Makindo Medical Notes

Hormones are chemical messengers produced by glands in the endocrine system. They regulate various functions in the body, including growth, metabolism, reproduction, and mood. These hormones are secreted directly into the bloodstream and travel to target organs, ensuring the body functions properly.

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• Hormones are chemical messengers produced by various glands in the body that regulate a wide range of physiological processes, including growth, metabolism, reproduction, mood, and immune function. Hormones are secreted directly into the bloodstream, where they travel to target organs and tissues to exert their effects.
• There are several types of hormones, each with a specific mechanism of action, which can be classified based on their structure and function.

Types of Hormones

• Steroid Hormones: Steroid hormones are derived from cholesterol and are lipid-soluble, meaning they can pass through cell membranes. They include hormones such as cortisol, aldosterone, testosterone, and estrogen. Steroid hormones primarily regulate processes such as metabolism, salt and water balance, inflammation, and reproductive functions.
  • Mechanism of action: Steroid hormones pass through the lipid bilayer of the target cell’s membrane and bind to specific intracellular receptors in the cytoplasm or nucleus. The hormone-receptor complex then binds to DNA, activating or repressing specific genes, leading to changes in protein synthesis. This results in long-term effects such as alterations in cell growth, metabolism, and reproduction.
  • Examples: Cortisol: Released by the adrenal glands, cortisol helps regulate metabolism, immune response, and stress adaptation. Testosterone and Estrogen: Sex hormones responsible for reproductive functions and secondary sexual characteristics.
• Peptide Hormones: Peptide hormones are made up of chains of amino acids and are water-soluble. They include hormones such as insulin, glucagon, growth hormone, and antidiuretic hormone (ADH). Peptide hormones play key roles in regulating metabolism, growth, fluid balance, and stress responses.
  • Mechanism of action: Peptide hormones cannot pass through the cell membrane due to their water solubility, so they bind to specific receptors on the surface of target cells. This binding activates second messenger systems (e.g., cAMP) inside the cell, leading to a cascade of intracellular events that result in rapid changes in cell function. The effects of peptide hormones tend to be fast but short-lived compared to steroid hormones.
  • Examples: Insulin: Produced by the pancreas, insulin helps regulate blood glucose levels by facilitating the uptake of glucose into cells. Glucagon: Also produced by the pancreas, glucagon increases blood sugar levels by promoting the release of glucose from stored glycogen in the liver.
• Amino Acid-Derived Hormones: Amino acid-derived hormones are small molecules derived from amino acids, such as tyrosine or tryptophan. These hormones include thyroid hormones and catecholamines (e.g., epinephrine and norepinephrine). They regulate metabolism, heart rate, blood pressure, and the fight-or-flight response.
  • Mechanism of action: Amino acid-derived hormones can either be lipid-soluble (like thyroid hormones) or water-soluble (like catecholamines). Lipid-soluble hormones act similarly to steroid hormones by crossing the cell membrane and binding to intracellular receptors, while water-soluble hormones bind to surface receptors and activate second messenger pathways.
  • Examples: Thyroid hormones (T3 and T4): Produced by the thyroid gland, these hormones regulate metabolism, heart rate, and energy levels. Epinephrine (Adrenaline): Secreted by the adrenal medulla, epinephrine is involved in the fight-or-flight response, increasing heart rate, blood pressure, and glucose availability during stress.
• Eicosanoids: Eicosanoids are hormones derived from fatty acids, such as arachidonic acid. They act as local signaling molecules and include prostaglandins, thromboxanes, and leukotrienes. Eicosanoids are involved in regulating inflammation, blood clotting, and immune responses.
  • Mechanism of action: Eicosanoids act primarily on nearby cells (paracrine signaling) rather than traveling through the bloodstream. They bind to cell surface receptors and activate signaling pathways that modulate inflammatory and immune processes. Their effects are usually short-lived, due to rapid degradation.
  • Examples: Prostaglandins: Involved in inflammation, pain, and fever responses. Non-steroidal anti-inflammatory drugs (NSAIDs) inhibit prostaglandin synthesis to reduce inflammation and pain. Thromboxanes: Promote blood clot formation by causing platelet aggregation and vasoconstriction.
• Glycoprotein Hormones: Glycoprotein hormones are peptide hormones that have a carbohydrate group attached. They include hormones such as follicle-stimulating hormone (FSH), luteinizing hormone (LH), and thyroid-stimulating hormone (TSH). These hormones are involved in regulating reproductive functions and thyroid activity.
  • Mechanism of action: Like other peptide hormones, glycoprotein hormones bind to receptors on the surface of target cells and initiate signaling cascades via second messengers. Their effects include stimulating the production of sex hormones, regulating the menstrual cycle, and controlling thyroid hormone release.
  • Examples: Follicle-stimulating hormone (FSH): Stimulates the maturation of ovarian follicles in women and spermatogenesis in men. Thyroid-stimulating hormone (TSH): Stimulates the thyroid gland to produce thyroid hormones, regulating metabolism and energy use.

