The term 'Big Blood Vessels' refers to the aortae and aortas, which are two of the largest arteries in the human body. The aortae are the two main arteries that branch off from the heart and carry oxygen-rich blood to the rest of the body. The aortas are the two main veins that return oxygen-depleted blood back to the heart. Together, these two vessels form the circulatory system, which is responsible for transporting oxygen and nutrients throughout the body.