Final answer:
In a low airflow condition, the low partial pressure of O2 causes blood vessels to constrict, reducing blood flow. Vasoconstriction adjusts blood pressure and flow, particularly during stress or exposure to cold, as seen in penguins.
Step-by-step explanation:
When there is little airflow, or a shunt-like condition, the low partial pressure of O2 will act on smooth muscle to constrict blood vessels, which will decrease blood flow. This process, known as vasoconstriction, involves the narrowing of the lumen of blood vessels and is primarily controlled by the autonomic nervous system in response to various stimuli. Vasoconstriction occurs to maintain adequate pressure in the vessels and to regulate the distribution of blood to areas where it is most needed. For example, in response to cold temperatures, penguins constrict the arterioles leading to their feet to significantly decrease blood flow and minimize heat loss.
During exercise or stress, arterioles may constrict in response to inadequate oxygenation or other factors. This can decrease the availability of oxygen to the tissues, hence why during exercise with a partially occluded artery, the blood flow may become insufficient. The body's response to low blood flow, depending on the circumstances, can either be relaxation of the smooth muscles to allow vessel dilation and increased blood flow, or contraction to prompt vasoconstriction and reduce blood flow.