Adrenal glands          

Introduction

Adrenal hormones are well known for fight or flight response. They are closely linked with the human emotions such as anger and anxiety. The hormones are secreted by the pair of adrenal glands is called adrenaline , an endocrinal hormone otherwise known as the supra renal hormone as they are situated just above each kidney. Suprarenal glands produce a number of hormones performing a wide range of functions but their main secretions are the adrenaline and the steroids (aldosterone and cortisol). Adrenal gland is made up of an outer cortex producing steroid hormones and an inner medulla that secretes catecholamines (adrenaline and noradrenaline). About 20% of noradrenaline (norepinephrine) and 80% adrenaline (epinephrine) are secreted in the medulla region. Anatomically, the right adrenal gland lies close to inferior vena cava along with the right lobe of the liver  whereas, the  left adrenal gland is adhered to  stomach, pancreas and spleen.

 

Structure of adrenal gland (suprarenal gland)

 

Adrenal Cortex

Adrenal cortex is the outermost layer of adrenal gland constituted by the 3 thin layers from periphery to the center. They are ; zona glomerulosa, zona fasciculata and the zona reticularis. Zona glomerulosa is the outermost zone of the adrenal cortex that assist in producing the aldosterone. Aldosterone is a mineralocorticoid produced with the help of aldosterone synthase which participates in the long-term regulation of blood pressure. The adrenal medulla continue along with fasciculata which is located in between the zona glomerulosa and zona reticularis. This is the largest of the 3 layers producing glucocorticoids such as cortisol. Zona Reticularis is the innermost cortical layer located just adjacent to the medulla help in producing androgens. 

 

Adrenal medulla

It is located at the center of each  gland and is enclosed by the adrenal cortex from outside. The medullary part of the adrenal gland consists of irregularly shaped cells grouped in between many tiny blood vessels. Cells of the adrenal gland is closely linked to the sympathetic division of the autonomic nervous system (ANS). In general, adrenal medullary cells are the modified post ganglionic neurons, and preganglionic autonomic nerve fibers channeled directly from the central nervous system.Therefore, medulla can directly affects available energy, heart rate, and metabolism. Adrenal medulla is well-known for flight or fight response through the secretion of epinephrine (adrenaline) and norepinephrine (noradrenaline).

 

Blood supply and nerve supply

The suprarenal glands receives the arterial supply by 3 arteries, the superior suprarenal artery, middle suprarenal artery and the inferior suprarenal artery and the venous blood is drained out through the suprarenal veins found one for each gland (Where the right suprarenal vein drains into the inferior vena cava and the left suprarenal vein drains into the left renal vein or the left inferior phrenic vein. The adrenal glands are supplied by the coeliac plexus and greater splanchnic nerves. Myelinated pre-synaptic fibres, mostly from the T10 to L1 spinal cord segments provide the sympathetic nerve supply to the adrenal medulla. Nerve supply is important for the normal response and secretory ability of the glands.

 

Adrenal secretions and their functions

Adrenal gland secretes a variety of hormones depending upon the cell types (cortex or medulla). The most common secretions are minerocorticoids, catecholamines,and androgens.Carticoids are generally called steroid hormones secreted by the adrenal cortex. Corticosteroids are of 2 types; mineralocorticoids and glucocorticoids. Mineralocorticoids help in the salt (mineral) balance and the blood volume. Whereas the glucocorticoids (cortisol) regulate the metabolism of proteins, fats and glucose

1. Mineralocorticoids

They act on the DCT (distal convoluted tubules)  and the collecting ducts of the kidneys by rising the sodium reabsorption, besides the excretion of potassium and hydrogen ions. Increased sodium reabsorption results in a state of hypertonic(highly concentrated) urine formation which in turn assist in retaining more water and minerals within the body. In addition, the amount of sodium retained back in the body participates in regulating blood pressure.

2Glucocorticoids

Adrenal secretions are controlled by the adrenocorticotropic hormone (ACTH) released from the anterior pituitary.  Therefore, as  the ACTH secretions rise, the coracoids levels also increase and vice versa. Glucocorticoids are generally called as cortisol. Cortisol help in influencing the metabolism of various nutrients. They act by increasing the circulating glucose levels within the body and also,they participate in the amino acids synthesis, absorption of proteins and lipids in the body.

3. Catecholamines

Catecholamines  are also known as epinephrine and norepinephrine (adrenaline and noradrenaline). They are the  water-soluble compounds regulating the blood pressure and heart rate. They also help in the fight or flight response during a crisis  by increasing the catecholamines secretion. The catecholamines function by increasing the breathingheart rate, and blood pressure as a result of constriction of blood vessels.

4. Androgens

Androgens  are the sex hormones secreted in the adrenal medulla. Since the male androgens are primarily secreted and controlled by testosterone, the adrenal secretions do not impact on the overall process of  sexuality and sexual characteristics.

 

Abnormalities in the adrenal secretions

Like many other glands, adrenal glands are at risk for structural and functional variations which can result in either excess or decreased secretion. The disease can be manifested by the damage or malfunctioning of cortex or medulla of the adrenaline. Increased secretion of adrenaline hormone is known as adrenocortical hyperfunction. This can be congenital (by birth) or acquired by tumors and other glandular infections. Adrenocortical hyperfunction is characterized by an excess of cortisol levels resulting in a condition called Cushing syndrome. On the other hand, the decreased secretion of adrenal hormones is known as adrenal insufficiency.

 

Read More

1           

Antidiuretic Hormone   

2

Pituitary gland and Pituitary hormones

3

Endocrinal control and coordination

4

Coordination in plants

 

Check Your Understanding

  1. Name the condition caused by adrenocortical hyperfunction.

  2. Which part of the adrenal gland secretes maximum secretions?

  3. Explain the mechanism of control of ADH on the urinary system

  4. What is cortisol?

 

 



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