RESPIRATORY PHYSIOLOGY.pptx

6/14/2023 By F.B 1
RESPIRATORY PHYSIOLOGY

Outlines
Structures and functions
The mechanics of breathing
The respiratory centre
Arterial and venous blood gases
Ventilation and perfusion
Dead space
Oxygen
Hypoxia
Carbon Dioxide
6/14/2023 By F.B 2

Respiration
•Ventilation:
•Action of breathing with muscles and lungs
•Gas exchange:
•Between air and capillaries in the lungs.
•Between systemic capillaries and tissues of the
body
•02 utilization:
•Cellular respiration in mitochondria
6/14/2023 By F.B 3

Functions of the Respiratory System
• Basically the respiratory system
serves a respiratory and non-
respiratory function.
• The prime function of
respiration is transferring of O2
to the body cells and removal
of CO2 from the body to the
exterior
• Non-respiratory role:
a. Metabolic function:
Angiotensin I to Angiot. II
b. Defense: has alveolar
macrophages for defense
c. Removal of excess fluid and
protein from the lung by its
lymphatic vessels
d.Acid-base balance
e.Vocalization (speech)
6/14/2023 By F.B 4

Inspiratory Movements
6/14/2023 By F.B 5

The Mechanics Of Breathing
• The lungs are contained in the thoracic cage, which is formed of ribs and
muscles.
• The cage is triangular, its base formed by the diaphragm.
• The side walls are formed by the ribs and the intercostal muscles.
• Two layers of pleura.
6/14/2023 By F.B 6

• Gas: the more volume, the less pressure (Boyle’s)
• Inspiration:
• lung volume increases ->
•decrease in intrapulmonary pressure, to just
below atmospheric pressure ->
•air goes in!
• Expiration: viceversa
6/14/2023 By F.B 7

• Compliance:
• Ability of the lungs to stretch during inspiration
• lungs can stretch when under tension.
• Elasticity:
• Ability of the lungs to recoil to their original collapsed shape during expiration
• Elastin in the lungs helps recoil
6/14/2023 By F.B 8

6/14/2023 By F.B 9

The Mechanics Of Breathing...
• The outer layer parietal of the pleura is in contact with the chest wall; the inner
layer visceral is in contact with the lungs.
• The space between the two layers is called the intrapleural space.
• The pressure in this space is negative (i.e. below atmospheric), while the
pressure in the lungs is higher, close to atmospheric pressure.
• Since the pressure within the lungs is higher than the pressure outside, the lungs
are kept expanded.
6/14/2023 By F.B 10

The chest cavity enlarges during inspiration because the muscles
contract.
Inspiration is therefore an active process.
The enlargement of the chest is in three directions:-
• Antero-posteriorly, i.e. from behind forwards.
This is produced by the intercostal muscles drawing the ribs and the
sternum forwards.
6/14/2023 By F.B 11

• Laterally: by the intercostal muscles drawing the lower ribs upwards
and outwards.
• Vertically (from above downwards): produced by the contraction of
the diaphragm.
The diaphragm accounts for 75% of the tidal volume.
Expiration is passive.
The elastic recoil of the lungs brings them back also to the resting
position.
6/14/2023 By F.B 12

6/14/2023 By F.B 13

The Respiratory Centre
•Basic rhythm of ventilation controlled by medullary
rhythmicity area (medulla oblongata)
•Inspiratory area (Dorsal Resp.Group)
• determines basic rhythm of breathing
• causes contraction of diaphragm and external intercostals
•Expiratory area (Ventral Resp. Group)
• Inactive during normal quiet breathing
• Activated by inspiratory area during forceful breathing
• Causes contraction of internal intercostals and abdominal
muscles
6/14/2023 By F.B 14

The Respiratory Centre...
•Transition between inhalation and exhalation
controlled by:
•Pneumotaxic area
• located in pons
• inhibits inspiratory area of medulla to stop inhalation
• Breathing more rapid when pneumotaxic area active
•Apneustic area
• located in pons
• stimulates inspiratory area of medulla to prolong inhalation
6/14/2023 By F.B 15

Arterial and venous blood gases
• The function of the lungs is to maintain a normal pressure of gases in
the arterial blood.This is achieved by ventilation.
• In this process of ventilation, fresh oxygen is brought into the lung
and excess carbon dioxide removed.
6/14/2023 By F.B 16

Exchange of oxygen
• The venous blood from the tissues reaches the lungs.
• Pressure of oxygen in the venous blood (40 mmHg )is less than the
pressure of oxygen in the alveoli.
• Gases move from an area of high pressure to an area of low
pressure, so the oxygen diffuses out from the alveoli to the venous
blood oxygenating it.
• The blood thus becomes arterial.
6/14/2023 By F.B 17

