The adrenal (or suprarenal) glands are a pair of endocrine glands situated above the medial aspect of the upper pole of each kidney. They secrete steroid and catecholamine hormones directly into the bloodstream and have various anatomical locations, structures and vascular supplies.
Anatomically, the adrenal glands are located in the posterior abdominal area, between the superomedial kidney and the diaphragm. The right gland is pyramidal in shape while the left gland is semi-lunar. The perinephric (or renal) fascia encircles the adrenal glands, separating them from the kidneys by the perirenal fat. Furthermore, the adrenal glands lie in close proximity to many other anatomical structures in the abdomen.
The structure of the adrenal glands includes an outer connective tissue capsule, a cortex and a medulla. Arteries and nerves enter the gland at numerous sites while veins and lymphatics leave it via the hilum. The cortex and medulla are the functional parts of the gland and they secrete different hormones. The cortex secretes two cholesterol-derived hormones, the corticosteroids and androgens while the medulla secretes catecholamines such as adrenaline and enkephalins.
The adrenal glands are supplied with blood via three main arteries – the superior adrenal artery, which arises from the inferior phrenic artery, the middle adrenal artery which arises from the abdominal aorta and the inferior adrenal artery which arises from the renal arteries. The right and left adrenal veins drain the glands with the right adrenal vein draining into the inferior vena cava and the left adrenal vein draining into the left renal vein.
The adrenal glands are further innervated by the coeliac plexus and greater splanchnic nerves. Sympathetic innervation to the adrenal medulla is mainly by myelinated pre-synaptic fibres from the T10 to the L1 spinal cord segments. Lymph drainage is to the lumbar lymph nodes by adrenal lymphatic vessels, which originate from two lymphatic plexuses, one deep to the capsule and the other in the medulla.
Clinically, a pheochromocytoma is a tumour of the adrenal medulla or preganglionic sympathetic neurones.
The adrenal glands are endocrine glands located superiorly to the kidneys and consist of two distinct regions – an outer cortex and an inner medulla. Both regions are of great functional significance, as the cortex secretes hormones such as corticosteroids and androgens, while the medulla is mainly responsible for the secretion of catecholamines like adrenaline in response to stress.
Structurally, the cortex is yellowish in colour and can be divided into three parts (superficial to deep) – the zona glomerulosa, zona fasciculata and zona reticularis. The glomerulosa produces and secretes the mineralocorticoid aldosterone, while the fasciculata and reticularis both secrete corticosteroids and androgens. The medulla, on the other hand, is dark brown in colour and contains chromaffin cells that secrete pain-controlling enkephalins alongside the aforementioned catecholamines.
The adrenal glands have a rich blood supply via three main arteries – the superior, middle and inferior adrenal arteries, as well as two adrenal veins that drain either into the inferior vena cava or the left renal vein. As for innervation, the glands are connected to the greater splanchnic nerve and the coeliac plexus, with the sympathetic innervation of the medulla coming from pre-synaptic fibres originating in the T10 to L1 spinal cord segments. Furthermore, the adrenal glands are drained by lymphatic vessels into the lumbar lymph nodes.
The anatomy and functions of the adrenal glands are of great clinical relevance, with a specific focus being on pheochromocytomas – tumours of the adrenal medulla that secrete adrenaline and noradrenaline uncontrolled, subsequently causing blood pressure to sharply increase. Patients may experience palpitations, headaches and profuse sweating, and the condition can be treated with phenoxybenzamine, a competitive, irreversible antagonist of adrenaline that binds to the receptors, reducing the action of adrenaline.
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