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Mawad
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For gas exchange to be efficient, __________ must match ___________.
alveolar perfusion (Q.); alveolar ventilation (V.A)
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Regional changes in __(3)__ in disease can cause marked ventilation/perfusion mismatch, leading to... (2)
lung compliance, airway resistance, and vascular resistance; ventilated alveoli with no blood supply or blood flowing through unventilated alveoli.
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Alveolar ventilation without perfusion (Q=0).
dead space ventilation
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Perfusion without alveolar ventilation (V.A=0).
right to left shunt- arterial blood does not come into contact with alveolus containing fresh gas
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____________ is the most common cause of hypoxemia (reduced systemic Pa O2).
Ventilation/perfusion mismatch
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Perfusion is better in the ________ portions of the lungs; ventilation is better in the _________ portions of the lungs.
lower; upper
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What local reflexes are present in lung that can limit ventilation/perfusion mismatching? (2)
hypoxic vasoconstriction and hypocapnic (state of reduced CO2 in the blood) bronchoconstriction
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What is hypoxic vasoconstriction?
localized hypoventilation (reduced airflow to alveoli), reducing local PA O2--> causes vasoconstriction of pulmonary arteriole supplying that alveolus, redirecting blood flow towards better-ventilated areas of the lungs
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What is hypocapnic bronchoconstriction?
occurs when airways are exposed to gas with lower than normal PA CO2 (b/c there is no movement of CO2 from blood to alveoli when no perfusion is occurring)--> leads to local bronchoconstriction to redirect airflow toward better-perfused lung regions
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What are the 2 forms of O2 carried in blood?
combined with hemoglobin (measured as %oxyHb or %saturation), dissolved in plasma (measured as PO2- ABG)
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O2 content of arterial blood (Ca O2) is the sum of...
oxyHb and amount of dissolve O2
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What are the 2 forms of hemoglobin?
oxyhemoglobin (HbO2- oxidized Fe+++), deoxyhemoglobin (Hb- reduced Fe++)
O2 + Hb ⇋ HbO2
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What is Hb saturation? What is its main determinant?
fraction of all Hb that is oxyHb; mainly determined by Pa O2 (b/c the higher the arterial PO2, the more oxygen there is, the more can be taken up by Hb)
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The total amount of O2 carried by hemoglobin depends on... (2)
Hb saturation (which depends on Pa O2) and [Hb] in blood
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Loss of Hb due to anemia reduces... (2)
amount of O2 carried and O2 saturation
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Why isn't an oxygen-hemoglobin dissociation curve a linear relationship?
reaction of the first Hb subunit with O2 FACILITATES reaction of the next with O2 [causes altered conformation by freeing up subsequent Hb subunits]
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Describe the oxygen-hemoglobin dissociation curve, and how it is affected by differing PO2 values.
at low PO2, there is a steep curve (so with small increases in PO2, there are major increases in %oxyHb); at high PO2, the curve is plateaued
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At low PO2, how is oxygen-Hb affected?
O2 and Hb rapidly combine/dissociate (need to be able to rapidly release it to the tissues)
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At PO2 above 60mmHg, how is oxygen-Hb affected?
further increases in PO2 produce only a small increase in %oxyHb saturation
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In a normal mammal, hyperventilation or supplemental O2 add __________ O2 to blood because...
- very little additional; Hb is 100% saturated at 100mmHg so above this, cannot carry more.
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What dictates Pa O2? What does not?
O2 diffusion from alveolus is only governed by O2 dissolved in plasma; O2 bound to Hb DOES NOT contribute to Pa O2
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Even though oxyHb does NOT contribute to Pa O2, it helps determine the total amount of O2 that diffuses across the alveolus because...
Hb is an O2 sink--> O2 diffuses from alveolar air to plasma--> plasma PO2 increases, promoting O2 diffusion into RBCs--> increased RBC PO2 promotes O2 binding to Hb--> effectively lowering dissolved O2, leading to a decreased Pa O2--> that causes an increased gradient for O2 diffusion into plasma
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Plasma PO2 does not reach PA O2 until...
all Hb is saturated.
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How does the Hb sink exchange O2 in the tissues?
Tissue PO2 is lower than systemic PaO2--> O2 diffuses from plasma to interstitial fluid--> IF PO2 increases, which promotes O2 diffusion into cells --> lowers PO2 of IF, promoting diffusion of O2 from plasma--> decrease in plasma PO2--> dissolved O2 diffuses from RBC to plasma--> facilitates dissociation of Hb and O2 [overall consequence is transfer of O2 from Hb to plasma to IF to cells by diffusion]
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At any given PO2, Hb saturation is modified by... (4)
blood PCO2, blood [H+], blood temperature, 2,3-DPG
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An increase in blood PCO2 shifts the oxyHb dissociation curve to the ____________.
right (Hb decreased affinity for O2- more easily delivered to the tissues)
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An increase in blood [H+] shifts the oxyHb dissociation curve to the ____________.
right(Hb decreased affinity for O2- more easily delivered to the tissues)
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An increase in blood temperature shifts the oxyHb dissociation curve to the ____________.
right(Hb decreased affinity for O2- more easily delivered to the tissues)
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An increase in 2,3-DPG shifts the oxyHb dissociation curve to the ____________.
right(Hb decreased affinity for O2- more easily delivered to the tissues)
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How does altered metabolism and increased production of CO2 alter affinity of Hb for O2?
