This document summarizes the physiology of respiration. It describes pulmonary ventilation as the process of breathing involving air exchange between the lungs and atmosphere. Inspiration occurs when intercostal muscles and diaphragm contract, enlarging the thoracic cavity and decreasing pressure to draw air into the lungs. Expiration is passive, as muscles relax and lungs recoil, increasing pressure to expel air. It also defines lung volumes like tidal volume and inspiratory reserve volume.

1. RESPIRATORY
SYSTEM (CONTD)
THE PHYSIOLOGY OF
RESPIRATION
BY M. SHAWA

2. LESSON OBJECTIVES
Describe pulmonary ventilation
Explain the mechanism of pulmonary
ventilation
Relate the Boyle’s law to events of
inspiration and expiration
Mention physical factors influencing
pulmonary ventilation
Identify the various pulmonary
volumes and capacity

3. RESPIRATORY PHYSIOLOGY
Respiration is the process of exchange
of gases in the body.
It involves the following processes:
1. External respiration
2. Transport of gases by blood
3. Internal respiration
4. Overall regulation of respiration.

4. PULMONARY VENTILATION
The process of gas exchange
(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 air and the alveoli of the lungs

5. PULMONARY VENTILATION (CONTD..)
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 the pulmonary
capillary blood gains oxygen and
losses carbon dioxide.

6. PULMONARY VENTILATION (CONTD..)
3. Internal (tissue) respiration – is the
exchange of gases between blood in
the systemic capillaries and tissue
cells.
In this step blood loses oxygen and
gains carbon dioxide.
Pulmonary ventilation depends on
volume changes in the thoracic cavity.

7. MECHANISM OF PULMONARY
VENTILATION
Primary principle of ventilation:
Air moves into lungs when air pressure
inside lungs is less than the air in the
atmosphere.
Air moves out of the lungs when air
pressure inside lungs is greater than
air pressure in the atmosphere.

8. MECHANISM OF PULMONARY VENTILATION
Before each inhalation, air
pressure in the lungs is equal to
the atmospheric air pressure.
Under standard conditions
atmospheric air exerts a pressure
of 760mm Hg.

9. MECHANISM OF PULMONARY
VENTILATION (CONTD..)
Alveolar air at the end of one
expiration, and before the
beginning of another inspiration
also exerts 760mm Hg.

10. MECHANISM OF PULMONARY VENTILATION
– (CONTD..)
When atmospheric pressure is
greater than pressure within the
lungs, air flows down the pressure
gradient from the atmosphere into
the lungs.
This is when inspiration occurs.

11. MECHANISM OF PULMONARY VENTILATION
– (CONTD..)
When pressure in the lungs
becomes greater than
atmospheric pressure, air moves
down the pressure gradient from
the lungs into the atmosphere.
During this time expiration occurs.

12. MECHANISM OF PULMONARY VENTILATION
– (CONTD..)
Pulmonary ventilation mechanism
establishes the two gas pressure
gradient that produce respiration.
(These are - one in which alveolar
pressure is lower than atmospheric
pressure to produce inspiration and on
in which it is higher than atmospheric
pressure to produce expiration)

13. MECHANISM OF PULMONARY
VENTILATION – (CONTD..)
These pressure gradients are
established by:
1. changes in the size of the thoracic
cavity which are produced by
contraction and
2. relaxation of respiratory muscles.

14. GAS LAWS
 Gas laws are statements about the
nature of gases.
They are based on the concept of the
‘ideal gas’ – i.e. a gas whose
molecules are so far apart that its
molecules rarely collide with one
another.

15. GAS LAWS (CONTD..)
Gas laws are also based on the
assumption that gas molecules
continually collide with the
container and thus producing a
force against it called gas
pressure.

16. GAS LAWS (CONTD..)
Some of the gas laws are:
1. Boyle’s law
2. Charles’ law
3. Dalton’s law (law of partial
pressure)
4. Henry’s law

17. BOYLE’S LAW
This law states that at constant
temperature a gas’s volume is
inversely proportional to its pressure.
When the volume of a container
increases, the pressure of the gas
inside it decreases and when the
volume decreases the gas pressure
increases.

18. BOYLES LAW AND VENTILATION
In ventilation, when thoracic volume
increases, air pressure in the airways
decreases ( allowing air to move
inward) and when thoracic volume
decreases air pressure in the airways
increases (allowing air to move
outward).

19. INSPIRATION
Contraction of the diaphragm alone or
contraction of both the diaphragm and
the external inter-costal muscles
produces quiet respiration.
When the diaphragm contracts, it
descends and makes the thoracic
cavity longer.

20. INSPIRATION (CONTD..)
Contraction of the external inter-costal
muscles pulls the anterior end of each
rib up and out.
This also elevates the sternum and
enlarges the thorax from front to back
and from side to side.

