BIOLOGY Volume 2 - A Guide to General Biology - 2004

17. COORDINATION AND REGULATION IN ANIMALS

17.6. The Endocrine System

The Endocrine System is formed mainly by Glands of Internal Secretion, which are also called Endocrine glands (from the Greek éndon — within, krinō — separate). A gland is a Structure that secretes a specific substance (secretion) or substances. In the body of humans and animals, Two Types of glands are distinguished: exocrine and endocrine. Exocrine glands (from the Greek éxō — outside), or glands of external secretion, possess ducts. An example of such glands is Sweat Glands, located deep in the Skin and releasing their secretion (sweat) onto its surface. Endocrine glands are characterized by the following features:

1) they secrete substances called Hormones;

2) they have no ducts, and hormones are secreted directly into the Blood;

3) they are richly supplied with blood, i.e., permeated by a relatively large number of Blood Vessels.

Some glands combine endocrine and exocrine Functions, such as the Pancreas, which, In addition to the hormones Insulin and Glucagon, secretes pancreatic juice containing digestive Enzymes that enters the duodenum via a duct.

A hormone is a chemical messenger with the following properties:

1) it is transported by the bloodstream;

2) it acts on target Organs distant from its site of synthesis;

3) it binds to target receptor molecules due to its precise structural fit (like a key fitting a lock), i.e., it acts highly specifically;

4) it is a relatively small, soluble organic molecule;

5) it is effective at low concentrations.

The endocrine and nervous systems act in a coordinated manner to maintain a constant internal environment (Homeostasis) within the body. A comparison of these two systems is presented in Table 17.1. As will be shown below, the Hypothalamus, or more precisely, the hypothalamo-hypophyseal system, plays an important role in neuroendocrine interaction (see also Section 17.2.4). Despite obvious differences, both systems are similar in that they use chemical messengers for cellular communication. It is believed that both systems arose and evolved in parallel as The Need for communication between different PARTS OF THE body increased with its growing size and complexity. In both cases, The primary function of these systems is to coordinate and control vital physiological processes.

The major human endocrine glands are shown in Fig. 17.46, and their hormones and effects are listed in Table 17.9.

Class="center">

Fig. 17.46. Location OF THE major endocrine glands in The Human Body.

Table 17.9. Major human endocrine glands, their functions, and factors regulating their secretion

