THE DIGESTIVE SYSTEM

THE HUMAN DIGESTIVE SYSTEM IS PRIMARILY...

THE DIGESTIVE SYSTEM: Every cell of the body needs nourishment, yet most cells cannot leave their position in the body and travel to a food source, so the food must be converted to a usable form and delivered to these cells. A healthy digestive system is therefore essential to maintaining life. The digestive system is a complex and vital system in the human body responsible for breaking down and absorbing nutrients from the food we eat. It involves a series of organs and processes working together to convert food into usable components for energy, growth, and maintenance of body functions.

The Components of the Digestive System 

MOUTH

Digestion begins in the mouth, where the process of mechanical and chemical digestion starts. Teeth chew the food, breaking it into smaller pieces, while salivary glands produce saliva containing enzymes that initiate the breakdown of carbohydrates.

PHARYNX and ESOPHAGUS

After chewing and swallowing, the food passes through the pharynx (throat) and into the esophagus, a muscular tube that carries food to the stomach through a coordinated movement called peristalsis.

STOMACH

In the stomach, the food mixes with gastric juices containing hydrochloric acid and enzymes, such as pepsin, to further break down proteins and create a semi-liquid mixture called chyme.

SMALL INTESTINE

The chyme enters the small intestine, where most digestion and nutrient absorption occur. The pancreas secretes digestive enzymes into the small intestine, helping to break down carbohydrates, proteins, and fats. The liver produces bile, which is stored in the gallbladder and released into the small intestine to aid in fat digestion.

SMALL INTESTINE SECTIONS

The small intestine is divided into three sections: duodenum, jejunum, and ileum. The majority of nutrient absorption occurs in the first two sections, while the ileum absorbs bile salts and vitamin B12.

VILLI AND MICROVILLI

The inner lining of the small intestine is covered in finger-like projections called villi, and each villus contains even smaller projections called microvilli. These structures increase the surface area for nutrient absorption, allowing efficient absorption of nutrients into the bloodstream.

LARGE INTESTINE(COLON)

The remaining undigested and unabsorbed food, along with waste products, moves into the large intestine. Here, water and some nutrients are absorbed, and the waste material is formed into feces.Feces are stored in the rectum until they are ready to be eliminated. The process of elimination occurs through the anus when the rectal muscles relax, and waste is expelled from the body.

MICROBIOME

The large intestine houses a diverse community of beneficial bacteria known as the gut microbiome. These bacteria aid in digesting certain substances, producing vitamins, and supporting overall gut heal.

FUNCTIONS OF THE LARGE INTESTINE

• Provision of Water, Electrolytes and Nutrients to the body
• Storage
• Secretion
• Absorption
• Motility
• Excretion

Provision of water, electrolytes and nutrients is essential for the normal functioning of body. Nutrients in foods is only of benefit to the body after digestion and absorption.

STORAGE FUNCTIONS OF THE DIGESTIVE SYSTEM

STOMACH

Food is stored here during first stage of digestion.

COLON/RECTUM

Digestion of residues, faeces and absorption of water during storage.

SECRETION

Secretions is another functions of the digestive system which include Exocrine Secretions, Endocrine Secretions and Paracrine Secretions

EXOCRINE SECRETIONS

Exocrine secretion in the digestive system refers to the process of releasing digestive enzymes, acids, mucus, and other substances produced by specialized cells into the lumen (cavity) of the digestive tract or onto its surface. These secretions aid in the breakdown of food, facilitate nutrient absorption, and protect the lining of the digestive organs. Exocrine glands are an integral part of the digestive system and are present in various organs along the digestive tract.

Exocrine Secretions and their Functions in the Digestive System

SALIVA

Salivary glands in the mouth secrete saliva, which contains enzymes like amylase that begin the digestion of carbohydrates. Saliva also helps in lubricating the food, making it easier to swallow.

GASTRIC JUICES

In the stomach, specialized cells secrete gastric juices that contain hydrochloric acid and pepsinogen. Hydrochloric acid lowers the pH of the stomach, creating an acidic environment that activates pepsinogen to become pepsin. Pepsin is an enzyme that starts the breakdown of proteins into smaller peptides.

PANCREATIC ENZYMES

The pancreas is a key exocrine gland that produces and releases digestive enzymes into the small intestine. These enzymes include pancreatic amylase (for carbohydrate digestion), lipase (for fat digestion), and various proteases (for protein digestion).

BILE

The liver produces bile, which is stored and concentrated in the gallbladder before being released into the small intestine. Bile aids in the emulsification and digestion of fats, making them more accessible to the action of lipase.

MUCUS

Mucus-secreting cells in the stomach and intestines produce mucus that coats the lining of these organs, protecting them from the acidic environment and mechanical wear and tear.

INTESTINAL ENZYMES

The cells lining the small intestine secrete various enzymes, such as maltase, lactase, and sucrase, which help break down complex carbohydrates into simple sugars for absorption.

