The Respiratory System (2)
Dr Nihal AbdAllah Ibrahim, MD
MBBCh, MSC, PhD
Transport of Respiratory Gases by
Blood
1- Oxygen transport:
The total Oxygen carrying capacity of blood is 200
ml/L blood. This amount is carried in 2 forms:
1) Bound to Hb in the RBCs: 99% (=197ml of O2/L)
2) Dissolved in the plasma : only 1% (= 3ml of O2/L).
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O2 and Hemoglobin
• Oxyhemoglobin (HbO2): hemoglobin-O2
combination
• Reduced hemoglobin (HHb): hemoglobin that
has released O2
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O2 and Hemoglobin
• Loading and unloading of O2 is facilitated by
change in shape of Hb
– As O2 binds, Hb affinity for O2 increases
– As O2 is released, Hb affinity for O2 decreases
• Fully (100%) saturated if all four heme groups
carry O2
• Partially saturated when one to three hemes
carry O2
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The Oxygen-hemoglobin dissociation
curve
• Hemoglobin saturation plotted against PO2 is not
linear but it is an S-shaped curve.
• It shows how binding and release of O2 is influenced
by the PO2.
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Factors affecting loading (saturation) of Hb with O2
Factors Influencing Hemoglobin Saturation
• Rate of loading and unloading of O2 is regulated by:
– PO2
– Temperature
– Blood pH
– PCO2
– Concentration of DPG
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Factors Influencing Hemoglobin Saturation
• Decrease Hb affinity for O2 (Shift the O2-hemoglobin
dissociation curve to the right by:
Increases in : temperature, H+, PCO2, and DPG
• Decreases in these factors increase the affinity of Hb to
O2 (shift the curve to the left)
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Decreased carbon dioxide
(PCO2 20 mm Hg) or H+ (pH 7.6)
10°C
20°C
38°C
43°C
Normal arterial
carbon dioxide
(PCO2 40 mm Hg)
or H+ (pH 7.4)
Normal body
temperature
Increased carbon dioxide
(PCO2 80 mm Hg)
or H+ (pH 7.2)
(a)
(b)
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PO (mm Hg)
2
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Carbon Dioxide Transport
• Normally, active body cells produce about 200ml of CO2 each
minute which is exactly the amount excreted by the lungs each
minute. CO2 is transported in 3 forms:
1- As bicarbonate ion in plasma (70%): most of CO2 entering the
plasma quickly enters the RBCs, where CO2 is transferred into
bicarbonate ions. This reaction is catalyzed by the Carbonic
Anhydrase enzyme(CA).
2- Chemically bound to Hb (23%) as “carbamino-hemoglobin” (HbCO2). The CO2 combines with the globin part of Hb (not with
iron) & therefore, it does not interfere with the O2 transport by
Hb.
3- Dissolved in plasma (7%).
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CO2 Transport
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Pulmonary Gas Exchange
• During this process, the dark venous blood present in
the pulmonary circulation is changed to bright red
oxygenated blood & will return to the left atrium
through the 4 pulmonary veins, to be distributed by the
left ventricle through the Aorta to all parts of the body.
• The process includes passive diffusion of O2 from
alveolar air into the blood & CO2 from blood to alveolar
air due to difference in partial pressures of these gases.
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•
•
•
•
PO2 (Oxygen pressure) in alveolar air = 100mmHg.
PO2 in pulmonary blood= 40mmHg.
PCO2 in venous blood = 46mmHg.
PCO2 in in alveolar air = 40mmHg.
So at the lungs:
O2 moves from alveoli (100mmHg) to blood (40mmHg)
CO2 moves from blood (46mmHg) to alveoli (40 mmHg)
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Gaseous Exchange between Lung alveoli
& Blood
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Gaseous Exchange between tissue cells &
Blood
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Control of Respiration
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Hypoxia
• It is inadequate delivery of O2 to the tissues. If the
level of Hb saturation with O2 drops below 75% it
causes bluish discoloration of the skin known as
cyanosis.
• Types:
1- Anemic Hypoxia: due to decreases RBCs count or Hb
content in blood.
2- Hypoxic Hypoxia: due to reduced pO2 in arterial blood
due to lung disease or breathing air containing low
amounts of O2.
3- Histotoxic Hypoxia: due to inability of the cells to utilize
O2 even if its amount is normal. It occurs in cyanide
poisoning.
4- Stagnant Hypoxia: results when blood circulation is
impaired or blocked by thrombi
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