21. INSPIRATION (CONTD..)
During forceful respiration additional
muscles aid in elevation of sternum
and rib cage.
These additional muscles are
sternocleidomastoid, scalenes,
pectoralis minor, and serratus anterior
muscles

22. INSPIRATION (CONTD..)
As the size of the thorax increases, the
intra-pleural (intra-thoracic) and
alveolar pressures decreases (Boyle’s
law) and inspiration occurs.
As the thorax enlarges, it pulls the
lungs along with it because of the
cohesion between two moist pleura
covering the lungs and the thorax.

23. INSPIRATION (CONTD..)
The lungs expands and pressure in
the tubes and the alveoli decrease.
Air then moves into the lungs until a
pressure equilibrium is established
between atmosphere and the alveoli,
then the flow of air stops.

24. INSPIRATION (CONTD..)
The ability of the lungs and thorax to
stretch is known as ‘Compliance’ and
is essential for normal breathing.
If compliance is reduced due to
disease or injury inspiration becomes
difficult.

25. PROCESS OF BREATHING
 Boyle’s law:
p1V1=p2V2
Marieb, Human Anatomy &
Physiology, 7th edition

26. EXPIRATION
Quiet expiration in a healthy person is
a passive process and depends more
on the elasticity of the lungs than on
muscle contraction.
It begins when the pressure gradients
that resulted in inspiration are
reversed.

27. EXPIRATION (CONTD..)
As the inspiratory muscles relax, the
size of the thorax decreases.
The rib cage descends and the
lungs recoil.
Both the thoracic and
intrapulmonary volume decrease.

28. EXPIRATION (CONTD..)
The decreased volume
compresses the alveoli and
increases its pressure above the
atmospheric pressure.
The pressure gradient forces
gases to flow out of the lungs.

29. EXPIRATION (CONTD..)
Forced expiration is an active process
produced by contraction of abdominal
wall muscles, primarily the oblique and
transversus muscles.
These contractions increase the intra-
abdominal pressure forcing the
abdominal organs superiorly against
the diaphragm and also depress the
rib cage.

30. EXPIRATION (CONTD..)
The tendency of the lungs to return to
their pre-inspiration volume is called
‘elastic recoil’.
If a disease condition reduces the
elasticity of pulmonary tissue,
expiration becomes forced even at
rest.

31. PHYSICAL FACTORS INFLUENCING
PULMONARY VENTILATION
Lungs stretch during inspiration and
recoil passively during expiration.
Energy is also used to overcome
various factors that hinder air passage
and pulmonary ventilation

32. PHYSICAL FACTORS INFLUENCING (CONTD..)
The factors include:
1. Airway resistance – gas flow
decreases as resistance increases.
 Resistance in respiratory tree is
determined mainly by the diameter of
tubes.

33. PHYSICAL FACTORS INFLUENCING (CONTD..)
1. Alveolar surface tension – Surfactant
( a liquid film coating the alveoli)
decreases the surface tension of
alveoli and prevents alveolar
collapse.

34. PHYSICAL FACTORS INFLUENCING (CONTD..)
Lung compliance – healthy lungs are
stretchy and distensible.
Lung compliance is determined by
distensibility of lung tissue and
alveolar surface tension
Lung disease like tuberculosis can
replace lung tissue with scar tissue
and decrease compliance.

35. PULMONARY AIR VOLUME AND
CAPACITY
At rest, a healthy adult takes an
average of 12 breaths a minute,
moving 500ml of air into and out of the
lungs with each inhalation and
exhalation.
The volume of one breath is called the
Tidal volume (VT).

36. PULMONARY AIR VOLUME AND
CAPACITY (CONTD..)
The minute ventilation (MV) – the
total volume of air inhaled and
exhaled each minute – is
respiratory rate multiplied by tidal
volume:
MV= 12 breaths/min x 500ml/breath

37. PULMONARY AIR VOLUME AND
CAPACITY (CONTD..)
A lower than normal minute ventilation
is a sign of pulmonary malfunction.
The apparatus used to measure the
volume of air exchanged during
breathing and respiratory rate is a
Spirometer or respirometer and the
record is called spirogram.

38. PULMONARY VOLUMES AND CAPACITIES
Marieb, Human Anatomy & Physiology, 7th edition

39. TIDAL VOLUME
Tidal volume varies from one person to
another and in the same person at
different times.
In a typical adult, about 70% of tidal
volume (350ml) actually reaches the
respiratory zone and participate in
external respiration.

40. TIDAL VOLUME (CONTD..)
The other 30% (150ml) remains in the
conducting zone of the nose, pharynx,
larynx, trachea, bronchi bronchioles.
Collectively the conducting airways
with air that does not undergo
respiratory exchange are known as
anatomic (respiratory) dead space.

