When a person is standing, blood flow in the lungs is highest at which zone?

In the context of pulmonary blood flow and the concept of lung zones, the correct answer is Zone 3. This zone is characterized by the highest blood flow due to the gravitational effects when a person is standing.

In Zone 3, the alveolar pressure is lower than both the arterial and venous pressures, which allows for continuous blood flow through the pulmonary capillaries. This results in optimal gas exchange because the capillaries are fully perfused, facilitating the efficient transfer of oxygen and carbon dioxide.

The distribution of blood flow in the lungs is influenced significantly by gravity. When a person is upright, blood preferentially collects in the lower parts of the lungs (Zone 3), where hydrostatic pressure is greatest. This increased pressure enhances blood flow through these areas, compared to the upper regions of the lungs (Zone 1), where the pressures can effectively occlude or diminish blood flow.

Understanding these zones is crucial for comprehending how the physiology of lung perfusion operates under varying conditions, such as posture, and highlights the importance of gravity in respiratory function.