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666 seCtioN iii RespiRatoRy ` RESPIRATORY—PhYSIOlOgY RespiRatoRy ` RESPIRATORY—PhYSIOlOgY
Oxygen content of O content = (1.34 × Hb × Sao ) + (0.003 × Pao )
2
2
2
blood Hb = hemoglobin concentration; Sao = arterial O saturation
2
2
Pao = partial pressure of O in arterial blood
2 2
Normally 1 g Hb can bind 1.34 mL O ; normal Hb amount in blood is 15 g/dL.
2
O binding capacity ≈ 20 mL O /dL of blood.
2
2
With Hb there is O content of arterial blood, but no change in O saturation and Pao .
2
2
2
O delivery to tissues = cardiac output × O content of blood.
2 2
hb CONCENTRATION % O 2 SAT OF hb DISSOlVED O 2 (Pao 2 ) TOTAl O 2 CONTENT
CO poisoning Normal (CO competes Normal
with O )
2
Anemia Normal Normal
Polycythemia Normal Normal
Methemoglobin Iron in Hb is normally in a reduced state Nitrites (eg, from dietary intake or polluted/
2+
(ferrous Fe ; “just the 2 of us”). high-altitude water sources) and benzocaine
3+
Oxidized form of Hb (ferric, Fe ) does not cause poisoning by oxidizing Fe to Fe .
2+
3+
bind O as readily as Fe , but has affinity for Methemoglobinemia can be treated with
2+
2
cyanide tissue hypoxia from O saturation methylene blue and vitamin C.
2
and O content.
2
Methemoglobinemia may present with cyanosis
and chocolate-colored blood.
Oxygen-hemoglobin ODC has sigmoidal shape due to positive
dissociation curve cooperativity (ie, tetrameric Hb molecule can 100 Hemoglobin
bind 4 O molecules and has higher affinity 90 Myoglobin
2
for each subsequent O molecule bound). 80
2
Myoglobin is monomeric and thus does 70
Right
Left
not show positive cooperativity; curve lacks 60 (↓ P 50 ) (↑ P 50 )
sigmoidal appearance. Hb saturation (%)
Shifting ODC to the right Hb affinity 50 P 50
for O (facilitates unloading of O to tissue) 40
2
2
P (higher Po required to maintain 50% 30
2
50
saturation). 20
Shifting ODC to the left O unloading 10 Venous blood Arterial blood
2
renal hypoxia EPO synthesis 0 (deoxygenated) (oxygenated)
compensatory erythrocytosis. 0 10 20 30 40 50 60 70 80 90 100
Fetal Hb (2 α and 2 γ subunits) has higher PO 2 (mm Hg)
Right shift
Left shift
affinity for O than adult Hb (due to affinity ( O₂ unloading to tissue) ( O₂ unloading to tissues)
2
for 2,3-BPG) dissociation curve is shifted Left = Lower ACE BATs right handed
+
left, driving diffusion of O across the placenta H ( pH, base) H ( pH, Acid)
+
2
from mother to fetus. PCO₂ PCO₂
2,3–BPG Exercise
Temperature 2,3–BPG
CO High Altitude
MetHb Temperature
HbF
FAS1_2019_16-Respiratory.indd 666 11/8/19 7:34 AM
