Gases in the bloodstream

Alveolar ventilation without perfusion (Q̇= 0)

Air goes in and out but no O2 and CO2 is exchanged because no blood is present

High V/Q mismatch

What is dead space ventilation

Perfusion (Q̇) without alveolar ventilation (V̇A= 0)

Arterial blood does not come into contact with alveolus containing fresh gas

Over time, build up of CO2 and reduction of O2

Low V/Q mismatch

What is a right to left shunt

V̇A/Q̇ mismatch

What is the most common cause of hypoxemia

Perfusion is better in lower portions of lungs (blood sinks)

Ventilation better in upper portions of lung (air rises)

Rate of change in Q̇ with lung height is greater than rate of change of V̇A

V̇A/Q̇ in the upper lung > V̇A/Q̇ in the lower lung

What are the regional differences in V̇A/Q̇ matching

Hypoxic vasoconstriction

Hypocapnic bronchoconstriction

What 2 local reflexes in the lungs limit V̇A/Q̇ mismatch

Hypoventilation reduces PAO2

PAO2 < 73 mmHg causes vasoconstriction of arteriole to that alveolus

Increased resistance= reduced blood flow to under-ventilated alveoli

Redirects blood flow towards better-ventilated regions of the lung

What is hypoxic vasoconstriction

Airways exposed to gas with abnormally low PACO2 (CO2 not being removed)

No perfusion occurring= no movement of CO2 from blood to alveoli= decreased PACO2

Decreased airway PCO2 induces local bronchoconstriction

Increases local airway resistance

Redirects air flow towards better-perfused lung regions

What is hypocapnic bronchoconstriction

Intrapulmonary: blood is traveling by the alveolus but no O2 is being picked up because no ventilation is occurring

Extrapulmonary: vessel is not traveling in any area where it can exchange gas. No opportunity to pick up O2

What is the difference between an intrapulmonary shunt and an extrapulmonary shunt

Combined with Hb (% oxyHb)

Dissolved in plasma (PO2)

What are the 2 forms in which O2 is carried in blood

Each Hb has 4 protein subunits

Each subunit has a globin and a HEME group

Each heme has one Iron ion

Each Fe can bind 1 O2 molecule

Each Hb molecule can carry 4 O2 molecules

Can exist as:

Oxyhemoglobin (HbO2)

Deoxyhemoglobin (Hb)

What is the structure of Hemoglobin

Increasing PaO2 increases combination of O2 with Hb

How does PaO2 impact Hb saturation

Binding of O2 to heme increases Hb affinity for O2 (will bind others more readily)

Steep between 10-60 and flat between 70-100

O2 and Hb rapidly combine/dissociate at low PO2 (90% Hb saturation at 60 mmHg PO2)

Above PO2 of 60 mmHg, further increases in PO2 produce only a small increase in Hb saturation

What is the O2-Hb dissociation curve

Hb saturation does not decline unless PAO2< 60 mmHg

At what point will impaired alveolar ventilation impact hemoglobin saturation

If lungs are impaired by disease: supplemental O2 will increase Hb saturation and CaO2

Normal mammal: no effect. Hb already fully saturated under normal conditions

In what situations will supplemental O2 assist in Hb saturation

75% Hb saturation

PaO2 of 40 mmHg

What is the normal hemoglobin saturation of pulmonary arterial blood

Pulmonary PaO2= 40 mmHg

PAO2= 105 mmHg

O2 diffuses from alveolar air into plasma, and instead of raising PaO2 immediately, binds to Hb

Lack of change in PaO2 maintains diffusion gradient between alveolus and capillary so that more O2 can be transferred into the bloodstream

How does Hb act as a sink during O2 exchange in the alveolus

Systemic PaO2= 95 mmHg

Tissue PO2= 75 mmHg

O2 diffuses from plasma to IF

PaO2 decreasing causes O2 to diffuse from RBC to plasma-> promotes dissociation of HbO2

O2 from Hb-> plasma-> IF-> cells

Hb in blood leaving tissue capillaries still 75% saturated at rest

What is the process of Hb unloading at tissues

Blood PCO2

Blood [H+]

Blood temperature

2,3-diphosphoglycerate (DPG)

Increase in any shifts curve to right-> Hb has reduced affinity for O2

What factors affect Hb saturation at any given PO2

Solubility in plasma

PO2 (capillary, arterial, venous)

What 2 factors determine the amount of O2 dissolved in plasma

0.3 mL/100mL plasma

What is the amount of dissolved O2 at PaO2 of 100 mmHg

Amount of hemoglobin in blood

Carrying capacity of Hb

PO2 in the blood

Shape of the O2-Hb dissociation curve

What 4 factors determine the total amount of O2 in the blood

Carrying capacity of Hb for O2

Concentration of Hb in blood

% Hb saturation (depends on PO2)

What 3 factors determine the amount of O2 combined with Hb in the blood

98%

What is the % Hb saturation at PaO2 of 100 mmHg

20 mL O2/ 100mL blood

What is the normal CaO2 (total amount of O2 in arterial blood) at PaO2 of 100 mmHg

CaO2 (total amount of oxygen in arterial blood

Cardiac output (so also depends on HR, SV, and tissue metabolism)

DaO2= CaO2 x CO

DaO2 must be > 100mL/O2/min at rest to sustain life

What determines delivery of O2 to tissues

There is always a greater flow of CO2 across the alveolus despite a smaller partial pressure difference between blood and alveolus

Why is alveolar ventilation so important for elimination of CO2

Dissolved in plasma: 10%

Converted to bicarbonate: 60%

As Carbamino Hb: 30%

What are the 3 forms in which CO2 is transported in the blood

As O2 saturation of Hb increases, CO2 content of blood decreases for the same PCO2

Compete for binding to Hb. If O2 is binding, CO2 can't. So for any given PaCO2, total CO2 carrying capacity is reduced in presence of increased PaO2

How does Hb saturation by O2 relate to CO2 content of the blood

Maximizes offloading of O2 from HbO2 in tissues, and offloading of CO2 from carbaminoHb in lungs

Exposure to air-> HbO2 increases-> CO2 cannot bind to Hb-> CO2 released from carbaminoHb into plasma-> PcCO2 increases-> diffusion gradient increases-> CO2 moves into alveolus

Increased PCO2 in tissues from metabolism-> increased formation of carbaminoHb-> curve shifts right-> O2 unloaded from HbO2-> PaO2 increases-> diffusion gradient increases-> O2 moves into cells

How does competitive binding of Hb by O2 and CO2 assist in transfer of gases between the lungs and tissues

More O2 is consumed in tissues

Tissue PO2 falls

Diffusion gradient from blood to tissue increases

Increased dissociation of HbO2

Muscle can extract almost all O2 delivered to it in blood

How does exercise affect O2 exchange in tissues