Mixing of venous blood with arterial blood (a right-to-left shunt) occurs normally because of which of the following? 1. Congenital cardiac defects 2. Bronchial venous drainage 3. Thebesian venous drainage in the heart

a. 1, 2, and 3
b. 2 and 3 only
c. 1 only
d. 2 only

ANS: B
Because the thebesian veins bypass, or shunt, around the pulmonary circulation, this phenomenon is called an anatomical shunt. When combined with a similar bypass in the bronchial circulation (see Chapter 7), these normal anatomical shunts account for approximately 2% to 3% of the total cardiac output.