ENDOCRINE SECRETIONS

Hormones secreted by the endocrine glands play an important role in regulating various aspects of the digestive process. These hormones are released into the bloodstream and have systemic effects on several organs and tissues involved in the digestive process.

Here are some of the major endocrine secretions and their roles in digestion:

GASTRIN

Gastrin is produced by G cells in the lining of the stomach. It stimulates secretion of gastric acid and enzymes in the stomach, helping to break down food.

CHOLECYSTOKININ(CCK)

CCK is produced by enteroendocrine cells in the small intestine. It is released in response to the presence of fats and proteins in the small intestine. CCK stimulates the gallbladder to secrete bile and the pancreas to secrete digestive enzymes, which aid in the digestion of fats and proteins.

 

SECRETIN

Secretin is also produced by enteroendocrine cells in the small intestine. It is released in response to acid chyme entering the duodenum. Secretin stimulates the pancreas to release bicarbonate which helps to neutralize stomach acid and create an optimal pH for digestive enzymes.

GHRELIN

Ghrelin is produced by stomach cells and acts as an appetite-stimulating hormone. It is released when the stomach is empty, signaling to the brain that it is time to eat. 
Insulin and Glucagon: Produced by the pancreas, insulin and glucagon are important for blood sugar regulation. Insulin facilitates the uptake of glucose into cells, while glucagon raises blood sugar when it’s too low.

MOTILIN

Produced by cells in the small intestine, motilin helps regulate the peristalsis (movement) of the digestive tract, aiding the movement of food through the digestive system.

INTESTINAL VASOACTIVE PEPTIDE(VIP)

 VIP is produced in different parts of the gastrointestinal tract. It affects smooth muscle relaxation, water and electrolyte excretion, and blood flow in the digestive system.
These endocrine secretions work together to ensure digestion, absorption of nutrients, and maintenance of the overall function of the digestive system. 

PARACRINE SECRETIONS

Paracrine secretion in the digestive system refers to the release of chemical signaling molecules, called paracrine factors, by a cell type, which then act on nearby target cells in the gastrointestinal tract, same tissue or organ. Unlike endocrine secretion, which involves the release of hormones into the bloodstream to act on distant target cells, parasecretory signaling affects neighboring cells in a localized area. As part of the digestive system, parasecretory secretion plays an important role in coordinating various digestive processes and regulating local functions. Here are some examples of paracrine secretion in the digestive system:

GASTRIN

Gastrin is a paracrine factor produced by certain cells in the stomach lining. It stimulates the secretion of gastric juices, including hydrochloric acid and pepsinogen, to aid in the digestion of proteins.

SOMATOSTATIN

Somatostatin is another paracrine factor secreted by specialized cells in the stomach and small intestine. It inhibits the release of gastric acid and other digestive enzymes, helping to regulate the digestive process.

HISTAMINE

In the stomach, histamine acts as a paracrine factor released by certain cells called enterochromaffin-like cells. It plays a role in stimulating the release of gastric acid.

SECRETIN

Secretin is produced and released by cells in the small intestine. It acts as a paracrine factor to stimulate the pancreas to secrete bicarbonate-rich fluid, which helps neutralize acidic chyme from the stomach.

CHOLECYSTOKININ(CCK)

CCK is a paracrine factor produced in the small intestine. It stimulates the gallbladder to contract, releasing stored bile into the small intestine to aid in the digestion and absorption of fats.

These paracrine factors act locally in the gastrointestinal tract to regulate the secretion of digestive juices, enzymes, and other substances necessary for proper digestion and absorption of nutrients. They help maintain the correct balance of digestive processes and contribute to overall digestive health.

WHAT IS THE DIFFERENCE BETWEEN ENDOCRINE SECRETION AND PARACRINE SECRETIONS?

The key difference between paracrine and endocrine secretions related to the digestive system is how they target and affect the cells and tissues involved in digestion:

PARACRINE SECRETIONS

Paracrine secretions are chemical messengers that are secreted by cells and act locally on neighboring target cells in the same tissue or organ.
These secretions have a relatively short range and do not enter the bloodstream.
Paracrine secretions signals affect neighboring cells, helping to coordinate and regulate specific processes in a local area. For example, the histamine secreted by gastric mucosal cells has the effect of stimulating the secretion of gastric acid in the vicinity to create favorable conditions for the digestive process.

ENDOCRINE SECRETIONS

Endocrine secretions are hormones secreted by specialized glands into the bloodstream. These hormones travel through the bloodstream to reach distant target cells in different parts of the body, not just the digestive system.
Endocrine signals have a broader field of influence and can affect cells and tissues distant from the secretory source.
For example, hormones like insulin and glucagon, produced by the pancreas, regulate blood sugar levels throughout the body, including those involved in digestion

In summary, paracrine secretions target adjacent cells and have local effects in a particular tissue or organ while endocrine secretions target distant cells and have systemic effects on different parts of the body including the digestive system. Both types of secretion play an important role in coordinating and regulating complex digestive processes and maintaining the body’s overall homeostasis.