BIOLOGY Volume 2 - A Guide to General Biology - 2004

17. COORDINATION AND REGULATION IN ANIMALS

17.6. The Endocrine System

17.6.5. The Adrenal Glands

Human Adrenal Glands are paired Organs, with each gland resting on the upper pole of a Kidney (Fig. 17.46). The adrenals consist of two distinct layers that have different embryological origins and function independently of one another. The outer layer, the adrenal cortex, accounts for 80% of the gland's mass. The inner layer, the adrenal medδulla [або medulla], is very similar in origin and function to the sympathetic Nervous system.

The Adrenal Cortex

The adrenal cortex secretes Two Types of Steroid Hormones, known as corticosteroids (Table 17.12). All of these hormones are derived from a common precursor, Cholesterol, which can be synthesized within the cortex itself or absorbed from the bloodstream when ingested in food. As fat-soluble compounds, steroid hormones diffuse readily across Cell membranes and bind to receptor Proteins in the Cytoplasm. The resulting hormone-receptor complexes are then transported into The Nucleus, where they bind to specific chromosomal sites, effectively "turning on" or "turning off" particular genes.

The size of the adrenal glands correlates with ACTH secretion and the body's capacity to cope with stress. These glands enlarge during prolonged periods of stress. Behavioral studies of animals under stress have shown that as population density increases, so does the secretion of Adrenocortical Hormones. Furthermore, in social species with established hierarchies, There is a direct correlation between an individual's social rank and the size of its adrenal glands.

Class="center">Table 17.12. Hormones secreted by the adrenal cortex and their Functions

Hormones

Function

Notes

Mineralocorticoids (aldosterone, etc.)

Regulate the body's Water and Electrolyte balance by activating cation pumps in cell membranes, which retains Na+ and Cl- while excreting K+. They prevent excessive loss of Na+ via sweat, saliva, and urine, and maintain a constant osmotic concentration in Body Fluids

Renin, secreted by the juxtaglomerular apparatus of the Kidneys, catalyzes The formation of angiotensin, which stimulates the release of aldosterone. Aldosterone enhances renal absorption of Na+ and triggers the release of ADH, which increases water reabsorption in the renal tubules. ACTH has no effect on aldosterone secretion

Glucocorticoids (cortisol, etc.)

A. Effects on Carbohydrate METABOLISM:

1) promote Gluconeogenesis;

2) stimulate hepatic Glycogen formation;

3) elevate Blood glucose levels

B. Protein metabolism:

1) stimulate The breakdown of Plasma Proteins;

2) increase the pool of free Amino Acids available for hepatic Protein Synthesis

C. Other functions:

1) suppress inflammatory and allergic responses;

2) reduce antibody production

Hypersecretion of these hormones leads to Cushing's syndrome (characterized by abdominal fat deposition, Muscle wasting, Hypertension, diabetes, and excessive Hair growth). Hypersecretion of ACTH by the adenohypophysis is known as Cushing's Disease. Hyposecretion of the adrenal cortex results in Addison's disease, manifested by muscle weakness, hypotension, lowered resistance to infection, fatigue, and Skin pigmentation

Regulation of corticosteroid secretion

As shown in Table 17.12, mineralocorticoid secretion is stimulated by The activity of renin and angiotensin. Glucocorticoids are released under The Influence of adrenocorticotropic hormone (ACTH), whose role in cortisol (hydrocortisone) release is illustrated in Fig. 17.47. This is a classic example of a cascade effect that amplifies The amount of hormone secreted through a chain of reactions.

ACTH is a polypeptide consisting of 39 amino acid residues. By binding to receptors On the surface of target cortical Cells, it activates adenylate cyclase, which converts ATP to cAMP (Section 17.6.1). This activates protein Kinases, ultimately leading to The conversion of cholesterol into glucocorticoids.

Adrenal medulla

The adrenal medulla forms the core of the Adrenal gland; it is densely innervated and richly supplied with Blood Vessels. The Cells of the medulla are modified sympathetic Neurons. In response to stimulation, they secrete adrenaline and noradrenaline in a 4:1 ratio (Section 17.6.1). Noradrenaline is also released as a neurotransmitter at the synapses of the sympathetic nervous system. Fundamentally, the adrenal medulla simply reinforces the effects of the sympathetic nervous system and is therefore not a vital gland. Medullary hormones act on A wide variety of organs, preparing the animal for rapid, vigorous responses such as fight or flight. In addition, these hormones help the body cope with stressful situations associated with, for example, intense physical exertion, pain, nervous Shock, cold, drops in blood sugar (hypoglycemia) and blood pressure (hypotension), anger, passion, and excitement. The sympathetic nervous system exhibits analogous functions.

Noradrenaline (norepinephrine) and adrenaline (epinephrine) are synthesized from The amino acid Tyrosine and belong to a group of BIOLOGICALLY ACTIVE SUBSTANCES known as catecholamines (Fig. 17.55). As seen in Table 17.13, the effects of both hormones are largely identical, except for their action on blood vessels. Noradrenaline causes vasoconstriction throughout the body, whereas adrenaline constricts blood vessels supplying the skin and digestive tract while dilating those supplying the Muscles and Brain. Both hormones bind to two types of so-called adrenergic receptors (α and β) on target cells. This activates adenylate cyclase, leading to the synthesis of cAMP and triggering the specific tissue responses listed in Table 17.13. Many organs contain both α- and β-receptors, with α-receptors appearing to have a higher affinity for noradrenaline, and β-receptors for adrenaline.

Fig. 17.55. Structural formulas of noradrenaline and adrenaline.

Table 17.13. Physiological effects of noradrenaline and adrenaline

Dilate pupils

Cause hair to stand on end (piloerection)

Dilate bronchioles, thereby improving pulmonary ventilation

Inhibit peristalsis

Suppress Digestion

Inhibit bladder contraction

Increase Heart rate and myocardial contractility

Cause generalized vasoconstriction

Raise blood pressure

Stimulate the conversion of glycogen to glucose in the Liver

Lower sensory thresholds

Increase alertness


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