Explain the physiological mechanisms behind blood flow redistribution during exercise and stress.

Redistribution of blood flow: This refers to the body changing where blood goes.

• To muscles: During exercise, blood flow to muscles increases significantly (from 20% to 85% of Cardiac Output - CO). This happens because active muscles produce substances like adenosine, CO₂, K⁺, and lactate, which cause blood vessels to widen (metabolic vasodilation), allowing more blood to reach them.
• To viscera/kidneys: At the same time, blood flow to internal organs like the intestines and kidneys decreases sharply (from 30% to 5%). This is due to the sympathetic nervous system causing blood vessels in these areas to narrow (sympathetic vasoconstriction), diverting blood away from less active organs.
• To skin: Initially, blood flow to the skin decreases to prioritize active muscles. However, as the body heats up from activity, blood flow to the skin increases to help release heat and regulate body temperature (thermoregulation).

Venoconstriction: This is when veins narrow. Veins act as a large blood reservoir. When they constrict, they push blood stored in them back towards the heart, which helps increase the amount of blood the heart can pump out.

Muscle pump: This mechanism involves skeletal muscles. When muscles contract, they squeeze the veins running through them. This squeezing action helps push blood in the veins back towards the heart, aiding venous return and preventing blood from pooling in the limbs.