PY 105 Chapter Notes - Chapter 11: Carbonic Anhydrase, Endothelium, Cooperative Binding
Transport of Gases
Oxygen
• Oxygen is too hydrophobic to dissolve in the plasma
• RBCs are used to bind and carry O2
• RBCs are able to carry oxygen because they contain millions of molecules of hemoglobin
o Composed of 4 polypeptide subunits
o Each subunit contains one molecule of heme, which is a large multiring structure that has a
single iron bound at its center
o The role of heme with its iron atom is to bind O2
o Each molecule of hemoglobin can carry four molecules of oxygen
o When none of the subunits have oxygen bound, all four subunits assume a tense
conformation that has a relatively low affinity for oxygen
o When one of the subunits binds oxygen, its conformation changes from a tense to a relaxed
state that has a higher affinity for oxygen
o Hemoglobin is said to bind oxygen cooperatively
o The level of O2 in active tissues is very low
o In the tissues, hemoglobin has low affinity for oxygen and tends to release any oxygen which
it carries
o The level of O2 in the lungs is of course very high
o When a RBC is passing through a capillary in the lungs, the hemoglobin will have a higher
affinity due to cooperative binding and will tend to bind oxygen strongly
o Certain factors stabilize the tense configuration (low O2 affinity):
• Decreased pH
• Increased pCO2 (level of CO2 in the blood)
• Increased temperature
o The fact that these factors stabilize tense hemoglobin and thus reduce its oxygen affinity is
known as Bohr effect
o The affinity of hemoglobin for oxygen can be quantified by measurement of the fraction of
O2 binding sites which have bound O2
• Percent saturation (% sat) = (# of O2 molecules bound) / (# of O2 binding sites) x 100%
Carbon Dioxide
• Transported in the blood in three ways:
o 73% of CO2 transport is accomplished by the conversion of CO2 to carbonic acid, which
dissociates into bicarbonate and a proton
• Water soluble and easily carried in the blood
• Conversion of CO2 into carbonic acid is catalyzed by an RBC enzyme called carbonic
anhydrase
o Some CO2 is transported by being stuck onto hemoglobin
• Binds to other sites on hemoglobin
• Important in Bohr effect because it stabilizes tense Hb
o Can be dissolved into blood and carried from tissues to lungs
Exchange of Substances Across the Capillary Wall
• Capillaries are the site of exchange between the blood and tissues
• Capillaries have walls of only a single layer of flattened endothelial cells, and there are spaces
(intercellular clefts) between the endothelial cells which make up the capillary wall
• Nutrients, wastes and white blood cells must be able to pass through the clefts
• Three main types of nutrients: amino acids, glucose and lipids
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