The term 'main veins' refers to the blood vessels that carry deoxygenated blood from the body parts back to the heart to be pumped into the lungs for oxygenation. However, the phrase "main veins" can be replaced with a number of synonyms that refer to the same concept such as the "large veins", "major veins" or "principal veins". Other possible alternatives include "primary vessels", "crucial conduits" and "key pathways". The use of such synonyms can help in avoiding repetition and enhance the writing experience. It can also add an extra layer of precision to any medical or biological writing.