Gas exchange in the

lungs. Ventilation to

perfusion ratio

Dariusz Nowak

Respiration

• Provides O

2

to the tissues and removes CO

2

• Precisely regulated to maintain pAO

2

85-100 mmHg and

pACO

2

35-45 mmHg

Under these conditions
97 % (and more) of Hb is saturated with O

2

100 ml of blood contains about 21 ml of O

2

(0.3 ml dissolved)

pH of blood is 7.35-7.45
• Can be divided into four steps:
- pulmonary ventilation
- Diffusion of O

2

and CO

2

between the alveoli and the blood

- Transport of O

2

and CO

2

in the blood and body fluids

- Regulation of ventilation

Respiration

• Inspiration –active , requires contraction of inspiratory

muscles – increase in chest cage volume

• Diaphragm – most important inspiratory muscle , but

does not increase the chest cage volume

• External intercostals
• Additional inspiratory muscles: sternocleidomastoid ,

anterior serati, scaleni

Quiet expiration – passive due to elastic recoil of lungs

and chest wall

Deep expiration – active
Abdominal recti, internal intercostals

Respiration

Flow = ∆P/R
∆P = increase in pressure , R = airways resistance
Pleural cavity, pleural fluid – lubricant
• Pleural pressure – pressure of the fluid in the space

between lung pleura and the chest wall pleura

• Expressed in cm of water against atmospheric pressure

(difference)

• 1 atmosphere = 10000 cm of water
• Alveolar pressure – pressure of the air inside the alveoli
Open glottis and no airflow – alv. pressure is equal to atm.

pressure

Respiration

Normal breathing

Inspiration 0.5 l of air in 2s
Expiration 0.5 l of air in 2-3s

Transpulmonary pressure (recoil pr.) = alveolar

pressure –

-

intrapleural pressure

•

measure of elastic forces in the lungs that tend to

collapse the lungs

•

intrapleural pressure = pressure in the oesophagus

Compliance of the lungs

Compliance = ∆ V / ∆ P (V – air spaces volume, P –

pressure)

• Increases with age
• Increased in pulmonary emphysema
• Decreased in pulmonary fibrosis

What influences lung compliance ?
• Elastic forces of the lung tissue, elastin fibers, collagen

fibers

• Surface tension of the alveolar-lining fluid and

epithelial-lining fluid in other air spaces

Surface tension

P = 2xα /R
R – radius of the alveolus , α – surface tension
Surfactant
• Produced by type II pneumocytes
• Degraded by alveolar macrophages
• Components: phospholipid dipalmitoylphosphatidylcholine
Surfactant apoproteins, Ca

2+

• Reduces the surface tension
• Antibacterial activity
• Lack of surfactant –premature babies –infant respiratory

distress syndrome (atelectasis and lower airways infections)

• Excess of surfactant – no degradation by alveolar

macrophages – pulmonary proteinosis

Why alveoli do not collapse at the

end of expiration ?

• Presence of surfactant
• Tension of alveolar capillaries
• Closing of small airways at the end

of forced expiration

Work of breathing

Work of inspiration (3 fractions)
1- required to expand lungs against the lung and

chest elastic forces – elastic work (compliance
work)

2- required to overcome the viscosity of lungs

and other chest wall structures – tissue
resistance work

3- required to overcome the airways resistance

during the movement of air into the lungs –
airway resistance work

Pulmonary fibrosis
Bronchial asthma

Energy required for respiration

Normal quiet respiration
3-5% of the total energy consumed by the body
Intensive exercise
Increases as much as 50-times

Severe chronic obstructive pulmonary disease

(COPD)

Work of breathing is very high and causes

cachexia

Pulonary volumes and capacities

Tidal volume = 500 ml (quiet respiration)
All pulmonary volumes and capacities depend on

sex , age and height

How to express results of spirometric tests and

how to compare results obtained for various
subjects

% of predicted value

Ventilation

Minute respiratory volume – amount of new air

moved into the airways each minute

Quiet respiration:
Respiratory rate = 12/min
TV = 500 ml
12x0,5 l = 6 l/min
Maximal voluntary ventilation = 200 l/min
Scotoma, dizzines, hyperventilation tetany
Why ?
Decrease in blood CO

2

and Ca

2+

level

Ventilation

Dead space – the space in the respiratory passages where

no gas exchange takes place.

