17.10 Organs with Secondary Endocrine Functions

The endocrine system isn't just about dedicated glands. Many organs play a secondary role in hormone production, contributing to the body's complex chemical messaging network. These organs, including the heart, gut, and kidneys, secrete hormones that regulate vital functions.

Beyond organs, other tissues like bone, fat, and skin also have endocrine roles. They produce hormones that influence metabolism, energy balance, and calcium regulation. This widespread hormone production highlights the interconnected nature of the body's regulatory systems.

Organs with Secondary Endocrine Functions

Endocrine System Overview

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• The endocrine system consists of glands and organs that produce and secrete hormones
• Hormones are chemical messengers that regulate various physiological processes
• Homeostasis is maintained through hormone actions on target cells
• Target cells have specific receptors that recognize and respond to hormones
• Feedback mechanisms (negative feedback and positive feedback) control hormone release and effects

Hormones from secondary endocrine organs

• Heart secretes hormones that regulate blood pressure and volume
  • Atrial natriuretic peptide (ANP) released by atrial cardiomyocytes when blood volume and pressure increase, promoting sodium and water excretion by the kidneys (natriuresis and diuresis) to reduce blood pressure and volume
  • Brain natriuretic peptide (BNP) secreted by ventricular cardiomyocytes in response to increased ventricular pressure and volume, having similar effects as ANP on natriuresis, diuresis, and blood pressure regulation
• Gastrointestinal tract produces hormones that control digestion and appetite
  • Gastrin released by G cells in the stomach and duodenum, stimulating gastric acid secretion, gastric motility, and growth of the gastric mucosa (lining)
  • Secretin secreted by S cells in the duodenum when acidic chyme enters, stimulating bicarbonate release from the pancreas and bile secretion from the liver while inhibiting gastric acid secretion and motility
  • Cholecystokinin (CCK) released by I cells in the duodenum and jejunum in response to fatty acids and amino acids, stimulating pancreatic enzyme secretion, gallbladder contraction, and promoting satiety (fullness) to reduce food intake
• Kidneys secrete hormones that regulate red blood cell production and blood pressure
  • Erythropoietin (EPO) produced by peritubular capillary endothelial cells when oxygen levels are low (hypoxia), stimulating red blood cell production (erythropoiesis) in the bone marrow
  • Renin released by juxtaglomerular cells when renal perfusion pressure decreases or sympathetic stimulation occurs, activating the ###Renin-Angiotensin-Aldosterone_System_0### (RAAS) which leads to increased blood pressure and volume through the actions of angiotensin II and aldosterone

Endocrine roles of non-glandular tissues

• Skeleton contributes to endocrine regulation through hormones secreted by bone cells
  • Osteocalcin released by osteoblasts during bone formation, promoting insulin secretion and sensitivity to enhance glucose homeostasis while also stimulating testosterone production in the testes
  • Fibroblast growth factor 23 (FGF23) secreted by osteocytes and osteoblasts when serum phosphate levels rise, reducing phosphate reabsorption in the kidneys and inhibiting the synthesis of active vitamin D (1,25-dihydroxyvitamin D) to regulate phosphate and vitamin D balance
• Adipose tissue secretes hormones that regulate energy balance and metabolism
  • Leptin released by adipocytes in proportion to fat stores, acting on the hypothalamus to suppress appetite and increase energy expenditure, regulating long-term energy balance and body weight
  • Adiponectin secreted by adipocytes, with levels inversely related to body fat percentage, enhancing insulin sensitivity and glucose uptake in target tissues while possessing anti-inflammatory and cardioprotective properties
• Skin synthesizes vitamin D, a hormone that regulates calcium and phosphate balance
  • Vitamin D produced in the skin when exposed to ultraviolet B (UVB) radiation, then converted to its active form (1,25-dihydroxyvitamin D) in the liver and kidneys, regulating calcium and phosphate homeostasis to promote bone mineralization while also modulating immune function and cell differentiation

Thymus vs liver hormone functions

• Thymus secretes hormones that support T lymphocyte development and immune function
  1. Thymosin released by thymic epithelial cells, stimulating the differentiation and maturation of T lymphocytes to enhance cell-mediated immunity
  2. Thymulin secreted by thymic epithelial cells, promoting T lymphocyte differentiation and function while modulating immune responses and inflammation
• Liver produces hormones that regulate growth, metabolism, and blood pressure
  • Insulin-like growth factor-1 (IGF-1) secreted by hepatocytes when stimulated by growth hormone, promoting cell growth, differentiation, and survival in various tissues while regulating glucose and lipid metabolism to enhance insulin sensitivity
  • Angiotensinogen released by hepatocytes and cleaved by renin to form angiotensin I, serving as a precursor to angiotensin II (a potent vasoconstrictor and aldosterone stimulator) as part of the renin-angiotensin-aldosterone system (RAAS) that regulates blood pressure and volume
  • Thrombopoietin secreted by hepatocytes and kidney cells, stimulating megakaryocyte differentiation and platelet production in the bone marrow to regulate platelet count and hemostasis (blood clotting)