Gland	Hormones	Functions	Factors regulating secretion
Hypothalamus	Releasing hormones (liberins) and inhibiting hormones (statins) Hormones of the posterior pituitary are also synthesized here	Regulation of Hormone secretion by the anterior pituitary	Secretion is regulated by hormone and metabolite levels via feedback mechanisms
Posterior pituitary	No hormone synthesis occurs; the following Hypothalamic hormones are stored and secreted here: Oxytocin	Milk ejection by the mammary gland, uterine contractions during childbirth	Negative feedback mechanisms involving hormones and The Nervous system
	Antidiuretic hormone (ADH, vasopressin)	Reduction of urine secretion by the Kidneys	Osmotic potential of the blood
Anterior pituitary	Follicle-stimulating hormone (FSH, follitropin)	In males — stimulation of Spermatogenesis In females — follicle growth in the Ovary	Blood levels of estrogens and testosterone; act via the Hypothalamus and Pituitary
	Luteinizing hormone (LH, lutropin)	In males — testosterone secretion	Blood testosterone level; acts via the hypothalamus and pituitary
		In females — secretion of estrogens and progesterone, ovulation, and Maintenance of the corpus luteum	Blood estrogen levels; act via the hypothalamus and pituitary
	Prolactin	Stimulation of milk production and secretion	Hypothalamic hormones
	Thyroid-stimulating hormone (TSH, thyrotropin)	Synthesis and Secretion of THYROID HORMONES and growth of The Thyroid Gland	Blood thyroxine level; acts via the hypothalamus and pituitary
	Adrenocorticotropic hormone (ACTH, corticotropin)	Synthesis and secretion of Adrenal Cortex Hormones, growth of these glands	Blood ACTH level; acts via the hypothalamus
	Growth Hormone (GH, somatotropin)	Protein Synthesis; growth, especially of limb bones	Hypothalamic hormones
Parathyroid gland	Parathyroid hormone (PTH, parathormone)	Increases blood calcium levels and decreases blood phosphate levels	Blood levels of Ca²⁺ and PO₄^3-
Thyroid gland	Triiodothyronine (T₃) and thyroxine (T₄)	Regulation of basal metabolic rate, growth, and development	TSH
	Calcitonin	Decreases blood calcium levels	Blood Ca²⁺ level
Adrenal cortex	Glucocorticoids (cortisol, etc.)	Protein breakdown, glucose/Glycogen synthesis, adaptation to stress, anti-inflammatory/anti-allergic effects	ACTH
	Mineralocorticoids (aldosterone, etc.)	Na⁺ retention by the kidneys; maintenance of the Na⁺/K⁺ ratio in extracellular and intracellular fluids; elevation of blood pressure	Blood levels of Na⁺ and K⁺; low blood pressure
Adrenal medulla	Adrenaline (epinephrine)	Increase in Heart rate and force of contraction; constriction of skin and gastrointestinal capillaries. Dilation of arterioles in The Heart and skeletal Muscles, elevation of blood glucose levels	Sympathetic nervous system
	Noradrenaline (norepinephrine)	General constriction of small Arteries, elevating blood pressure	Nervous system
Islets of Langerhans	Insulin (beta Cells)	Decreases blood glucose levels; increases uptake and utilization of glucose and Amino Acids by cells	Blood levels of glucose and amino acids
	Glucagon (alpha cells)	Increases blood glucose levels; enhances breakdown of glycogen to glucose in the Liver	Blood Glucose Level
Stomach	Gastrin	Secretion of gastric juice	Presence of food in The Stomach
Duodenum	Secretin	Secretion of pancreatic juice; inhibition of gastric juice secretion	Presence of acidic food in the duodenum
	Cholecystokinin (pancreozymin)	Emptying of the Gallbladder and secretion of pancreatic juice into the duodenum	Presence of Fatty acids and amino acids in the duodenum
Kidneys	Renin	Conversion of angiotensinogen to angiotensin	Blood Na⁺ level, low blood pressure
Ovary	Estrogens (17β-estradiol, etc.)	Development of female secondary sexual characteristics; Regulation of the Menstrual cycle	FSH and LH
	Progesterone	Pregnancy, inhibition of ovulation	LH
Corpus luteum	Progesterone and estrogens	GROWTH AND DEVELOPMENT of the Uterus	LH
	Progesterone and estrogens	Fetal development	Developing fetus
Placenta	Chorionic gonadotropin	Maintenance of the corpus luteum	Developing fetus
	Placental lactogen	Stimulation of mammary gland growth	Developing fetus
Testis	Testosterone	Development of male secondary sexual characteristics	LH and FSH

17.6.1. Mechanism of hormone action

As shown in Table 17.10, Vertebrate Hormones can be divided into four groups based on their chemical nature: 1) Peptides and Proteins; 2) amine derivatives (e.g., Tyrosine); 3) Steroids; and 4) fatty acids.

Table 17.10. Chemical Nature of major human hormones

Chemical group	Hormone	Main source
Peptides and proteins	Releasing and inhibiting hormones	Hypothalamus
	Growth hormone Follicle-stimulating hormone Luteinizing hormone Prolactin Thyroid-stimulating hormone Adrenocorticotropic hormone	Anterior pituitary
	Oxytocin ADH (vasopressin)	Posterior pituitary
	Parathyroid hormone	Parathyroid gland
	Calcitonin	Thyroid gland
	Insulin Glucagon	Islets of Langerhans (pancreas)
	Gastrin	Gastric mucosa
	Secretin	Duodenal mucosa
Amines	Adrenaline	Adrenal medulla
	Noradrenaline	Adrenal medulla and sympathetic nervous system
	Thyroxine Triiodothyronine	Thyroid gland
Steroids	Testosterone	Testes
	Estrogens Progesterone	Ovaries and placenta
	Corticosteroids	Adrenal cortex
Fatty acids	Prostaglandins	Many Tissues

Hormone release

The mechanisms regulating hormone release by endocrine glands are listed below: 1. The presence of a specific metabolite in the blood. For example, an excess of glucose in the blood triggers the secretion of insulin by the pancreas, which lowers blood glucose levels. 2. The presence of another hormone in the blood. For example, many hormones secreted by the anterior Pituitary Gland are 'trophic' hormones (tropins), meaning they stimulate hormone secretion by other endocrine glands. 3. Stimulation by the sympathetic nervous system. For example, in Response to nerve impulses generated during stressful situations, The adrenal medulla secretes adrenaline and noradrenaline. Negative feedback In the first two cases described above, the timing and amount of hormone released are regulated by feedback mechanisms, usually negative ('the less, the more' and vice versa). This type of control is discussed in detail in Section 17.6.4 using thyroxine function as an example. Cascade effect Hormone release in response to the presence of another hormone in the blood is typically controlled by the hypothalamus and pituitary gland, and the final effect is often the result of the secretion of three different hormones. A good example is the ACTH-mediated release of cortisol, shown schematically in Fig. 17.47. This mechanism, known as a cascade effect, is important because the action of a small amount of 'trigger' hormone is amplified at each stage, leading to significant Changes in the final target organ. Thus, cortisol in Fig. 17.47 is an anti-stress factor secreted by the outer part (cortex) of the Adrenal Glands. It belongs to the glucocorticoids, which, among other things, raise blood sugar levels when the body requires increased Energy Expenditure (see also Section 17.6.5).

