Final answer:
Normal pulmonary shunting occurs due to bronchial circulation and thebesian veins, not from pulmonary veins or alveolar capillaries, which are typically involved in efficient gas exchange where blood gets oxygenated before entering the systemic circulation.
Step-by-step explanation:
The normal contributors to pulmonary shunting or anatomic shunt include bronchial circulation and thebesian veins. Unlike oxygen-rich blood vessels, bronchial circulation can contribute to anatomic shunting by providing venous blood that can bypass the gas exchange process occurring in the alveoli. Similarly, thebesian veins drain directly into the left side of the heart, adding a small volume of deoxygenated blood to the oxygenated blood coming from the pulmonary veins. It is not typically pulmonary veins or alveolar capillaries that contribute to shunting under normal conditions because they are generally part of the efficient gas exchange process where oxygen is picked up and carbon dioxide is released.
Ordinarily, blood in the pulmonary artery is deoxygenated and is transported to the lungs where it receives oxygen. After this exchange, the now oxygen-rich blood travels back to the heart via the pulmonary veins to be circulated to the rest of the body. The oxygenated blood flows into the aorta to be distributed systemically. Therefore, the contributors to a normal pulmonary shunt would be associated with systemic venous return that bypasses alveolar oxygenation, primarily through bronchial circulation and not the normally functioning alveolar or pulmonary vascular system.