Chemical Coordination and Integration

Learning Outcomes:

1. Understand the role of endocrine glands and hormones in the body.
2. Comprehend the structure and function of the human endocrine system.
3. Analyze the mechanisms of hormone action.
4. Recognize the importance of chemical messengers in maintaining body functions.

Endocrine Glands and Hormones

Endocrine glands, unlike exocrine glands, are ductless and release their secretions, known as hormones, directly into the bloodstream. Hormones function as intercellular messengers, produced in trace amounts and regulating distant target organs. The classic definition of a hormone—a chemical produced by endocrine glands and carried by blood to a distant target organ—has evolved. Now, a hormone is defined as a non-nutrient chemical messenger produced in small amounts. The human body, along with vertebrates, possesses complex endocrine systems involving numerous hormones, while invertebrates have simpler systems.

Key Points about Hormones:

1. They are non-nutrient chemicals acting as messengers.
2. Hormones are produced in trace amounts and can influence distant organs.
3. In vertebrates, hormones regulate a variety of biological processes.

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The Human Endocrine System

The human endocrine system consists of organized endocrine glands and hormone-producing tissues located throughout the body. These organs include the pituitary, pineal, thyroid, adrenal, pancreas, parathyroid, thymus, and gonads (testis in males and ovary in females). Several other organs, such as the gastrointestinal tract, liver, kidney, and heart, also secrete hormones.

The Hypothalamus

The hypothalamus, situated at the base of the diencephalon in the forebrain, plays a key role in regulating a wide range of bodily functions. It contains clusters of neurosecretory cells (nuclei) that produce releasing and inhibiting hormones to regulate pituitary hormone synthesis and secretion.

Functions of Hypothalamic Hormones:

1. Gonadotrophin-releasing hormone (GnRH) stimulates pituitary release of gonadotrophins.
2. Somatostatin inhibits growth hormone release from the pituitary.
3. Releasing hormones regulate pituitary secretion, while inhibiting hormones suppress it.

The Pituitary Gland

The pituitary gland is enclosed in a bony structure called the sella turcica, attached to the hypothalamus. It is divided into two anatomical parts: adenohypophysis (anterior pituitary) and neurohypophysis (posterior pituitary).

Key Functions of Pituitary Hormones:

1. The anterior pituitary produces growth hormone (GH), prolactin (PRL), thyroid-stimulating hormone (TSH), adrenocorticotrophic hormone (ACTH), luteinizing hormone (LH), and follicle-stimulating hormone (FSH).
2. Oxytocin and vasopressin are stored and released by the posterior pituitary, although they are synthesized in the hypothalamus.

Hormonal Disorders:

1. Over-secretion of GH can lead to gigantism, while under-secretion causes dwarfism.
2. Excess secretion of GH in adulthood causes acromegaly, characterized by abnormal growth, especially of facial features.
3. Lack of vasopressin (ADH) results in Diabetes Insipidus, causing excessive loss of water.

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The Pineal Gland

The pineal gland, located in the forebrain, secretes melatonin, a hormone crucial in maintaining the body’s 24-hour cycle (circadian rhythm).

Functions of Melatonin:

1. Regulates the sleep-wake cycle and body temperature.
2. Influences metabolism, pigmentation, and the menstrual cycle.

Thyroid Gland

The thyroid gland has two lobes on either side of the trachea, connected by a tissue called the isthmus. It secretes two main hormones: thyroxine (T4) and triiodothyronine (T3). Iodine is essential for thyroid hormone synthesis, and its deficiency leads to hypothyroidism and conditions like goiter.

Effects of Thyroid Hormones:

1. Regulate the basal metabolic rate.
2. Stimulate red blood cell formation.
3. Control carbohydrate, protein, and fat metabolism.

Thyroid Disorders:

1. Hypothyroidism leads to conditions such as cretinism and irregular menstrual cycles.
2. Hyperthyroidism can cause Graves’ disease, characterized by goiter, exophthalmos, and weight loss.

Parathyroid Gland

Parathyroid glands, located behind the thyroid, secrete parathyroid hormone (PTH), which regulates calcium homeostasis.