Exchange of carbon dioxide
•The venous blood arriving in the lung is rich in CO2.
•The pressure of CO2 in the venous blood is 46
mmHg.
•The alveolar pressure is 40 mmHg.
•The CO2 diffuses from the venous blood to the
alveoli.
•The arterial blood leaves the lung with an oxygen
tension of 100 mmHg and a CO2 tension of 40
mmHg.
6/14/2023 By F.B 18

Lung volumes and capacities
Lung volumes are important parameters in
respiratory
physiology and clinical practice and.
The sum of all of the named lung volumes
equals the maximum to which the lung can
be inflated.
Lung capacities are clinically useful
measurements that represent a combination
of two or more volumes.
6/14/2023 By F.B 19

Definitions of lung volumes
• TV: is the volume of gas passing into or out of the lungs per breath
(10ml/kg or approximately 500 ml in the average adult).
• MV: is the volume of gas passing in or out of the lungs per minute. It is
equal to the tidal volume multiplied by the number of breaths per minute.
Mv = Tv  RR
• Dead space: is the part of the tidal volume that does not take part in
ventilation (2ml/kg). It could be considered wasted ventilation in that it
serves no useful purpose where gaseous exchange is concerned.
6/14/2023 By F.B 20

Lung volumes and capacities, explanation
6/14/2023 By F.B 21

Lung volumes and capacities, explanation
6/14/2023 By F.B 22

Definitions Of Lung Volumes...
• Alveolar ventilation is that part of the tidal volume that actually
takes part in gaseous exchange.
• Tidal volume = dead space + alveolar ventilation
500 ml= 150 ml + 350 ml (in 75kg adult).
• Vital capacity is the greatest amount of air that can be expired after a
person has breathed in as much as possible (60–70 mL/kg).
6/14/2023 By F.B 23

• FRC: volume of air in the lungs at the end of a normal expiration.
• It is important during anaesthesia because it contains a reservoir of
oxygen (e.g. after pre-oxygenation and full nitrogen wash-out) which
can maintain blood levels of oxygen during apnoea.
• It is decreased by up to 25% in the supine position and also reduced
in obesity, pregnancy and under anaesthesia.
6/14/2023 By F.B 24

Factors known to alter the FRC include the following:
• Body habitus: FRC is directly proportional to height. Obesity,
however, can markedly decrease FRC (primarily as a result of reduced
chest compliance).
• Sex: FRC is reduced by about 10% in females compared with males.
• Posture: FRC decreases as a patient is moved from an upright to a
supine or prone position.
6/14/2023 By F.B 25

• Lung disease: Decreased compliance of the lung, chest, or both is
characteristic of restrictive pulmonary disorders all of which are
associated with a low FRC.
• Diaphragmatic tone: This normally contributes to FRC.
6/14/2023 By F.B 26

Closing Capacity: The volume at which airways begin to close in
dependent areas of the lung.
Small airways lacking cartilaginous support depend on radial traction
caused by the elastic recoil of surrounding tissue to keep them open.
At lower lung volumes, alveoli in dependent areas continue to be
perfused but are no longer ventilated; intrapulmonary shunting of
deoxygenated blood promotes hypoxemia.
6/14/2023 By F.B 27

Closing capacity is normally well below, but rises steadily with age.
At an average age of 44 years, closing capacity equals FRC in the supine
position; by age 66, closing capacity equals or exceeds FRC in the
upright position in most individuals.
Unlike FRC, closing capacity is unaffected by posture.
6/14/2023 By F.B 28

• Expiratory reserve volume (ERV) is the amount of air that can be
forcefully expired at the end of a normal expiration.
• Residual volume (RV) is the amount of air remaining in the lungs at
the end of a maximum forced expiration.
6/14/2023 By F.B 29

Diffusion :is the passage of fluid or gas from an area of high pressure
across a membrane. The gases in the alveoli pass into the blood in
the capillaries by this process of diffusion. The gases have to pass
through three layers:
• The alveolar lining or epithelium
• Interstitial fluid
• The capillary lining or endothelium
6/14/2023 By F.B 30

If any of these layers is thickened then diffusion is impeded.
The alveolar lining may be thickened by disease of the lung.
The interstitial fluid may be increased in heart disease.
6/14/2023 By F.B 31

• Airway resistance denotes the measure of obstruction to the flow of
gas in the air passages.
• Most of the resistance to air flow is in bronchi greater than 2 mm in
diameter.
• Airway resistance is increased in asthma, emphysema, pulmonary
congestion etc.
• In these conditions the work of breathing is increased.
6/14/2023 By F.B 32

