The outer cortex, comprising ninety per cent of the gland, surrounds the central medulla. The cortex is a zonated structure and is covered by a thin capsule, below which isolated groups of glomerulosa cells are found. Most of the cortex is made up of the zona fasciculata and the zona reticularis, the latter adjoining the medulla. The blood supply is derived from the inferior phrenic artery, the aorta, and the renal arteries, and is largely subcapsular. The venous circulation within the gland is complex and is thought to play an important role in regulating steroid synthesis. The central vein from the right adrenal is short and enters the inferior vena cava directly, whereas that from the left adrenal drains into the left renal vein.
