Final answer:
Experiencing vasoconstriction in the legs due to cold increases peripheral resistance, which typically causes blood pressure to rise, as the heart must work harder to circulate blood through the narrower pathways.
Step-by-step explanation:
When you enter a cold lake up to your knees and experience vasoconstriction due to the activation of the sympathetic nervous system, your blood pressure is likely to rise. This is because vasoconstriction, the narrowing of the blood vessels, increases peripheral resistance, which in turn causes an increase in blood pressure.
The role of peripheral resistance is to influence blood pressure and blood flow within the body. When the blood vessels in your legs constrict, less blood is able to flow through the constricted space at any given time, causing the heart to work harder to pump blood through these narrowed vessels, thereby increasing the blood pressure.