Ventilation And Perfusion
Ventilation (V)
•The lower part ventilates better than the
upper.
•The lower part of the lung, due to its weight
and the effect of gravity tends to be less
distended in the resting position than the
upper part.
•Therefore during inspiration the lower part of
the lung has a greater capacity to fill or
distend.
6/14/2023 By F.B 33

Ventilation And Perfusion...
Perfusion (Q)
• The pulmonary circulation is affected by gravity.
• As with ventilation, perfusion is greater at the base than at the apex
and so the lower zone has a better blood flow than the middle or
upper zone.
• However, the overall result is that ventilation is relatively greater
than blood flow at the apex and relatively less than blood flow at the
base.
6/14/2023 By F.B 34

The V/Q ratio
• Expresses the relationship between the regional blood flow and the
regional ventilation in the lung.
• Ventilation/Perfusion ratios are not uniform throughout the lung but
in the healthy person ventilation and perfusion are reasonably well
matched with an overall ratio of 0.9.
• The V/Q ratio is higher at the apex and lower at the base.
6/14/2023 By F.B 35

The V/Q ratio...
• In those areas of the lung where the V/Q ratio is high, the capillary
blood gases returning from the lung approach those in alveolar air
(high O2 low CO2).
• In those areas where the V/Q ratio is low the capillary blood gas
tensions approach those of venous blood (low O2 high CO2).
• When pulmonary blood flow is high but ventilation is poor it is
referred to as pulmonary shunting.
6/14/2023 By F.B 36

• Regional ventilation perfusion ratios can be altered in pulmonary
disease.
• In pulmonary embolism there is under perfusion.
• In certain lung disease there may be under-ventilation, e.g.
pneumonia.
• These conditions affect gas exchange.
6/14/2023 By F.B 37

Dead Space
Defined as that part of the tidal volume that does not take part in gas
exchange.
Three types of dead space will be considered:
Anatomical dead space
This is the volume of gas that fills the respiratory passages but which is
not involved in gas exchange.
6/14/2023 By F.B 38

It extends from the nostrils and mouth down to, but not including, the
alveoli
In the average adult this is about 150 ml (2ml/kg body weight).
The anatomical dead space is reduced by tracheostomy and
endotracheal intubation.
6/14/2023 By F.B 39

Physiological dead space
This includes all the air (that for various reasons) does not take part in
gas exchange.
It includes the anatomical dead space plus air in any of the alveoli into
which the blood does not flow and any air in over distended alveoli
where gaseous exchange is impaired.
Physiological dead space is increased in lung disease.
6/14/2023 By F.B 40

Anaesthetic dead space
Anaesthetic dead space is that part of the tidal volume that occupies the
anaesthetic apparatus and is hence not available for gas exchange.
The exact volume of anaesthetic dead space depends on the circuit used.
Consider the "T piece" used for children.
6/14/2023 By F.B 41

• The anaesthetic dead space is the volume between the patient and
the inflow of fresh gases into the "T piece".
• If the circle absorber is used, then the anaesthetic dead space is the
volume between the patient and the junction of the inspiratory and
expiratory tubing.
6/14/2023 By F.B 42

• If a face mask is used, the dead space is increased considerably
because part of the patient's tidal volume occupies the space under
the mask and is hence wasted.
• The use of an endotracheal tube or a tracheostomy tube reduces the
dead space.
6/14/2023 By F.B 43

Oxygen
Exchange between the lungs and the atmosphere
The amount of O2 in inspired atmospheric air is 21%.
(158 mmHg).
The inspired air is drawn into the alveoli where it mixes with the air left from
the previous respiration.
Therefore, the oxygen is diluted, the CO2 raised and water vapour is added.
The pressure of O2 in the alveolar air becomes 103 mmHg.
6/14/2023 By F.B 44

Oxygen...
Exchange between the blood and the lungs
• The venous blood arrives in the lung (from the tissues) with an O2
pressure of 40 mmHg.
• Because of the difference in pressure, O2 passes from the alveoli into
the blood stream.
• This transfer occurs by the process of diffusion.
• The O2 in solution determines the oxygen tension
6/14/2023 By F.B 45

Oxygen...
It moves from the alveoli where the tension is high to the capillary
blood where the tension is low.
The capillary blood leaves the lungs after picking up O2 and giving up
CO2.
It is high in O2 (both dissolved O2 and O2 in combination with
haemoglobin).
The arterial blood leaves with an oxygen tension of 100 mmHg.
6/14/2023 By F.B 46

