusion proceeds
4. Keep exterior O₂ concentration high
5. Results in higher realized value of R, rate of diffusion

3. Increasing the diffusion surface area
1. More advanced invertebrates and vertebrates possess respiratory organs
1. Increase surface area over which diffusion occurs
2. Provides contact between external environment and internal circulating fluids
3. Increase A and decreasing d

2. Atmospheric Pressure and Partial Pressures
1. Dry air = 78.09% N₂ + 20.95% O₂ + 0.93% (argon + inert gases) + 0.03% CO₂
2. Amount of air present decreases at high altitudes fig 50.3
3. At sea level, air pressure measures 760 mm of mercury fig 50.3
1. Equals the barometric pressure of air
2. Equivalent to one atmosphere of pressure

4. Each gas within the air exerts a partial pressure(Px) = 760 x % gas
1. Nitrogen (PN₂) = 760 x 79.02% = 600.6 mmHg
2. Oxygen (PO₂) = 760 x 20.95% = 159.2 mmHg
3. Carbon dioxide (PCO₂) = 760 x 0.03% = 0.2 mmHg

5. Less air, therefore less oxygen present at high altitudes
1. Barometric pressure above 6000 meters = 380 mmHg
2. PO₂ = 380 x 20.95% = 80 mmHg
3. Only half the oxygen is available compared to sea level

50.2 Gills are used for respiration by aquatic vertebrates

1. The Gill as a Respiratory Structure
1. Basic Structure of Gills
1. Aquatic organs (gills) project from body into water
1. Simple gills like papulae of echinoderms fig 50.2c
2. Convoluted gills of fish fig 50.2e

2. Increase in diffusion surface area enables aquatic organisms to extract more oxygen
3. External gills
1. Provide greatly increased surface area
2. Include gills of fish and larval amphibians and neotenic amphibians fig 50.4

4. Disadvantages of external gills
1. Difficult to constantly circulate water past diffusion surface
2. Inefficient, highly branched gills offer resistance against movement

5. Special branchial chambers in other organisms pump water past gills
1. Internal mantle cavity of mollusks opens to outside, contains gills
2. Contraction of muscular walls draws water in and expels it
3. Crustacean cavity lies between body and hard exoskeleton
4. Movement of limbs draws water through branchial chamber

2. The Gills of Bony Fishes
1. Gills are located between mouth (buccal cavity) and opercular cavity fig 50.5
1. Buccal cavity opens and closes with movement of mouth
2. Opercular cavity opened and closed by moving operculum, gill cover

2. Two sets of cavities serve as pumps
1. Expand alternately to move water into mouth, through gills, out operculum One-way flow of water over gills

3. Continuously swimming fish have nearly immovable gill covers
1. Water constantly forced over gills as fish swim
2. Process is a form of ram ventilation

4. Most bony fish have flexible gill covers, permit pumping action

3. Effects of Gill Construction on Parameters of Diffusion
1. Structure of gills fig 50.6
1. Four gill arches on each side of head
2. Each gill composed of two rows of gill filaments
3. Filaments divided into thin lamellae that project into flow of water
4. Movement of water across lamellae occurs in only one direction
5. Direction of blood circulation runs opposite that of water flow
6. Countercurrent flow maximizes p between water and blood

2. Advantage of countercurrent exchange fig 50.7a
1. Least oxygenated blood meets least oxygenated water at back of gill
2. Most oxygenated blood meets most oxygenated water at front of gill
3. Diffusion occurs along entire length of gill

3. If water and blood flowed in the same direction, concurrent flow
1. Oxygen-free blood would meet highly oxygenated water
2. Diffusion would initially be high fig 50.7b
3. Oxygenated blood would meet less oxygenated water at back of gill
4. Diffusion would cease, only front part of gill would be functional

4. Fish gills are most efficient respiratory organs

50.3 Lungs are used for respiration by terrestrial vertebrates

1. Respiration in Air-Breathing Animals
1. Gills Not Adaptable for Terrestrial Use
1. Air is less buoyant than water
1. Lamellae lack structural support, collapse without water buoyancy
2. More oxygen present in air than in water
1. Water = 5-10 ml O₂ per 1 liter water
2. Air = 210 ml of O₂ per 1 liter air

