15
nervous system, as well as induce metabolic actions, including glycogen breakdown
in skeletal muscle and liver, increased plasma lactate, mobilization of free fatty
acids, and increased metabolic rate. Due to extensive vasoconstriction, noradrenaline causes a significant increase in peripheral vascular resistance, while adrenaline
causes vasoconstriction of visceral organs and skin but vasodilation of the skeletal
muscle that ultimately decreases total peripheral resistance (TPR). Thus, collective
action of both results in increased myocardial rate, but the intake of noradrenaline
is responsible for reflex bradycardia that occurs as a result of a significant increase
in mean arterial and peripheral resistance. Even though adrenaline often causes
anxiety and fear in humans, they increase alertness [6].
Adrenal Cortex
Adrenal cortex is accountable for the release of mineralocorticoids, androgens (sex
hormones), and glucocorticoids. Glucocorticoids have effect on metabolism of carbohydrates, fats, and proteins and are essential in facilitating responses to stress as
well as fasting. Mineralocorticoids are important in order to keep balance for sodium
and thus extracellular fluid balance. Compared with the pituitary hormones FSH
and LH, androgen has little effect on reproductive function. Adrenal cortex is histologically divided into three distinct layers. The extreme layer contains zona glomerulosa, central layer (major layer) consists of zona fasciculate cells, and the
innermost layer comprised of zona reticularis. Corticosterone is secreted in all three
regions, whereas biosynthesis of aldosterone occurs in zona glomerulosa. The enzymatic mechanisms that form cortisol (hydrocortisone) and androgens are mainly
present in two internal regions.
Glucocorticoids
The adrenocorticotropic hormone (ACTH) secreted by the anterior lobe of pituitary
gland regulates the release of glucocorticoids. It is regulated via hypothalamic discharge of corticotropin releasing hormone (CRH) into hypophyseal portal system.
Cortisol secretion induces a negative feedback influence on the anterior lobe of
pituitary gland as well as hypothalamus. Cortisol amount in plasma was in diurnal
mode and maximum amount is observed just before awaking in the morning
(Fig. 1.5). The circulating corticosteroids primarily bind to plasma proteins, i.e.,
corticosteroid binding globulin (transcortin) as well as albumin. Hormones are
deactivated mainly in the liver by binding with sulfate or glucuronic acid and are
excreted into the urine. Glucocorticoids play a vital role in controlling metabolism
of carbohydrates, proteins, and fats. They stimulate the storage of liver glycogen.
During fasting, they trigger formation of glucose in the liver in order to supply glucose for brain metabolism. They have an antagonistic effect on insulin, leading to an
increase in blood sugar. Glucocorticoids enhance the vasoconstriction of catecholamines and reduce the permeation of vascular endothelium that is crucial for
1 Physiology of Endocrine System and Related Metabolic Disorders
nervous system, as well as induce metabolic actions, including glycogen breakdown
in skeletal muscle and liver, increased plasma lactate, mobilization of free fatty
acids, and increased metabolic rate. Due to extensive vasoconstriction, noradrenaline causes a significant increase in peripheral vascular resistance, while adrenaline
causes vasoconstriction of visceral organs and skin but vasodilation of the skeletal
muscle that ultimately decreases total peripheral resistance (TPR). Thus, collective
action of both results in increased myocardial rate, but the intake of noradrenaline
is responsible for reflex bradycardia that occurs as a result of a significant increase
in mean arterial and peripheral resistance. Even though adrenaline often causes
anxiety and fear in humans, they increase alertness [6].
Adrenal Cortex
Adrenal cortex is accountable for the release of mineralocorticoids, androgens (sex
hormones), and glucocorticoids. Glucocorticoids have effect on metabolism of carbohydrates, fats, and proteins and are essential in facilitating responses to stress as
well as fasting. Mineralocorticoids are important in order to keep balance for sodium
and thus extracellular fluid balance. Compared with the pituitary hormones FSH
and LH, androgen has little effect on reproductive function. Adrenal cortex is histologically divided into three distinct layers. The extreme layer contains zona glomerulosa, central layer (major layer) consists of zona fasciculate cells, and the
innermost layer comprised of zona reticularis. Corticosterone is secreted in all three
regions, whereas biosynthesis of aldosterone occurs in zona glomerulosa. The enzymatic mechanisms that form cortisol (hydrocortisone) and androgens are mainly
present in two internal regions.
Glucocorticoids
The adrenocorticotropic hormone (ACTH) secreted by the anterior lobe of pituitary
gland regulates the release of glucocorticoids. It is regulated via hypothalamic discharge of corticotropin releasing hormone (CRH) into hypophyseal portal system.
Cortisol secretion induces a negative feedback influence on the anterior lobe of
pituitary gland as well as hypothalamus. Cortisol amount in plasma was in diurnal
mode and maximum amount is observed just before awaking in the morning
(Fig. 1.5). The circulating corticosteroids primarily bind to plasma proteins, i.e.,
corticosteroid binding globulin (transcortin) as well as albumin. Hormones are
deactivated mainly in the liver by binding with sulfate or glucuronic acid and are
excreted into the urine. Glucocorticoids play a vital role in controlling metabolism
of carbohydrates, proteins, and fats. They stimulate the storage of liver glycogen.
During fasting, they trigger formation of glucose in the liver in order to supply glucose for brain metabolism. They have an antagonistic effect on insulin, leading to an
increase in blood sugar. Glucocorticoids enhance the vasoconstriction of catecholamines and reduce the permeation of vascular endothelium that is crucial for
1 Physiology of Endocrine System and Related Metabolic Disorders