Lecture 27

Question 1

explain how gas in solution depends on solubility and partial pressure

Answer 1

• at equilibrium the partial pressures in air and solution are equal
• concentrations in air and solution are unlikely to be equal
• CO2 is 20x more soluble than O2
  -> capacity of plasma for O2 is very low

Question 2

explain gas transport in the blood for O2 (hemoglobin)

Answer 2

• total O2 in the blood = amount dissolved in plasma + amount bound to hemoglobin
• hemoglobin -> found in RBC
  -> reversibly binds O2
  -> each Hb molecule binds 4 O2 molecules
• Hb bound to O2 (HbO2) = oxyhemoglobin
  Hb + O2 -> HbO2

Question 3

explain how hemoglobin increases bloods O2 carrying capacity

Answer 3

• oxygen transport in blood without hemoglobin (alveolar PO2 = arterial PO2)
• oxygen dissolves in plasma’
• O2 content of RBC = O
• Oxygen transport at normal PO2 in blood with hemoglobin
• RBC with Hb are carrying 98% of their maximum load of oxygen
• O2 content of RBC = 197 mL O2/L blood
• increase in total O2 carrying capacity

Question 4

explain the O2-Hb dissociation curve

Answer 4

• plateau portion = 60-100 mmHg (alveoli)
• steep portion = 0-40 mmHg (resting cell)
• x axis = PO2 (mm Hg)
• y axis = hemoglobin saturation

Question 5

why is the O2-Hb dissociation curve sigmoidal

Answer 5

• cooperative binding of O2
• hemoglobin has 4 binding sites for O2
• if O2 binding to each subunit was independent the O2-Hb dissociation curve would be hyperbolic
• since cooperative, the binding of O2 molecules increases binding affinity of the remaining sites
• in the steep region, a small change in PO2 can result in large change in %Hb saturation

Question 6

P50 is the oxygen partial pressure at which hemoglobin is 50% saturated with O2, as the P50 increases hemoglobin’s affinity for oxygen______ and why

Answer 6

• decreases
• Higher P50 = need more oxygen to reach 50% saturation → affinity is lower.

Question 7

explain the effect of changing pH for oxygen binding by Hb

Answer 7

• low pH reduces O2 carrying capacity of Hb
• O2 dissociates more readily at tissues where pH is lower
• low pH = curve shifts to the right (decreases affinity)

Question 8

explain the effect of changing PCO2 for oxygen binding by Hb

Answer 8

• high PCO2 shifts O2 carrying capacity of Hb to the right (decreased affinity)
• O2 dissociates more readily at tissues where PCO2 is higher
• less saturation at the tissues = release more O2

Question 9

explain the transport of O2 in the blood

Answer 9

• total blood O2 = dissolved O2 + HbO2
• in lungs PO2 is high
  -> drives O2 exchange into plasma
  -> high plasma PO2 drives O2 binding to Hb
• in tissues PO2 is low
  -> drives O2 exchange out of plasma
  -> low plasma PO2 drives O2 release from Hb

Question 10

explain CO2 transport in blood and the 3 mechanisms

Answer 10

• CO2 + Hb = not the same binding site
  -> allosteric binding site
• 7% transported as dissolved gas in plasma
• 23% transported as HbCO2
• 70% transported as bicarbonate dissolved in plasma
  CO2 + H2O -> H2CO3 -> H+ + HCO3-
  -> carbonic anhydrase
• normally H+ is buffered by Hb in red blood cells
• excess H+ present = respiratory acidosis

Question 11

how are HCO3- levels regulated

Answer 11

• antiporter
• HCO3- out to plasma (70%), Cl- into RBC
• HCO3- in from plasma, Cl- out from RBC

Question 12

what drives regulation of ventilation

Answer 12

• emotions and voluntary control
• CO2
• O2 and pH

Question 13

explain chemoreceptors, what they do, where they are located

Answer 13

• sensory receptors convert chemical signals to action potentials
• central chemoreceptors located in the medulla
  -> increased activity in response to elevated PCO2
  (results in increased rate and depth of respiration)
• peripheral chemoreceptors located in the carotid sinus and aortic arch
  -> increased activity in response to elevated PCO2 and [H+] or decreased PO2
  -> afferent signals back to respiratory control center of medulla oblongata
  (results in increased rate and depth of respiration)

Question 14

what is the pathway for regulation of ventilation

Answer 14

• increased plasma, PCO2, or H+ or decreased PO2 = carotid and aortic chemoreceptors -> sensory neuron -> respiratory control center in medulla oblongata -> increased AP in somatic motor neurons -> muscles of ventilation -> increased rate and depth of breathing -> decreased plasma PCO2
• increased plasma PCO2 -> central chemoreceptors - then the same