Chapter 18a
Gas Exchange and
Transport
About this Chapter
• Diffusion and
solubility of gases
• Gas exchange in
lungs and tissues
• Gas transport in the
blood
• Regulation of
ventilation
Overview
• Oxygen and carbon dioxide move into and
out of the blood at pulmonary and systemic
capillaries
• Internal respiration
CO2
O2
Airways
Alveoli of lungs
6 CO2 exchange
at alveolar-capillary
interface
CO2 O2
1 Oxygen exchange
at alveolar-capillary
interface
O2
CO2
Pulmonary
circulation
2 Oxygen transport
5 CO2 transport
Systemic
circulation
CO2
O2
4 CO2 exchange
at cells
3 Oxygen exchange
at cells
CO2
Cells
O2
Cellular
respiration
Nutrients
ATP
Figure 18-1
Diffusion and Solubility
• Constants influencing diffusion in the lungs
• Surface area
• Contact between air and blood
• Membrane thickness
• Alveoli and endothelium
• Diffusion distance
• Distance between blood and air
• Concentration gradient
• Most important factor as others usually are
constant
Movement of Gases
• Pressure gradient
• Partial pressure change
• Solubility
• Gas into liquid
• Temperature
• Higher faster
Gases in Solution
PO2 = 100 mm Hg
PO2 =
100 mm Hg
[O2] =
5.20 mmol/L
PO2 = 0 mm Hg
PO2 =
100 mm Hg
[O2] =
0.15 mmol/L
(a)
(b)
(c)
Figure 18-2a–c
Gases in Solution
PO =
2
100 mm Hg
[O2] =
5.20 mmol/L
PCO =
2
100 mm Hg
[CO2] =
5.20 mmol/L
PO =
2
100 mm Hg
[O2] =
0.15 mmol/L
(c)
PCO =
2
100 mm Hg
[CO2] =
3.00 mmol/L
(d)
Solubility difference between O2
and CO2
Figure 18-2c–d
Gas Diffusion
Alveoli
Alveoli
PO2 = 100 mm Hg
PCO2 = 40 mm Hg
PO2 = 40
PO2 = 100
PCO2 = 46
Circulatory
system
PO2 = 40
PCO2 = 40
Circulatory
system
PO2 = 100
PCO2 = 46
PCO2 = 40
PO2 <
– 40 mm Hg
PCO2 >– 46 mm Hg
Peripheral tissue
Peripheral tissue
(a) Oxygen diffusion
(b) CO2 diffusion
Figure 18-3
Gas Exchange
PLAY
Interactive Physiology® Animation: Respiratory System:
Gas Exchange
Partial Pressures
Table 18-1
Gas Exchange
Table 18-2
Causes of Low Alveolar PO2
1. Inspired air has abnormally low oxygen
content
•
Altitude
2. Alveolar ventilation is inadequate
•
•
•
Decreased lung compliance
Increased airway resistance
Overdose of drugs
•
What types??
Alveolar Ventilation
• Pathological conditions
that reduce alveolar
ventilation and gas
exchange
Figure 18-4a
Alveolar Ventilation
Fewer
alveoli
Figure 18-4b
Alveolar Ventilation
Low
compliance
Figure 18-4c
Alveolar Ventilation
Figure 18-4d
Alveolar Ventilation
Figure 18-4e
Gas Exchange
• Oxygen diffuses across alveolar epithelial
cells and capillary endothelial cells to enter
the plasma – respiratory membrane
Surfactant
Alveolar
epithelium
Fused basement
membranes
Nucleus of
endothelial
cell
O2
Alveolar
air space
0.1–1.5 m
O2
Capillary
lumen
Plasma
RBC
Figure 18-5
Gas Exchange
• Pathological changes
• Decrease in amount of alveolar surface area
• emphysema
• Increase in thickness of alveolar membrane
• fibrosis
• Increase in diffusion distance between alveoli
and blood
• pneumonia
Oxygen Transport
Capillary
endothelium
ARTERIAL BLOOD
O2 dissolved in plasma (~PO2) < 2%
O2
O2 + Hb
Hb•O2
> 98%
Red blood cell
Alveolus
Alveolar
membrane
Transport
to cells
Hb•O2
Hb + O2
O2 dissolved in plasma
Cells
O2
Used in
cellular
respiration
Figure 18-6
Oxygen Transport
(a) Oxygen transport in blood without
hemoglobin. Alveolar PO2 = arterial PO2
• Hemoglobin increases
oxygen transport by
blood
PO2 = 100 mm Hg
Alveoli
Arterial
plasma
O2 molecule
PO2 = 100 mm Hg
Oxygen dissolves in plasma.
