Physiology of organs and systems

Humoral regulation and pancreatic hormones

Humoral regulation is a phylogenetically early part of the neurohumoral system that manages vital processes through body fluid media via ions, metabolites, eicosanoids, and hormones.

Pancreatic hormones play a key role in carbohydrate and lipid metabolism:

• Glucagon is synthesized by $\alpha$-cells of the islets of Langerhans. It acts as a hyperglycemic factor: it activates glycogenolysis, inhibits its synthesis, stimulates gluconeogenesis, and also triggers lipolysis and ketogenesis. Glucagon secretion is stimulated by hypoglycemia, stress, and sympathetic influences, while being suppressed by insulin and somatostatin.
• Somatostatin is produced by $\delta$-cells, activates the inhibitory cAMP G-protein system, and paracrine-inhibits the secretion of insulin and glucagon.

Pineal gland and circadian rhythms

The pineal gland is of neuroglial origin and synthesizes biogenic amines and peptide hormones, among which melatonin occupies a crucial place.

• The synthesis of melatonin directly depends on illumination: bright light inhibits the conversion of serotonin, while darkness activates it.
• Cyclic hormone production ensures the daily adaptation of the body.
• Effects: melatonin inhibits metabolism by suppressing the secretion of ACTH, TSH, and GH, affects the ovarian-menstrual cycle, and induces a sedative effect, facilitating the transition to sleep.
• Secretion regulation occurs reflexively via the sympathetic nervous system based on signals from the retina.

Calcium-regulating system and endocrine role of the kidneys

The parathyroid glands secrete parathyroid hormone (PTH), which maintains a constant level of calcium in the blood:

• In bone tissue, PTH activates osteoclasts, causing demineralization and calcium release.
• In the kidneys, it enhances calcium reabsorption, stimulates the synthesis of the active form of vitamin D (calcitriol), and accelerates phosphate excretion.
• PTH secretion is governed exclusively by calcium concentration via a feedback mechanism.

The kidneys lack specialized glands, but they synthesize calcitriol and erythropoietin. Additionally, they produce renin, an enzyme that triggers the renin-angiotensin-aldosterone system.

Physiology of hunger and satiety

The emotions of hunger and satiety play a vital protective and motivational role in regulating metabolism.

• Hunger is accompanied by unpleasant subjective sensations (epigastric pain, weakness) and acts as an integral signal of the need for nutrients.
• Pros of the integral signal: rapidity of assessing needs and initiating food-seeking behavior without analyzing specific nutrients.
• Cons: risk of excessive food intake and unnecessary strain on the digestive apparatus.
• Satiety signals the cessation of eating, allowing the feeding process to be completed on time. The evolutionary meaning of the system is the maximum speed of forming feeding behavior.

Class and exam questions

What else to study on this topic

• Mechanisms of action of somatostatin at the cellular level
• Paracrine regulation in the pancreatic islets
• The role of the kidneys in blood pressure regulation via the RAAS
• The role of calcitriol and vitamin D in metabolism
• Neural regulation of circadian rhythms of the pineal gland

Other topics in the section «Hormonal regulation of physiological functions»

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