Diagram

Introduction to Hormonal Regulation

Hormonal regulation refers to the mechanisms by which the body controls the production, release, and action of hormones. The endocrine system, comprising various glands such as the pituitary, thyroid, adrenal, and pancreas, is responsible for secreting hormones into the bloodstream. Hormones are essential for regulating processes such as growth, metabolism, reproduction, and stress responses. The release and action of hormones are tightly regulated to maintain homeostasis, ensuring that physiological processes function correctly.

Negative Feedback Mechanism

One of the primary methods of hormonal regulation is through negative feedback loops. In this mechanism, the body reduces the production of a hormone when its levels are too high, and increases production when levels are too low. This system works similarly to a thermostat that maintains a constant temperature.

• Example: Regulation of Thyroid Hormones (T3 and T4): The hypothalamus secretes thyroid-releasing hormone (TRH), which stimulates the anterior pituitary gland to release thyroid-stimulating hormone (TSH). TSH then acts on the thyroid gland to produce the thyroid hormones T3 and T4. As levels of T3 and T4 rise in the blood, they exert negative feedback on both the hypothalamus and pituitary gland to reduce the secretion of TRH and TSH. This feedback loop helps maintain stable levels of thyroid hormones. /li>
• Example: Regulation of Blood Glucose by Insulin and Glucagon: When blood glucose levels rise after a meal, the pancreas releases insulin, which promotes the uptake of glucose into cells, lowering blood glucose levels. As glucose levels drop, insulin secretion decreases. Conversely, when blood glucose levels fall, the pancreas secretes glucagon, which stimulates the liver to release stored glucose into the bloodstream, raising blood glucose levels. This is an example of negative feedback regulating blood glucose.

Positive Feedback Mechanism

Unlike negative feedback, positive feedback amplifies the original stimulus, leading to an increase in hormone production. Positive feedback loops are less common but are important in certain biological processes.

• Example: Oxytocin During Childbirth: During labor, the hormone oxytocin is released from the pituitary gland, stimulating uterine contractions. These contractions push the baby toward the cervix, which sends signals to the brain to release more oxytocin. This increases the strength and frequency of the contractions, further amplifying oxytocin release. This positive feedback loop continues until the baby is delivered.
• Example: Blood Clotting: When a blood vessel is injured, platelets begin to adhere to the site and release chemicals that attract more platelets. This creates a positive feedback loop that accelerates the clotting process, forming a blood clot to seal the injury.

Hormonal Rhythms

Some hormones are regulated by circadian rhythms or other time-based cycles. Hormones like cortisol and melatonin follow a daily rhythm, rising and falling at specific times of day to align with the body's needs.

• Cortisol: Cortisol, produced by the adrenal glands, follows a circadian rhythm. Levels are highest in the early morning, helping to wake the body and provide energy for the day. Cortisol levels gradually decline throughout the day and reach their lowest point at night. This pattern is essential for regulating metabolism, immune function, and the body's stress response.
• Melatonin: Melatonin is a hormone produced by the pineal gland in response to darkness. It regulates the sleep-wake cycle by promoting sleep. Melatonin levels are high at night and low during the day. Disruptions in this rhythm, such as those caused by shift work or jet lag, can affect sleep and overall health.

Hormonal Regulation via Receptor Sensitivity

Hormones exert their effects by binding to specific receptors on target cells. The sensitivity of these receptors can be

• Downregulation of Receptors: Prolonged exposure to high levels of a hormone can lead to a decrease in the number of receptors for that hormone on the target cell, reducing the cell's sensitivity. For example, continuous exposure to high insulin levels (as seen in type 2 diabetes) can result in insulin resistance, where the cells no longer respond effectively to insulin.
• Upregulation of Receptors: In contrast, a decrease in hormone levels may lead to an increase in the number of receptors, enhancing the cell’s sensitivity to that hormone. For example, when estrogen levels rise during pregnancy, there is an increase in oxytocin receptors in the uterus, preparing the body for labour.

