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Hormones are molecules produced in one body part that affect cells in other parts. They are released directly into interstitial fluid by endocrine glands, skipping ducts, and then enter the bloodstream through capillaries. This distribution reaches target cells across the body, through the blood stream. Despite needing small amounts, hormone levels in circulation are generally low. In addition to conventional endocrine glands, various body tissues such as the gastrointestinal tract, placenta, kidneys, skin, and heart also contain endocrine tissue and secrete hormones.
Eicosanoids and Growth Factors
Eicosanoids, including prostaglandins and leukotrienes, which are synthesized from arachidonic acid act as local hormones in response to stimuli. Eicosanoids bind to cell receptors, influencing processes like smooth muscle contraction, blood flow, immune responses, and inflammation.
Growth factors include hormones like insulin-like growth factor, thymosin, and others. These growth factors stimulate cell growth and division, playing crucial roles in tissue development, growth, and repair, often acting as autocrine or paracrine regulators.
Hormones of the Heart
Atrial natriuretic factor (ANF) or atrial natriuretic peptide (ANP) is secreted by the heart walls. An increase in blood volume triggers the secretion of ANP from the heart. While the precise role of ANP in regular tubular function remains uncertain, it can impede the reabsorption of sodium (Na+) and water in the proximal convoluted tubule and collecting duct. Additionally, ANP inhibits the release of aldosterone and antidiuretic hormone (ADH). These actions amplify the elimination of sodium in urine (natriuresis) and elevate urine production (diuresis), consequently reducing both blood volume and blood pressure.
Hormones of the Kidney
- Erythropoietin (EPO)
EPO is synthesized by cells in the peritubular interstitial region of the kidneys. Its role involves stimulating the production of precursor cells for red blood cells. When tissues experience hypoxia, a deficiency in oxygen, the kidneys respond by increasing the release of erythropoietin. This, in turn, accelerates the transformation of proerythroblasts into reticulocytes within the red bone marrow. The elevated count of circulating red blood cells leads to improved oxygen delivery to body tissues.
- Renin
When blood volume decreases or there’s reduced blood flow to the kidneys, juxtaglomerular cells within the kidneys release renin into the bloodstream. Subsequently, renin combines with angiotensin-converting enzyme (ACE) to create the active hormone angiotensin II. This hormone raises blood pressure through two mechanisms. Firstly, angiotensin II is a potent vasoconstrictor, increasing systemic vascular resistance and consequently elevating blood pressure. Secondly, it triggers the release of aldosterone, which enhances the reabsorption of sodium ions (Na+) and water by the kidneys. This elevated water reabsorption leads to an overall increase in blood volume, resulting in heightened blood pressure.
Hormones of the GI tract
Gastrin is secreted by the gastric glands. The small intestine’s intestinal glands secrete the hormones secretin, cholecystokinin (CCK), and gastric inhibitory peptide (GIP).
- Gastrin
Gastrin is released by G cells in the gastric glands, triggered by multiple factors including stomach distension by chyme (partially digested food mixed with gastric juices), partially digested proteins in chyme, high chyme pH resulting from food presence, caffeine, and acetylcholine from parasympathetic neurons. Upon release, gastrin enters the bloodstream, circulates through the body, and eventually reaches its target digestive organs. Gastrin prompts gastric glands to produce significant amounts of gastric juice. It also reinforces the contraction of the lower oesophageal sphincter, preventing acid chyme reflux into the oesophagus, enhances stomach motility, and relaxes the pyloric sphincter, promoting gastric emptying. Gastrin secretion is dampened when gastric juice pH falls below 2.0 and stimulated when pH rises. This negative feedback loop ensures an optimal low pH for pepsin function, microbial control, and protein denaturation in the stomach.
- Cholecystokinin
The intestinal phase of digestion is influenced by two key hormones released by the small intestine: cholecystokinin and secretin. Cholecystokinin (CCK) is produced by CCK cells within the small intestine’s intestinal glands, prompted by chyme containing amino acids from partially digested proteins and fatty acids from partially digested triglycerides. CCK triggers the secretion of enzyme-rich pancreatic juice and induces the gallbladder wall to contract, releasing stored bile into the common bile duct via the cystic duct. Additionally, CCK relaxes the sphincter of the hepatopancreatic ampulla, permitting the flow of pancreatic juice and bile into the duodenum. This hormone also curbs gastric emptying by causing the pyloric sphincter to contract, induces a sense of fullness by influencing the hypothalamus, supports pancreas health, and amplifies secretin’s effects.
