The sexual hormones....????
nope the blood veins :)
or blood vessels

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The major parts of circulatory system are heart and blood vessels. 
the blood vessels are of three types-1. arteries 
2. veins 
3. capillaries 
the arteries carry blood mainly oxygenated blood from heart to different parts of the body. 
the veins carry deoxygenated blood from all parts of the body to heart. 
capillaries connect the arteries and veins...they are thin walled and allow the exchange of gases to occur across their walls.
The heart, blood, and the blood vessels