The Respiratory System

Chapter 49

Gas Exchange

• One of the major physiological challenges facing all multicellular animals is obtaining sufficient oxygen and disposing of excess carbon dioxide

• In vertebrates, the gases diffuse into the aqueous layer covering the epithelial cells that line the respiratory organs

• Diffusion is passive, driven only by the difference in O2 and CO2 concentrations on the two sides of the membranes and their relative solubilities in the plasma membrane

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Gas Exchange

• Rate of diffusion between two regions is governed by Fick’s Law of Diffusion

• R = Rate of diffusion • D = Diffusion constant• A = Area over which diffusion takes place• p = Pressure difference between two sides• d = Distance over which diffusion occurs

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R =DA p

d

Gas Exchange

• Evolutionary changes have occurred to optimize the rate of diffusion R– Increase surface area A– Decrease distance d– Increase concentration difference p

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Gas Exchange

• Gases diffuse directly into unicellular organisms• However, most multicellular animals require

system adaptations to enhance gas exchange• Amphibians respire across their skin• Echinoderms have protruding papulae• Insects have an extensive tracheal system• Fish use gills• Mammals have a large network of alveoli

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Gills

• Specialized extensions of tissue that project into water

• Increase surface area for diffusion• External gills are not enclosed within body

structures– Found in immature fish and amphibians– Two main disadvantages

• Must be constantly moved to ensure contact with oxygen-rich fresh water

• Are easily damaged9

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Gills

• Within each lamella, blood flows opposite to direction of water movement– Countercurrent flow– Maximizes oxygenation of blood– Increases p

• Fish gills are the most efficient of all respiratory organs

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Gills

• Many amphibians use cutaneous respiration for gas exchange

• In terrestrial arthropods, the respiratory system consists of air ducts called trachea, which branch into very small tracheoles – Tracheoles are in direct contact with individual

cells– Spiracles (openings in the exoskeleton) can be

opened or closed by valves

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Lungs

• Gills were replaced in terrestrial animals because– Air is less supportive than water– Water evaporates

• The lung minimizes evaporation by moving air through a branched tubular passage

• A two-way flow system – Except birds

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Lungs

• Air exerts a pressure downward, due to gravity

• A pressure of 760 mm Hg is defined as one atmosphere (1.0 atm) of pressure

• Partial pressure is the pressure contributed by a gas to the total atmospheric pressure

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• Partial pressures are based on the % of the gas in dry air

• At sea level or 1.0 atm– PN2 = 760 x 79.02% = 600.6 mm Hg

– PO2 = 760 x 20.95% = 159.2 mm Hg

– PCO2 = 760 x 0.03% = 0.2 mm Hg

• At 6000 m the atmospheric pressure is 380 mm Hg– PO2 = 380 x 20.95% = 80 mm Hg

*Point is, at higher altitudes there is less pressure, so less air gets into your lungs

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Lungs

• Frogs have positive pressure breathing– Force air into their lungs by creating a positive

pressure in the buccal cavity

• Reptiles have negative pressure breathing– Expand rib cages by muscular contractions,

creating lower pressure inside the lungs

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Lungs

• Lungs of mammals are packed with millions of alveoli (sites of gas exchange)

• Inhaled air passes through the larynx, glottis, and trachea

• Bifurcates into the right and left bronchi, which enter each lung and further subdivide into bronchioles

• Alveoli are surrounded by an extensive capillary network

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Lungs

Lungs

• Lungs of birds channel air through very tiny air vessels called parabronchi

• Unidirectional flow• Achieved through the action of anterior and

posterior sacs (unique to birds)• When expanded during inhalation, they take

in air• When compressed during exhalation, they

push air in and through lungs21

Lungs

• Respiration in birds occurs in two cycles– Cycle 1 = Inhaled air is drawn from the trachea

into posterior air sacs, and exhaled into the lungs– Cycle 2 = Air is drawn from the lungs into anterior

air sacs, and exhaled through the trachea

• Blood flow runs 90o to the air flow– Crosscurrent flow– Not as efficient as countercurrent flow