- Secretin
The entry of acidic chyme into the duodenum prompts the release of secretin from S cells in the small intestine’s glands. Secretin stimulates the flow of bicarbonate-rich pancreatic juice to neutralise the chyme’s acidity. Besides this primary role, secretin inhibits gastric juice secretion, supports pancreas well-being, and enhances CCK’s effects. Overall, secretin contributes to neutralising chyme acidity in the duodenum and reducing stomach acid production.
- GIP (Glucose-dependent insulinotropic peptide)
Initially referred to as gastrointestinal inhibitory peptide (GIP) or gastric inhibitory peptide, this hormone was thought to reduce stomach acid secretion, protect the small intestine from acid damage, slow down stomach food transfer rate, and hinder gastrointestinal (GI) motility and acid secretion. However, it was later found that these effects occur only at higher-than-normal physiological levels and that similar outcomes are naturally facilitated by another hormone, secretin. Presently, GIP is understood to primarily induce insulin secretion, which is triggered mainly by glucose hyperosmolarity in the duodenum. Subsequent to this revelation, some researchers prefer the term “glucose-dependent insulinotropic peptide,” maintaining the acronym “GIP.” After a meal, GIP, increased blood glucose, and certain amino acids prompt insulin release from pancreatic beta cells. Insulin enhances anabolic enzyme activity for synthesis and decreases catabolic enzyme activity. It facilitates glucose and amino acid entry into cells, converts glucose to glycogen, and promotes triglyceride synthesis in the liver and adipose tissue. Protein synthesis is also stimulated. Glucose enters cells through GLUT transporters, with insulin increasing GLUT4 insertion for quicker transport. Inside cells, glucose becomes phosphorylated, trapping it. This process involves both anabolic and catabolic reactions during the absorptive state.
Summary
Hormones are signaling molecules produced in one part of the body that influence the functioning of cells in other parts. They are secreted into the interstitial fluid by endocrine glands, bypassing ducts, and then enter the bloodstream via capillaries, enabling their distribution to target cells throughout the body. These hormones are essential for regulating various physiological processes. Apart from traditional endocrine glands, other body tissues like the gastrointestinal tract, placenta, kidneys, skin, and heart also contain endocrine tissue and secrete hormones. These hormones can act as local regulators or growth factors, stimulating cell growth and division. Specific hormones, such as atrial natriuretic peptide (ANP) from the heart, erythropoietin (EPO) and renin from the kidneys, and various hormones from the gastrointestinal tract, play significant roles in maintaining homeostasis and physiological balance within the body.
FAQs on Hormones of Heart, Kidney & GIT
What are hormones?
Hormones are chemical messengers produced by various glands and tissues in the body. They travel through the bloodstream to target cells and regulate various physiological processes.
How do hormones reach their target cells?
Hormones are released into the bloodstream and carried to target cells throughout the body. They interact with specific receptors on the surface or within the cells to initiate a response.
What is the role of endocrine glands in hormone production?
Endocrine glands, such as the thyroid, adrenal, and pituitary glands, specialize in producing hormones. These hormones help regulate growth, metabolism, reproduction, and other bodily functions.
Are hormones only produced by endocrine glands?
No, hormones can also be produced by tissues and organs that are not traditionally classified as endocrine glands. Examples include the heart, kidneys, gastrointestinal tract, and skin.
What are eicosanoids?
Eicosanoids are local hormones derived from arachidonic acid. They include prostaglandins and leukotrienes, which play roles in inflammation, blood flow regulation, and immune responses.
How do growth factors influence cell growth and division?
Growth factors are hormones that stimulate cell growth and division. They play important roles in tissue development, growth, and repair, often acting as autocrine or paracrine regulators.
What is the role of hormones in the heart?
Hormones like atrial natriuretic peptide (ANP) from the heart regulate blood volume and pressure by affecting the reabsorption of sodium and water in the kidneys.
How does erythropoietin (EPO) influence red blood cell production?
EPO, produced by the kidneys, stimulates the production of red blood cell precursors in response to low oxygen levels. This helps maintain adequate oxygen delivery to tissues.
What is the function of renin in the kidneys?
Renin is released by the kidneys in response to decreased blood volume or blood flow. It plays a role in raising blood pressure by initiating the production of angiotensin II and aldosterone.
How do hormones regulate digestion?
Hormones like gastrin, secretin, and cholecystokinin (CCK) play roles in regulating gastric acid secretion, pancreatic enzyme release, gallbladder contraction, and digestive processes.
What is the significance of glucose-dependent insulinotropic peptide (GIP)?
GIP, originally thought to inhibit stomach acid secretion, is now understood to primarily induce insulin secretion in response to elevated glucose levels in the duodenum.
