The document summarizes the physiology of the respiratory system. It discusses: 1) The mechanism of breathing including inspiration through contraction of the diaphragm and intercostal muscles, and expiration which is usually passive. 2) Gas exchange which occurs through diffusion across the alveolar-capillary membrane in the lungs, with oxygen diffusing into blood and carbon dioxide diffusing out. 3) Transport of oxygen which is carried in both dissolved and chemically bound forms in blood, with deoxygenated blood releasing carbon dioxide as it passes through the pulmonary circulation to be exhaled.

1. PHYSIOLOGY OF THE
RESPIRATORY SYSTEM
HUMAN PHYSIOLOGY
BY- HAILEMARIAM Z.
LIDETA COLLAGE

2. Outline
Introduction
Mechanism of breathing
Diffusion and gas transport (O2 and
CO2)
Regulation of breathing
2

3. Introduction
The goals of respiration are to provide oxygen to the tissues
and to remove carbon dioxide.
In addition to functioning in gas exchange, the respiratory
system also participates in regulating blood pH, contains
receptors for the sense of smell, filters inspired air,
produces sounds, and rids the body of some water and heat
in exhaled air.
3

4. Respiratory system anatomy review
Parts of the respiratory system can be classified
according to either structure or function.
Structurally, the respiratory system consists of two
parts: upper respiratory system and lower
respiratory system
4

5. 5
(1) The upper respiratory system
includes the nose, nasal cavity, pharynx,
and associated structures.
(2) The lower respiratory system includes
the larynx, trachea, bronchi, and lungs.

6. 6

7. Cont…
Functionally, the respiratory system also
consists of two parts:
◦the conducting zone and
◦the respiratory zone
7

8. 8

9. Cont…
The conducting zone consists of a series of
interconnecting cavities and tubes both outside and
within the lungs.
These include the nose, nasal cavity, pharynx, larynx,
trachea, bronchi, bronchioles, and terminal bronchioles;
their function is to filter, warm, and moisten air and
conduct it into the lungs.
9

10. Cont…
2 The respiratory zone consists of tubes and
tissues within the lungs where gas exchange
occurs.
These include the respiratory bronchioles, alveolar
ducts, alveolar sacs, and alveoli and are the main
sites of gas exchange between air and blood.
10

11. General Function of the
respiratory system
A. Gas Exchange
While air passing through the lungs, the atmospheric air
delivers O2 to the blood in the pulmonary capillaries and in
exchange takes away CO2 from the blood.
B. Phonation
Enable speech production
Movement of air past vocal folds makes sound & speech.
11

12. Cont…
C. Pulmonary defense
◦ Filters inspired air/Prevent entrance of
microorganisms.
D. Acid-base balance
E. Synthesis
◦ Formation of ACE through its vascular
endothelium.
F. Smell
12

13. Breathing
During normal breathing, inspiration is an active process
and expiration is a passive process.
Expiration is usually passive, and normally occurs by recoil
of stretched tissue of lungs and thorax.
High rates of ventilation or airway obstruction may cause
the muscles of expiration to actively contract.
13

14. Cont…
Breathing can be:
A. Abdominal Breathing
◦ Occur due to diaphragmatic movement.
◦ Downward movement of diaphragm
◦ displacement of abdominal viscera and abdominal
wall abdominal breathing.
14

15. Cont…
B. Thoracic Breathing
◦ Occur due to movement of chest wall thoracic breathing
NB. The work of breathing is higher in thoracic than in
abdominal breathing.
At rest, abdominal breathing accounts for 70% of the
pulmonary ventilation while thoracic breathing 30%.
15

16. Cont…
 In pregnancy and ascites, the movement of the
diaphragm is limited, and breathing becomes
mainly thoracic.
 During deep breathing both abdominal and
thoracic breathing are equal in magnitude.
16

17. Mechanism of breathing
The process of gas exchange in the body, called respiration,
has three basic steps:
1. Pulmonary ventilation or breathing, is the inhalation
(inflow) and exhalation (outflow) of air and involves the
exchange of air between the atmosphere and the alveoli of
the lungs.
17

