The adrenal glands are covered with a thick connective tissue capsule from which trabeculae extend into the parenchyma, carrying blood vessels and nerves. The secretory parenchymal tissue is organized into two distinct regions (Fig. 21.18):  The cortex is the steroid-secreting portion. It lies beneath the capsule and constitutes nearly 90% of the gland by weight.  The medulla is the catecholamine-secreting portion. It lies deep to the cortex and forms the center of the gland.
Parenchymal cells of the cortex and medulla are of different embryologic origin.
Embryologically, the cortical cells originate from mesodermal mesenchyme, whereas the medulla originates from neural crest cells that migrate into the developing gland (Fig. 21.19). Although embryologically distinct, the two portions of the adrenal gland are functionally related (see below). The parenchymal cells of the adrenal cortex are controlled in part by the anterior lobe of the pituitary gland and function in regulating metabolism and maintaining normal electrolyte balance (Table 21.9).
The adrenal glands are supplied with blood by the superior, middle, and inferior suprarenal arteries. These vessels branch before entering the capsule, to produce many small arteries that penetrate the capsule. In the capsule, the arteries branch to give rise to three principal patterns of blood distribution (Figs. 21.20 and 21.21). The vessels form a system that consists of  capsular capillaries that supply the capsule.
the cortex and then drain into the fenestrated medullary capillary sinusoids.  medullary arterioles that traverse the cortex, traveling within the trabeculae, and bring arterial blood to the medullary capillary sinusoids.
sympathetic ganglion (from neural crest) cortical primordium aorta of fetal cortex chromaffin aorta cells of future medulla
FIGURE21.18 • Photomicrograph of the adrenal gland. This low-power micrograph of an H&E–stained specimen shows the full thickness of the adrenal gland with the cortex seen on both surfaces and a central region containing the medulla. Within the medulla are profiles of the central vein. Note that the deeper portion of the cortex stains darker than the outer portion, a reflection of the washed-out lipid in the zona glomerulosa and outer region of the zona fasciculata. This section also includes a cross section of the adrenal vein, which is characterized by the longitudinally arranged bundles of smooth muscle in its wall. 20.
The medulla thus has a dual blood supply: arterial blood from the medullary arterioles and “venous” blood from the cortical sinusoidal capillaries that have already supplied the cortex. The venules that arise from the cortical and medullary sinusoids drain into the small adrenomedullary collecting veins that join to form the large central adrenomedullary vein, which then drains directly into the inferior vena cava on the right side and into the left renal vein on the left side. In humans, the central adrenomedullary vein and its tributaries are unusual in that they have a tunica media containing conspicuous, longitudinally oriented bundles of smooth muscle cells. Synchronous contraction of longitudinal smooth muscle bundles along the central adrenomedullary vein and its tributaries cause the volume of the adrenal gland to decrease. This volume decrease enhances the efflux of hormones from the adrenal medulla into the circulation, an action comparable to squeezing a wet sponge.
Lymphatic vessels are present in the capsule and the connective tissue around the larger blood vessels in the gland.