Adrenal glands          

Introduction

Adrenal glands and its hormones are well known for fight or flight response. Adrenaline is an endocrinal hormone and they are otherwise known as suprarenal glands as they are situated just above each kidney. There are 2 glands (paired) and they are located just above each kidney; the left and right adrenal glands. They produce a number of hormones performing a wide range of functions. They secrete adrenaline and the steroids (aldosterone and cortisol).  Each adrenal gland has an outer cortex producing steroid hormones and an inner medulla secrete catecholamines (adrenaline and noradrenaline). About 20% noradrenaline (norepinephrine) and 80% adrenaline (epinephrine) are secreted in the medulla. The right adrenal gland is close to inferior vena cava and the right lobe of the liver. Whereas left adrenal gland is attached to the important organs such as stomach, pancreas and spleen.

 

Table Of Contents

1. Introduction

2. Structure of adrenal gland (suprarenal gland)

    2.1 Adrenal Cortex

    2.2 Adrenal medulla

    2.3 Blood supply and nerve supply

3. Adrenal secretions and their functions

    3.1 Corticosteroids

    3.2 Abnormalities in the adrenal secretions

Structure of adrenal gland (suprarenal gland)

 

Adrenal Cortex

This is the outermost layer having 3 subdivisions in the form of thin layers. The 3 layers from the surface towards the centre are zona glomerulosa, zona fasciculata and the zona reticularis Medulla continue along with fasciculata. Zona glomerulosa is the outermost zone of the adrenal cortex assist in producing the aldosterone. Aldosterone is a mineralocorticoid produced with the help of aldosterone synthase. Aldosterone participates in the long-term regulation of blood pressure. Zona fasciculata is located in between the zona glomerulosa and zona reticularis. This layer is the largest of the 3 layers producing glucocorticoids such as cortisol. Zona Reticularis is the innermost cortical layer locate just adjacent to the medulla. Zona reticularis produces androgens. The adrenal glands sit in close proximity to many other structures in the abdomen:

 

Adrenal medulla

It is found at the centre of each adrenal gland and is surrounded by the adrenal cortex. The medulla is the inner part of an adrenal gland help in the flight or fight response through the secretion of epinephrine (adrenaline) and norepinephrine (noradrenaline).

 

Blood supply and nerve supply

The suprarenal glands receive the arterial supply by 3 arteries, the superior suprarenal artery, middle suprarenal artery and the inferior suprarenal artery. The adrenal 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 have nerve supply 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 discussed below

 

Corticosteroids

They are generally known by the term steroid hormones secreted by the adrenal cortex. Corticosteroids can be mineralocorticoids and glucocorticoids. Mineralocorticoids help in the salt (mineral) balance and the blood volume. Whereas the glucocorticoids like cortisol regulate the metabolism of proteins, fats and glucose. Mineralocorticoids acts on the DCT (distal convoluted tubules)  and the collecting ducts by rising the sodium reabsorption and 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. However, the amount of sodium retained back in the body is closely linked with blood pressure. Glucocorticoids are generally called as cortisol that has an influence on the metabolism. They act by increasing the circulating glucose levels within the body. They also have effects on the amino acids synthesis, absorption of proteins and lipids in the body. Adrenal secretions are controlled by the adrenocorticotropic hormone (ACTH) from the anterior pituitary.  Hence if the ACTH rise, automatically the corticoids levels also increase and vice versa.

  1. Catecholamines are also known as epinephrine and norepinephrine (adrenaline and noradrenaline). Catecholamines are water-soluble compounds regulating the blood pressure and heart rate and also to generate the fight or flight response. Whenever an individual face threat or a panic situation, catecholamines secretion will suddenly increase resulting in the quickening of breathing and heart rate, an increase in blood pressure as a result of constriction of blood vessels.

  2. Androgens are secreted in the adrenal medulla. They are also known as male sex hormones. Since the male androgens are primarily secreted and controlled by testosterone, the adrenal secretions do not have an overall effect on the sexuality and sexual characteristics within the male body.

 

Abnormalities in the adrenal secretions

Like all other glands, adrenal glands can result in the abnormal structure and functional variations resulting in either excess or decreased secretion of hormones. The disease conditions can occur in 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 tumours 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.



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