The Endocrine System

General Information

• The endocrine system is one of two major control systems in the body, the other being the nervous system.
• This system comprises ductless glands, known as endocrine glands, that release hormones.
• Unlike exocrine glands, which secrete products into ducts, endocrine glands release hormones directly into the bloodstream;
• Hormones act as chemical messengers, traveling from secretion sites to target cells.
• Endocrinology involves the analysis of hormone structures, functions, and hormonal systems.
• Hormone actions are governed by ligand-receptor interactions and cell signaling
• Hormones link various organ systems
• Homeostasis maintenance is a coordination of multiple organ system functions.
• Multiple regulatory systems often working in opposition control most physiological functions
• Hormone binding to carrier proteins and receptors shows chemistry and physics influence physiological processes
• The hypothalamus and anterior pituitary connections determines function.
• Controlled exchange of materials that occurs that synthesize hormones.

Hormones and Endocrine Glands

• Endocrine glands are ductless, releasing hormones into the bloodstream, first into interstitial fluid then diffused into the blood.
• The endocrine system's components are not physically connected but functionally form a system.
• Organs like the heart can be part of the endocrine system alongside primary functions by having hormone-secreting cells.
• The hypothalamus releases neurohormones, functioning as hormones by entering the bloodstream rather than affecting adjacent cells as neurotransmitters.

Hormone Structures and Synthesis

• Hormones fall into three classes: amines, peptides/proteins, and steroids.
• Amine hormones are tyrosine derivatives, including thyroid hormones, catecholamines (epinephrine/norepinephrine), and dopamine.
• The adrenal gland secretes catecholamines from the adrenal medulla and steroid hormones from the adrenal cortex.
• The adrenal medulla is a modified sympathetic ganglion that releases secretions into the blood.
• Dopamine, another catecholamine, is synthesized by hypothalamic neurons and inhibits activity in pituitary endocrine cells.
• Peptide hormones are generally polypeptides synthesized as preprohormones, then cleaved into prohormones, and finally processed into active hormones.
• Steroid hormones are derived from cholesterol either synthesized by the cells or uptaken from extracellular fluid.
• Steroid hormone production involves anterior pituitary hormone binding to plasma membrane receptors leading to cAMP production and protein phosphorylation.
• Steroid hormones diffuse across the plasma membrane to bind with carrier proteins.
• Five hormones are secreted by the adrenal cortex: aldosterone, cortisol, corticosterone, DHEA, and androstenedione.
• Aldosterone or mineralocorticoid that regulates salt balance by impacting sodium, potassium, and hydrogen ions in the kidneys controlled by Angiotensin II.
• Cortisol, a glucocorticoid, mainly affects glucose and other nutrient metabolism, as well as impacting the immune system.
• Adrenal androgens (DHEA/androstenedione) have less potency than testosterone but function in adult females and in both sexes in the fetus and at puberty.
• The adrenal cortex has three layers: zona glomerulosa producing aldosterone, and zona fasciculata and reticularis producing cortisol and androgens.
• The gonads produce steroid hormones but not aldosterone and cortisol.
• Testes produce testosterone, and ovaries produce estrogens (mainly estradiol) and progesterone.
• Corpus luteum cells produce progesterone, also synthesized in the placenta and adrenal cortex.

Hormone Transport, Metabolism, and Excretion

• Peptide and catecholamine hormones are water-soluble, transported dissolved in plasma.
• Steroid and thyroid hormones are poorly soluble, transported mainly bound to plasma proteins.
• Dissolved (free) hormones are in equilibrium alongside bound counterparts.
• Total hormone concentration = free + bound fractions.
• Only free hormones can encounter target cells.
• Protein binding affects hormone metabolism and excretion.
• Hormone's concentration in plasma depends on secretion rate and removal rate.
• Clearance of hormone happens via excretion or transformation.
• Liver and kidneys are major organs for metabolism/excretion.
• Some peptide hormones are metabolized by target cells via hormone-receptor complex endocytosis.
• Catecholamine and peptide hormones are rapidly broken down, active for minutes to an hour.
• Protein-bound hormones are protected and remain active for longer, steroid/thyroid hormones can be active for hours or days.
• Metabolism of some hormones will activate instead of inactivate them.

Mechanisms of Hormone Action

• Hormones are transported through the blood and reach all tissue but trigger responses from specific target cells.
• The ability to trigger a response depends on hormone receptors that can be found on or in the target cells.
• Water-soluble messenger receptors (peptide hormones and catecholamines) are proteins on the surfaces of plasma membranes.
• Lipid-soluble messenger receptors (steroid and thyroid hormones) are proteins that can be found in the target cells.
• Hormones regulate cell response through those hormone receptors.
• Receptor modulation include up-regulation, that means receptors increase and are resulted from low hormone concentrations.
• Down-regulation means receptors have decreased, often as a results from exposure that have high concentration, decreasing responsiveness in order to prevent overstimulation.
• Hormones can regulate both its receptor and the receptors for all other hormones too.
• Permissiveness means hormone A must be present to make hormone B to have big effect (hormone A ability to upregulate B's receptor).

Events Elicited by Hormone-Receptor Binding

• Hormones triggers cell responses using a process, such as the one in Chapter 5.
• Receptors for peptide hormones and catecholamines are located on cells' outer surface, triggering signal transduction upon activation; this results in many activated, specific receptors.
• These include triggering the activity of enzymes, janus kinases or G proteins.

Inputs That Control Hormone Secretion

• Control is by change in mineral or nutrients concentrations, release of neurotransmitters, or the impact other hormone.
• Hormone rate will have output based on the relative amounts with stimulatory and inhibitory inputs.
• Neural input from the autonomic nervous system controls a number of hormones.
• This includes inputs from sympathetic and parasympathetic nerve systems.

Types of Endocrine Disorders

• Can be classified as: -Too little: hyposecretion -Too much: hypersecretion -Decreased responsivemess: hyporesponsiveness -Increased responsiveness: hyperresponsiveness
• Primary hyposecretion results from the gland not functioning correctly -This causes reduced hormone secretion.
• Secondary is from glands that are recieving not enought stimulation.
• Most hormone hyposecretion is treated by giving artifical hormones

Hypothalamus and Pituitary Gland

• Pituitary gland, lies in the pocket called Sella Turcica at the base of the brain Just below the brain it connect the hypothalamus by infundibuum
• The pituitary lobes include the anterior lobe and posterioe pituitary.