Gas exchange

1
Gas exchange
Airway
no gas exchange
O2
CO2
flow
2
Anatomical dead space
VD Anat 1 ml/lb body wt.
room air
start insp
end insp
end exp
O2
CO2
No gas exchange (white) in anatomical dead space
flow
3
Minute Ventilation
VE breathing frequency (f) x tidal
volume (VT)
5 L/min 10/min x 500 ml
Correction for VD Anat VE f x (VT VD
anat) 3.5 L/min 10/min x (500 150)
ml
4
Estimating anatomical dead space
Volume x Fraction Mass of Gas
Mass of Gas of VT sum of gas masses of VA
and VD
VD
VT x FECO2 VA x FACO2 VD X FDCO2
VT
VA
Since FDCO2 0
VA (VT x FECO2) / ( FACO2)
Since VA VT - VD
5
Partial pressure of a gas
Daltons Law in a mixture of gases, partial
pressure (Pgas) of each gas contributes
additively to total pressure (PB) in proportion
to the gass fractional volume (Fgas)
Pgas PB x Fgas
6
Alveolar PO2 is lt Inspired PO2
dry room air inspired air alveolar gas PO2
160 150 100 PCO2 0 0 40 PH2O
(37oC) 0 47 47 PN2 600 563
573 Total (mmHg) 760 760 760
7
Henrys Law at equilibrium, gas pressure above a
liquid equals gas pressure in the liquid
PO2, gas 100 mmHg
PO2, blood 100
8
Alveolar gas partial pressures determine blood
gas tensions
9
Blood gas tension determines blood gas content
Concentration of gas dissolved per liter of blood
(C) depends on gas solubility (a) and Pgas in
blood
CO2 a x PO2, blood
a, Solubility coefficient (ml O2/liter.mmHg)
CO2 0.03 x 100 3 ml O2/liter of
blood 0.3 ml O2/100ml of blood
Dissolved gas
10
The oxyhemoglobin dissociation curve
100
HbO2 saturation ()
50
0
0 20 30 40 50 60 70 80 90
100 110 120
PO2, blood
11
The O2 carrying capacity of blood Hgb-bound O2
dissolved O2
PO2, alv
dissolved O2 in blood
PO2, blood
HgbO2 saturation
1.34 ml at 100
O2 content in 1 liter of blood at PO2, blood of
100 mmHg (HgbO2 sat x 1.34 x Hgb) (.03
x PO2, blood) (0.98 x 1.34 x 150 g/l) 3
200 ml O2/liter of blood

12
How tissues get O2
alveolus
PO2, alv 100
O2
flow
PO2, blood 100
tissue
PO2, tissue 10
O2
40
PO2, blood
40
50
100
13
Low pH unloads O2 better
alveolus
PO2, alv 100
O2
flow
PO2, blood 100
tissue
PO2, tissue 10
O2
40
PO2, blood
40
50
100
14
The oxyhemoglobin dissociation curve
100
HbO2 saturation ()
pH
pH
50
0
0 20 30 40 50 60 70 80 90
100 110 120
PO2, blood
15
The pulmonary circulation
O2
CO2
Pulmonary capillaries
Pulmonary artery
Pulmonary vein
LA
RV
tissue
16
PVR is ltltlt SVR
PVR (Ppa Pla) / CO 2 units
O2
CO2
Pulmonary capillaries
Ppa 20
Pla 10
LA
Pao 200
RV
Pra 0
CO 5
SVR (Pao Pra) / CO 40 units
tissue
Note pressures are mean and in cmH2O. CO is 5
L/min.
17
The lung has low vascular resistance
18
The lung has low vascular tone
pressure
lung
flow
kidney
flow autoregulation
time
19
Systemic capillaries
20
Lung capillaries
Alveolar wall
21
Lung capillaries
Alveolus
22

• The lungs low vascular resistance is due to
• Low vascular tone
• Large capillary compliance

23
PA enters mid lung height
PA
24
Gravity determines highest blood flow at lung base
End expiration
Ppa Pla (cmH2O)
-10 cm
Palv 0
10 5
0 cm
20 10
30 20
10 cm
25
Hypoxic pulmonary vasoconstriction
PO2 100 mmHg
hypoxia
100
40
100
40
100
40
40
PO2
26
Capillary filtration determines lung water content
27
The Starling equation describes capillary
filtration
FR Lp x S (Pc Pi) s (Pc Pi)
FR filtration rate S capillary surface area
Pc capillary pressure Pi interstitial
pressure s reflection coefficient Pc plasma
colloid osmotic pressure Pi interstitial colloid
osmotic pressure
28
Keeping the alveoli dry Large capillary
pressure drop
20
Pressure (cmH2O)
15
s (Pc Pi)
10
PA
Capillary bed
LA
s (Pc Pi) .8 (30 12)
29
Keeping the alveoli dryPerivascular cuff
formation
30
Perivascular cuffs in early pulmonary edema
cuff
Normal lung
Early pulmonary edema
31
The ultimate insult alveolar flooding
32
Keeping the alveoli dryactive transport
removes alveolar liquid
NaCl
alveolar space
NaCl transporter
active liquid transport
Na-K pump
Na
K
Cl
interstitium
33
SUMMARY
Features of the pulmonary circulation designed
for efficient gas exchange
1. Accommodate the cardiac output low
vascular tone high capillary compliance
34
SUMMARY
Features of the pulmonary circulation designed
for efficient gas exchange
2. Keep filtration low near alveoli low
Pc vascular interstitial sump
35
SUMMARY
Features of the pulmonary circulation designed
for efficient gas exchange
3. Keep liquid out of the alveoli active
transport high resistance epithelium
36
Control of Breathing Central neurons determine
minute ventilation (VE) by regulating tidal
volume (VT) and breathing frequency (f). VE
VT x f
37
Neural Control of Breathing Respiratory neurons
VRG
pons medulla
DRG
38
Neural Control of Breathing The efferent pathway
muscle supply
autonomic
Phrenic n.
39
Control of Breathing by Central chemoreceptors
CO2, H
Central chemoreceptors
major regulators of breathing
CO2, H
Central chemoreceptors
Resp neurons
VE
40
Chemical Control of Breathing Peripheral chemore
ceptors
IXth n.
Carotid body
O2
10 contribution to breathing
CO2
pH
41
CO2 drives ventilation
50
hypercapnia
VE
(L/min)
quiet breathing
5
35 40 45
CO2 tension in blood (mm Hg)
42
Hypoxia is a weak ventilatory stimulus
blood pO2 50 mmHg
50
blood pO2 100 mmHg
VE
(L/min)
5
35 40 45
CO2 tension in blood (mm Hg)
43
Reflex Control of Breathing Neural receptors
Airway Receptors Slowly adapting (stretch - ends
inspiration) Rapidly adapting (irritants -
cough) Bronchial c-fiber (vascular congestion -
bronchoconstriction)
Xth n.
afferents
Parenchymal c-fiber (irritants -
bronchoconstriction)