Functions of PTH:

1. Increases calcium levels by stimulating bone resorption.
2. Enhances calcium reabsorption in the kidneys.
3. Promotes calcium absorption from food.

Thymus

The thymus gland, located between the lungs, is crucial for the development of the immune system. It secretes thymosins, which aid in the differentiation of T-lymphocytes.

Functions of Thymosins:

1. Provide cell-mediated immunity.
2. Promote antibody production for humoral immunity.

The thymus undergoes degeneration in older individuals, leading to weakened immune responses.

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Adrenal Gland

The adrenal glands, located above each kidney, consist of the adrenal medulla and the adrenal cortex.

Adrenal Medulla Hormones:

1. Adrenaline (epinephrine) and noradrenaline (norepinephrine) are secreted during stress and emergency situations.
2. These hormones are known as emergency hormones or fight-or-flight hormones.
3. They increase alertness, heart rate, respiration, and glycogen breakdown.

Adrenal Cortex Hormones:

1. The cortex secretes glucocorticoids and mineralocorticoids.
2. Glucocorticoids (e.g., cortisol) regulate carbohydrate metabolism, immune response, and cardiovascular function.
3. Mineralocorticoids (e.g., aldosterone) control water and electrolyte balance, influencing blood pressure.

Adrenal Disorders:

1. Underproduction of cortical hormones leads to Addison’s disease, causing weakness and fatigue.

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Pancreas

The pancreas functions both as an exocrine and endocrine gland. The endocrine portion, composed of the Islets of Langerhans, contains alpha and beta cells that secrete glucagon and insulin respectively.

Functions of Pancreatic Hormones:

1. Glucagon increases blood glucose levels by promoting glycogenolysis and gluconeogenesis.
2. Insulin decreases blood glucose by enhancing cellular glucose uptake and stimulating glycogenesis.

Pancreatic Disorders:

1. Lack of insulin or insulin resistance results in diabetes mellitus, characterized by hyperglycemia and glucose loss in urine.

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Testis

The testis, located in the scrotum of males, is responsible for sperm production and secretion of androgens like testosterone.

Functions of Androgens:

1. Regulate the development of male secondary sex characteristics (e.g., deep voice, facial hair).
2. Stimulate spermatogenesis and influence male sexual behavior.

Ovary

The ovaries of females produce estrogen and progesterone.

Functions of Ovarian Hormones:

1. Estrogen regulates the growth of female secondary sexual characteristics and supports the menstrual cycle.
2. Progesterone is crucial for pregnancy maintenance and mammary gland development.

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Hormones of the Heart, Kidney, and Gastrointestinal Tract

Several non-endocrine organs also produce hormones. For example, the heart secretes atrial natriuretic factor (ANF), which reduces blood pressure by dilating blood vessels. The kidneys produce erythropoietin, which stimulates red blood cell production (erythropoiesis). The gastrointestinal tract secretes hormones such as **gastrin

**, *secretin*, *cholecystokinin (CCK)*, and *gastric inhibitory peptide (GIP)* to regulate digestive processes.

Mechanism of Hormone Action

Hormones exert their effects by binding to specific hormone receptors in target tissues. These receptors are either membrane-bound or intracellular. Hormones that bind to membrane-bound receptors generate second messengers like cyclic AMP to regulate cell metabolism. Hormones interacting with intracellular receptors, such as steroid hormones, regulate gene expression and influence developmental processes.

Important Note:

1. Hormone receptors are specific to each hormone, ensuring precise control over physiological processes.

Types of Hormones Based on Chemical Nature:

1. Peptide hormones (e.g., insulin).
2. Steroid hormones (e.g., testosterone).
3. Amino acid derivatives (e.g., epinephrine).
4. Iodothyronines (e.g., thyroid hormones).

MCQ:
Which hormone is secreted by the posterior pituitary but synthesized by the hypothalamus?
Answer: Oxytocin

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Comparison Table: Adrenal Cortex vs. Adrenal Medulla

Feature	Adrenal Cortex	Adrenal Medulla
Hormones Secreted	Glucocorticoids, Mineralocorticoids	Adrenaline, Noradrenaline
Function	Regulate metabolism, blood pressure	Control fight-or-flight responses
Disorder Associated	Addison’s Disease	Stress-related reactions