Origin. Adrenal gland have mesoderm and ectoderm. This gland also called suprarenal gland or ‘life-saving glands’ or essential endocrine glands. It is because the absence of adreno-cortical hormones causes death within 3 to 15 days and absence of adreno-medullary hormones, decrease the resistance to mental and physical stress.
Adrenal gland Location. The adrenal gland are paired glands placed on the top of the kidneys.
Adrenal gland Structure. The adrenal gland are conical, yellowish bodies. Each has two distinct regions having different embryonic origin, structure and function. The two regions are called adrenal cortex and adrenal medulla.
The adrenal cortex is an external firm, pale yellowish-pink tissue formed from mesoderm. It further shows three concentric regions outer, middle and inner. Each region produces its own set of hormones. The adrenal cortex is essential for life. Its destruction or removal causes death of the animal. The adrenal cortex secretes three groups of steroid hormones :
(i) Mineralocorticoids. These hormones are secreted by the outer region of the adrenal cortex. They regulate mineral metabolism, and control the sodium and potassium ratio in the extracellular and intracellular fluids. Their secretion is stimulated by a fall in the Na+ concentration in the plasma, or a rise in the K+ concentration in the plasma, or a fall in the volume of blood. The major mineralocorticoid is aldosterone, commonly callled salt-retaining hormone. It tends to increase the reabsorption of sodium from urine, saliva, bile and sweat to reduce its loss from the body. It also increases the elimination of potassium in these fluids in exchange for the reabsorbed sodium. Aldosterone also increases the reabsorption of water from the urine by raising the osmotic pressure of the blood through reabsorption of Na+ into it. Reabsorption of water increases the volume of blood and other extracellular fluids. In short, mineralocorticoids maintain water and electrolyte balance and blood volume in the body.
(ii) Glucocorticoids. These hormones are secreted by the middle region of the adrenal cortex. They regulate the metabolism of carbohydrates, proteins and fats. They increase the blood-glucose level by converting proteins and fats into carbohydrates which are, in turn, converted to glucose. They transport a special plasma protein known as glucocorticoids- binding globulin/transcortin. In case of excessive bleeding, glucocorticoids constrict blood vessels to offset the drop in blood pressure due to blood loss. They have anti-inflammatory and antiallergic effects. The most important glucocorticoid is cortisol.
(i) Sexcorticoids or sex hormones. These hormones are secreted by both the middle and the inner regions of the adrenal cortex. They include small amounts of both male and female sex hormones. However, more male sex hormone (testosterone) is produced than female sex hormones (oestrogen and progesterone).Testosterone stimulates the development of male secondary sexual characters such as distribution of body hair, deepening of voice. Oestrogen stimulates the appearance of female secondary sexual characters such as enlargement of breasts, onset of menstruction, etc.
Activation. The adrenal cortex is stimulated to secrete its hormones by the adrenocorticotrophic hormone from the anterior pituitary (described ahead).
Adrenal gland Disorders
Improper functioning of adrenal cortex produces many disorders.
(i) Addison’s Disease. This is caused by deficiency of mineralocorticoids. It is characterized by ion imbalance which lowers water retention. Therefore, the person has low blood sugar, low water Na+ concentration in plasma, high K+ concentration in plasma, and increased loss of Na+ and water in urine (dehydration). The symptoms include weight loss, weakness, nausea, vomiting and diarrhoea.
Deficiency of glucocorticoids causes a bronze-like pigmentation of skin.
(ii) Cushing’s Syndrome. It is caused by excess of cortisol. It is characterized by high blood sugar, excretion of sugar in urine, obesity, wasting of limb muscles, high Na+ concentration in plasma, low K+ concentration in plasma, and rise in blood volume and blood pressure.
(iii) Aldosteronism. It is caused by excess of aldosterone. Its symptoms are high Na” concentration in plasma, low K+ concentration in plasma, and rise in blood volume and blood pressure.
(iv) Adrenal Virilism. It is caused by excess of sexcorticoids in a female. She develops male secondary sexual characters such as beard, moustaches and hoarse voice. Excess in early age may produce early abnormal sexual maturity in male.
The adrenal medulla is an internal soft, dark reddish brown tissue derived from the ectoderm. It enables the animal to face physical and emotional stress. Still it is not vital for survival. Its removal does not cause death as it simply augments the sympathetic neryous system. The renal medulla secretes two similar hormones : adrenaline (epinephrine) and noradrenaline (norepinephrine) that have almost the same basic effect.
Activation. At the time of stress or danger, the CNS stimulates the adrenal medulla to release epinephrine and norepinephrine. In addition, the sympathetic nervous system is also activated to release more norepinephrine.
Adrenal gland function
The function of adrenal gland secretes a number of different hormones that help regulate your metabolism, immune system, blood pressure, response to stress and are metabolised by enzymes either within the gland or in other parts of the body. These hormones are involved in a number of essential biological functions.
ROLE OF EPINEPHRINE AND NOREPINEPHRINE
Epinephrine and Norepinephrine whip up metabolism for preparing the animals to face speacial conditions created by physical stress such as fall in blood pressure or blood sugar, muscular exertion, pain, cold and injury ; and by emotional stress such as anger, fear, passion, shock and grief. All these conditions require more energy which is provided by the increasing heart beat, blood pressure, respiratory rate, sugar level of blood, blood supply of heart and skeletal muscles and brain through dilation of their small arteries, and oxidative metabolism. The hormones also cause contraction of spleen to squeeze nerve out the reserve supply of blood, contraction of small arteries of skin and viscera to divert blood to muscles, decrease in clotting time of blood, dilation of pupil, and contraction of arrector pili muscles of the skin to cause gooseflesh. They also stimulate the breakdown of liver to provide glucose for respiration.
FIGHT OR FLIGHT RESPONSE
The above role of adrenaline and noradrenaline is often called “fight or flight reaction.” It prepares the body to face stress or danger. A warm red face, cold sweating hands and faster heart beats are the symptoms often noticeable before a stage performance or an examination. They are a part of the “fight or flight” , reaction that prepares us to meet the stress.
Glands of Emergency. Because of the above role of their hormones in “fight or flight” reaction, the adrenals are known as the “glands of emergency.”
Sympathetico-adrenal System. Adrenal medulla is stimulated to secrete its hormones by nerve impulses reaching through the sympathetic nerve fibres, and adrenaline reinforces the action of sympathetic nerves. Both the hormones of the adrenal medulla and the sympathetic nervous system act on the same organs and produce similar of effects on them. In fact, the adrenal medulla and the sympathetic nervous system function as an integrated system, often called sympathetico-adrenal system.