Suprarenal gland

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The suprarenal (or adrenal) glands are paired retroperitoneal endocrine glands situated over the medial aspects of the upper poles of each kidney. They play a vital role in the body's fight or flight response by generating stress hormones that activate physiological adaptations to counteract changes in the external environment.

The suprarenal glands are covered with a thick connective tissue capsule from which trabeculae extend into the parenchyma.

Deep to the capsule, is the suprarenal cortex, which is the steroid-secreting portion of the gland. The cortex can be subdivided into the zona glomerulosa, the zona fasciculata and the zona reticularis.

• The zona glomerulosa is comprised of small rounded cells that are responsible for secreting mineralocorticoids such as aldosterone. Aldosterone regulates the uptake of water in the distal convoluted tubules, which consequently alters the body’s blood pressure.

• The zona fasciculata is significantly thicker than the other two cortical layers. It is made up of pale staining vacuolated cells arranged in parallel rows. This layer is responsible for secreting glucocorticoids to increase the overall blood glucose level in an effort to provide more energy for a system under stress.

• Finally, the zona reticularis consists of smaller cells that stain darker relative to cells of the aforementioned layers. Here, suprarenal androgens are produced, which serve as precursors for testosterone.

A thin, grey medulla can be found at the center of the suprarenal gland. Chromaffin cells, ganglion cells and dilated capillaries reside here.
Chromaffin cells are responsible for the production of catecholamines, namely epinephrine (adrenaline), norepinephrine (noradrenaline) and dopamine. Epinephrine is released directly into the medullary capillaries and carried to their site of action via systemic circulation. Ganglion cells modulate the secretory activity of the suprarenal cortex and their axons extend to the splanchnic nerves to innervate abdominal organs.

The suprarenal glands have an elaborate blood distribution which consists of:

• capsular capillaries supplying the capsule.
• fenestrated cortical sinusoidal capillaries supplying the suprarenal cortex and draining into the fenestrated medullary capillary sinusoids.
• medullary arterioles traveling within the trabeculare and traversing the suprarenal cortex to bring arterial blood to the medullary capillary sinusoids.

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