18. 18

19. Cont…
2. External (pulmonary) respiration is the exchange
of gases between the alveoli of the lungs and the
blood in pulmonary capillaries across the
respiratory membrane.
In this process, pulmonary capillary blood gains O2
and loses CO2.
19

20. Cont…
3. Internal (tissue) respiration is the exchange of
gases between blood in systemic capillaries and
tissue cells. In this step the blood loses O2 and
gains CO2. Within cells, the metabolic reactions
that consume O2 and give off CO2 during the
production of ATP are termed cellular respiration
20

21. PULMONARY VENTILATION
Pulmonary ventilation, or breathing, consists of inhalation
and exhalation.
The movement of air into and out of the lungs depends on
pressure changes governed in part by Boyle’s law, which
states that the volume of a gas varies inversely with
pressure, assuming that temperature remains constant.
21

22. Inhalation
Just before each inhalation, the air pressure inside the lungs is
equal to the air pressure of the atmosphere, which at sea level is
about 760 millimeters of mercury (mmHg).
For air to flow into the lungs, the pressure inside the alveoli must
become lower than the atmospheric pressure.
 This condition is achieved by increasing the size of the lungs.
22

23. Cont…
Differences in pressure caused by changes in lung volume
force air into our lungs when we inhale and out when we
exhale.
For inhalation to occur, the lungs must expand, which
increases lung volume and thus decreases the pressure in
the lungs to below atmospheric pressure.
23

24. Cont…
The first step in expanding the lungs during normal
quiet inhalation involves contraction of the main
muscles of inhalation, the diaphragm and external
intercostals
24

25. 25

26. Cont…
During normal quiet inhalation, the diaphragm descends
about 1 cm, producing a pressure difference of 1–3
mmHg and the inhalation of about 500 mL of air.
In strenuous breathing, the diaphragm may descend 10
cm, which produces a pressure difference of 100 mmHg
and the inhalation of 2–3 liters of air.
26

27. Cont…
Contraction of the diaphragm is responsible for
about 75% of the air that enters the lungs during
quiet breathing.
Advanced pregnancy, excessive obesity, or
confining abdominal clothing can prevent complete
descent of the diaphragm.
27

28. Cont…
The next most important muscles of inhalation are
the external intercostals. When these muscles
contract, they elevate the ribs.
Contraction of the external intercostals is
responsible for about 25% of the air that enters the
lungs during normal quiet breathing.
28

29. Cont…
During quiet inhalations, the pressure between the two
pleural layers in the pleural cavity, called intrapleural
(intrathoracic) pressure, is always subatmospheric (lower
than atmospheric pressure).
Just before inhalation, it is about 4 mmHg less than the
atmospheric pressure, or about 756 mmHg at an
atmospheric pressure of 760 mmHg
29

30. 30

31. 31

32. Cont…
As the diaphragm and external intercostals contract and the overall size
of the thoracic cavity increases, the volume of the pleural cavity also
increases, which causes intrapleural pressure to decrease to about 754
mmHg.
During expansion of the thorax, the parietal and visceral pleurae
normally adhere tightly because of the sub atmospheric pressure
between them and because of the surface tension created by the moist
adjoining surfaces.
32

33. Cont…
As the volume of the lungs increases in this way, the
pressure inside the lungs, called the alveolar (intrapulmonic)
pressure, drops from 760 to 758 mmHg.
 pressure difference is thus established between the
atmosphere and the alveoli. Because air always flows from a
region of higher pressure to a region of lower pressure,
inhalation takes place
33

34. Cont…
Air continues to flow into the lungs as
long as a pressure difference exists.
During forceful inhalation, accessory
muscles of inhalation
(sternocleidomastoids, scalenes, and
pectoralis minors) are also used.
34

35. 35

36. 36

37. Exhalation
Breathing out, called exhalation (expiration), is also due to a
pressure gradient, but in this case the gradient is in the
opposite direction: The pressure in the lungs is greater than
the pressure of the atmosphere.
Normal exhalation during quiet breathing, unlike
inhalation(i.e inhalation is active process), is a passive
process because no muscular contractions are involved.
37

