Adrenal gland

Adrenal glandAbira Rajasingam

Also known as suprarenal glandsEndocrine glands that are retroperitoneal and are attached to the superior border of each kidney by a dense fibrous capsuleThe right adrenal gland is triangular shaped whereas the left is semi-lunar shapedFound at the level of the 12th ribA typical adrenal gland weighs about 5.0 g but its size can greatly vary according to its secretory demands

Adrenal gland

Divided into 2 parts: a superficial cortex and an inner medullaEmbryologically, the cortex develops from mesodermal tissue whereas the medulla develops from neuronal tissue Adrenal cortex has 3 distinct regions: . Zona Glomerulosa . Zona Fasciculata . Zona Reticularis

Adrenal gland

Adrenal gland layers

Accounts for 15% of the volume of the adrenal cortexProduces mineralocorticoids of which aldosterone is the main hormoneThis hormone is released in response to a rise in potassium level and it also occurs in response to Angiotensin 2Aldosterone causes the retention of sodium ions at the kidneys, sweat glands, salivary glands and pancreas and is also involved in elimination of K+As a secondary effect, the reabsorption of Na+ enhances the osmotic reabsorption of water at the kidneys, sweat glands, salivary glands and pancreas Zona Glomerulosa

Renin-AAS pathway

Stimuli that initiate the renin-angiotensin-aldosterone pathway include dehydration, Na+ deficiency, or haemorrhage These conditions cause a decrease in blood volume Decreased blood volume leads to a decreased blood pressure Lowered blood pressure stimulate certain cells of the kidneys, called juxtaglomerular cells, to secrete the enzyme renin The level of renin in the blood increases Renin converts angiotensinogen into angiotensin I Blood containing increased levels of Angiotensin I circulates in the body ACE converts angiotensin I into the hormone angiotensin II Blood level of angiotensin II increase Angiotensin II stimulates the adrenal cortex to secrete aldosterone Blood containing increased levels of aldosterone circulates to the kidney Aldosterone increases reabsorption of Na+, which in turn causes reabsorption of water by osmosis. As a result, less water is lost in the urine. Aldosterone stimulates the kidney to increase secretion of K+ and H+ into the urine With increased water reabsorption by the kidneys, blood volume increases As blood volume increases, blood pressure increases to normal Angiotensin II also stimulate contraction of smooth muscle in the walls of arterioles. The resulting vasoconstriction of the arterioles increases blood pressure and thus helps raise blood pressure to normal.Besides angiotensin II, a second stimulator of aldosterone secretion is an increase in the K concentration of blood (or interstitial fluid). A decrease in the blood K level has the opposite effect.

RAA pathway

Secretes glucocorticoids- which regulates metabolism and resistance to stress Cortisol, corticosterone and cortisone 95% of glucocorticoid activity Low levels of glucocorticoids- mainly cortisol- stimulate neurosecretory cells in the hypothalamus to secrete corticotropin-releasing hormone (CRH) CRH promotes the release of ACTH from the anterior pituarity ACTH flows in the blood to the adrenal cortex, where it stimulates glucocorticoid secretionACTH also stimulates secretion of aldosterone Zona Fasciculata

Protein breakdown:GCs increase the rate of protein breakdown- mainly in muscle fibers- increases the liberation of amino acids into the bloodstream.Glucose formation: Liver cells may convert certain amino acids or lactic acid to glucose, which neurons and other cells can use for ATP production = gluconeogenesis Lipolysis: Depression of immune response: Given for organ transplant Resistance to stress:Additional glucose supplied by the liver cells provides tissues with a ready source of ATP to combat a range of stresses, including exercise, fasting, fright, temperature extremes, high altitude, bleeding, infection, surgery, trauma, and diseaseHigh doses can cause severe mental disturbances Used in the treatment of chronic inflammatory diseases e.g. RAGlucocorticoids

The major androgen secreted by the adrenal gland is dehydroepiandrosterone After puberty in males, the androgen testosterone is also released in much greater quantity by the testes. Thus, the amount of androgens secreted by the adrenal gland in males is usually so low that their effects are insignificant.In females, however, adrenal androgens play important roles.They promote libido (sex drive) and are converted into oestrogens (feminizing sex steroids) by other body tissues. After menopause, when ovarian secretion of oestrogens ceases, all female oestrogens come from conversion of adrenal androgens. Adrenal androgens also stimulate growth of axillary and pubic hair in boys and girls and contribute to the prepubertal growth spurt. Although control of adrenal androgen secretion is not fully understood, the main hormone that stimulates its secretion is ACTH.Androgens

Inner region of the adrenal gland Modified sympathetic ganglion of the ANS Develops from the same embryonic tissue as all other sympathetic ganglia, but its cells, which lack axons, form clusters around large blood vesselInstead of releasing neurotransmitters- they secrete hormones- from Chromaffin cells Innervated by sympathetic preganglionic neurons of the ANS. Because the ANS exerts direct control over the Chromaffin cells, hormone release can occur very quickly.

Adrenal medulla

Epinephrine- 80%Norepinephrine- 20%Hormones of the adrenal medulla intensify sympathetic responses that occurs in other parts of the body In stressful situations and during exercise, impulses from the hypothalamus stimulate sympathetic preganglionic neurons stimulate the Chromaffin cells to secrete epinephrine and norepinephrine. By increasing heart rate and force of contraction, epinephrine and norepinephrine increase the output of the heart, which increases blood pressure. They increase blood flow to the heart, liver, skeletal muscle, and adipose tissue, dilate airways to the lungs; and increase blood levels of glucose and fatty acids.

Adrenal medulla

Blood supply

Like other endocrine glands, the adrenal glands are highly vascularised. There are usually 3 arteries that supply each adrenal gland:Superior adrenal artery provided by the inferior phrenic artery The middle adrenal artery is provided by the abdominal aortaThe inferior adrenal artery is provided by the renal arteryVenous drainage of the adrenal glands is achieved by the adrenal veins:The right adrenal vein drains into the IVCThe left adrenal vein drains into the left renal vein or the left inferior phrenic vein The adrenal glands and the thyroid gland are the organs that have the greatest blood supply per gram of tissue. Blood supply

Innervated by sympathetic nervous systemThe nerves supplying the adrenal glands are derived from the coeliac plexus and thoracic splanchnic nervesAdrenal medullary cells are modified postganglionic neurons, where preganglionic fibers synapse on neuroendocrine cells that secrete epinephrine, norepinephrine into the bloodstreamNerve supply