Fig. 17.47. An example of a cascade effect in the REGULATION OF GLUCOSE-to-glycogen conversion through the release of ACTH-releasing factor (corticotropin-releasing hormone). The overall 'Amplification' in this case is 56,000-fold (data from: Bradley, 1976).

Action on target cells Each hormone is highly specific and acts only on particular target cells that carry the corresponding protein receptors on their surface. Non-target cells lack these receptors, so the hormone has no effect on them. Upon binding to its receptor, a hormone can alter the function of various target Cell structures. These primarily include: 1) The Cell membrane; 2) membrane-bound enzymes (second messenger mechanism); 3) genes. Examples of these effects are given below. CELL MEMBRANE. One of the effects of insulin—increased cellular glucose uptake—occurs because the hormone, upon binding to receptors, increases the permeability of the cell membrane to glucose. Adrenaline acts on smooth Muscle cells by opening or closing their sodium, potassium, or both channels, which alters the Membrane Potential and consequently stimulates contraction or relaxation. SECOND MESSENGER MECHANISM. Adrenaline and many Peptide Hormones, upon binding to outer Membrane Receptors, cannot penetrate the cell; however, they trigger the intracellular release of a 'second messenger,' which initiates a cascade of enzymatic reactions leading to the desired effect. In many cases, the second messenger is the nucleotide cyclic adenosine monophosphate (cAMP), whose simplified MECHANISM OF ACTION is shown in Fig. 17.48. When a hormone binds to a receptor protein, this protein is activated and becomes the enzyme adenylate cyclase, which catalyzes The conversion of ATP to cAMP. In principle, cAMP can initiate a wide range of responses depending on the cell type in which it is formed.

Fig. 17.48. A simplified diagram showing how adrenaline stimulates glucose release from a liver cell. Activation of membrane-bound adenylate cyclase leads to The formation of cAMP, which activates enzyme systems that ultimately break down (phosphorolyze) glycogen into glucose. Glucose then diffuses out of the cell into the blood.

Thus, in the case of adrenaline, cAMP activates the enzyme protein kinase, which in turn activates another enzyme, phosphorylase, which converts glycogen to phosphorylated glucose. The ultimate effect of adrenaline in this scenario is the release of glucose from the cell. Amplification occurs at each step of this process: only a few molecules of adenylate cyclase are needed to activate many protein kinase molecules, and so on. This is another example of a cascade effect. Other Hormones that use cAMP as a second messenger include ADH (Sections 17.6.2 and 20.6), TSH (Section 17.6.4), ACTH (Section 17.6.5), glucagon (Section 17.6.6), LH and FSH (Sections 21.7.4 and 21.7.6), and most hypothalamic releasing hormones. GENES. Steroid Hormones (Sex Hormones and hormones secreted by the adrenal cortex) pass through the outer membrane of target cells and bind to a receptor protein in the Cytoplasm. The resulting complex enters the Cell Nucleus, where the hormone acts directly on the Chromosomes, 'switching on' specific genes by triggering their METABOLISM/31.html">Transcription—the synthesis of Messenger RNA (mRNA). This RNA moves into the cytoplasm, where it is translated, leading to the synthesis of new proteins, such as enzymes that perform specific functions. The hormone thyroxine, upon entering the cell, binds directly to receptor proteins on the chromosomes, producing a similar effect.

Last update: 06/08/2026

Editorial and Educational Adaptation: This material has been compiled based on the primary/original source text. The project team performed an editorial review, corrected technical inaccuracies, structured sections, and adapted the content for an educational format.

What was processed:

elimination of formatting defects (OCR errors, structural breaks, corrupted characters);
editorial organization of content;
standardization of terminology in accordance with academic sources;
verification of factual statements against the original source text.

All mentions of the author, publication year, and origin of the primary text have been preserved in accordance with the source.