Carriage of oxygen in the blood..
O2 is also carried in combination with the haemoglobin in the red
blood cells.
1 gm of haemoglobin can carry 1.34 ml of O2.
The normal adult with 14.8 g of Hb/100 ml of blood will therefore
carry 19 ml of O2 per 100 ml of blood.
The Hb is almost fully saturated with this 19 ml O2/100 ml blood.
6/14/2023 By F.B 47

Carriage of oxygen in the blood..
• This O2 is in the red blood cells.
• Each 100 ml of blood gives off 5 ml of O2 as it passes through the
tissues and returns to the lungs as venous blood carrying 14 ml of O2
per 100 ml of blood.
• When the Hb combines with O2 it is termed oxyhaemoglobin (oxyHb).
• When it gives up the O2 in the tissues it is termed reduced
haemoglobin.
6/14/2023 By F.B 48

Carriage of oxygen in the blood...
The passage of oxygen from the blood to the tissues
In the tissues the capillary blood is exposed to a low O2 tension.
The oxygen diffuses out of the blood into the tissues.
Initially the dissolved O2 in the plasma diffuses out first.
6/14/2023 By F.B 49

• When this happens the O2 in combination with the haemoglobin is
no longer in equilibrium with the dissolved O2.
• The O2 passes from the red blood cells (haemoglobin) into the
plasma and thence into the tissues.
• The O2 is thus unloaded into the tissues.
6/14/2023 By F.B 50

There is a relationship between the O2 tension in the plasma and the O2 in
combination with the haemoglobin.
If the O2 tension is low the O2 content of the haemoglobin will be low.
If the O2 tension is high the O2 content of the haemoglobin will be high.
The graph below shows the relationship. It is referred to as the
oxyhaemoglobin curve.
6/14/2023 By F.B 51

Oxyhemoglobin Dissociation Curve
6/14/2023 By F.B 52

The following situations shift the oxyhaemoglobin dissociation curve to
the right and make it release oxygen more readily.
• An increase in the CO2 tension in the blood.
• An increase in the H+ ion concentration.
• An increase in temperature.
• An increase in 2,3 DPG (Diphosphoglycerate).
6/14/2023 By F.B 53

• 2,3 DPG is found in red blood cells.
It is increased in chronic anaemia, hypoxia and thyrotoxicosis, and
results in more O2 being delivered to the tissues.
• It is decreased in polycythaemia, myxoedema and stored blood and
inhibits the release of O2 to the tissues.
6/14/2023 By F.B 54

A rightward shift in the oxygen–hemoglobin dissociation curve lowers
O2 affinity, displaces O2 from hemoglobin, and makes more O2
available to tissues; a left ward shift increases hemoglobin’s affinity
for O2 , reducing its availability to tissues.
6/14/2023 By F.B 55

Factors that shift the O2-Hb dissociation curve
to the right and
6/14/2023 By F.B 56

Hypoxia
Hypoxia means a lack of O2 in the body (tissues).
Hypoxaemia is a lack of O2 in the blood.
This may reveal itself as central cyanosis which is a blueness of the
tissues of the lips, tongue and mucous membranes of the mouth.
Oxygen saturation below 90% is a reasonable definition.
6/14/2023 By F.B 57

Hypoxia
There are various types of hypoxia:
• Hypoxic hypoxia
• Anaemic hypoxia
• Stagnant hypoxia
• Histotoxic hypoxia
• Diffusion hypoxia
6/14/2023 By F.B 58

Effects of oxygen lack
The central nervous system:
• The blood flow to the brain is first increased. This is followed by oedema
or swelling of the brain and unconsciousness. Irreversible brain damage
will occur within 4 minutes if severe hypoxia is not treated.
•The respiratory system: Increase in respiration in conscious
patients( reflex stimulation of the respiratory centre via the
chemoreceptors).
6/14/2023 By F.B 59

• The cardiovascular system: The pulse rate and the blood pressure
rise at first and then fall as the hypoxia increases.
• Other tissues: The liver cells are damaged.
6/14/2023 By F.B 60

If a patient becomes hypoxic under anaesthesia, then:
Increase the inspired oxygen concentration to 100%.
Check oxygen analyser, if available, or oxygen cylinder levels, to
exclude a failed oxygen supply.
Check that ventilation is adequate:
6/14/2023 By F.B 61

Check end-tidal CO2, if available. Note that oesophageal intubation
must be excluded early as this needs to be corrected promptly.
Switch to hand ventilation to assess pulmonary compliance and
auscultate both sides of the lung to exclude endobronchial
intubation.
6/14/2023 By F.B 62