38. 38

39. Cont…
Exhalation starts when the inspiratory muscles
relax. As the diaphragm relaxes, its dome moves
superiorly owing to its elasticity. As the external
intercostals relax, the ribs are depressed, which
decreases lung volume.
In turn, the alveolar pressure increases to about
762 mmHg. Air then flows from the area of higher
pressure in the alveoli to the area of lower pressure
in the atmosphere.
39

40. 40

41. 41

42. Cont…
Exhalation becomes active only during
forceful breathing, as occurs while
playing a wind instrument or during
exercise.
Internal intercostal and abdominal
muscles are the main muscles for
forceful exhalation.
42

43. 43

44. 44

45. Cont…
Air pressure differences drive airflow during
inhalation and exhalation.
However, three other factors affect the rate
of airflow and the ease of pulmonary
ventilation:
◦ surface tension of the alveolar fluid,
◦ compliance of the lungs, and
◦ airway resistance.
45

46. Surface tension
The surface of the alveolar cells is moist, and air-
filled sacs lined with water.
surface tension is a force generated at air– water
interface by the attractive forces between the
water molecules.
If surface tension increases, ventilation will
decrease.
The surface tension exerted by alveolar fluid is
decreased by the presence of surfactant.
46

47. Surfactant
Pulmonary surfactant is a mixture of phospholipids and
protein.
•produced by alveolar type II cells.
•Lowers surface tension.
•Reduces attractive forces of hydrogen bonding by
becoming interspersed between H20 molecules.
•As alveoli radius decreases, surfactant’s ability to lower
surface tension increases.
Decreases Surface tension in alveoli
47

48. Compliance
Compliance is the ease with which the lungs and
thoracic wall can expand.
Lung compliance is determined by the elastic
connective tissues of the lungs and the surface
tension of the fluid lining the alveoli.
If compliance decreases, then ventilation
decreases.
48

49. Airway resistance
Airway resistance determines how much air flows
into the lungs at any given pressure difference
between atmosphere and alveoli.
 The major determinants of airway resistance are
the radii of the airways.
Airway resistance increases ventilation decreases
49

50. Exchange of Gases in Alveoli
and Tissues
•Consists of respiratory zones:
• Respiratory bronchioles,
• Alveoli,
• Alveolar duct,&
• Alveolar sac.
•Exchange of O2 for CO2 occurs through
alveolo-capillary (respiratory) membrane.
•Alveolo-capillary membrane is made of
several layers & consists of fluid film lining
the alveoli and capillary endothelial cells.
50

51. Alveoli
Are microscopic thin-walled air sacs within
the lungs, where all gas exchange takes
place.
Provide an enormous surface area for gas
diffusion.
51

52. Cont…
Alveoli walls are made up of:
1. Type I Pneumocytes - Simple thin squamous epithelium
major lining cells, mainly responsible for gas exchange.
2. Type II Pneumocytes - that are less in number and constitute
thicker granulocytes responsible for the production of
surfactants (a substance that prevents the alveoli from
collapsing by reducing the surface tension of the fluids that
line them.)
52

53. 53

54. Cont…
3. Type III Pneumocytes- large Phagocytic macrophage cells found
in alveolar cavities.
These cells keep alveolar surfaces sterile by removing debris and
microbes.
Exchange of gases in lungs and tissues is by diffusion as a result
of differences in partial pressures. Gases diffuse from a region of
higher partial pressure to one of lower partial pressure.
54

55. Cont…
Normal alveolar gas pressure for oxygen is 105
mmHg and for carbon dioxide is 40 mmHg.
At any given inspired PO2 , the ratio of oxygen
consumption to alveolar ventilation determines
alveolar PO2 —the higher the ratio, the lower the
alveolar PO2 .
The higher the ratio of carbon dioxide production
to alveolar ventilation, the higher the alveolar
PCO2 .
55

56. …
The average value at rest for systemic venous PO2
is 40 mmHg and for PCO2 is 46 mmHg.
As systemic venous blood flows through the
pulmonary capillaries, there is net diffusion of
oxygen from alveoli to blood and of carbon dioxide
from blood to alveoli.
By the end of each pulmonary capillary, the blood
gas pressures have become equal to those in the
alveoli.
56

