1. The Respiratory System
Chapter 49

2. 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

3. 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
Dp = Pressure difference between two sides
d = Distance over which diffusion occurs
R =
DA Dp d

4. Gas Exchange
Evolutionary changes have occurred to optimize the rate of diffusion R
Increase surface area A
Decrease distance d
Increase concentration difference Dp

5. 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

9. 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 damaged

11. Gills
Within each lamella, blood flows opposite to direction of water movement
Countercurrent flow
Maximizes oxygenation of blood
Increases Dp
Fish gills are the most efficient of all respiratory organs

13. 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

14. 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

15. 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

16. Partial pressures are based on the % of the gas in dry air
At sea level or 1.0 atm
PN2 = 760 x 79.02% = mm Hg
PO2 = 760 x 20.95% = 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

17. 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

19. 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

20. Lungs

21. 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 lungs

22. 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

23. Lungs

24. 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

26. 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

27. 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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30. 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

31. 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

32. 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)

34. Hemoglobin Consists of four polypeptide chains: two a and two b
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

35. 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

36. Hemoglobin

37. 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 effect

38. 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”

39. 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)