Check blood pressure and pulse.
Correct the cause:
Deficiency in inspired O2.
Respiratory obstruction.i.e. laryngospasm or inadequately cleared
airway (try Guedel airway, nasal airway or laryngeal mask airway).
6/14/2023 By F.B 63

Inadequate ventilation (check capnograph).
If difficulty in ventilation always consider a blocked endotracheal tube i.e.
sputum plug or foreign body as well as the more common causes.
Blocked endotracheal tube is excluded by the free passage of a suction
catheter down the tube.
Bronchospasm.
Aspiration.
Pneumothorax.
Pulmonary Embolus.
6/14/2023 By F.B 64

Carbon dioxide
Levels of carbon dioxide (CO2)
Atmospheric air contains about 0.04% of CO2.
Expired air contains 4.5% of CO2.
The tension or pressure of CO2 in arterial blood is 40 mmHg .
The tension or pressure of CO2 in venous blood is 46 mmHg .
6/14/2023 By F.B 65

Transport of CO2 in the blood
The tissues produce CO2 which is taken up by the blood.
The blood arrives in the lungs with a CO2 pressure of 46 mmHg.
The pressure of CO2 in the alveoli of the lungs is 40mmHg.
There is a difference of 6 mmHg, sufficient to enable the CO2 to diffuse from
the venous blood into the alveoli of the lung, from where it is carried to the
exterior with the expired gases.
6/14/2023 By F.B 66

The carbon dioxide is carried in the blood in three forms:
• In simple solution
• In combination with the Hb
• As bicarbonate
The greatest part of the CO2 is carried as bicarbonate.
6/14/2023 By F.B 67

Carbon Dioxide
• CO2 + H2O <->H2CO3<->H+ + HCO3-
• Enzyme is Carbonic Anhydrase
• Chloride shift to compensate for bicarbonate moving in and out of
RBC
6/14/2023 By F.B 68

•Chloride shift:
•Chloride ions help maintain electroneutrality.
•HC03
- from RBC diffuses out into plasma.
•RBC becomes more +.
•Cl- attracted in (Cl- shift).
•H+ released buffered by combining with
oxyhemoglobin.
•Reverse in pulmonary capillaries
6/14/2023 By F.B 69

6/14/2023 By F.B 70

Effects of carbon dioxide excess
CNS:
There is an increase in the blood flow to the brain with a
corresponding increase in cerebral volume and raised intracranial
pressure.
The sequence of events is:
• Headache
• Nausea
• Vomiting
• Coma
6/14/2023 By F.B 71

Respiratory system:
The respiratory centre is stimulated both directly and reflexly via the
chemoreceptors.
If the pressure of CO2 in the blood stream rises by 1.5 mmHg the tidal
volume is doubled.
Severe hypercapnia produces respiratory failure.
Bohr effect: the oxyhaemoglobin dissociation curve shifts to the right.
This means more O2 is given up at tissue level.
6/14/2023 By F.B 72

Cardiovascular system: Cardiac output is increased by direct effect of
CO2 and by the effect of the increase in circulating catecholamines.
• Heart rate increases and becomes irregular.
• Peripheral resistance increases.
• Blood pressure may rise at first, especially in the unanaesthetised
person.
6/14/2023 By F.B 73

Causes of CO2 excess
• Respiratory obstruction causing inadequate ventilation.
• Inadequate ventilation, due to depression of the respiratory centre or
the action of muscle relaxants.
• Faulty CO2 absorption in circle absorbers.
• Accidental administration of CO2.
6/14/2023 By F.B 74

Low CO2
Excessive ventilation (hyperventilation) reduces the tension of CO2 in
the blood.
The patient becomes hypocapnic (low CO2).
Hyperventilation is best avoided during general anaesthesia.
It can result in cerebral vasoconstriction and is therefore dangerous,
especially in older people.
6/14/2023 By F.B 75

Low CO2
• It may also result in foetal asphyxia if used during a Caesarean
section.
• Hypocapnia may delay the onset of breathing at the end of an
anaesthetic.
• Hypocapnia shifts the oxyhaemoglobin dissociation curve to the left
and reduces the oxygen released to the tissues.
6/14/2023 By F.B 76

Acid-Base Balance
•Normal blood pH: 7.40 (7.35- 7.45)
•Alkalosis: pH up
•Acidosis: pH down
• Hypoventilation:
• PC02 rises, pH falls (acidosis).
• Hyperventilation:
• PC02 falls, pH rises (alkalosis).
6/14/2023 By F.B 77

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