57. Cont…
Inadequate gas exchange between alveoli
and pulmonary capillaries may occur when
the alveolar-capillary surface area is
decreased, when the alveolar walls thicken,
or when there are ventilation–perfusion
inequalities.
57

58. Diffusion of gases
O2 and CO2 are transferred across
the respiratory membrane by
diffusion.
 O2 diffuses into the blood and
CO2 diffuses into the alveoli.
58

59. Cont…
The diffusion of these gases depends on
the following factors:
1. Pressure gradient
2. Solubility of gas
3. Surface area of respiratory membrane
4. Molecular weight of the gas
5. Thickness of the respiratory membrane
59

60. Transport of Oxygen
O2 delivery to particular tissue depends on
◦Amount of O2 entering the lungs
◦Adequate pulmonary blood flow
◦Adequate gas exchange
◦O2 carrying capacity of blood
◦Adequate blood flow to the tissue
60

61. Cont…
Takes place in 3 steps
1.Diffusion of O2.
2.Transport of O2 in the blood.
3.Delivery of O2 to the tissues.
61

62. 1. Diffusion of O2
O2 is diffused from atmosphere to
alveoli, because of pressure gradient
62

63. Venous Blood
Arterial Blood
Alveoli
160 mm Hg
Pulmonary capillary
PO2 =
63
PCO2 =
0.3 mm Hg
104 mm Hg
PO2 =
PCO2 =
40 mm Hg
40 mm Hg
PO2 =
PCO2 =
45 mm Hg
10 4mm Hg
PO2 =
PCO2 =
40 mm Hg

64. Transport of gases
2. Transport of O2 in Blood:
1. Dissolved form – 3% in the plasma
2. Chemical form (HbO2) – 97%
 Chemical form: 97% is the main form of oxygen transport
O2
+ Hb Hbo2
O2 Combines with iron part of Hb
NB!
 1 gm of Hb can carryout 1.34 ml of O2
64

65. Transport of gases
3. Delivery of O2 to the tissues
Tissue
Cells
Tissue capillary
PO2 =95mmHg
Arterial blood Venous
blood
65
PCO2 = 40mmHg
PO2 = 40
PCO2 = 45
PO2 = 40
PCO2 = 45

66. Transport of gases
Transport of CO2
 Takes place in 3 steps
1. Diffusion of CO2
2. Transport of CO2 in blood
3. Carriage of CO2 to the lungs
1. Diffusion of CO2: some amount of Co2 is
transported
from tissues to blood , because of
pressure gradient.
66

67. Transport of gases
2. Transport of CO2 in blood
a. Dissolved form
Tissue Plasma Lungs
CO2 CO2 CO2
7 %
67

68. Transport of gases
b. Carbamino form
Tissue R.B.C Lungs
Co2 Co2 + Amino group of Hb
Carbamino Hb
23 %
68

69. Transport of gases
C. Bicarbonate form: major transported form, 70 %
Tissue Plasma R.B.C
CO2 CO2 + H2O H2CO3
HCO3- + H+
Alveoli
H2CO3
At the tissue level
At the lung level
CO2
CO2
H2O
69

70. Transport of gases
3. Carriage of CO2 to the lungs:
 Because of pressure gradient CO2 is diffused
from blood to
the lungs & eliminated from the lungs.
70

71. 71

72. REGULATION OF RESPIRATION
Our body regulates respiration in different way
mainly through:
◦ Control by Nervous system and
◦ Control by concentration of gases/humoural regulation
72

73. Nervous system control of
respiration
The neural signals that control respiration are generated in
respiratory control regions located in the brainstem(i.e. medulla
and pons).
The primary respiratory control center is the medullary
respiratory center.
In addition, two other respiratory centers lie higher in the brain
stem in the pons—the pneumotaxic center and apneustic center.
73

74. Respiratory centers
Respiratory center on medulla
◦ Inspiratory center
◦ Expiratory center
◦ Rhythm control center
Respiratory center on pon
◦ Pneumotaxic center
◦ Apneustic center
74

75. 75

76. Medullary respiratory center
 Are the primary respiratory center
 initiate and maintain spontaneous respiratory
patterns.
 include electrically excitable cell populations
that induce sustained inspiration or forced
expiration.
Include
◦ Inspiratory center
◦ Expiratory center
◦ Rhythm control center
76