CO2 and H+ combine with globin and alter the conformation of Hb, decreasing its affinity for O2
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How does altered metabolism and increased blood temperature alter affinity of Hb for O2?
increase temp alters conformation of Hb, decreasing affinity for O2
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How does altered metabolism and DPG production by glycolysis alter affinity of Hb for O2?
DPG binds reversibly to Hb, reducing its affinity for O2
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What is Henry's Law?
amount of gas dissolves in a liquid at equilibrium is directly proportional to the partial pressure of the gas with which the liquid is in equilibrium [at equilibrium, partial pressure of gas in liquid and gas phases are identical]
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When PO2 is higher in gas phase than liquid phase, O2 will _____________ until equilibrium is reached.
diffuse into liquid [henry's law]
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The concentration of a gas in plasma depends on... (2)
its partial pressure, its solubility in plasma
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__________ solubility of a gas in plasma means greater concentration at any given partial pressure.
Higher
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If plasma is exposed to 2 gases of differing solubility but the same partial pressure, at equilibrium, ....
both will have identical partial pressure in the liquid but differing concentrations in the liquid [CO2 is way more soluble than O2- if there's the same partial pressure of each, the conc of CO2 will be way higher]
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Quantity of dissolved O2 in blood depends on... (2)
solubility, partial pressure
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Quantity of combined O2 (oxyHb) in blood depends on... (4)
amount of Hb in blood, carrying capacity of Hb, PO2 in blood, shape of the oxyHb dissociation curve
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The O2 content of arterial blood (Ca O2) is the sum of ____________; this is important because...
dissolves O2 and Hb-bound O2; since Hb is 98% saturated at 100mmHg PO2, O2 supplementation will primarily result in an increase in dissolved O2 (which is insignificant int he grand scheme...you're not doing much)
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The rate of O2 delivery to the tissues (Da O2) depends on.... (3)
metabolic needs of tissues, heart rate, stroke volume
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Cyanosis results from...
excess amount of deoxyHb in skin blood vessels (esp. mucous membranes); usually due to an impairment in ventilation (rarely problems with Hb itself).
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Why don't you get cyanosis with anemia?
b/c there is less Hb, so all the Hb that is there is saturated with O2 and therefore red
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Why is there a greater flow of CO2 than O2 across the alveolus despite a smaller difference in the pressure gradient?
because CO2 is so much more soluble
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What happens to CO2 in blood? (3)
small amount carried dissolved in the plasma, most of it is converted to bicarb by erythrocytes, some forms carbamino-Hb
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As PCO2 increases in blood, the oxy-Hb dissociation curve shifts to the ________ because...
right (decreased Hb affinity for O2-easier to deliver O2 to tissues); there is a need for increased oxygen delivery.
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Hypoventilation results in a relative _________ in Pa CO2, leading to an increase in _______, _________ arterial pH, and ___________.
increase; [H+]; lower; respiratory acidosis
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Hyperventilation results in a relative _________ in Pa CO2, leading to a decrease in _______, _________ arterial pH, and ___________.
decrease; [H+]; increased; respiratory alkalosis
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In exercising muscle, more O2 is consumed, therefore...
tissue PO2 falls, increasing the diffusion gradient from blood to tissue and increasing the dissociation of Hb-O2.
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What is the difference b/w hypoxemia and hypoxia?
hypoxemia is low Pa O2, while hypoxia is O2 deficiency at the tissue level; hypoxemia is a cause of hypoxia, but it si not the only cause
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What are causes of hypoxia? (4)
hypoxemia, anemia, ischemia, histotoxicity
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What is histotoxicity?
inability of tissue to utilize O2, even though O2 is being delivered at a normal rate (ex. cyanide poisoning, Vit B deficiency)
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What are causes of hypoxemia? (5)
low atmospheric or inspired oxygen (fire, improper anesthesia), hypoventilation, impaired diffusion, vascular shunt, V/Q mismatch (most common)
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What are potential causes of hypoventilation, leading to hypoxemia? (3)
defects i neurologic mechanisms of ventilation, thoracic cage abnormalities, upper airway obstruction
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With hypoxemia due to hypoventilation, reduced Pa O2 is ALWAYS accompanied by ________.
elevated Pa CO2
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With hypoxemia caused by a vascular shunt, _________ does not increase because...
Pa CO2; hypoxemia stimulates increased ventilation.
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What are the effects of severe tissue hypoxia?
cell death
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Mild hypoxia results in... (2)
depressed mental activity (maybe coma), reduced work capacity of muscles
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O2 therapy for hypoxia is effective for... (3)
atmospheric hypoxia (high altitude), hypoventilation due to increased airway resistance, hypoventilation due to pulmonary edema
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O2 therapy for hypoxia is ineffective for... (2)
hypoxia caused by reduced O2 transport by blood or resulting from inadequate tissue O2 utilization
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What are the compensatory mechanisms for acclimation to high altitude? (3)
peripheral chemoreceptors stimulate ventilation (at low PaO2), kidneys release erythropoietin in response to hypoxemia, increased DPG synthesis (reduced affinity b/w Hb and O2)
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Hypercapnia usually only occurs in association with hypoxia caused by... (2)
hypoventilation or circulatory deficiency.
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Hypercapnia is NOT associated with...
reduce atmospheric oxygen, anemia, poisoning of oxidative enzymes (b/c no CO2 even being produced in these circumstances), increased alveolar perfusion distance (CO2 is still diffusible enough to equilibrate)
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What are clinical signs of respiratory compromise? (11)
lethargy/exercise intolerance, increased f or VT, altered posture, open-mouthed breathing, cyanosis, noisy breathing, excessive nasal secretions, loss of symmetry, cough, sneeze, fever
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