33 - Blood Flow in the Lungs and Ventilation-Perfusion Relationship

Question 1

1) Understand the characteristics of pulmonary blood vessels

Answer 1

• Circulation:
o Right ventricle Pulmonary arteries  pulmonary capillaries  pulmonary veins

• Very compliant vessels (they have to accommodate for the entire Cardiac output)

• Pulmonary Veins
o High in number and thin walled
o Because of this they can either ↑ or ↓ their capacitance to provide constant left ventricular output in the face of variable pulmonary arterial flow.

• Capillaries
o Have some closed/compressed that can be open during exercise and can enlarge as internal pressure rises.
o Left heart failure can lead to lungs fill with blood.

• Larger pulmonary arteries/veins have smooth muscle which regulate diameter and resistance to blood flow.

Question 2

2) Understand the bronchial circulation and its contribution to systemic blood PO2

Bronchial ARTERIES

Answer 2

• Bronchial arteries
o Provide a source of oxygenated blood to the lungs.
o It receives about 1-2% of the cardiac output.
o Cystic fibrosis
 Bronchial arteries increase in size and about 10-20% of CO can go to lungs.

Question 3

2) Understand the bronchial circulation and its contribution to systemic blood PO2

Bronchial VEINS

Answer 3

• Bronchial veins
o About 1/3 of the blood returns to the right atrium through bronchial veins
o The remainder drains into the left atrium via pulmonary veins = Physiological shunt of blood
 This is what decreases systemic PO2 from 104  95 mm Hg

Question 4

3) Describe hypoxic vasoconstriction and its implication for pulmonary physiology. Effect of long-term hypoxic pulmonary vasoconstriction

Answer 4

• Hypoxic vasoconstriction:
o ↓ in alveolar pressure results in pulmonary vasoconstriction around that particular alveoli
o This is the most important factor for vasoconstriction.
o Increased resistance comes from constriction of small precapillary vessels.
o This is unique to the lungs. (In other systemic areas, hypoxia often leads to vasodilation).
o This helps match perfusion to ventilation.

• Long-term:
o Persistent hypoxic vasoconstriction and associated vascular remodeling are the main mechanisms for sustained pulmonary arterial hypertension.

Question 5

4) List the factors that control the dynamics of fluids between capillaries, interstitium, and alveoli. Changes that lead to pulmonary edema.

Answer 5

• Primary capillaries
o Relatively leaky (colloid osmotic pressure 2x peripheral tissue)

• Pulmonary capillary pressure
o Low in comparison to the periphery (systemic capillaries)

• Interstitial fluid pressure
o Also low comparably to the rest of the body

• If there is positive pressure in the interstitial layer, there will be dumping of fluid into alveoli because alveolar walls are so thin.

• Lymphatic system
o Most fluid that goes into the interstitium is pumped out by the lymphatic system, but some of it is evaporated thru the alveoli

• Pulmonary edema
o Any factor that increases fluid filtration out of the pulmonary capillaries or that impedes pulmonary lymphatic drainage.

Question 6

5) Understand the concept of ventilation-perfusion matching and the changes in V/Q, including dead space (pulmonary embolism, high, low V/Q, left-to-right, right-to-left shunt

Answer 6

• V/Q on average is 0.8. (V = ventilation, Q = perfusion)

* In an upright person, both V and Q fall from base to apex, but perfusion falls faster.

Question 7

Mismatches - dead space

Answer 7

o Dead Space: V/Q = ∞
 Ventilation occurs but not perfusion.
 This can happen in a pulmonary embolism where blood flow is occluded to a certain region of the lung
 This is likely coming from thrombi in the leg or a vein
 Clinical pictures  chest pain, hypoxemia, hypotension

Question 8

Mismatches - High V/Q

Answer 8

o High V/Q
 These regions have high ventilation compared to perfusion.
 There is some blood flow but very little.
 Pulmonary capillary blood here has a high pressure of oxygen and low pressure of carbon dioxide.

Question 9

Mismatches - Low V/Q

Answer 9

o Low V/Q
 These regions have low ventilation but high perfusion.
 Pulmonary capillary blood here has low oxygen pressure and high carbon dioxide pressure.

Question 10

Shunts

Answer 10

V/Q = 0

Question 11

Physiologic Shunt:

Answer 11

• 2% of cardiac output bypasses the alveoli.
• Part of this is the bronchial blood flow, serving the metabolic function of the bronchi.
• Other is the small amount of coronary blood flow that drains directly into the left ventricle thru the thebesian veins.
• Arterial oxygen pressure less than alveolar oxygen pressure here?

Question 12

Right-to-left shunt:

Answer 12

• Defect in wall between the ventricles.
• Hypoxemia always occurs with this because a lot of the cardiac output is not delivered to the lungs.
• This cannot be corrected by having a person breath a high O2.

Question 13

Left-to-right shunt:

Answer 13

• More common and do not cause hypoxemia.
• These are due to patent ductus arteriosis and traumatic injury.
• Pulmonary blood flow becomes higher than systemic blood flow.
• The pressure of oxygenated blood in the right side of the heart will be elevated.