77. Respiratory control areas on
medulla
Three respiratory control site
1. The dorsal respiratory group (DRG ): is the primarily
inspiratory control area
2. The ventral respiratory group (VRG) is mainly for control
of expiration.
3. The pre-Bötzinger complex is site for generation of
respiratory rhythm
77

78. 78

79. The Pontine respiratory center
The respiratory centers in the pons exert “fine-tuning”
influences over the medullary center to help produce
normal, smooth inspirations and expirations.
There are 2 areas on the pons involved in the control of
respiration
◦ The pneumotaxic center
◦ The apneustic center
79

80. The Pontine respiratory center
 Apneustic center:
◦Promotes inspiration by stimulating
the Inspiratory neurons in the
medulla.
◦Controls gasping
Pneumotaxic center:
◦Antagonizes the apneustic center.
◦Inhibits inspiration.
80

81. Voluntary Control of Breathing
Cerebral cortex
◦ Involved for voluntary control of breathing
◦ does not act on the respiratory center in the
brain stem but instead sends impulses directly to
the motor neurons in the spinal cord that supply
the respiratory muscles.
81

82. Cont’d
This voluntary control of respiration cannot be
maintained when the involuntary stimuli, such as an
elevated PCO2 or H concentration, become intense.
 An example is the inability to hold your breath for very
long.
Besides the obvious forms of voluntary control,
respiration must also be controlled during such
complex actions as speaking, singing, whistling and
swallowing.
82

83. REGULATION OF RESPIRATION BY PO2, PCO2 ,
AND H+ CONCENTRATION
The arterial blood gas content in the body needs to
be regulated; meaning the level of PO2, PCO2, and
in turn the level of H+ in the arterial blood need to
remain constant.
If any change of concentration of those gases
occur, it will be sensed by chemoreceptor which in
turn stimulate the medullary respiratory center in
order to readjust.
83

84. Chemoreceptor
Chemoreceptors monitor partial pressures of oxygen and
carbon dioxide in arterial blood and relay this information to
the respiratory control center, so that it can adjust ventilation
in response to changes in these variables.
Chemoreceptors involved in the control of breathing are
classified depending on their location as
◦ Peripheral Chemoreceptor or
◦ Central Chemoreceptor
84

85. 85

86. Peripheral Chemoreceptor
Carotid bodies - located high in the neck at the
bifurcation of the common carotid arteries and are
important to monitor oxygen supply to the brain
The carotid body input is the predominant
peripheral chemoreceptor involved in the control
of respiration.
Aortic bodies – located in the thorax on the arch of
the aorta
86

87. Cont’d
The peripheral chemoreceptors are
composed of specialized receptor cells
stimulated
◦ Mainly by significant decreased PO2 (hypoxia)
◦ Increased H concentration (metabolic acidosis)
◦ Increased PCO2 (respiratory acidosis)
87

88. Central Chemoreceptor
located in the medulla oblongata
respond to changes in the brain extracellular fluid.
They are stimulated by increased PCO2 via associated
changes in H concentration or
In other words they are stimulated by an increase in the H
concentration of the brain’s extracellular fluid.
Like the peripheral chemoreceptors, provide excitatory
synaptic input to the medullary inspiratory neurons.
88

89. 89

90. 90

91. 91

92. 92

93. Protective Reflexes
The cough and the sneeze reflexes are
responses that protect the respiratory
system from irritant materials, which
originate in sensory receptors located
between airway epithelial cells.
The receptors for the sneeze reflex are in
the nose or pharynx; those for cough are in
the larynx, trachea, and bronchi.
93

94. Cont’d
When the receptors initiating a cough are stimulated,
the medullary respiratory neurons reflexively cause a
deep inspiration and a violent expiration.
In this manner, particles and secretions are moved
from smaller to larger airways and aspiration of
materials into the lungs is also prevented.
Alcohol inhibits the cough reflex, which may partially
explain the susceptibility of alcoholics to choking and
pneumonia.
94

95. Advantage of nose breathing
over mouth breathing
95

96. THANK YOU!!!
96