When an area of the body is injured, infected or exposed to an allergen, the immune system can respond by releasing histamine. This substance binds to receptors in the blood vessels that increase the permeability of these vessels, allowing fluids and white blood cells to flow out into the affected tissue. The increased permeability can allow more fluids to build up in the area than are normally present, resulting in swelling. A number of different conditions can contribute to swelling of the lower posterior portion of the leg, known as the calf.
Torn Calf Muscle
There are two separate layers, or compartments, containing the muscles of the calf. They are the superficial posterior compartment, which houses the gastrocnemius and soleus muscles, and the deep posterior compartment, containing the tibialis posterior and flexor muscles. A tear in any of the calf muscles due to trauma or overexertion can cause swelling as the immune system's inflammatory response releases fluids into the affected area. Pain and bruising can accompany the swelling, according to FamilyDoctor.org.
Deep Venous Thrombosis
Clotting in the blood vessels of the leg can result in swelling of the calf. This condition, called deep venous thrombosis, can be fatal if the blood clot breaks free and lodges in the lungs, according to FamilDoctor.org. Deep venous thrombosis is often associated with periods of prolonged inactivity, and is commonly seen in patients convalescing from hip fractures. Any signs of this life-threatening condition should immediately be reported to a physician.
Varicose Veins
The legs and feet are the most common location where varicose veins occur, according to MayoClinic.com. Though typically painless, this condition manifests as darkly colored veins that bulge from the skin and appear twisted. In instances where varicose veins in the calf do cause pain, it can be accompanied by swelling in the calf as well. Varicose veins are thought to be the result of an inability of the valves in the blood vessels to cope with the volume of blood that is passing through the veins.
Achilles Tendon Tear
The Achilles tendon is a tough, fibrous band that attaches the calf muscle to the heel. It is subject to injury when stretched too far, and can rupture partially or completely. This type of injury causes severe pain, according to Cedars-Sinai, and swelling frequently accompanies this pain. A person with an Achilles tendon muscle typically is unable to walk, and complete rupture makes it impossible to stand on tiptoe.
