Hormones of Heart, Kidney and GI Tract: Hormones are chemical messengers secreted directly into the bloodstream, where they are carried to the body’s organs and tissues to perform their activities. Hormones affect many different areas of physiological functioning and processes. Here are a few examples: Growth and development. Where do we get the hormones from? The endocrine system of course.

We all know that the endocrine system is critical for maintaining homeostasis and the chemical balance in our bodies. It is essential for a variety of life processes, and small imbalances in our hormone production can cause serious health effects. Human Endocrine System does this by secreting chemicals that diffuse throughout the body and can influence all the cells of the target. However, did you know that even other organs like the heart,  that are not classified as endocrine glands can also secrete hormones under special circumstances? Read on more to find out.

Which Organs in the Body Produce Hormones?

Several organs produce hormones under special circumstances. Let us discuss them one by one.

1. Heart

When the body’s blood volume or pressure rises, the cells of the atrial wall of the heart stretch. Specialised cells in the atria’s wall generate and secrete the peptide hormone atrial natriuretic peptide in response (ANP). ANP tells the kidneys to lower salt reabsorption, which reduces the quantity of water reabsorbed from the urine filtrate and lowers blood volume. ANP also inhibits renin secretion and initiates the renin-angiotensin-aldosterone system (RAAS), as well as causing vasodilation. As a result, ANP helps lower blood pressure, blood volume, and sodium levels in the blood.

2. Gastrointestinal Tract

The GI tract’s endocrine cells are found in the stomach and small intestine mucosa. Some of these hormones are released in reaction to a meal, and they help indigestion. Gastrin, a peptide hormone produced in response to stomach enlargement that increases the release of Hydrochloric acid, is an example of a hormone secreted by stomach cells.

As acidic chyme (partially digested food and fluid) passes from the stomach, the small intestine secretes secretin, a peptide hormone. It causes the pancreas to secrete bicarbonate, which buffers the acidic chyme, and it prevents the stomach from secreting more Hydrochloric acid.

Another peptide hormone produced by the small intestine is cholecystokinin (CCK). It stimulates the release of pancreatic enzymes and bile from the gallbladder, which aid digestion. Other hormones released by intestinal cells improve glucose metabolism by activating pancreatic beta cells to secrete insulin, decreasing glucagon secretion by alpha cells, or improving cellular insulin sensitivity.

3. Kidneys

The kidneys are a part of several intricate endocrine networks and can produce hormones. For instance, a decrease in blood flow to the kidneys causes them to release the enzyme renin. This activates the renin-angiotensin-aldosterone (RAAS) system, promoting sodium and water reabsorption. Eventually, the blood flow and blood pressure increase due to such increased reabsorption.

The kidneys can also help regulate blood calcium levels similarly by producing calcitriol from vitamin D3, which is released in response to parathyroid hormone secretion (PTH).

Lastly, in reaction to low oxygen levels, the kidneys also create the hormone erythropoietin (EPO). EPO increases oxygen delivery to tissues by stimulating the creation of red blood cells (erythrocytes) in the bone marrow.

4. Skeleton

Researchers have recently discovered that our skeleton also produces at least two hormones. In reaction to elevated blood levels of vitamin D3 or phosphate, bone cells can release fibroblast growth factor 23 (FGF23).

This subsequently causes the kidneys to reduce calcitriol production from vitamin D3 and increase phosphorus excretion, thereby reducing the phosphate levels. Osteocalcin, produced by osteoblasts, is another hormone that encourages pancreatic beta cells to make more insulin. It can also affect peripheral tissues, potentially increasing their insulin sensitivity and glucose utilisation.

5. Adipose Tissue

We now know that several hormones involved in fat metabolism and storage are produced and secreted by fatty tissue in the body. The satiety hormone, Leptin, a protein produced by adipose cells and circulated in the blood in amounts that are exactly proportional to body fat levels, is a good example of such a process. Leptin is a hormone that is released in response to food consumption. Its mode of action is attached to brain neurons that control energy intake and expenditure.

Thus, after a meal, leptin binding causes a sensation of satiety and reduces hunger. Leptin binding to brain receptors may also cause the sympathetic nervous system to modulate bone metabolism, thereby resulting in an increased cortical bone deposition.

Yet another example is the hormone produced by adipose cells, adiponectin, which appears to lower cellular insulin resistance while also protecting blood arteries from inflammation and atherosclerosis

6. Skin

By creating the inactive form of vitamin D3, cholecalciferol, the skin can act as an endocrine organ. When the cholesterol in the epidermis is exposed to UV light, it gets transformed to cholecalciferol, which is then absorbed into the bloodstream, downstream cholecalciferol is transformed in the liver into an intermediate that is transported to the kidneys and processed to calcitriol, the active form of vitamin D3.

