The Anatomy of the Superior Vena Cava

Veins are tubular, hollow structures that form part of the circulatory system of the body; in most instances, veins carry deoxygenated blood toward the heart. The superior vena cava (SVC) is one of the two largest veins in the body and is considered one of the many systemic veins. Systemic veins are those carrying deoxygenated blood from various areas of the body to the heart and are vital to the function of the cardiovascular system.

Anatomy

The importance of the superior vena cava is its function in the cardiovascular system. More specifically, the superior vena cava transports unoxygenated blood from the upper half of the body to the right atrium of the heart. Next, the blood is circulated through the heart to the right ventricle, then pumped out of the heart through pulmonary arteries to the lungs to be oxygenated.

Why Is Blood Important?

To fully understand the significance of the superior vena cava, it’s important to know a little about what comprises blood and why it's so important. Blood:

• Transports oxygen to and from the lungs
• Removes waste products, such as carbon dioxide
• Acts as the medium that delivers nutrients from the digestive system
• Circulates disease-fighting entities (such as white blood cells) that are an important part of the immune system.

Without the continuous circulation of blood, the body could not live. The vena cava plays a vital role in helping to circulate deoxygenated blood from the upper half of the body, draining it into the heart, so the blood can continue to the lungs to be reoxygenated. 

Structure

The superior vena cava is relatively large and measures .78 inches in diameter and 2.7 inches in length.

All blood vessels—including veins and arteries—have basically the same structure. Namely, they are hollow tubes with a lumen (open inner space). The width and length of each blood vessel may vary, but they all have a hollow opening through which blood flows through.

The walls of arteries and veins have the same three layers, including: 

• The tunica intima: The innermost, thinnest layer of the vein, consisting of a single layer of endothelial cells (the type of cells found inside of the lining of the heart, lymph vessels, and blood vessels). This layer’s function is to reduce friction on the inside of the vessel, caused by the transport of blood.
• The tunica media: The middle layer of the veins; in an artery, this layer is the thickest because it is comprised of muscle tissue, but in veins, there is much less elastic tissue and more collagen fibers. Collagen fibers are comprised of a fibrous protein that makes up the supporting element of connective tissue.
• The tunica externa (also called tunica adventitia): The outermost layer of the vein, comprised of connective tissue and nerves; in veins, this is the thickest layer. It protects and strengthens the vein and serves to attach the vein to surrounding structures.

Most veins contain valves that help keep the blood moving in the direction of the heart, preventing the blood from backflowing away from the heart. The superior vena cava, however, is one of a few valveless veins in the body.

The mechanism that prevents blood from backing up into the superior vena cava from the right atrium during its contraction period (called systole) is part of a muscle that comprises the atrial walls, which wraps around the site of the entrance of the vena cava. When the heart contracts, this muscle closes almost completely over the opening of the superior vena cava, preventing the backflow of blood from the atrium.

Location

The superior vena cava is formed by the left and right brachiocephalic veins—also referred to as the innominate veins—on the right side of the upper chest, posterior (behind) to the lower border of the first costal cartilage.

The superior vena cava is located in the thorax (chest), more specifically, it is in the anterior (front) right, superior (above) mediastinum. 

The superior vena cava starts at the lower border of the first costal cartilage. It is located posterior (behind) this first costal cartilage on the right side and descends vertically to the right of the trachea (windpipe) and the aorta (the largest artery in the body). This large vein drains into the right atrium of the heart at the superior cavoatrial junction (located at the level of the third costal cartilage).

The lower half of the superior vena cava is covered by pericardium—a sac that encloses the heart. The superior vena cava ends at the junction of the superior vena cava and the left atrium, emptying into the upper aspect of the right atrium at the level of the third costal cartilage.

Anatomical Variations

Anatomical variations of the superior vena cava include:

Congenital Defects of the Superior Vena Cava

A congenital defect is an anomaly that occurs during fetal development. These are present at birth but may be diagnosed prenatally (before birth). It's also possible that these defects are not diagnosed until later in life.

According to the World Health Organization (WHO), heart defects (such as those which commonly occur at the same time as anomalies of the SVC) are among the most common and severe birth defects.

There are a few common congenital conditions of the superior vena cava including:

• Persistent left superior vena cava (persistent LSVC): is a common anomaly of the systemic veins.  It occurs most commonly with congenital heart disease. (CHD).
• Superior vena cava syndrome (SVCA): is a condition involving a compressed or partial blockage of the superior vena cava. Often, the underlying cause is lung cancer, non-Hodgkin lymphoma, or cancer that spreads to the chest.

Function

The superior vena cava is a vital structure in the human circulatory system that helps drain large amounts of deoxygenated blood from the head, eyes, neck, and upper limbs into the upper left chamber (atrium) of the heart.

Clinical Significance

The superior vena cave is a vessel with thin walls and low pressure, which makes it susceptible to conditions that increase the venous pressure. This is an important factor when it comes to conditions of the superior vena cava. Anything that causes hypertension (high blood pressure) in the vessels of the right side of the heart—or the pulmonary circulation—can majorly impact the superior vena cava.