Blood Flow - Trachte

Question 1

What is the blood pressure in the pulmonary arteries?

Answer 1

LOW

(normally 25/8 mmHg)

(mean pressure ~15 mmHg)

Question 2

What conditions could significantly alter the pulmonary artery pressure?

Answer 2

• HTN
• CHF
• Increased altitude
• Anything that causes back of blood or deficient left heart function

Question 3

What is the resistance in the pulmonary circulation? Versus systemic resistence?

Answer 3

• Pulmonary Resistance = 2
  
  • 1/10 of systemic
• Systemic Resistance = 20

Question 4

How do alveolar and thoracic pressures impact blood flow?

Answer 4

• Pulmonary capillaries are completely surrounded by alveoli
  
  • therefore → they have alveolar pressure
• Pulmonary vessels are pulled open during inspiration → decreased intrathoracic pressure

Question 5

What condition radically changes the pressure surrounding the pulmonary capillaries?

Answer 5

Tension Pneumothorax

• crushes veins and prevents venous return
• positive pressure ventilation

Question 6

Why does pulmonary artery resistance decrease with an increase in pulmonary artery pressure?

Answer 6

• Recruitment of additional capillaries in conducting blood flow.
• Distention of capillaries conducting blood to allow more blood flow.
• Expansion of the lung also reduces resistance.
  *

Question 7

How do you calculate pulmonary blood flow using the Fick Principle?

Answer 7

• O₂ Consumption = CO (C_AO2 – C_VO2)
• Therefore,
  
  • Pulmonary blood flow = VO₂/(C_AO2 – C_VO2)

Question 8

What is the influence of gravity and alveolar pressure on blood flow?

Answer 8

• More pulmonary blood flow to the base of the lung than to the apex because of gravity
  
  • bottom of lung is closer to level of the heart
• Alveolar pressure greatest at the apex of the lung
  
  • decreases blood flow

Question 9

What are the three potential zones of perfusion in the lung?

Answer 9

• Zone 1: Apex
  
  • P_Alveolar > P_arterial > P_venous
  • No flow occurs in this region → no gas exchange
  • Alveolar dead space (physiologic dead space)
• Zone 2: Apical region
  
  • P_arterial > P_Alveolar > P_venous
  • Blood flow in this region is determined by pressure differential between arteries and Alveoli
  • Venous pressure does not influence flow
• Zone 3: Midregions - Base
  
  • P_arterial > P_venous > P_Alveolar
  • Flow is dependent on the arterial/venous pressure difference

Question 10

What is the influence of alveolar hypoxia on blood flow?

Answer 10

• Alveolar hypoxia → constricts blood vessels perfusing the hypoxic region of the lung
  
  • mechanism is independent of nerves

Question 11

What is the mechanism that causes hypoxic pulmonary vascular constriction?

Answer 11

• Low O₂ → alveoli → inhibits voltage gated K⁺ channels
  
  • traps K⁺ in cell → increased positive membrane potential (hypopolarization/depolarization)
  • reach threshold → causes action potential
  • vascular smooth muscle constriction!

Question 12

What is the impact of low hydrostatic pressure on fluid filtration?

Answer 12

• Fluid movement out of the pulmonary vasculature is controlled by Starling forces (15-25 = -10)
  
  • low hydrostatic pressure than in systemic circulations ⇒ favors reabsorption
  • this is good for keeping the lungs dry

Question 13

In what situations is there increased fluid movement out of the pulmonary vasculature?

Answer 13

• Increased hydrostatic pressure:
  
  • CHF
  • Pulmonary HTN
• Decreased oncotic pressure:
  
  • Liver failure

Question 14

What are the metabolic functions of the lung?

Answer 14

• Activates angiotensin I → angiotensin II
  
  • mediated by ACE
• Inactivates bradykinin
  
  • mediated by ACE
• Accumulates serotonin to remove it from the circulation
• Metabolizes prostaglandin E₁, E₂ and F_2-alpha
• Synthesizes both prostaglandins and leukotrienes
  
  • bronchoconstrictors
  • also removes leukotrienes from the circulation