As you know, Vitamin D is required for a number of physiological activities, a few examples of which are calcium absorption in the intestine and immune system function. In several studies, low vitamin D levels have been linked to an increased risk of cancer, severe asthma, and multiple sclerosis in some studies.

It is also well established that vitamin D insufficiency causes rickets in children and osteomalacia in adults, both of which induce bone degradation. Thus, the skin plays an important role in maintaining the levels of a very important vitamin in our bloodstream

7. Thymus

The thymus is an immune system organ that is larger and more active during early childhood and then begins to atrophy as we get older. Its endocrine function is to produce a group of hormones known as thymosins, which aid in developing and differentiating immune cells called T lymphocytes.

Although the exact function of thymosins is unknown, what we do understand is that they definitely have a role in the immunological response. However, thymosins have been detected in organs other than the thymus and show a wide range of functions, therefore, they can’t be classified as thymic hormones strictly.

8. Liver

At least four major hormones or hormone precursors are secreted by the liver: insulin-like growth factor (somatomedin), angiotensinogen, thrombopoietin, and hepcidin. Insulin-like growth factor-1 is the body’s immediate stimulation for triggering any kind of development, in particular bone growth.

Angiotensinogen, similarly, is a precursor to the hormone angiotensin, which raises blood pressure. Thrombopoietin, another hormone released by the liver, promotes platelet formation in the blood. Lastly, Hepcidins serve to manage iron homeostasis in our body fluids by blocking the release of iron from cells in the body.

Summary

To Summarise, kidneys secrete Atrial natriuretic peptide (ANP) which tells the kidneys to lower salt reabsorption, which in turn reduces the quantity of water reabsorbed from the urine filtrate. ANP also inhibits renin secretion and initiates the renin-angiotensin-aldosterone system.

Bone cells produce FGF23, which reduces calcitriol production from vitamin D3 and increases phosphorus excretion. Osteocalcin, produced by osteoblasts, encourages pancreatic beta cells to make more insulin. Several hormones involved in fat metabolism and storage are produced and secreted by adipose tissue.

Here is a quick table to recall all you’ve learned so far in an easy way.

Organ	Hormones secreted	Effects of the hormone
Heart	Atrial natriuretic peptide (ANP)	Reduces blood volume, blood pressure, and Na+ concentration
Gastrointestinal tract	Gastrin, secretin, and cholecystokinin	Aid digestion of food and buffering of stomach acids
	Glucose-dependent insulinotropic peptide (GIP) and glucagon-like peptide 1 (GLP-1)	Stimulate beta cells of the pancreas to release insulin
Kidneys	Renin	Stimulates release of aldosterone
	Calcitriol	Aids in the absorption of Ca2+
	Erythropoietin	Triggers the formation of red blood cells in the bone marrow
Skeleton	FGF23	Inhibits production of calcitriol and increases phosphate excretion
	Osteocalcin	Increases insulin production
Adipose tissue	Leptin	Promotes satiety signals in the brain
	Adiponectin	Reduces insulin resistance
Skin	Cholecalciferol	Modified to form vitamin D
Thymus (and other organs)	Thymosins	Among other things, aids in the development of T lymphocytes of the immune system
Liver	Insulin-like growth factor-1	Stimulates bodily growth
	Angiotensinogen	Raises blood pressure
	Thrombopoietin	Causes increase in platelets
	Hepcidin	Blocks release of iron into body fluids

FAQs on Hormones of Heart, Kidney and GI Tract

Students might be having many questions with respect to the Hormones of Heart, Kidney and GI Tract. Here are a few commonly asked questions and answers.

Q.1. What are the major hormones secreted by the heart?
Ans: Specialized cells in the atria’s wall generate and secrete the peptide hormone ANP (atrial natriuretic peptide) in response. It tells the kidneys to lower salt reabsorption, which reduces the quantity of water reabsorbed from the urine filtrate and lowers blood volume.

Q.2. What are the major hormones secreted by the liver?
Ans: At least four major hormones or hormone precursors are secreted by the liver: insulin-like growth factor (somatomedin), angiotensinogen, thrombopoietin, and hepcidin.

Q.3. What are the major hormones secreted by the kidney?
Ans: Renin is the major hormone secreted by the kidney. Other hormones from the kidney are calcitriol and erythropoietin.

Q.4. What are the major hormones secreted by the GI tract?
Ans: Gastrin, a peptide hormone produced in response to stomach distention that increases the release of Hydrochloric acid, is an example of a hormone secreted by stomach cells.

Q.5. What is RAAS and what does it do?
Ans: The renin-angiotensin-aldosterone is the RAAS system and it promotes sodium and water reabsorption.

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