Veins Without Valves

Generally veins have valves. This is to prevent the backward flow of blood within veins. Arteries carry blood from the heart to peripheral tissues. At tissue levels, the arteries branch out and finally give rise to capillaries. The capillaries then unit and forms the veins. Veins carry blood from peripheral tissues to the heart. As the vessels branch out in the above manner, the pressure within those vessels gradually decline. When it comes to the capillaries, the pressure is very low compared to that of major vessels such as the aorta. The pressure within the veins is even more less. Therefore, there is less force to pump blood back to the heart. 

Usually, the body takes the help of muscle contraction to pump blood up the veins. So, when the muscles contract, they squeeze blood up in the veins. When this happens, there should be mechanism to prevent blood from going back. This is where the valves come in to play. They prevent backward flow of blood. 

Not all the veins in the body contain valves. There are some veins that do not have veins. They are,

• The pulmonary veins
• Superior vena cave
• Left and right bracheocephalic trunks
• Azygos system of veins