Endocrine Gland Control

The activity of many endocrine glands is regulated by the hypothalamus-pituitary axis. The hypothalamus releases hormones that either stimulate or inhibit the release of pituitary hormones, which in turn regulate the function of other endocrine glands such as the thyroid, adrenal glands, and gonads.

• Example: Hypothalamic-Pituitary-Adrenal (HPA) Axis: The HPA axis controls the release of cortisol from the adrenal glands. The hypothalamus secretes corticotropin-releasing hormone (CRH), which stimulates the pituitary gland to release adrenocorticotropic hormone (ACTH). ACTH then triggers the adrenal glands to release cortisol. This axis plays a central role in regulating the body's response to stress.
• Example: Hypothalamic-Pituitary-Gonadal (HPG) Axis: The HPG axis regulates reproductive hormones. The hypothalamus secretes gonadotropin-releasing hormone (GnRH), which stimulates the pituitary gland to release luteinizing hormone (LH) and follicle-stimulating hormone (FSH). These hormones regulate the production of sex hormones (estrogen, progesterone, and testosterone) from the ovaries and testes.

Conclusion

Hormonal regulation is essential for maintaining homeostasis in the body. Through mechanisms such as negative and positive feedback loops, circadian rhythms, receptor sensitivity, and control by the hypothalamus-pituitary axis, the body carefully modulates hormone levels to meet its physiological needs. Disruptions in these regulatory systems can lead to endocrine disorders such as diabetes, hypothyroidism, or adrenal insufficiency, underscoring the importance of balanced hormonal control.

Anterior Pituitary Hormones

The anterior pituitary gland, often called the "master gland," secretes several vital hormones:

• Growth hormone (GH): Stimulates growth of bones and tissues, helps regulate metabolism.
• Thyroid-stimulating hormone (TSH): Stimulates the thyroid gland to release thyroid hormones.
• Adrenocorticotropic hormone (ACTH): Stimulates the adrenal cortex to produce cortisol.
• Follicle-stimulating hormone (FSH): Promotes the development of reproductive cells (spermatogenesis in males, follicular growth in females).
• Luteinizing hormone (LH): Triggers ovulation in females and stimulates testosterone production in males.
• Prolactin: Stimulates milk production in females during lactation.

Adrenal Cortex Hormones

The adrenal cortex produces hormones crucial for maintaining homeostasis, including:

• Mineralocorticoids: Regulate electrolyte and water balance. Examples include aldosterone and 11-deoxycorticosterone.
• Glucocorticoids: Help regulate metabolism, reduce inflammation, and manage stress. Examples include cortisol and corticosterone.
• Sex hormones: Small amounts of androgens, oestrogen, and progesterone are produced here, influencing reproductive functions.

Posterior Pituitary Hormones

The posterior pituitary stores and releases hormones made in the hypothalamus:

• Antidiuretic hormone (ADH): Helps regulate water balance by reducing urine production and promoting water reabsorption in the kidneys.
• Oxytocin: Stimulates uterine contractions during childbirth and milk ejection during breastfeeding.

Thyroid Gland Hormones

The thyroid gland regulates metabolism and calcium levels through the secretion of:

• Thyroxine (T4): Increases metabolic rate and influences growth and development.
• Triiodothyronine (T3): More potent than T4, it also increases metabolic rate and regulates numerous bodily functions.
• Calcitonin: Lowers blood calcium levels by inhibiting bone resorption.

Parathyroid Gland Hormone

Parathormone (PTH): Raises blood calcium levels by promoting calcium release from bones and increasing calcium reabsorption in the kidneys.

Adrenal Medulla Hormones

The adrenal medulla secretes catecholamines, which prepare the body for the fight-or-flight response:

• Adrenaline (Epinephrine): Increases heart rate, blood pressure, and energy supply.
• Noradrenaline (Norepinephrine): Similar to adrenaline, but primarily affects blood vessels, increasing blood pressure.
• Dopamine: Influences pleasure, motivation, and motor control, and acts as a precursor to adrenaline and noradrenaline.

Conclusion

In summary, hormones play a vital role in regulating almost every physiological process in the human body. The balance and proper functioning of these hormones are crucial for maintaining homeostasis and overall health. Understanding how these hormones work helps in diagnosing and treating various endocrine disorders.