Gas exchange
Airway
no gas
exchange
CO2
flow
O2
Anatomical dead space
VD Anat = 1 ml/lb body wt.
airway
room air
CO2
flow
O2
start insp
end insp
end exp
No gas exchange (white) in
anatomical dead space
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
Estimating anatomical dead
space
V
xF
= Mass of Gas
olume
raction
Mass of Gas of VT = sum of gas masses
of VA and VD
VD
VT
VA
VT x FECO2 = VA x FACO2 + VD X FDCO2
Since FDCO2 = 0
VA = (VT x FECO2) / ( FACO2)
Since VA = VT - VD
(FACO2 - FECO2)
VD =
end exp
VT
FACO2
Partial pressure of a gas
Dalton’s Law: in a mixture of gases, partial
pressure (Pgas) of each gas contributes additively
to total pressure (PB) in proportion to the gas’s
fractional volume (Fgas)
Pgas = PB x Fgas
Alveolar PO2 is < Inspired PO2
N2
CO2
H2O
O2
14%
21%
dry room air
inspired air
alveolar gas
PO2
PCO2
PH2O (37oC)
PN2
160
0
0
600
150
0
47
563
100
40
47
573
Total (mmHg)
760
760
760
Henry’s Law: at equilibrium, gas
pressure above a liquid equals gas
pressure in the liquid
PO2, gas = 100 mmHg
PO2, blood = 100
Alveolar gas partial pressures
determine blood gas tensions
alveolus
PCO2, alv =40
PO2, alv =100
O2
CO2
flow
PCO2, blood =40
PO2, blood =100
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
CO2
= 0.03 x 100
= 3 ml O2/liter of blood
= 0.3 ml O2/100ml of blood
Dissolved gas
a, Solubility coefficient
(ml O2/[liter.mmHg])
The oxyhemoglobin
dissociation curve
100
HbO2
saturation
50
(%)
0
0
20
30
40
50 60 70 80 90 100 110 120
PO2, blood
The O2 carrying capacity of blood
= Hgb-bound O2 + dissolved O2
PO2, alv
PO2, blood
dissolved O2 in 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
How tissues get O2
alveolus
PO2, alv =100
100
~30%
HgbO2
2
saturation
(%)
flow
50
PO2, blood =100
O
alveolus
tissue
tissue
PO2, tissue
10
0
O2
50
PO2, blood
40
40 50
0
100 PO2, blood
100
Low pH unloads O2 better
alveolus
PO2, alv =100
O2
flow
PO2, blood =100
100
HbO2
saturation
(%)
50
tissue
~50%
pH
tissue
0
PO2, tissue
10
0
O2
40
40 50
50
PO2, blood
pH
100 PO2, blood
alveolus
100
The oxyhemoglobin
dissociation curve
100
HbO2
saturation
50
(%)
pH
pH
0
0
20
30
40
50 60 70 80 90 100 110 120
PO2, blood
The pulmonary circulation
O2
CO2
Pulmonary
capillaries
Pulmonary
artery
Pulmonary
vein
LA
RV
tissue
PVR is <<< SVR
PVR
= (Ppa – Pla) / CO CO2
= 2 units
O2
Pulmonary
capillaries
Ppa = 20
Pla = 10
LA
Pra = 0
Pao = 200
RV
CO = 5
Note: pressures are ‘mean’
and in cmH2O. CO is 5 L/min.
tissue
SVR
= (Pao – Pra) / CO
= 40 units
The lung has low vascular resistance
The lung has low vascular tone
pressure
lung
flow
kidney
flow autoregulation
time
Systemic capillaries
Lung capillaries
Alveolar wall
Lung capillaries
Alveolus
The lung’s low vascular resistance
is due to
1. Low vascular tone
2. Large capillary compliance
PA enters mid lung height
PA
30 cm
Gravity determines highest
blood flow at lung base
End expiration
Ppa
Pla
(cmH2O)
-10 cm
Palv = 0
10
5
20
10
0 cm
30
20
+10 cm
Hypoxic pulmonary
vasoconstriction
PO2 = 100
mmHg
hypoxia
100
40
100
40
100
40
PO2
40
Capillary filtration determines
lung water content
alveolus
capillary
lymphatic
The Starling equation
describes capillary filtration
FR = Lp x S [ (Pc – Pi) – s (Pc – Pi) ]
FR
S
Pc
Pi
alveolus
Pi
Pc
Pi
Pc
filtration rate
capillary surface area
capillary pressure
interstitial pressure
s reflection coefficient
Pc plasma colloid osmotic pressure
Pi interstitial colloid osmotic pressure
Keeping the alveoli “dry”:
Large capillary pressure drop
20
Pressure
(cmH2O)
s (Pc – Pi)
15
10
PA
Capillary
bed
LA
s (Pc – Pi) = .8 (30 – 12)
Keeping the alveoli “dry”:
Perivascular cuff formation
alveolus
vascular
cuffing
Perivascular cuffs in early
pulmonary edema
cuff
Normal
lung
Early pulmonary
edema
The ultimate insult: alveolar
flooding
alveolus
Keeping the alveoli “dry”:
active transport removes
alveolar liquid
alveolar
space
NaCl
NaCl
transporter
active liquid
transport
Na-K pump
Cl
Na K
interstitium
SUMMARY
Features of the pulmonary
circulation designed for efficient
gas exchange:
1. Accommodate the cardiac
output
* low vascular tone
* high capillary compliance
SUMMARY
Features of the pulmonary
circulation designed for efficient
gas exchange:
2. Keep filtration low near alveoli
* low Pc
* vascular interstitial sump
SUMMARY
Features of the pulmonary
circulation designed for efficient
gas exchange:
3. Keep liquid out of the alveoli
* active transport
* high resistance epithelium
Control of Breathing
Central neurons determine minute
ventilation (VE) by regulating tidal
volume (VT) and breathing
frequency (f).
VE = VT x f
VRG
pons
medulla
DRG
Neural Control
of Breathing –
Respiratory
neurons
Neural Control
of Breathing –
The efferent
pathway
muscle supply
autonomic
Phrenic n.
Central
chemoreceptors
CO2, H+
Control of
Breathing by
Central
chemoreceptors
major regulators of
breathing
CO2, H+
Central
chemoreceptors
Resp
neurons
VE
IXth n.
Carotid
body
~10% contribution
to breathing
O2
CO2
pH
Chemical
Control of
Breathing –
Peripheral
chemoreceptors
CO2 drives ventilation
50
hypercapnia
VE
(L/min)
5
quiet breathing
35
40
45
CO2 tension in blood
(mm Hg)
Hypoxia is a weak ventilatory
stimulus
50
blood pO2 =
50 mmHg
blood pO2 =
100 mmHg
VE
(L/min)
5
35
40
45
CO2 tension in blood
(mm Hg)
Reflex Control of
Breathing –
Neural receptors
Xth n.
afferents
Airway Receptors:
Slowly adapting (stretch - ends
inspiration)
Rapidly adapting (irritants cough)
Bronchial c-fiber (vascular
congestion - bronchoconstriction)
Parenchymal c-fiber (irritants bronchoconstriction)