A.Pressures and cardiac output into the pulmonary circulation

1.  Pressures

  • are much lower in the pulmonary circulation than in the systemic circulation.
  • For example, pulmonary arterial pressure in 15 mm Hg (compared with aortic pressure of 100 mm Hg).

2. Resistance

It is also much lower in the pulmonary circulation than in the systemic circulation.

3.  The cardiac output of the right ventricle

  • is pulmonary blood flow.
  • is equal to the cardiac output of the left ventricle.

Although pressures in the pulmonary circulation are low, they are sufficient to pump the cardiac output because the resistance of the pulmonary circulation is proportionately low.

B.  Distribution of the pulmonary bold flow.

  • When a person is supine, blood flow is nearly uniform throughout the lung. 
  • When an individual is standing, blood flow is unevenly circulated because of the impact of gravity. Blood flow is lowest at the zenith of the lung (zone 1) and highest at the base of the lung (zone 3).

1.  Zone 1__blood flow is lowest

  • Alveolar pressure > arterial pressure > venous pressure.
    The high alveolar pressure may lessen the capillaries and diminish blood flow in zone 1. This situation can happen if arterial blood pressure is declined as a result of hemorrhage or if alveolar pressure is increased because of positive pressure ventilation

2.  Zone 2__blood flow is medium.

  • Arterial pressure > alveolar pressure > venous pressure.
  • Moving down the lung, arterial pressure progressively increases because of gravitational effects on arterial pressure.
  • Arterial pressure is greater than alveolar pressure in zone 2, and blood flow is driven by the difference between arterial pressure and alveolar pressure.

3.  Zone 3__blood flow is highest.

  • Arterial pressure > venous pressure > alveolar pressure.
  • moving down toward the base of the lung, arterial pressure is highest because of gravitational effects, and venous pressure finally increases to the point where it exceeds alveolar pressure.
  • In zone 3, blood flow is driven by the difference between arterial and venous pressure as in most vascular beds.

C.  Regulation of pulmonary blood flow __hyoxic vasoconstriction

  • In the lungs, hypoxia causes vasoconstriction.
  • This response is the opposite of that in other organs, where hypoxic causes vasodilation. 
  • Physiologically, this impact is significant because local vasoconstriction redirects blood away from ineffectively ventilated, hypoxic regions of the lung and toward well-ventilated regions.
  • Fetal pulmonary vascular resistance is very high because of generalized hypoxic vasoconstriction; as result blood flow through the fetal lungs is low, with the first breath, the alveoli of the neonate are oxygenated, pulmonary vascular resistance decreases, and pulmonary blood flow increases and becomes equal to cardiac output (as occurs in the adult).

D.  Shunts

1.   Right-to-left shunts

  • Ordinarily, occur to a small extent because of 2% of the cardiac output bypasses the lungs. Maybe as great as 50% of cardiac output in certain congenital abnormalities.
  • are seen is tetralogy of Fallot.
  • always result in a decrease in arterial PO₂ because of the admixture of venous blood with arterial blood.
  • The magnitude of a right-to-left shunt can be evaluated by having the patient inhale 100%
  •   O₂ and measuring the degree of dilution of oxygenated arterial blood by nonoxygenated shunted (venous) blood.

2.  Left-to-right shunts

  • are more common than are right-to-left shunts because pressures are higher on the left side of the heart.
  • are usually caused by congenital abnormalities (e.g., patent ductus arteriosus) or traumatic injury.
  • do not result in a decrease in arterial PO₂. Instead, PO will be elevated on the right side of the heart because there has been an admixture of arterial blood with venous blood.


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