Physiology of the Endocrine System

Summary

This lecture delves into the endocrine system's physiology, explaining the chemical classification of hormones and their mechanisms of release. It covers the functions of 10 major endocrine glands and their hormones, including the hypothalamus-pituitary relationship and the roles of insulin, glucagon, estrogen, progesterone, and testosterone. The lecture also discusses feedback loops and the impact of hormones on growth, metabolism, and reproduction, highlighting the endocrine system's complexity and importance.

Takeaways

• 🔬 The endocrine system lecture covers the physiology of 10 major endocrine glands and their hormones, including their chemical classification and mechanisms of release.
• 🧪 Hormones are chemically classified into lipid-soluble (steroid hormones derived from cholesterol) and water-soluble (derived from amino acids or peptides), affecting their transport and function in the body.
• 🚀 Lipid-soluble hormones can diffuse through the cell membrane and bind to intracellular receptors, while water-soluble hormones bind to cell surface receptors and act through secondary messengers.
• 🔄 Hormones are released by three main mechanisms: neural stimulation, humoral stimulation, and hormonal stimulation, each involving different triggers and feedback loops.
• 🌟 The hypothalamus-pituitary axis is central to the endocrine system, with the hypothalamus producing hormones that regulate the pituitary gland, which in turn controls other endocrine glands.
• 🌱 Growth hormone (HGH) stimulates the production of insulin-like growth factors, promoting cell growth and affecting blood glucose levels, regulated by growth hormone releasing and inhibiting hormones.
• 🍼 Prolactin initiates and maintains milk production in mammary glands, with its secretion regulated by the hypothalamus and influenced by infant suckling.
• 🔄 The thyroid gland produces T3 and T4 hormones, which increase the basal metabolic rate and affect cell growth and body heat production, regulated by thyroid stimulating hormone (TSH).
• 🏋️‍♂️ Adrenaline and cortisol from the adrenal cortex respond to stress and regulate metabolism, with cortisol having immunosuppressive effects and aldosterone affecting blood pressure and volume.
• 💊 The pancreas produces insulin and glucagon, which work antagonistically to regulate blood glucose levels, with insulin lowering and glucagon raising them.
• 👶🏻 The ovaries and testes produce sex hormones like estrogen, progesterone, and testosterone, which are crucial for sexual development, reproduction, and secondary sexual characteristics, regulated by follicle stimulating hormone (FSH) and luteinizing hormone (LH).

Q & A

What are the two broad chemical classes of hormones?

Hormones can be divided into two broad classes: those that are lipid-soluble (mostly derived from cholesterol, also known as steroid hormones) and those that are water-soluble (mainly derived from a single amino acid or a peptide).

How do lipid-soluble hormones differ in their transport and duration of action compared to water-soluble hormones?

Lipid-soluble hormones are transported in the blood by transport proteins due to their inability to dissolve in water, and they remain in circulation much longer than water-soluble hormones, which can dissolve in the watery blood and circulate freely. This results in a longer duration of action for lipid-soluble hormones.

What is the primary function of water-soluble hormones in the bloodstream?

Water-soluble hormones, because they can dissolve in the watery blood, circulate as free hormones not attached to proteins. They have a shorter half-life and are often degraded by enzymes in the blood.

How do lipid-soluble hormones interact with their target cells?

Lipid-soluble hormones can diffuse through the phospholipid bilayer of the cell membrane due to their solubility in lipids. Their receptors are located inside the cell, allowing the hormones to bind and alter gene expression within the target cell.

What is the mechanism by which water-soluble hormones bind to their receptors and initiate a response?

Water-soluble hormones, unable to pass through the phospholipid bilayer, bind to receptors located on the surface of the plasma membrane. This binding activates a G protein, which in turn activates adenylate cyclase, leading to a cascade of events that alter the target cell's activity.

What is the exception to the classification of water-soluble hormones, and how do they behave like lipid-soluble hormones?

The thyroid hormones T3 and T4 are an exception. Although water-soluble and derived from the amino acid tyrosine, they are transported in the blood by a transport protein and can diffuse through the plasma membrane to bind to receptors inside their target cells, similar to lipid-soluble hormones.

What are the three main mechanisms that control hormone release from endocrine glands?

The three main mechanisms are neural stimulation, where an action potential stimulates an endocrine gland; humoral stimulation, where the concentration of a chemical in the blood triggers hormone release; and hormonal stimulation, where a hormone is regulated by the presence or absence of another hormone in the blood.

Can you explain the role of the hypothalamus in the regulation of the anterior pituitary gland?

The hypothalamus produces hormones that regulate the secretion of the anterior pituitary gland. It is connected to the anterior pituitary by a portal system, allowing direct secretion of hypothalamic hormones into the blood vessels that supply the anterior pituitary, influencing the release of its hormones.

How does the posterior pituitary gland release its hormones?

The posterior pituitary gland does not produce its own hormones but stores and releases two hormones produced in the hypothalamus: oxytocin and antidiuretic hormone (ADH). These hormones are transported to the posterior pituitary via nerve axons and released into the bloodstream upon stimulation.

What are the main functions of the hormones released by the anterior pituitary gland?

The anterior pituitary gland releases hormones that regulate a wide range of bodily functions. These include human growth hormone (HGH), thyroid-stimulating hormone (TSH), follicle-stimulating hormone (FSH), luteinizing hormone (LH), prolactin, and adrenocorticotropic hormone (ACTH). They act as tropic hormones, controlling other endocrine glands or various body processes.

How does the regulation of thyroid-stimulating hormone (TSH) work in relation to the hypothalamus and the thyroid gland?

The release of TSH from the anterior pituitary is stimulated by thyrotropin-releasing hormone (TRH) from the hypothalamus. TRH release depends on blood levels of thyroid hormones T3 and T4. When thyroid hormone levels are low, TRH is released, stimulating TSH production, which in turn stimulates the thyroid gland to produce more thyroid hormones. High levels of thyroid hormones create a negative feedback loop, inhibiting further release of TRH and TSH.