How Do Veins Work? Understanding Leg Vein Anatomy and Valve Function

What Do Veins Do?

Veins are blood vessels responsible for returning blood from the body's tissues back to the heart. After oxygen and nutrients have been delivered by the arteries, veins collect the oxygen-depleted blood so that it can be pumped to the lungs, where it is reoxygenated before circulating around the body again.

Unlike arteries, which carry blood under high pressure directly from the heart, veins operate under much lower pressure. In the legs, this presents a unique challenge — blood must travel upwards against gravity over a considerable distance. To overcome this, the venous system relies on three key components working together:

• Healthy one-way valves, which prevent blood from flowing backwards.
• The calf muscle pump, often called the "second heart", which contracts during walking and squeezes blood upwards through the deep veins.
• Normal vein walls, which provide structural support and maintain efficient blood flow.

The Three Vein Systems in the Legs

The veins in the legs are organised into three interconnected systems:

• Deep veins lie within the muscles and carry approximately 90% of the blood returning from the legs. These veins are the main pathway back to the heart and are responsible for most of the venous circulation.
• Superficial veins lie just beneath the skin and carry the remaining 10% of blood. These are the veins that may become enlarged and visible as varicose veins when their valves fail.
• Perforator veins connect the superficial veins to the deep veins. Their valves ensure that blood flows in one direction — from the superficial system into the deep system — where it can be efficiently pumped back towards the heart.

In a healthy leg, blood flows from the superficial veins through the perforator veins into the deep veins, before returning to the heart. When the valves within the superficial or perforator veins become incompetent, blood flows backwards (venous reflux), increasing pressure within the superficial veins. Over time, this sustained pressure causes the veins to enlarge, become tortuous and eventually develop into varicose veins.

Understanding how the normal venous system works helps explain why modern treatment focuses on correcting the underlying venous reflux rather than simply removing the visible veins. A duplex ultrasound scan allows the vascular specialist to identify exactly which veins are affected and determine the most appropriate treatment strategy.

How Do Vein Valves Work?

The veins in your legs contain tiny one-way valves that ensure blood flows in the correct direction — upwards towards the heart. These delicate valves open to allow blood to pass through and close immediately afterwards to prevent it from flowing backwards under the force of gravity.

Every time you walk, your calf muscles contract and compress the deep veins within the leg. This action, known as the calf muscle pump, propels blood upwards towards the heart. As blood is pushed forward, the vein valves open. When the muscles relax, the valves close, preventing blood from falling back down the leg. This highly efficient system repeats with every step you take, allowing blood to circulate effectively despite gravity.

What Happens When Valves Fail?

Varicose veins develop when these valves become incompetent, meaning they no longer close completely. Instead of flowing only towards the heart, some blood leaks backwards into the vein — a process known as venous reflux. This reflux increases pressure within the vein (venous hypertension), causing the vein wall to gradually stretch, enlarge and become tortuous. Over time, the increased pressure may also affect the surrounding skin and tissues, leading to symptoms such as aching, heaviness, swelling and, in advanced cases, skin changes or venous leg ulcers.

Contrary to popular belief, the visible bulging vein is not the primary problem — it is the result of the underlying valve failure. This is why simply removing or injecting the visible veins without first identifying the source of reflux may result in incomplete treatment or early recurrence.

How Is Valve Function Assessed?

A duplex ultrasound scan is the gold standard investigation for assessing vein valve function. Performed while you are standing, it allows the vascular specialist to identify exactly which valves are leaking, map the pattern of venous reflux, and determine the most appropriate treatment. Modern treatments such as endovenous laser ablation (EVLA), radiofrequency ablation (RFA), mechanochemical ablation (MOCA) and cyanoacrylate closure (VenaSeal™) are all designed to treat the underlying reflux rather than simply improving the appearance of the veins.

Understanding how vein valves work — and what happens when they fail — explains why modern varicose vein treatment focuses on correcting the underlying cause of the disease, providing durable symptom relief and reducing the risk of future complications.