The adrenal glands, also known as suprarenal glands, are vital endocrine glands responsible for producing hormones that regulate metabolism, immunity, blood pressure, and stress responses. Found in vertebrates, their structural organization reflects the complexity of their functions. Understanding the structural anatomy of the adrenals is crucial for students, educators, and researchers in Zoology and related life sciences.
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Infographic displaying structural layers of adrenal gland |
Introduction
The adrenal glands are small, triangular-shaped glands located on top of each kidney. Despite their size, they have immense physiological significance. Structurally, each gland consists of two distinct regions—the adrenal cortex and the adrenal medulla—each performing specific endocrine functions.
Each adrenal gland is located on the upper pole of each kidney, hence the name "suprarenal gland." The right adrenal gland is pyramidal in shape, while the left is semilunar or crescent-shaped. These glands are relatively small, measuring about 4 to 6 centimeters in length and weighing approximately 4 to 5 grams each. They appear yellowish in color due to the high lipid content in the adrenal cortex, which is involved in the synthesis of steroid hormones.
Internal Structure:
The adrenal cortex is the outer layer and comprises about 80–90% of the gland. The adrenal cortex is divided into three distinct zones, each responsible for producing specific hormones. The zona glomerulosa is the outermost layer of the cortex. It contains columnar or pyramidal-shaped cells and primarily secretes mineralocorticoids, especially aldosterone, which helps regulate electrolyte and water balance by controlling sodium and potassium levels in the body.
The middle and widest zone of the cortex is known as zona fasciculata. This region consists of polyhedral cells arranged in long cords and is rich in lipids. It produces glucocorticoids, mainly cortisol, which play a vital role in glucose metabolism, immune system regulation, and stress response.
The zona reticularis is the innermost layer of the cortex. Its cells are smaller, darker-staining, and arranged in a network-like structure. This zone secretes sex steroids, mainly androgens, which supplement gonadal hormones and are especially important during puberty and in the development of secondary sexual characteristics.
The adrenal medulla forms the central part of the adrenal gland and is functionally linked to the sympathetic nervous system. It contains chromaffin cells, which are modified postganglionic neurons. These cells secrete adrenaline (epinephrine), accounting for about 80%, and noradrenaline (norepinephrine), about 20%. These hormones are essential for the fight-or-flight response, increasing heart rate, blood pressure, and blood glucose levels during stress.
From an embryological perspective, the adrenal cortex originates from the mesoderm, while the medulla arises from neural crest cells of the ectoderm. This dual origin explains their distinct roles—steroid hormone production by the cortex and catecholamine secretion by the medulla.
Functional Coordination
Although structurally and functionally distinct, both cortex and medulla work in synchrony during stress. Cortisol from the cortex enhances the effects of adrenaline from the medulla, optimizing the body's stress response.
Frequently Asked Questions (FAQs)
Q1. What are the two main parts of the adrenal gland?
Ans: The adrenal cortex and adrenal medulla.
Q2. Which hormones are produced by the adrenal cortex?
Ans: Mineralocorticoids, glucocorticoids, and androgens.
Q3. What is the role of adrenaline?
Ans: It prepares the body for fight-or-flight responses by increasing heart rate, blood flow, and glucose levels.
Q4. Which part of the adrenal gland is involved in long-term stress response?
Ans: The adrenal cortex, primarily through the release of cortisol.
Q5. Is the adrenal medulla under hormonal or neural control?
Ans: Neural control via the sympathetic nervous system.
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