Ch. 6 Gas Transport by the Blood
Oxygen
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Carried in the blood in 2 forms
o Dissolved in plasma
o Bound to Hb in RBC
Henry’s Law: Amt. of dissolved O2 is relative to the partial pressure
O2 content = 1.35 (Hb) (O2 Sat.) + 0.003 (PaO2)
o 18.7 = 1.35 (14)(.975) + 0.003 (100) --- Normal
o Note: west uses 1.39 for this, 1.35 is avg. of difft. Literature
o West says normal O2 content of blood is about 20.8 ml O2 per 100 ml blood
Hemoglobin
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4 polypeptide chains; 2 alpha, 2 beta
Hb A: Normal adult Hb
o Normal ferrous (2+) iron can be oxidized to ferric (3+) form by various drugs
o Ferric Hb = methemoglobin  toxic effects
Hb F: infant Hb (gradually replaced over 1st year)
Hb S: sickle cell (has valine substituted for glutamic acid in beta chains)
o Hb S results in a right shift of dissociation curve
 Deoxygenated form crystallizes in the RBC  thrombus
Anemic pt w/ Hb conc. Of 10g per 100 ml blood, w/ PO2 of 100 mm Hg
O2 dissociation curve
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Capacity: max amt. that O2 that Hb can carry (i.e. 4 x O2 per 1 molecule of Hb)
O2 saturation:
o ( O2 combined with Hb / O2 capacity ) x 100
o Normal O2 saturation of 97.5% w/ a PaO2 of 100 mm Hg
Change in Hb from saturated state to unsaturated state (deoxy) is accompanied by
conformational change in molecule
Oxygenated form = R state (relaxed)
Deoxygenated form = T state (tense)
Sigmoidal curve means:
o Flat upper portion – so even if PAO2 drops some, no biggie
o Diffusion process is increased d/t difference of partial pressure of O2 b/t alveoli and cap
o Steep lower part of curve means that peripheral tissues can withdraw large amounts of
O2 for only a small drop in capillary PO2
Cyanosis: reduced Hb is purple – low arterial O2 saturation
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Right Shift Caused by: (increased tissue metabolism increased O2 unloading)
o Increased: temp, CO2, DPG, H+
o Decreased: pH
Left Shift Caused by: (i.e. cooling down a trauma victim to reduce O2 unloading)
o Increased: pH
o Decreased: temp, CO2, DPG, H+
o Carbon Monoxide
DPG – metabolite of glycolytic pathway; increased by chronic hypoxia, altitude, chronic lung
disease
Bohr Effect: at lower pH, O2 will bind to Hb with lower affinity
PO2 (50%) – 27 mm Hg ((normal)) [increased in rt. Shift, or decreased in lt. shift]
Carbon Monoxide (CO)
o Irreversibly (essentially) binds to Hb and forms carboxyhemoglobin (COHb)
o Little goes a long way: PCO of 0.16 mm Hg = 75% Hb bound as COHb
o Shifts curve to left
Oxygen Dissociation Curve
o PO2: 100 (arterial)
o SO2: 97% (arterial)
o PO2: 40 (from tissues)
o SO2: 75% (from tissues)
Carbon Dioxide CO2
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Carried as: dissolved, bicarbonate (HCO3), combo w/ proteins (carbamino compounds)
Normal = 24 mmol/L
Dissolved CO2: obey’s Henry’s Law
o Henry’s Law: At a constant temperature, the amount of a given gas dissolved in a given
type and volume of liquid is directly proportional to the partial pressure of that gas in
equilibrium with that liquid
o About 20 times more soluble than O2
Bicarbonate Prod’n (HCO2)
o Main form of CO2 in blood
o CO2 + H2O <-> H2CO3 <-> H+ + HCO2
o HCO2: bicarbonate
o H2CO3: carbonic acid
o Carbonic anhydrase: converts CO2 + H2O  H2CO3
o H+ cannot easily diffuse out of RBC
Chloride Shift: To maintain neutrality in RBC, Cl- ions diffuse into the RBC from plasma
Some of H+ ions liberated are bound to reduced Hb
o H+ + HbO2 <-> H+ ∙Hb + O2
Haldane Effect: the fact that deoxygenation of the blood increases its ability to carry CO2
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The lower the saturation of Hb with O2, the larger the CO2 concentration for a given
PCO2
Uptake of CO2 by RBC increases osmolarity and therefore water enters cell, increasing Vol.
