Lecture 23: Blood Gas Transport

Question 1

Hemoglobin

Answer 1

• Hemoglobin has a tetrameric structure made up of 4 globins.
• Each globin is attached to a protoporphyrin (heme) group.
• Each hemegroup has a ferrous group (Fe++) at its center.
• Each (Fe++) can bind 1 molecule (diatomic) of oxygen.

Question 2

Transport of Oxygen in The Blood

Answer 2

• Initial pressure difference that causes oxygen to diffuse into the pulmonary capillary =
• 104 - 40 = 64 mmHg ((PO2 in alveolus) - (PCO2 in venous blood))
• In the pulmonary capillary the blood PO2 rises close to that in the alveolar air by the time the blood has moved onethird the distance through the capillary.
• See Slide 7
• A person may require 20x the normal amount of oxygen during exercise.
• Diffusing capacity of oxygen increases 3x during exercise:
• There is increased surface area of capillaries participating in diffusion.
• There is a more nearly ideal V/Q ratio in the upper parts of the lungs.
• Factors that determine tissue PO2:
• Rate of oxygen transport to the tissues
• Rate of oxygen consumption by the tissues
• Normal intracellular PO2:
• Normal range = 5-40 mm Hg
• Mean of 23 mm Hg
• 1-3 mm Hg PO2 required for normal cellular respiration
• Normal intracellular PCO2:
• Normal = 46 mm Hg (compared to interstitial of 45 mm Hg).
• Arterial blood entering tissues = 40 mm Hg.
• Venous blood leaving tissues = 45 mm Hg.
• See Slide 9

Question 3

Review slides 11-19

Answer 3

• Upper limit of arterial blood flow is Limited by max PO2 in arterial Blood.
• Factors that determine tissue PO2:
• Rate of oxygen transport to the tissues
• Rate of oxygen consumption by the tissues
• Note that decrease in blood flow from normal increases peripheral tissue CO2
• Note that 6x increase in blood flow from normal decreases peripheral tissue CO2 to almost equal that in arterial blood
• Stare at slide 18-19 and see if any of that makes sense

Question 4

Oxygen Transport

Answer 4

• 15 grams Hb/dl blood
• 1 gram Hb can bind 1.34 ml O2
• 1 dl blood carries 20.1 ml O2 (1.34 x 15)
• 19.4 –14.4 = 5 ml O2/dl transported to tissues
• Utilization coefficient = percentage of blood that gives up its oxygen:
• 5/19.4 = 25%
• Strenuous exercise → 75 to 85%

Question 5

Hemoglobin as a “Tissue Oxygen Buffer” System

Answer 5

• In order to release 5 ml of oxygen/dl of blood:
• PO2 must fall to about 40 mm Hg:
• Tissue PO2 cannot rise above this level.
• When PO2 is high (pulmonary capillaries), oxygen binds with hemoglobin.
• When PO2 is low (tissue capillaries) oxygen is released from hemoglobin.

Question 6

See Slide 24-27

Answer 6

• Shift of oxygen-hemoglobin curve to right is (usually) caused by decrease in pH
• Increased pCO2 → ↓pH O2 is forced from the hemoglobin
• Normal [BPG] keeps dissociation curve slightly shifted to right all the time.

Question 7

The Bohr Effect

Answer 7

• Increase in blood [carbon dioxide] and H+ ions:
• Shifts oxygen-hemoglobin curve to right.
• Enhances release of oxygen from the blood in tissues
• Enhances oxygenation of blood in lungs
• Decrease in blood [carbon dioxide] and H+ ions:
• Shifts oxygen-hemoglobin curve to left
• Occurs in lungs
• See Slide 29
• When cellular pO2 is more than 1 mm Hg, [ADP] becomes the limiting factor in the rates of chemical reactions.

Question 8

Carbon Dioxide Transport

Answer 8

• Small amount is dissolved in the blood:
• 2.7 ml/dl at 45 mm Hg
• 2.4 ml/dl at 40 mm Hg
• Accounts for about 7% of carbon dioxide transported
• About 70% is transported as carbonic acid:
• Requires carbonic anhydrase
• Also employs a bicarbonate/chloride transporter
• Remainder is transported as carbamino hemoglobin

Question 9

Bohr Vs. Haldane Effects

Answer 9

• Bohr effect:
• Increase in blood carbon dioxide causes oxygen to be displaced from hemoglobin.
• Shifts oxygen-hemoglobin dissociation curve to right
• Haldane effect:
• Binding of oxygen with hemoglobin displaces carbon dioxide from blood:
• • Binding of oxygen causes hemoglobin to become a stronger acid.
• • More acidic hemoglobin has less of a tendency to bind with carbon dioxide.
• • Increased acidity of hemoglobin causes it to release hydrogen ions.
• See Slide 32-36

Question 10

• Explain why blood may be bright red in carbon monoxide poisoning.

Answer 10

• Carbon monoxide displaces oxygen on the hemoglobin molecule.
• Binds 250x stronger than oxygen.
• See Figure 41-12: Carbon monoxide-hemoglobin dissociation curve.
• In carbon monoxide poisoning, oxygen content of blood is greatly reduced, but PO2 of the blood may be normal. 40
• Therefore: Blood may be bright red.
• See Slide 39