41. TIDAL VOLUME (CONTD..)
Alveolar ventilation refers to the
volume of inspired air that actually
reaches or ‘ventilates’ the alveoli.
Only this volume of air takes part in
the exchange of gases between air
and blood.

42. OTHER LUNG VOLUMES
Several other lung volumes are
defined relative to forceful breathing.
These volumes are larger in males,
taller individuals and younger adults
They are smaller in females, shorter
individuals, and the elderly.

43. INSPIRATORY RESERVE VOLUME
When you take a very deep breath you
can inhale more than 500ml.
This additional air is called the
inspiratory reserve volume (IRV).
It is about 3100ml in an average adult
male and 1900ml in an adult female.

44. INSPIRATORY RESERVE VOLUME
Even more air can be inhaled if
inhalation follows forced
exhalation.

45. EXPIRATORY RESERVE VOLUME
After expiration of tidal air (500ml), an
individual can still force more air out of
the lungs.
The largest volume of air that one can
forcibly expire after expiring tidal air is
called expiratory reserve volume
(ERV)

46. EXPIRATORY RESERVE VOLUME (CONTD..)
An average adult normally has an ERV
of between 1000 and 1200ml.

47. RESIDUAL VOLUME
Even after forceful exhalation, a
considerable amount of air still
remains trapped in the alveoli.
This amount of air that cannot be
forcibly expired is known as
residual volume (RV).

48. RESIDUAL VOLUME (CONTD..)
Residual volume amounts to about
1200ml in males and 1100ml is
females.
Between breaths, an exchange of
oxygen and carbon dioxide occurs
between trapped residual air in the
alveoli and the blood.

49. PULMONARY CAPACITIES
A pulmonary capacities are a
combination sum of two or more
pulmonary volumes.
Pulmonary capacities include:
1. Vital capacity
2. Inspiratory capacity
3. Functional residual capacity
4. Total lung capacity.

50. VITAL CAPACITY (VC)
It represents the largest volume of air
an individual can move in and out of
the lungs.
It is determined by measuring the
largest possible expiration after the
largest possible inspiration.

51. VITAL CAPACITY (CONTD..)
VC is the sum of inspiratory reserve
volume, tidal volume and expiratory
reserve volume (VC = IRV + TV +
ERV).
It is about 4800ml in males and
3100ml in females.

52. VITAL CAPACITY (CONTD..)
The VC of an individual depends on
the size of the thoracic cavity, posture,
and other factors.
The VC is larger when standing erect
than when stooped over or lying down.
VC is decreased if lungs contain more
blood than normal e.g. In CCF.

53. INSPIRATORY CAPACITY (IC)
This is the maximum amount of air an
individual can inspire after a normal
expiration.
It is the sum of tidal volume and
inspiratory reserve volume (IC = TV +
IRV)

54. INSPIRATORY CAPACITY (IC)
It is (500ml + 3100ml = 3600ml) in
males and (500ml + 1900ml =
2400ml in females)

55. FUNCTIONAL RESIDUAL CAPACITY (FRC)
 This is the amount of air left in the lungs at
the end of a normal expiration.
 It is the sum of residual volume and
expiratory reserve volume (FRC = RV +
ERV).
 FRC is 2200 to 2400ml.

56. TOTAL LUNG CAPACITY (TLC)
This is the total volume of air a lung
can hold.
TLC is the sum of vital capacity and
residual volume (TLC = VC + RV)

57. PULMONARY GAS EXCHANGE
The exchange of O2 and CO2 between
alveolar air and pulmonary blood
occurs via passive diffusion.
It is governed by behaviour of gases
as described by two gas laws namely;
Dalton’s law and Henry’s law

58. DALTON’S LAW AND PULMONARY GAS
EXCHANGE
Daltons law states that each gas in a
mixture of gases exerts its own
pressure as if no other gases were
present.
This pressure of a specific gas in a
mixture is called partial pressure (Px);
the subscript is the chemical formula
of the gas.

59. ATMOSPHERIC AND PARTIAL PRESSURE
The total pressure of the mixture is
calculated by adding all the partial
pressures.
Atmospheric air is a mixture of
nitrogen N2, O2, H2O, and CO2 plus
other gases present in small
quantities.

60. ATMOSPHERIC AND PARTIAL PRESSURE
(CONTD..)
Atmospheric pressure is the sum of all
these gases:
Atmospheric pressure (760mmHg) =
PN + PO2+ PH2O +PCO2 + Pother gases
To determine the partial pressure
exerted by each gas in a mixture,
multiply the percentage of the gas by
the total pressure of the mixture.

