The adrenal glands also known as the suprarenal glands. Supra meaning above, and renal meaning kidneys. So these glands are situated on top of the kidneys. These are endocrine glands that produce a variety of hormones, but most notable adrenaline, and the steroids aldosterone and cortisol. Each gland has an outer cortex which is divided into three different zones and an inner medulla. The three zones of the cortex are the zone glomerulosa, zone fasciculate, and zone reticularis.
This article will go briefly touch on the structure of the adrenal gland, including each zone of the cortex. Then it will dive into the function of the adrenal gland and the hormones it produces along with their specific cellular target. Finally the article will conclude with an overview of adrenal insufficiency and cortisol overproduction and diseases that illustrate those two conditions.
As mentioned earlier, the gland is composed of an outer cortex, and an inner medulla. The outer cortex can be further divided into three zones that each have a specific function.
The zona fasciculata sits between the other two zones (zona glomerulosa, and zona reticularis) and consists of cells responsible for producing glucocorticoids such as cortisol. Its the largest of the three zones consisting of about 80% of the space in the cortex.
The zona glomerulosa is the outermost zone of the adrenal cortex. The cells that are situated in this zone are responsible for the production of mineralocorticoids such as aldosterone. Aldosterone is an important regulator of blood pressure. Review the article covering the Renin-Aldosterone system.
The zona reticular is the innermost cortical layer which is primarily responsible for producing androgens. Its main component synthesized is dehydroepiandrosterone (DHEA), and androstenedione, which is the precursor to testosterone.
The medulla is in the centre of each adrenal gland with the cortex around the entire periphery. The chromatin cells within the medulla are the bodies main source of catecholamines. Catecholamines produced in the medulla are adrenaline (epinephrine), and noradrenaline (norepinephrine). Regulation of the synthesis of these catecholamines is driven by the sympathetic nervous system via the preganglionic nerve fibers stemming from the thoracic spinal cord (T5-T11) to the adrenal glands. When the medulla gets stimulated to produce these hormones it secretes them directly into the cardiovascular circulation system, which is unusual of sympathetic innervation as they usually have distinct synapses on specialized cells.
Mineralocorticoids such as aldosterone are named according to its function. They regulate minerals, such as salt and regulate blood volume (blood pressure). Aldosterone, the most prominent mineralocorticoid acts on the distal convoluted tubules and the collecting ducts by increasing the reabsorption of sodium and the excretion of both potassium and hydrogen ions. The amount of salt present in the body affects the extracellular volume, which influences the blood pressure.
Glucocorticoids are also named due to its function. Cortisol is a prominent glucocorticoid that regulates the metabolism of proteins, fats and sugars (glucose). Cortisol increases the circulating level of glucose. They cause protein catabolism into amino acids and the synthesis of glucose from the amino acids in the liver. They also increase the concentration of fatty acids by increasing lipolysis (fat breakdown) which cells can use as an alternative energy source in situations of glucose absence. Glucocorticoids also play a role in suppression of the immune system. They induce a potent anti-inflammatory effect.
Cortisol is the prominent glucocorticoid produced by the adrenal gland. The adrenal gland secretes a basal level of cortisol depending on the time of day it is. Cortisol concentrations in the blood are highest in the early morning and lowest in the evening as part of the circadian rhythm of adrenalcorticotropic hormone (ACTH) secretion. The article on general endocrinology explains what ACTH is and how it affects the adrenal gland. Basically what happens is the hypothalamus secretes corticotropin releasing hormone that acts on the pituitary to produce ACTH that acts on the adrenal gland cortex to produce cortisol.
Androgens and Catecholamines
The primary androgen produced by the adrenal gland is DHEA, which is converted to more potent androgens such as testosterone, DHT, and estrogen in the gonads. DHEA acts as a precursor. Androgens drive sexual maturation.
Catecholamines are produced by the chromaffin cells from tyrosine. The enzyme tyrosine hydroxyls converts tyrosine to L-DOPA. L-DOPA is then converted to dopamine before it can be turned into norepinephrine. Norepinephrine is then converted to epinephrine by the enzyme phenylethanolamine N-methyltransferase (PNMT). Epinephrine and norepinephrine act as adrenoreceptors throughout the body, whose primary effect is to increase the blood pressure and cardiac output by way of vasoconstriction. Catecholamines play a huge role in the fight-or-flight response.
The normal function of the adrenal gland can be impaired from infections, tumors, autoimmune diseases, or from previous medical therapy such as radiation and chemotherapy. Cushing’s syndrome is the manifestation of glucocorticoid excess. Symptoms and sign are a direct result of chronic exposure to glucocorticoids. Diagnosis is difficult because the symptoms are often nonspecific and pathognomonic of the syndrome in isolation. Symptoms include proximal (distant) muscle weakness, wasting of the extremities, increased fat in the abdomen and face often leading to a moon face, bruising without trauma, and a buffalo hump. A buffalo hump is fat on the back of the neck and supraclavicular pads. In women, menstrual irregularities are common such as oligomenorrhea (infrequent menstrual periods), amenorrhea (absence of menstrual periods), and variable menses. Hyperpigmentation can occur by increased secretion of cortisol. Cortisol acts on the melanocyte-stimulating hormone receptors.
Glucose intolerance is common in Cushing’s syndrome. Primarily due to stimulation of gluconeogenesis by cortisol and insulin resistance caused by the obesity. This leads to hyperglycemia, which can exacerbate any diabetic patient.
Bone loss and osteoporosis is common in patients with Cushing’s syndrome because there is less intestinal calcium absorption. Calcium is vital to bone health and growth. The decrease in bone formation is coupled with an increased rate of bone reabsorption which can lead to more pathological fractures.
Addison’s disease is considered primary hypoadrenalism. There is an inherent deficiency of glucocorticoids and mineralocorticoids. Most commonly caused by an autoimmune condition. Autoimmune means that the body is attacking itself by production of antibodies against cells of the adrenal cortex. In cases of adrenal crisis due to autoimmune primary adrenal insufficiency clinical presentation is usually the patient presenting in a state of shock. Abdominal tenderness upon deep palpation is common. Patients present with hyperpigmentation due to chronic ACTH release by the pituitary. Proopiomelanocortin is overproduced which is a pro hormone that is cleaved into its biologically active hormones corticotropin and melanocyte-stimulating hormone (MSH). This causes increased melanin synthesis, causing the hyperpigmentation. Other non-specific symptoms such as lethargy, fatigue, weakness, confusion, anorexia, nausea, vomiting, or even coma can occur. One of the most commonly presented symptoms is fever and infection, which can be exaggerated by the hypocortisolemia.
Its important to take this article slowly. There a lot of different parts, but the aim was to look at the hormones themselves and how they physiologically act on the body, then take what was learned about those and apply them to two scenarios, hypo/hyperadrenalism and how it affects the body. Cushing’s syndrome is where there is hyperproduction of cortisol primarily leading to many disastrous effects on the body. Addisons disease is an autoimmune disease where the body produces antibodies against the cells of the adrenal cortex, causing destruction of the gland itself, again leading to detrimental effects on the body.