3. Collapse reduces diffusion surface area
4. Internal air passages remain open due to structural support

2. Water diffuses into air through evaporation
1. Terrestrial organisms constantly lose water to atmosphere
2. Gills provide an enormous surface area for water loss

2. Evolution of Two Kinds of Terrestrial Respiratory Organs
1. Both systems sacrifice efficiency to reduce water loss
2. Tracheae of insects fig 50.2d
1. Extensive series of air-filled passages within body
2. Oxygen diffuses directly from trachea to cells, no circulatory intervention
3. Openings close when CO₂ levels are below certain point to limit water loss

3. Lungs of terrestrial vertebrates
1. Respiratory process called ventilation
2. Air saturated with water vapor before reaching area of gas exchange
3. Air enters and exits through one tube, minimizes evaporation
4. Two-way flow of air replaces one-way flow (except in birds)

2. Respiration in Amphibians and Reptiles
1. Amphibians
1. Structure of the amphibian respiratory system fig 50.8
1. Lungs are sac-like outpouchings of gut with few folds fig 50.9
2. Less surface area than other terrestrial vertebrates
3. Connected to rear of oral cavity (pharynx)
4. Opening controlled by glottis

2. Breathing process different from other terrestrial vertebrates
1. Force air into lungs by creating positive pressure outside lungs
2. Fill pharynx with air, close mouth and nostrils
3. Elevate floor of oral cavity
4. Pushes air into lungs
5. Analogous to mouth-to-mouth resuscitation in humans

3. Breathing process of other vertebrates
1. Create negative pressure within lungs
2. Expand thoracic cavity by contractions of muscles

4. Supplemental oxygen obtained by diffusion across moist skin, cutaneous respiration
1. Skin is wet and well-vascularized
2. More important in winter when metabolism is low

2. Reptiles
1. Expand rib cages, exhibit negative pressure breathing
2. Lungs have greater surface area
3. Cannot obtain oxygen through watertight skin surface

3. Respiration in Mammals
1. Greater Oxygen Demand in Mammals
1. Metabolic demands greater due to endothermy
2. Lungs more highly branched with alveoli clusters fig 50.9
3. Structure of mammalian lung
1. Inhaled air brought in through larynx past glottis and vocal chords
2. Enters cartilage-supported trachea
3. Splits into right and left bronchi that enter lung
4. Further subdivide into numerous, small bronchioles
5. Bronchiole delivers air to blind-ended alveolar sac
6. Alveoli surrounded by extensive capillary network
7. All gas exchange occurs across walls of alveoli fig 50.3f

4. Branching and alveoli vastly increase total surface area
1. Humans have 300 million alveoli in two lungs
2. Area about 42 times the surface area of body

4. Respiration in Birds
1. Birds Possess a More Efficient Respiratory System
1. Birds possess parabronchi, tiny vessels through which air flows sfig 50.11a
1. Do not have blind-end alveoli like mammals
2. Gas exchange occurs in parabronchi

2. Bird air flow is unidirectional like in fish
1. Other terrestrial vertebrates mix new and old air
2. Only fresh air enters bird parabronchi, old air exits by different route

3. Birds have two unique groups of air sacs fig 50.11b
1. Anterior and posterior air sacs expand during inspiration
2. Sacs take in air
3. Sacs compressed during expiration
4. Air pushed into other elements of avian respiratory system

4. Avian respiration has two-cycles fig 50.11c
1. With inspiration both anterior and posterior air sacs expand
2. With inhalation fresh air passes into only posterior air sacs
3. Anterior air sacs fill with air from lungs
4. Air from anterior sacs flows out of the body with exhalation
5. With exhalation air in posterior sacs flows into lung
6. Process repeated in second cycle
7. Air flows through lungs in only one direction
8. Air finally exhaled at end of second cycle

5. Direction of air flow is different from the flow of blood fig 50.10
1. Flow of air and blood are at 90º angles to one another
2. Called cross-current flow
3. Less efficient than fish, more efficient than mammals