61. ATMOSPHERIC AND PARTIAL PRESSURE
(CONTD..)
 The partial pressure of the inhaled air is as
follows:
 PN2 = 0.786 x 760mmHg = 597.4mmHg
 PO2 = 0. 209 x 760mmHg = 158.8mmHg
 PH2O= 0.004 x 760mmHg = 3.0mmHg
 PCO2= 0.0004 x 760mmHg = 0.3mmHg
 Pother gases = 0.0006 x 760mmHg = 0.5mmHg
 Total = 760mmHg

62. PULMONARY GAS EXCHANGE (CONTD..)
These partial pressures determine the
movement of O2 and CO2 between the
atmosphere and the lungs, between
the lungs and the blood, and between
the blood and body cells.

63. PULMONARY GAS EXCHANGE (CONTD..)
Each gas diffuses across a
permeable respiratory membrane
from the area of greater partial
pressure to an area of less partial
pressure.
The greater the difference in
partial pressure, the faster the rate
of diffusion.

64. PULMONARY GAS EXCHANGE (CONTD..)
As compared to inhaled air, the
alveolar air has less O2 and more
CO2.
The gas exchange in the alveoli
increases the CO2 content and
decreases the O2 content of
alveolar air.

65. PULMONARY GAS EXCHANGE (CONTD..)
O2 enters the blood from the
alveolar air because the PO2 of
alveolar air is greater than the PO2
of incoming blood.
The gases diffuse down the
pressure gradient.

66. PULMONARY GAS EXCHANGE (CONTD..)
The two way exchange of gases
between alveolar air and
pulmonary blood converts
deoxygenated blood to
oxygenated blood.

67. PULMONARY GAS EXCHANGE
(CONTD..)
The amount of O2 diffusing into blood
each minute depends on:
1. The O2 pressure gradient between
alveolar air and incoming pulmonary
blood (alveolar PO2 – blood PO2).
2. The total functional surface area of
the respiratory membrane.

68. PULMONARY GAS EXCHANGE
(CONTD..)
3. The respiratory minute volume
(respiratory rate per minute times
the volume of air inspired per
respiration.
4. Alveolar ventilation.

69. PULMONARY GAS EXCHANGE
(CONTD..)
Anything that decreases these factors
causes a decreases in the oxygen
diffusion into the blood.

70. THE HENRY’S LAW
This law states that the
concentration of a gas in a
solution depends on the partial
pressure of the gas and solubility
of the gas as long as the
temperature remains constant.

71. THE HENRY’S LAW (CONTD..)
The ability of the gas to stay in
solution is greater when its partial
pressure is higher and when the it
has a high solubility in water.
The higher the partial pressure of a
gas over a liquid and the higher the
solubility, the more the gas will stay in
solution

72. THE HENRY’S LAW (CONTD..)
When a mixture of gases is in contact
with a liquid, each gas will dissolve in
the liquid in proportion to its partial
pressure gradient.
H2O H2O

73. GAS SOLUBILITY
 CO2 is highly soluble in water than O2
 O2 is poorly soluble
 N2 almost insoluble
H2O H2O H2O
Carbondioxide Oxygen Nitrogen

74. GAS SOLUBILITY
The solubility of CO2 is 24 times
greater than that of O2.
Therefore much more CO2 is
dissolved in blood plasma.
O2 is poorly soluble in water and
therefore only 1.5% is transported
in dissolved form.

75. TRANSPORTATION OF GASES (CONTD..)
Fluids can only hold small
amounts of gas in solution.
 This causes most O2 and CO2 to
rapidly bind with other molecules
such as plasma, protein, or water.

76. TRANSPORTATION OF GASES
(CONTD..)
The binding of gas molecules with
another molecule reduces its plasma
concentration and more gas can
diffuse into plasma.

77. TRANSPORTATION OF GASES
Blood transports O2 and CO2
either as solute of combine with
other chemicals.
Soon after entering the blood
stream O2 and CO2 dissolve in
plasma.

78. TRANSPORT OF OXYGEN
 1.5 % dissolved
 98.5 % bound to
haemoglobin
 Iron in Hb binds
to oxygen
 4 O2 molecules
per Hb molecule
Marieb, Human Anatomy & Physiology, 7th edition

79. TRANSPORT OF OXYGEN
The portion of haemoglobin contains
four atoms of iron, each capable of
binding to a molecule of O2.
 Each haemoglobin molecule carries 4
O2 molecules.
Each 100ml of oxygenated blood
contains the equivalent of 20ml of
gaseous O2.

80. TRANSPORT OF OXYGEN
O2 does not dissolve easily in
water.
Only about 1.5% of inhaled O2 is
dissolved in plasma.
98.5% of blood O2 is bound to
haemoglobin in RBCs.

81. TRANSPORT OF OXYGEN
O2