Is about 150 ml
Increases slightly with age
Anatomic dead space
Alveolar dead space – volume of alveoli with no or very

poor blood flow in adjacent capillaries

Physiologic dead space = anatomic + alveolar
• Minute alveolar ventilation= respiratory rate x (TV-V

D

)

TV – tidal volume
V

D

– physiologic dead space

airways

Factors causing constriction:
Parasympathetic nerves
Acetylocholine
Histamine
Cold air
SO

2

, smoke , dust

Inflammatory mediators : e.g PAF (platelet activating factor),

leukotrienes

Factors causing relaxation:
Sympathetic nerves
Adrenaline
Atropine (and derivatives)
Beta-2- agonists

Functions of the respiratory

passageways

• Warming
• Humidification
• Cleaning (filtration, precipitation)

Mucocilliary clearance
Alveolar macrophages

Pulmonary circulation

• Pulmonary vessels : pulmonary artery, right

and left main branches, arteries, arterioles,
capillaries, veins

Supply blood to lungs for gas exchange

• Bronchial vessels: arteries , capillaries, veins –

supply blood to supporting tissues of lung

• Lymphatic vessels – prevent pulmonary edema

Pulmonary circulation

• High flow = cardiac output = flow in systemic

circulation

• Low pressure
• Low resistance
Mean pulmonary arterial pressure = 15 mmHg
(systolic =25 , diastolic= 8 mmHg)

Blood volume in the lungs – about 450 ml
70 ml in pulmonary capillaries
Blood stays in pulmonary capillaries for 0.8 s
During exercise – 0.3 s

Control of pulmonary blood flow

distribution

• PO

2

in alveoli decreases below 73 mmHg –

alveolar hypoxia – adjacent blood vessels (small
arteries) constrict

• Blood flow decreases in hypoventilated areas of

the lung

• Large areas are hypoventilated – increase in

resistance of pulmonary circulation – increased
work of right ventricle- hypertrophy and
insufficiency of right ventricle.

• Body position

Gas exchange

•

CO

2

is 20-times more soluble than O

2

•

CO

2

diffuses 20-times faster through alveolar-blood

barrier thanO

2

•

Gas concentration gradient

Consequences for laboratory findings during development

of respiratory insufficiency

1.

Hypoxemia during exercise, CO

2

normal or decreased

due to compensatory hyperventilation

2.

Hypoxemia at rest

3.

Hypoxemia and hypercapnia at rest

Alveolar-blood barrier (respiratory

membrane)

1- alveolar lining fluid
2- alveolar epithelium
3- epithelial basement membrane
4- thin interstitial space
5- capillary basement membrane
6- capillary endothelium
What affects the rate of CO

2

and O

2

diffusion ?

1- thicknes of the membrane
2- surface area of the membrane
3- diffusion coefficient of the gas in the

substance of the membrane

4 – gas concentration gradient
5 – blood flow in capillaries

Lung diffusing capacity

For O

2

, CO

2

, CO , NO

For CO – important , used in clinical practice
Why ?
CO has 250-times higher affinity to hemoglobin than O

2

.

Its pressure in capillary blood is O mmHg

Gas mixture for determination of carbon monoxide lung

diffusing capacity (DL

CO

)

CO- 0.3%, helium or CH

4

, and N

2

at balance

Age ,sex, height, cigarette smoking , blood hemoglobine

level,

Correction for Hb and alveolar volume
< 80% of predicted – clinical significance

Ventilation-perfusion ratio

Va –alveolar ventilation
Q – blood flow
Va/Q = normal
Va/Q = O - shunt
Va/Q = ∞ - physiologic dead space

Physiologic shunt
Shunted blood – normally 2% of the cardiac

output

Ventilation-perfusion ratio

Normal person , upright position
Top of the lung Va/Q > 2.5 times than ideal value

(physiologic dead space)

Middle of the lung
Bottom of the lung Va/Q < 0.6 times than ideal value

(physiologic shunt)

Chronic obstrucive pulmonary disease (COPD)
Cigarette smoking
• Obstruction of small airways and destruction of alveolar

walls with capillaries

• Some lung areas exhibit seriuos shunt or serious dead

space

Transport of O

2

and CO

2

in the blood

Arterial blood :hemoglobin saturation with O

2

= 97% , PO

2

=95

mmHg

In venous blood: Hb saturation with O

2

= 75% , PO

2

= 40 mmHg

Normal subject
Hb level = 15 g/dl
1 g Hb binds about 1.34 ml O

2

15x 1.34 = 20.1 ml
0.3 ml O

2

dissolved in blood

Total amount of O

2

is 20.4 ml in 100 ml blood

Venous blood contains 14,4 ml O

2

/dl

20.1 – 14.4 = 5.7
About 5 to 6 ml of O

2

is transported to tissues by 100 ml of blood

Factors influencing binding of O

2

to

hemoglobin

• H

+

concentration (pH)

• Increased CO

2

concentration

• Increased temperature
• Increased 2,3-diphosphoglycerate (2,3-

DPG) concentraton in erythrocytes

• CO has 250-times higher affinity to Hb

than O

2

Transport of CO

2

in the blood

Resting conditions
4 ml CO

2

are transported from tissues to the lungs

in 100 ml of venous blood

1- dissolved - 7% of total
2- bicarbonate ions (enzyme carbonic anhydrase) –

70 %

3- combination with –NH

2

groups of Hb and plasma

proteins,

Carbaminohemoglobin - 23 %