O2 content of plasma = 3 mL O2/L blood
O2 content of red
blood cells
=0
Total O2 carrying
3 mL O2/L blood
capacity
Figure 18-7a
Oxygen Transport
(b) Oxygen transport at normal
PO2 in blood with hemoglobin
PO2 = 100 mm Hg
PO2 = 100 mm Hg
Red blood cells with hemoglobin are carrying 98% of their maximum load of oxygen.
O2 content of plasma = 003 mL O2/L blood
O2 content of red
blood cells
= 197 mL O2/L blood
Total O2 carrying
capacity
200 mL O2/L blood
Figure 18-7b
Oxygen Transport
(c) Oxygen transport at reduced PO2
in blood with hemoglobin
PO2 = 28 mm Hg
PO2 = 28 mm Hg
Red blood cells carrying 50% of their
maximum load of oxygen.
O2 content of plasma = 000.8 mL O2/L blood
O2 content of red
blood cells
= 099.5 mL O2/L blood
Total O2 carrying
capacity
100.3 mL O2/L blood
Figure 18-7c
The Hemoglobin Molecule
 Chain
• Hemoglobin
consists of 4
subunits, each
centered
around Fe2+
 Chain
Heme
group
(a)
Porphyrin
ring
(b)
R = additional C, H, O groups
Figure 18-8
Oxygen-Hemoglobin Dissociation Curve
Figure 18-9
Oxygen Binding
• Physical factors alter
hemoglobin’s affinity
for oxygen shift curve
right or left
• pH
• Temperature
• pCO2
• BPG
• RBCs produce during
hypoxia
• Hb type
• Fetal HbF
Figure 18-10a
Oxygen Binding
Figure 18-10b
Oxygen Binding
Figure 18-10c
Oxygen Binding
• 2,3-DPG decreases hemoglobin’s affinity for
oxygen
Figure 18-11
Oxygen Binding
• Maternal and fetal hemoglobin have different
oxygen-binding properties
Figure 18-12
Oxygen Binding
• The total oxygen content of arterial blood
depends on the amount of oxygen dissolved
in plasma and bound to hemoglobin
TOTAL ARTERIAL
O2 CONTENT
Oxygen dissolved in
plasma (PO2 of plasma)
is influenced by
Composition of
inspired air
Alveolar
ventilation
Oxygen
bound to Hb
helps
determine
Oxygen diffusion
between alveoli
and blood
Adequate
perfusion
of alveoli
% Saturation
of Hb
x
Total number of
binding sites
affected by
Rate and
depth of
breathing
Airway
resistance
Lung
compliance
Surface
area
Membrane
thickness
Diffusion
distance
PCO2
pH
Temperature
2,3–DPG
Hb content
per RBC
x
Number
of RBCs
Amount of
interstitial fluid
Figure 18-13
Carbon Dioxide Transport
• Dissolved: 7%
• Converted to bicarbonate ion: 70%
• Bound to hemoglobin: 23%
• Hemoglobin also binds H+
• Hb and CO2: carbaminohemoglobin
H20 + CO2
H2CO3
H+ + HCO3-
Carbon Dioxide Transport in the Blood
VENOUS BLOOD
CO2
Dissolved CO2
(7%)
Cellular
respiration
in
peripheral
tissues
Red blood cell
CO2 + Hb
CO2 + H2O
Hb•CO2 (23%)
CA
Cl–
HCO3–
H2CO3
H+ + Hb
Hb•H
HCO3– in
plasma (70%)
Capillary
endothelium
Cell membrane
Transport
to lungs
Dissolved CO2
Dissolved CO2
HCO3–
in
plasma
Hb + CO2
Hb•CO2
Cl–
CO2
Alveoli
HCO3–
Hb•H
H2CO3
CA
H2O + CO2
H+ + Hb
Figure 18-14
Summary of O2 and CO2 Exchange and Transport
Dry air = 760 mm Hg
PO2 = 160 mm Hg
PCO2 = 0.25 mm Hg
Alveoli
PO2 = 100 mm Hg
PCO2 = 40 mm Hg
CO2 O2
CO2 transport
HCO3– = 70%
Hb•CO2 = 23%
Dissolved CO2 = 7%
Arterial blood
PO2 = 100 mm Hg
PCO2 = 40 mm Hg
Pulmonary
circulation
O2 transport
Hb•O2 > 98%
Dissolved O2 < 2%
(~PO2)
Venous blood
PO2 = 40 mm Hg
PCO2 = 46 mm Hg
Systemic
circulation
O2
CO2
Cells
PO 2 <
– 40 mm Hg
PCO2 >
– 46 mm Hg
Figure 18-15
Gas Transport
PLAY
Interactive Physiology® Animation: Respiratory System:
Gas Transport