Carbamino compounds:
o Formed by combination of CO2 with terminal amine groups in blood proteins
o Most important protein: globin in Hb
o Hb ∙ NH2 + CO2 <-> Hb∙NH∙COOH
o Reduced Hb can bind more CO2 as carbamino Hb
CO2 dissociation curve
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Linear relationship (steeper)
PaCO2 = PACO2
Normal Ranges: 38-42 mm Hg
CO2 is carried as: dissolved, bicarbonate, carbamino
CO2 curve is right-shifted by increases in Saturation of O2
Acid-Base Status
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Lungs excrete huge amounts of carbonic acid (via CO2 and H2O)
Altering alveolar ventilation and thus elimination of CO2, the body has greater control over acidbase balance
Normal pH = 7.4
Hb acts as a buffer
If plasma bicarbonate concentration is altered by the kidney, the buffer is less effective
o Base Excess: Increase in bicarbonate conc.
o Base deficit: reduced bicarbonate conc.
Respiratory Acidosis
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Caused by an increase in PCO2 which reduces HCO3-/PCO2 ratio  decrease pH
If respiratory acidosis persists, kidney responds by conserving HCO3- (d/t increased PCO2)
Henderson Hassselbach:
o pH = 6.1 + log [(bicarbonate- mmol/L) / 0.03 x 60]
Respiratory Alkalosis
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caused by a decrease in PCO2
o can be d/t: hyperventilation, high altitude
Metabolic Acidosis
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ratio of HCO3- to PCO2 falls  decrease in pH
can be d/t DM, lactic acid
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increase in respiration acts to increase CO2 (le’ chatlier principle)
o stimulus is H+ ions in peripheral chemoreceptors
Metabolic Alkalosis
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increase in HCO3 raises the HCO3/PCO2 ratio and pH increases
can be d/t loss of gastric acid secretion, secretion by vomiting, reduction of alveolar ventilation
that raises PCO2
4 types of acid base disturbances
o pH = pK + log (HCO3- / 0.03 x PCO2)
Acidosis
Primary
Compensation
o Respiratory
PCO2 ++
HCO3- ++
o Metabolic
HCO3 -PCO2 –
Alkalosis
o Respiratory
PCO2 -HCO3 –
o Metabolic
HCO3 ++
n/c
Blood- Tissue Gas Exchange
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d/t diffusion
proportional to tissue area and difference in gas partial pressure b/t 2 sides
inversely proportional to thickness
thickness of BGB is < 0.5 µm
distance b/t open capillaries in resting muscle 50 µm
during exercise additional cap’s open, reducing the diffusion distance and increasing the area for
diffusion
CO2 diffuses x20 faster than O2; therefore O2 deliver is more difficult than CO2 removal
Critical situation: the intercapillary distance or the O2 consumption where PO2 = 0 mm Hg
Purpose of much higher PO2 in capillary blood is to ensure an adequate pressure for diffusion of
O2 to the mitochondria
o At sites of O2 utilization the PO2 is very low (( i.e. why O2 leaves RBC for low pressure in
mito))
Hypoxia: abnormally low PO2 in tissue
o Can be measured by cardiac output x arterial O2 conc.
o Q(dot) x CaO2
Tissue Hypoxia can be d/t:
o A low PaO2 (hypoxic hypoxia  or, hypoxemia)
o Reduced ability of blood to carry O2 as in anemia or CO poisoning (anemic hypoxia)
o Reduction in tissue blood flow i.e. hemorrhagic shock (circulatory hypoxia)
o Cyanide (f’s up ETC) – (hystotoxic hypoxia)
Refer to 6-1 table for more info