The superior vena cava (SVC) originates in the chest, behind the first right sternocostal articulation, from the confluence of two main collector vessels: the right and left brachiocephalic veins which receive the ipsilateral internal jugular and subclavian veins. It is located in the anterior mediastinum, on the right side. The internal jugular vein collects the blood from head and deep sections of the neck while the subclavian vein, from the superior limbs, superior chest and superficial head and neck. Several other veins from the cervical region, chest wall and mediastinum are directly received by the brachiocephalic veins. After the brachiocephalic convergence, the SVC follows the right lateral margin of the sternum in an inferoposterior direction. It displays a mild internal concavity due to the adjacent ascending aorta. Finally, it enters the pericardium superiorly and flows into the right atrium; no valve divides the SVC from right atrium. The SVC's length ranges from 6 to 8 cm. Its diameter is usually 20-22 mm. The total diameters of both brachiocephalic veins are wider than the SVC's caliber. The blood pressure ranges from -5 to 5 mmHg and the flow is discontinuous depending on the heart pulse cycle. The SVC can be classified anatomically in two sections: extrapericardial and intrapericardial. The extrapericardial segment is contiguous to the sternum, ribs, right lobe of the thymus, connective tissue, right mediastinal pleura, trachea, right bronchus, lymphnodes and ascending aorta. In the intrapericardial segment, the SVC enters the right atrium on the upper right face of the heart; in front it is close to the right main pulmonary artery. On the right side, the lung is in its proximity, separated only by mediastinal pleura. The right phrenic nerve runs next to the SVC for its entire course [1] (Figure 1). The SVC receives a single affluent vein: the azygos vein. The azygos vein joins the SVC from the right side, at its mid length, above the right bronchus. The Azygos vein constantly receives the superior intercostal vein, a large vessel which drains blood from the upper two or three right intercostal spaces. In the case of SVC obstruction, the azygos vein is responsible for the most important collateral circulation. According to the expected collateral pathways, the SVC can be divided into two segments: the supra-azygos or www.intechopen.com

Summary The results of calcium and parathormone loading tests in 6 individuals with Turner's syndrome indicate a certain degree of insensitive‐ness of the tissues, especially of the kidney, when compared with healthy individuals. A similar insensitiveness expressing itself by a decrease in hydrogen ion excretion was found after ammonium chloride loading. An analysis of the results suggest the existence in this syndrome of insensitiveness of renal tubules to regulatory factors. Osteoporosis and stunting of growth may be connected partly with intestinal calcium loss occurring at puberty, and partly with this insensitiveness to regulatory factors.