Human Anatomy - M.M. Kurepina, A.P. Ozhigova, A.A. Nikitina 2010

Internal Organs
System of Organs of Internal Secretion (Endocrine System)
Adrenal Glands

The Adrenal Glands (glandulae suprarenales) are paired, flattened Organs located near the superior pole of each Kidney at the level of the XI—XII thoracic vertebrae (Atl. Fig. 218). The right gland is triangular in shape and adjacent to the INFERIOR VENA CAVA and the Liver; it is largely uncovered by the Peritoneum. The left, more flattened gland is adjacent to The Stomach, Spleen, and Pancreas. It is covered by the peritoneum anteriorly. Posteriorly, the adrenal glands rest against the Diaphragm. The right Adrenal gland lies slightly higher than the left. In adults, the adrenal glands reach 5 cm in length and weigh from 10 to 20 g. A deep groove, the hilum, is located on their anteromedial surface. Veins and Lymphatic vessels pass through it. Arteries and nerves penetrate the adrenal glands from both the anterior and posterior surfaces.

Externally, the glands are covered by a fibrous capsule containing a small number of smooth Muscle Cells. Connective Tissue septa extend inward from the capsule. The adrenal glands consist of a cortex and a medulla, which differ in Structure, function, and origin. The medulla occupies the deep portions of the gland and consists of chromaffin tissue. Its cells form clusters around Blood Vessels and secrete epinephrine and norepinephrine. By origin, these cells are similar to Neurons of the sympathetic Nervous system. They are innervated by cholinergic preganglionic nerve fibers, unlike other secretory cells, which are innervated by postganglionic fibers. Under conditions of heightened excitement, pain, hypothermia, or stress, epinephrine is released from the medullary cells, reinforcing the effects of the sympathetic nervous system. Consequently, The Heart rate and strength of contractions increase, blood pressure rises, Blood supply to skeletal Muscles is enhanced, etc. Ganglion cells are also found in the medulla.

The cortex is located at the periphery of the adrenal gland and is more developed than the medulla (Atl. Fig. 219). The secretory Cells of the cortical parenchyma form three zones. Beneath the capsule, in the zona glomerulosa, cells are arranged in irregular clusters separated by blood capillaries. Deeper lies the zona fasciculata. Its cells form radially oriented cords 1—2 cells thick, which are also separated by capillaries. At the border with the medulla is the zona reticularis, formed by anastomosing cords of cells. Dilated capillaries are located between them.

Adrenal Cortex Hormones are Steroids (glucocorticoids, mineralocorticoids, and Sex Hormones). They are involved in METABOLISM, growth processes, and immune responses. Their Functions and Mechanisms of action are covered in detail in the physiology course. Cell functions and their rate of turnover are regulated by the adrenocorticotropic hormone of the Pituitary Gland. Most likely, the renewal of adrenal cortex cells occurs through their division in the zona glomerulosa and migration into the zona fasciculata.

The three adrenal arteries entering the gland branch into numerous vessels, forming a subcapsular plexus. From this plexus, some branches penetrate the capsule, while others run to the medulla, and the majority to the cortex, descending from the zona glomerulosa to the zona reticularis. Thus, medullary cells come into contact with both arterial capillaries and venules draining from the cortex. Blood from these vessels collects in the central vein, which eventually receives veins from the adrenal capsule and exits through the hilum.