Beachey: Respiratory Care Anatomy and Physiology, 2nd Edition

Answers to Concept Questions

Chapter 6: Pulmonary Blood Flow

6-1	The anatomical shunt through the bronchial venous circulation allows deoxygenated systemic venous blood to mix with and lower the PO2 of the blood flowing through pulmonary veins to the left ventricle and ultimately through the systemic arteries. If shunt were completely absent, pulmonary venous blood PO2 and ultimately systemic arterial blood PO2 would be identical to alveolar gas PO2. However, the normal presence of anatomical shunt causes systemic arterial PO2 to be lower than alveolar PO2, creating the normal PAO2  PaO2 difference.

6-2	A stenotic mitral valve impedes blood flow from the pulmonary veins to the left ventricle. This causes blood to dam up in the pulmonary veins and capillaries, raising their pressures along with the pulmonary capillary wedge pressure.

6-3	A pure increase in pulmonary vascular resistance (PVR) does not affect the left atrial pressure because the left atrium is downstream from the site of the resistance. Thus an increased PVR does not affect pulmonary capillary wedge pressure (PCWP). Because the pulmonary artery is upstream from the rest of the pulmonary vasculature, an increased PVR increases the pulmonary artery pressure (PAP). The difference between mean PAP and PCWP is a measure of PVR’s magnitude.

6-4	Heart failure (i.e., left ventricular “pump” failure) decreases the cardiac output. As a result, blood dams up in the left atrium and pulmonary circulation, increasing pulmonary vascular pressure and distending pulmonary capillaries.

6-5	Because nitric oxide synthase (NOS) fosters nitric oxide (NO) production and because NO dilates blood vessels, an NOS-inhibiting drug would have the opposite effect. In other words, an NOS inhibitor would promote vasoconstriction.

6-6	Regional alveolar hypoxia induces localized pulmonary vasoconstriction, which shows up on a perfusion scan of the lungs as a lack of blood flow in hypoxic lung regions. A pulmonary embolus blocks blood flow to certain lung regions, producing a similar perfusion scan. A simultaneous ventilation scan helps distinguish between these two conditions; reduced ventilation matched with reduced perfusion implies the presence of hypoxic pulmonary vasoconstriction.

6-7	High pulmonary capillary membrane permeability is the likely cause of pulmonary edema. A normal pulmonary capillary wedge pressure rules out hydrostatic edema, and a normal plasma protein concentration rules out osmotic pulmonary edema.