6. Birds can survive in much higher altitudes than mammals

50.4 Mammalian breathing is a dynamic process

1. The Structures and Mechanics of Breathing
1. The Structure of the Mammalian Lung
1. Bronchioles deliver air to alveoli where gas exchange occurs
1. Lines by epithelium only one cell layer thick
2. Alveoli are outpouchings surrounded by capillaries one cell layer thick
3. Distance between air and blood is two cell layers, 0.5 to 1.5 µm

2. Partial pressures of gases in the lungs fig 50.12
1. PO₂ of air in lungs normally about 100 mmHg
2. PO₂ of blood is about 105 mmHg
3. Returned blood has PO₂ of 40 mmHg
4. Blood PCO₂ also changes

3. Outside of lungs covered by visceral pleural membrane
4. Inner wall of thoracic cavity lined by parietal pleural membrane
5. Space between membranes called the pleural cavity
1. Normally small and filled with fluid
2. Fluid links membranes together like water film holds two sheets of glass together
3. Lungs held tight to thoracic cavity
4. Each lung has own pleural cavity, if one punctured other lung functional

2. Mechanics of Breathing
1. Air drawn into lung by formation of negative pressure
2. Relation to Boyle's Law of gasses
1. Volume of gas increases, pressure decreases
2. Volume of thorax increases with inspiration
3. Lungs expand due to adherence of visceral and parietal pleural membranes
4. Pressure is lower than atmospheric, air enters lungs

3. During inhalation or inspiration thoracic volume increases
1. Rib external intercostal muscles contract raising the ribs
2. Diaphragm contracts, lowers and flattens
3. Increases volume of thorax
4. Force deeper inspiration by contracting other thoracic muscles fig 50.13a

4. During exhalation or expiration
1. Thorax and lungs resist distention, recoil when force subsides
2. Expansion places structures under elastic tension
3. Diaphragm and external intercostal muscles relax in unforced expiration
4. Extra air can be forced out of lungs fig 50.13b
1. Contraction of abdominal muscles
2. Diaphragm pushed further up into thoracic cavity

3. Breathing Measurements
1. Tidal volume: Volume inspired and expired in a single breath
1. About 500 ml of air
2. Anatomical dead space: 150 ml within air passages
3. Birds have no "dead volume" of air remaining in lungs as do mammals

2. Vital capacity: Amount of air expired after forceful, maximum inspiration
1. Men = 4.6 liters, women = 3.1 liters
2. Emphysema reduces vital capacity
3. Alveoli destroyed by cigarette smoking

3. Normal breathing rates oxygenate blood, remove carbon dioxide
1. PO₂ and PCO₂ kept within normal ranges
2. Hypoventilation
1. Breathing insufficient to maintain normal rates
2. Elevated PCO₂ level

3. Hyperventilation
1. PCO₂ abnormally low

4. Not equal to increased breathing associated with exercise
1. Faster breathing matched to faster metabolism
2. Blood gas measurements remain normal

2. Mechanisms that Regulate Breathing
1. Breathing Initiated by Respiratory Center in Brain
1. Sends nerve signals to diaphragm and intercostal muscles
1. Stimulate muscles to contract
2. Contraction and expansion of chest causes inspiration
3. Expiration proceeds when neurons stop producing impulses

2. Breathing muscles are skeletal, but are under involuntary control
3. Can be voluntarily over-ridden in hypo- or hyperventilation

2. Reflex Pathway Prevents Life Threatening Alterations in Breathing
1. Holding breath causes increase in blood PCO₂
1. Causes increase in carbonic acid, lowers blood pH
2. Peripheral chemoreceptors in aortic and carotid bodies are sensitive to pH
3. Send impulses to respiratory control center to reinitiate breathing

2. Central chemoreceptors detect changes in pH of cerebrospinal fluid (CSF) fig 50.14
3. Peripheral chemoreceptors are responsible for immediate changes
4. Central chemoreceptors are responsible for sustained changes
5. Increased respiratory rate eliminates excess CO₂, pH returns to normal
6. Indefinite hyperventilation also prevented by chemoreceptors
1. Decrease in PCO₂ and increase in pH stop reflex drive to breathe
2. Constrict cerebral blood vessels, cause dizziness
3. Can hold breath longer by hyperventilating first

