THE RESPIRATORY SYSTEM
LECTURE 5:
GAS TRANSPORT
Dr. Eamonn O’Connor
Human Form & Function
Lecture Outline
1

Gas Transport
  Oxygen
  Carbon

Dioxide
Effect of changes in Gas concentrations
Physiology 11-12. The Respiratory System. Lecture 5.
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Oxygen Transport
2

2 Forms:
  1.5
% Dissolved in plasma
a PaO2 = 100 mmHg; 3 ml.litre-1blood (0.3 ml.dl-1)
  Resting VO2 = 250 ml.min-1
  Pulmonary blood flow (CO) = 5L.min-1 (O2 content = 21%)
  Therefore, O2 breathed = 1L.min-1
  15 ml.min-1 of O2 breathed dissolves
  NB: Only dissolved gas contributes to circulating partial pressures
  At
  98.5
% Bound to haemoglobin
binds 1.34 ml O2.g-1 (x130 – 150 g.litre-1)
  Provides 175 – 200 ml O2.litre-1 (17.5 – 20 ml O2.dl-1)
  Hb
Physiology 11-12. The Respiratory System. Lecture 5.
Haemoglobin (Hb)
3

Found only in red blood corpuscles (RBC’s)



Structure



150 g.L-1 in men (15 g.dl-1)
130 g.L-1 in women (13 g.dl-1)
Tetrameric globular protein
4 Haeme groups (each contain 1 Fe2+ ion)
Cooperative reversible binding of up to:


4 O2 molecules
4 CO2 molecules (normally 2 at most in venous blood)



Hb always remains partly oxygenated (>40% saturated)
Transports 98.5% of O2 in blood
Function to provide:


Greater oxygen carrying capacity
Improved efficiency by ↓ energy expenditure for VE
Physiology 11-12. The Respiratory System. Lecture 5.
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Hb-Oxygen Dissociation Curve
4
Physiology 11-12. The Respiratory System. Lecture 5.
Hb-O2 Dissociation Curve
5
Physiology 11-12. The Respiratory System. Lecture 5.
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Changes in Affinity of Hb for O2


Describes changing affinity of Hb
for O2
Right shifted

Tissue level unloading


Left shifted

Lung level loading


↑ PCO2, ↓ pH (↑ H+), ↑ Temperature
↓ PCO2,↑ pH (↓ H+),↓ Temperature
2,3-diphosphoglycerate

2,3-DPG → right shift

Metabolite of RBC glycolysis
Significant in hypoxia (altitude)

Physiology 11-12. The Respiratory System. Lecture 5.
Carbon Dioxide Transport
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Resting VCO2 = 200 ml.min-1
  3 Forms

  Dissolved
  10%
  Bound
to Hb
  30%
  Bicarbonate
  Mostly
(HCO3-)
in plasma
  60%
Physiology 11-12. The Respiratory System. Lecture 5.
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O2 & CO2 Transport
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Physiology 11-12. The Respiratory System. Lecture 5.
Hb-CO2 Dissociation Curve
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Physiology 11-12. The Respiratory System. Lecture 5.
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Hypoxia (Insufficient Cellular O2)
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
Hypoxic hypoxia

Low PaO2 → ↓ %Hb Sat
Inadequate gas exchange
↓ PB (altitude)
  Cyanosis (skin bluish tint) = 70% Hb Sat



Anaemic hypoxia

↓ total blood O2 content with normal PaO2
↓ circulating RBC’s
↓ RBC Hb content
  CO poisoning (no cyanosis – HbCO is pink)



Circulatory hypoxia

↓ supply of oxygenated blood with normal O2 content and
PaO2


Vessel blockage
Congestive heart failure
Physiology 11-12. The Respiratory System. Lecture 5.
Abnormal PaCO2
11

Hypercapnia

Excess PaCO2
via hypoventilation (under breathing)
Occurs with most lung diseases
  Occurs in conjunction with reduced PaO2



Hypocapnia

Below normal PaCO2
via hyperventilation (over breathing)
Occurs with anxiety & fear
  No impact on PaO2




Except at low PB (e.g. altitude) where low PaO2 stimulates a hyperventilation
NB: hyperpnea

increased breathing/ventilation to match metabolic demand
(e.g. exercise)
Physiology 11-12. The Respiratory System. Lecture 5.
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