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Lungs

Gas Exchange

• Gas exchange is driven by differences in partial pressures

• Blood returning from the systemic circulation, depleted in oxygen, has a partial oxygen pressure (PO2) of about 40 mm Hg

• By contrast, the PO2 in the alveoli is about 105 mm Hg

• The blood leaving the lungs, as a result of this gas exchange, normally contains a PO2 of about 100 mm

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Lung Structure and Function

• During inhalation, thoracic volume increases through contraction of two muscle sets– Contraction of the external intercostal

muscles expands the rib cage– Contraction of the diaphragm expands the

volume of thorax and lungs

• Produces negative pressure which draws air into the lungs

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Lung Structure and Function

• Thorax and lungs have a degree of elasticity

• Expansion during inhalation puts these structures under elastic tension

• Tension is released by the relaxation of the external intercostal muscles and diaphragm

• This produces unforced exhalation, allowing thorax and lungs to recoil

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Lung Structure and Function

• Tidal volume– Volume of air moving in and out of lungs in a person at

rest• Vital capacity

– Maximum amount of air that can be expired after a forceful inspiration

• Hypoventilation– Insufficient breathing– Blood has abnormally high PCO2

• Hyperventilation– Excessive breathing– Blood has abnormally low PCO2

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Lung Structure and Function

• Each breath is initiated by neurons in a respiratory control center in the medulla oblongata

• Stimulate external intercostal muscles and diaphragm to contract, causing inhalation

• When neurons stop producing impulses, respiratory muscles relax, and exhalation occurs

• Muscles of breathing usually controlled automatically– Can be voluntarily overridden – hold your breath

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Lung Structure and Function

• Neurons are sensitive to blood PCO2 changes

• A rise in PCO2 causes increased production of carbonic acid (H2CO3), lowering the blood pH

• Stimulates chemosensitive neurons in the aortic and carotid bodies

• Send impulses to respiratory control center to increase rate of breathing

• Brain also contains central chemoreceptors that are sensitive to changes in the pH of cerebrospinal fluid (CSF)

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Hemoglobin

• Consists of four polypeptide chains: two and two

• Each chain is associated with a heme group• Each heme group has a central iron atom that

can bind a molecule of O2

• Hemoglobin loads up with oxygen in the lungs, forming oxyhemoglobin

• Some molecules lose O2 as blood passes through capillaries, forming deoxyhemoglobin

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Hemoglobin

• At a blood PO2 of 100 mm Hg, hemoglobin is 97% saturated

• In a person at rest, the blood that returns to the lungs has a PO2 about 40 mm Hg less

• Leaves four-fifths of the oxygen in the blood as a reserve

• This reserve enables the blood to supply body’s oxygen needs during exertion

• Oxyhemoglobin dissociation curve is a graphic representation of these changes

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Hemoglobin

Hemoglobin

• Hemoglobin’s affinity for O2 is affected by pH and temperature

• The pH effect is known as the Bohr shift – Increased CO2 in blood increases H+

– Lower pH reduces hemoglobin’s affinity for O2

– Results in a shift of oxyhemoglobin dissociation curve to the right

– Facilitates oxygen unloading

• Increasing temperature has a similar effect37

Transportation of Carbon Dioxide

• About 8% of the CO2 in blood is dissolved in plasma

• 20% of the CO2 in blood is bound to hemoglobin

• Remaining 72% diffuses into red blood cells– Enzyme carbonic anhydrase combines CO2 with H2O to

form H2CO3

– H2CO3 dissociates into H+ and HCO3–

– H+ binds to deoxyhemoglobin

– HCO3– moves out of the blood and into plasma

– One Cl– exchanged for one HCO3– – “chloride shift”

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Transportation of Carbon Dioxide

• When the blood passes through pulmonary capillaries, these reactions are reversed

• The result is the production of CO2 gas, which is exhaled

• Other dissolved gases are also transported by hemoglobin– Nitric oxide (NO) and carbon monoxide (CO)

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