Calf Muscle..

The term "second heart" refers to the calf muscles, which act as a pump to return deoxygenated blood from the lower legs to the heart and lungs.

This calf muscle pump is vital for proper circulation, lymphatic drainage, and even helps with blood sugar regulation. 

It works by contracting and expanding, pushing blood up through one-way valves in the veins.   

Circulation:  
The primary function of the calf muscle pump is to assist the main heart by

•  moving blood against gravity from the lower legs back to the heart. 
• The calf pump works with one-way venous valves that open to allow blood to move up and close to prevent backflow, ensuring blood moves in the right direction.  
• When the calf muscles contract, they also stimulate lymphatic vessels, which helps drain waste and improves the immune system.