Gas Transport

Question 1

By what 2 methods is oxygen transported in the blood?

Answer 1

1. Bound to hemoglobin ( > 98%)
2. Dissolved in plasma ( < 2%)

Question 2

What is the key determinant of the hemoglobin oxygen saturation?

Answer 2

PaO2 –> the partial pressure of oxygen physically dissolved in the blood

Question 3

What is demonstrated by the Oxygen Dissociation Curve?

Answer 3

At a low oxygen pressure, Hb-O2 dissociates to drop off oxygen in tissue that needs it.

When oxygen pressure is high like in the lungs, Hb readily takes up O2 in order to transport it elsewhere.

Question 4

What is the importance of the plateau portion of the Oxy-Hb dissociation curve?

Answer 4

The plateau shows that we will maintain an adequate saturation of oxygen, despite a range of pressures of oxygen.

This is important because different altitudes have different oxygen levels.

Question 5

What are the Hb-O2 saturation levels at the following partial pressures of oxygen:

PO2 100

PO2 60

PO2 40

PO2 27.5

Answer 5

Question 6

What is Oxygen Delivery (DO2)?

Answer 6

Oxygen delivery:

Cardiac output x Oxygen content

DO2 = CO x CaO2

Oxygen uptake is VO2

Question 7

What occurs with a right shift of the Oxy-Hb dissociation curve?

What can cause a right shift?

Answer 7

Right shift:

• Lower Hb affinity for O2
• Facillitates unloading of O2
• Increased tissue O2 uptake

Causes: increased temperature, increased CO2, decreased pH, increased 2,3 DPG

Question 8

What occurs with a left shift of the Oxy-Hb dissociation curve?

What causes a left shift?

Answer 8

Left Shift:

• increased Hb affinity for O2
• less O2 unloading at tissues
• decreased tissue O2 uptake

Shifting left is better overall, and can occur due to respiratory alkalosis.

Question 10

What is carbon monoxide poisoning?

What happens to the PaO2 and O2 delivery?

Answer 10

CO binds to hemoglobin with 240X greater affinity than oxygen. So a small partial pressure of CO will outcompete a larger partial pressure of O2, causing asphyxiation.

CO also shifts the oxygen dissociation curve to the LEFT, which is bad because it means thebound O2 is not dropped off at the tissue.

The PaO2 remains the same, but the delivery of O2 (DO2) is decreased.

Question 11

How is carbon dioxide transported in the blood?

Answer 11

1. Carbamino compounds (~5%)
2. HCO3 (~90%)
3. Dissolved CO2 in plasma (~5%) - this is what PaCO2 measures

(CO2 is 20x more soluble than O2)

Question 12

How does the alveolar PaCO2 and PaO2 compare to the arterial PaO2 and PaCO2?

Answer 12

PAO2 does NOT equal PaO2 –> it’s the A-a gradient

However, PACO2 ~ PaCO2

Question 13

What is hypercapnia? What are it’s complications?

Answer 13

Hypercapnia is excessive carbon dioxide in the bloodstream, typically caused by inadequate respiration.

Hypercapnia can lead to:

• Life threatening acidosis
• Coma (CO2 narcosis)
• Hypoxemia

Question 14

What causes hypercapnia?

Answer 14

Central:

• CNS disease (stroke, trauma)
• Drugs (sedatives, narcotics)

Peripheral:

• Lung disease (severe VQ mismatch)
• Neuromuscular disease
• Chest wall (kyphoscoliosis, obesity, etc)

Question 15

What are the requirements for normal oxygen transfer to the blood?

Answer 15

Inspired air with adequate oxygen

Alveolus (ventilation)

Interstitium

Artery (perfusion)

Question 17

What is arterial hypoxemia?

Answer 17

Low partial pressure of oxygen in the arterial blood.

PaO2 < 60 mm Hg

Question 18

What 5 mechanisms can lead to arterial hypoxemia?

And do they have normal or increased A-a gradients?

Answer 18

1. Low inspired oxygen (normal) - usually due to decrease in barometric pressure (altitude)
2. Hypoventilation (normal)
3. Diffusion limitation (increased)
4. Shunt (increased)
5. V/Q mismatch (increased)

Question 19

What is the A-a gradient?

Answer 19

The difference between the alveolar and arterial partial pressure of O2.

It is normally between 8 and 15 mm Hg

Uses the alveolar O2 calculated from the Alveolar gas equation, and the measured arterial O2 from ABG.

It assesses the efficiency of gas exchange.