The Adrenal Glands
The adrenal (or suprarenal) glands are paired retroperitoneal (lying posterior to the peritoneum) endocrine glands situated over the medial aspects of the upper poles of each kidney.
They secrete steroid and catecholamine hormones directly into the blood.
This article will provide an overview of the adrenal glands in terms of their anatomical location and relations, structure, function, blood supply, lymphatics and neural innervation and will end with some clinical relevance.
The adrenal glands lie in the posterior abdomen, situated between the superomedial kidney and the diaphragm. They cover part of the anterior surface of each kidney.
The right gland is pyramidal in shape, contrasting with the semi-lunar shape of the left gland.
Perinephric (or renal) fascia encloses the adrenal glands and the kidneys. This fascia attaches the glands to the crura of the diaphragm. They are separated from the kidneys by the perirenal fat.
The adrenal glands consist of an outer connective tissue capsule, a cortex and a medulla. Veins and lymphatics leave each gland via the hilum, but arteries and nerves enter the glands at numerous sites.
The outer cortex and inner medulla are the functional portions of the gland. They are actually two separate endocrine glands, with different embryological origins:
- Cortex – derived from the embryonic mesoderm.
- Medulla – derived from the ectodermal neural crest cells.
Each is responsible for different secretions.
The cortex is yellowish in colour. It secretes two cholesterol derived hormones – corticosteroids and androgens. Functionally, the cortex can be divided into three regions which are (from superficial to deep):
- Zona glomerulosa – produces and secretes mineralocorticoids such as aldosterone.
- Zona fasciculata – produces and secretes corticosteroids such as cortisol. It also secretes a small amount of androgens.
- Zona reticularis – produces and secretes androgens such as dehydroepiandrosterone (DHES). It also secretes a small amount of corticosteroids.
The medulla lies in the centre of the gland, and is dark brown in colour. It contains chromaffin cells, which secrete catecholamines (such as adrenaline) into the bloodstream in response to stress. These hormones produce a ‘flight-or-fight‘ response.
Chromaffin cells also secrete enkephalins which function in pain control.
The adrenal glands sit in close proximity to many other structures in the abdomen, these are important to be aware of clinically.
Neurovascular and Lymphatic Supply
The adrenal glands have a rich blood supply, which is supplied via three arteries:
- Superior adrenal artery – arises from the inferior phrenic artery
- Middle adrenal artery – arises from the abdominal aorta.
- Inferior adrenal artery – arises from the renal arteries.
Right and left adrenal veins drain the glands. The right adrenal vein drains into the inferior vena cava, whereas the left adrenal vein drains into the left renal vein.
Lymph drainage is to the lumbar lymph nodes by adrenal lymphatic vessels. These vessels originate from two lymphatic plexuses – one deep to the capsule, and the other in the medulla.
The adrenal glands are innervated by the coeliac plexus and abdominopelvic splanchnic nerves. Sympathetic innervation to the adrenal medulla is via myelinated pre-synaptic fibres, mainly from the T10 to L1 spinal cord segments.
Clinical Relevance: Adrenal Endocrine Diseases
The adrenal glands are important organs in the endocrine system, secreting wide range of hormones. Thus, there are a wide range of clinical syndromes resulting from adrenal gland disease:
A pheochromocytoma is a tumour of the adrenal medulla or preganglionic sympathetic neurones. It secretes adrenaline and noradrenaline uncontrollably, causing blood pressure to greatly increase. Patients may present with palpitations, headaches and diaphoresis (profuse sweating).
Phenoxybenzamine, a competitive, irreversible antagonist of adrenaline, can be used in treatment to reduce blood pressure by binding to adrenaline receptors, making less available for adrenaline to act upon.
Addison’s disease is characterised by low cortisol and aldosterone levels, typically due to the autoimmune destruction of the adrenal cortex. Stress may exacerbate the condition, producing an Addisonian crisis which must be treated urgently with IV cortisol and administration of dextrose in normal saline as fluid replacement, in order to prevent death.
Cushing’s syndrome describes the signs and symptoms associated with chronically elevated cortisol levels in the blood. These include:
- Moon shaped face and buffalo hump
- Thin skin and easy bruising
There are various causes of a high glucocorticoid levels. If the anterior pituitary gland is the cause (e.g pituitary adenoma), this is known as Cushing’s disease.