Consuming liquid or food lowers oxygen to the head...
When we eat food, our whole digestive system becomes very active and the cells of this system require more oxygen for optimal work.
So the blood vessels supplying the abdomen become dilated to increase blood flow/oxygen to the stomach...
and consequently, the blood and oxygen supply to limbs and the head become constricted to help with this process.
The heart also starts beating faster to ensure ideal supply of blood to all the organs.