7. PO₂ is a stimulus for breathing only under special conditions
1. At high altitudes where PO₂ is low
2. In patients with emphysema

50.5 Blood transports oxygen and carbon dioxide

1. Hemoglobin and Oxygen Transport
1. Association of Respiratory and Circulatory Systems
1. Oxygen dissolved in blood dependent on PO₂ of air
1. Blood plasma can contain only 3 ml O₂/liter
2. Whole blood contains nearly 200 ml O₂/liter
3. Most oxygen bound to hemoglobin inside red blood cells

2. Hemoglobin: Oxygen carrier protein within the blood of most animals
1. Four polypeptide subunit protein
2. Each subunit combines with iron containing heme group fig 50.16
3. Hemoglobin picks up oxygen in lungs
1. Bright red color when bound with oxygen
2. Called oxyhemoglobin

4. Hemoglobin releases oxygen at tissues
1. Called deoxyhemoglobin
2. Dark red color, looks blue under skin

5. Hemoglobin widely distributed oxygen carrier protein throughout animal kingdom

3. Hemocyanin: Second carrier protein found in many invertebrates
1. Uses copper instead of iron
2. Does not occur within blood cells, exists free in hemolymph

2. Oxygen Transport
1. At sea level PO₂ is 105 mmHg
1. Less than that of atmosphere due to mixing of old air in anatomical dead space
2. Blood leaving alveoli slightly less due to inefficiency in lung function

2. At PO₂ of 100 mmHg, 97% bound to hemoglobin in red blood cells
3. Percent saturation in arterial blood is 97% at sea level
4. Extracellular fluid surrounding tissues has lower PO₂
1. Oxygen diffuses from capillaries into tissues
2. PO₂ of venous blood is 40 mmHg, percent saturation is 75%

5. Graphical representation is an oxyhemoglobin dissociation curve fig 50.17a
1. At rest, 22% (97-75) of the oxyhemoglobin releases oxygen to tissues
2. One fifth of oxygen unloaded in tissues, four-fifths in blood as reserve
1. Blood can additionally supply oxygen needs at exercise
1. If venous blood PO₂ is 20 mmHg, saturation is 35% fig 50.17b
2. Amount unloaded now 62% (97-35)

2. Blood contains reserves for 4-5 minutes without breathing

6. Presence of CO₂ at metabolizing tissues
1. Combines with water to form carbonic acid, lowers pH of blood
2. Occurs in red blood cells, hemoglobin has less affinity for oxygen
3. Hemoglobin releases oxygen more readily
4. Dissociation curve shifted to right, called Bohr effect fig 50.18

2. Carbon Dioxide and Nitric Oxide Transport
1. Carbon Dioxide Transport
1. As red blood cells unload oxygen, blood absorbs CO₂ from tissues
1. 8% dissolved in plasma, 20% binds to hemoglobin
2. 72% enters red blood cells
1. Carbonic anhydrase catalyzes formation of carbonic acid
2. Carbonic acid dissociates to form bicarbonate and hydrogen ions
3. CO₂ removed from plasma, allows loading of greater amounts

2. Blood carries CO₂ back to lungs
3. Lower PCO₂ of air in alveoli
1. Carbonic anhydrase reaction proceeds in reverse
2. Gaseous CO₂ released, diffuses into alveoli
3. Leaves body with next exhalation fig 50.19

2. Nitric Oxide Transport
1. Hemoglobin also holds and releases nitric oxide gas (NO)
2. Important regulatory gas acts on many cells to change shapes and functions
1. Presence of NO in blood vessels causes them to expand
2. Relaxes surrounding muscle cells
3. Blood flow and pressure regulated by NO in bloodstream

3. Hemoglobin carries NO as super nitric oxide, has extra electron
1. Binds to cysteine amino acid
2. Blood picks up super nitric oxide in lungs along with oxygen

4. At body capillaries O₂/CO₂ gas exchange occurs
1. Hemoglobin release of NO can increase blood flow, blood vessels expand
2. Hemoglobin traps excess NO on empty iron atoms, causes vessels to constrict

5. Blood returns to lungs to release CO₂ and regular NO bound to iron atoms in hemoglobin
6. Picks up O₂ and super NO to continue cycle