The Cardiovascular System – Veins

By: Alere Staff

We have learned in this continuing series that arteries carry oxygen and nutrient rich blood, delivering it to our organs and cells. How does the blood get back to the heart?  Veins are the vessels that carry the “used” blood back to the heart. The smaller veins empty into the largest veins, the Inferior and Superior Vena Cava. Blood then flows into the top chamber on the right side of the heart. It then gets pumped through the lungs to pick up fresh oxygen and nutrients to complete the cardiac cycle.

While arteries function as a high pressure system, our veins are a low pressure system. Veins are flexible and can expand when needed to meet the needs of circulation. Venous blood flow relies on gravity, muscle contraction and a series of one way valves to move blood. Blood clots most commonly occur in veins due to the low pressure and the ability for blood to accumulate in the veins during time of inactivity, especially in the deep veins. Veins need movement and muscle contraction to assist the flow back to the heart. 

Fun Fact: All blood is red whether it is full of oxygen or not. So why do our veins appear blue? Our skin and subcutaneous fat absorbs blue light all the way to the veins therefore this is the only color that is reflected back. Other colors in the light spectrum get absorbed before it gets to the level of the veins. Veins are closer to the surface and have thinner walls than arteries. If arteries were closer to the surface, they would appear blue in color as well.

Interested in learning more about the Cardiovascular System? Check out the other articles in our series:


  1. 2013. Function of Veins and Arteries in the Body. Retrieved May 20, 2015 from website:
  2. Bailey, R. Vein. About Education. Retrieved June 17, 2015 from website:
  3. Helmenstine, Anne Marie. Why Veins Look Blue. About Education. Retrieved June 17, 2015 from website: