Human Digestive System – Digestive Glands

Food is the basic necessity for living for all organisms present on this planet. Have you ever wondered how the food you eat gets digested and provides us energy and ensures our survival?

All we do is intake the food, but our body has several mechanisms and several organs that help to extract energy out of that food. Food takes about six to eight hours to pass through the stomach and small intestine.

But the entire digestive process takes about thirty-six long hours. Lets today discuss the human digestive system, various organs, and their functions.

What is Digestive System?

The digestive system is characterized by the gastrointestinal tract. It is a portal by which various nutrients enter our bodies. It plays a major role in the maintenance of the body. It starts from the mouth and ends in the anus.

It consists of various organs and structures – Oesophagus, stomach, small and large intestines. Along with these are present accessory organs which help in the process of digestion such as teeth, tongue, salivary glands, liver, pancreas.

The human digestive system basically involves the consumption of food and its conversion to energy. This energy is necessary for our survival. The conversion of food to energy is not a one-step process.

It is the process that involves the processing of the food to extract the nutrients from it and utilize it for energy generation. This whole process is what we call, digestion.

The process of digestion involves the functioning of the alimentary canal (GI tract), accessory organs, and some organ systems. The Monogastric nature of humans makes the process quite simple as compared to some other animals.

Humans have only one chamber in the stomach. Some animals have multiple chambers which makes the process complex. For example, Cows have 4 chambers, Camels have 3 chambers, and so on.

The nervous and circulatory system of humans also plays an important role in digestion. The union of nerves, blood, hormones, bacteria, and some other organs completes the process of digestion.

How Digestion of Food Takes Place?

• In the mouth, food masticates with the help of teeth, tongue, and mixed with the saliva to form bolus which can be easily swallowed. It then passes through the oesophagus (25 cm long). “Peristaltic” movements help food to pass from the oesophagus to the stomach. The entry of the oesophagus into the stomach is regulated by the lower oesophageal sphincter.
• From the stomach, food passes into the small intestine which consists of the duodenum, jejunum, and ileum. The entry of food from the stomach into the duodenum is controlled by the pyloric sphincter.
• Then it goes from the ileum to the large intestine consisting of three parts – Cecum, Colon, Rectum.
• The opening of the ileum into the caecum guards by the ileocaecal valve.
• The vermiform appendix(a vestigial organ) is also present with caecum. It does not play any role in the process of digestion. Rectum finally opens into the Anus which marks the end of the Gastrointestinal Tract.

Gastrointestinal Tract (Alimentary Canal)

• It is responsible for breaking down food and supplying the body with water, nutrients, and electrolytes needed to maintain life.
• All the food passes through this continuous canal or tract.
• The alimentary canal has various compartments such as –

1. Buccal cavity
2. Oesophagus
3. Stomach
4. Small intestine
5. Large intestine
6. Anus

• Neural and hormonal control of the activities of the canal ensures proper coordination of different parts.
• The secretion of saliva occurs due to the feelings of sight, smell, and presence of the food.
• Similarly, secretion of gastric and intestinal juices occurs due to neural signals.
• The nervous system plays a major role to control the muscular activities of the alimentary canal

These are the processes which occur in the GI tract –

• Ingestion: which involves mastication and deglutition.
• Digestion: Complex particles are broken down into simple absorbable forms-

1. Starch [amylases] into monosaccharides
2. Proteins [proteases] into Dipeptidases and Amino acid
3. Fats [lipases] into monoglycerides and free fatty acids

• Absorption: Nutrients, water, and electrolytes are transported from the Gastrointestinal Tract into circulation.
• Egestion: Undigested food is passed out through the body in the form of feces via the anus.

Organization of Gastrointestinal Tract

GI Tract has the following layers from outside inwards.

1. Serous Layer: It helps in the attachment of the Gastrointestinal Tract to surrounding structures.
2. Smooth Muscle Layer: It consists of an inner circular and outer longitudinal muscle layer. This layer helps in mixing and propulsion of food.
3. Submucosal layer: It consists of loose connective tissue, blood vessels, and lymphatics
4. Mucous layer: It is lined by epithelium and its stroma consists of glands and muscularis mucosae.

Two important plexuses are also present:

• Myenteric plexus (Auerbach Plexus): It lies between longitudinal and circular smooth muscle. It is mainly a motor function.
• Submucosal plexus (Meissner’s plexus): It lies between the submucosal layer and linear circular smooth layer. It is mainly sensory in function.

Buccal Cavity (Oral Cavity)

• The process of digestion starts with the ingestion of food through the mouth. The mouth leads to the oral cavity.
• The oral cavity consists of teeth and tongue.
• Human beings form two sets of teeth in their lifetime. One set of temporary milk teeth (deciduous teeth) in the early years and the other set of permanent or adult teeth.
• Normally, humans have 32 permanent teeth. Teeth are of four types, namely, Incisors (I), Canine (C), Premolars (PM), and Molars (M).
• There are 2 Incisors, 1 Canine, 2 Premolars, and 3 Molars (2123 dental formula) in both the upper and lower jaw.
• Salivary glands in our mouth secrete saliva which breaks down the starch into sugars.
• The saliva contains electrolytes (Na+, K+, Cl2-, HCOs) and enzymes, salivary amylase, and lysozyme.
• Salivary amylase hydrolyses about 30 percent of the starch into maltose. Maltose is a disaccharide.
• Lysozyme is an antibacterial agent to prevent infections.
• The fleshy muscular organ, i.e., tongue mixes the saliva with the food during chewing.
• Frenulum attaches the tongue with the floor of the buccal cavity.
• Papillae are the small projections on the upper surface of the tongue. Some of the papillae bear taste buds.

Oesophagus (Food Pipe)

• The Esophagus is a thin, long tube that passes through the neck, the thorax, and the diaphragm. It ends at the cardia of the stomach.
• The pharynx is present at the end of the oral cavity. Both air and food pass through the pharynx.
• The pharynx has two openings. One goes to the windpipe and the other goes to the food pipe.
• Epiglottis is a cartilaginous flap that prevents the food from entering into the glottis. Glottis is the opening of the windpipe.
• Mucus in saliva lubricates and binds the masticated food into a slippery bolus. Lubrication ensures that the bolus easily passes through the oesophagus by deglutition or swallowing.
• Successive waves of muscular contraction ensure that the bolus reaches the stomach. These waves are known as peristalsis. The gastro oesophageal sphincter is present at the end of oesophagus. The sphincter controls the passage of food into the stomach.

Stomach

• The stomach is present at the upper left portion of the abdominal cavity. It consists of three parts, namely, cardiac, fundus, and pyloric.
• The gastroesophageal sphincter is a muscular sphincter present at the end of oesophagus. It controls the passage of food into the stomach.
• The mucosa is the inner lining of the stomach. It secretes hydrochloric acid, mucous, and digestive juices.
• Hydrochloric acid breaks down the food. It also kills unnecessary bacteria.
• The mucus forms a protective coating for the inner lining of the stomach.
• The digestive juices split up the proteins to form simple substances.
• Types of cells in the stomach –

1. Chief or peptic or zymogen cells that secrete pepsinogen
2. Parietal or oxyntic cells – secrete HCl
3. G-cells/ gastrin cells release gastrin

Daily 2.5-3 liters of gastric juice is secreted. It has pH 1-2. It consists of electrolytes (cations and anions) along with Pepsin (digests protein), Rennin (curdles milk), Gastric lipase, Lysozyme, Urease, and Carbonic Anhydrase.

Small Intestine

• Small Intestine is the major site of absorption of carbohydrates, proteins, and fats. Intestinal Juice is also called succus entericus. It is a nearly 5 m long coiled tube.
• It consists of three parts, namely, a duodenum, a jejunum, and ileum.
• The stomach opens into the duodenum by the pyloric sphincter. The Ileum opens into the large intestine.
• The inner walls of the small intestines cover with villi. Villi are the finger-like outgrowths which increase the surface area for absorption.
• Transportation of absorbed molecules takes place with the help of blood vessels. The molecules move towards cells of different organs.
• This movement of molecules is what we call assimilation.
• The unabsorbed and undigested food moves towards the large intestine.

Daily 3 liters of Intestinal Juice is secret. Its pH is 7.6. It comprises of enterokinase (it activates trypsinogen), Proteolytic Enzymes (like aminopeptidase and dipeptidase), Nuclease (Nucleotide and Nucleoside), Invertase (Sucrase), Maltase, Lactase, Intestinal Lipase, Alkaline phosphatase

Sucrose                         [Sucrase]                    Glucose and Fructose

Maltose                          [Maltase]                   Two molecules of Glucose

Lactose                          [Lactase]                    Glucose and Galactose

Alpha (symbol)-limiting dextrins  [alpha-limiting dextrinase] Glucose

Fats                                 [Lipase]                         monoglycerides + Fatty Acids

Peptones and Polypeptides    [Erepsin]          Amino Acids

Nucleic Acids              [Nucleases]                Purine and pyrimidine

Large Intestine

• The main function of the large intestine is the absorption of water. The main sites of absorption are the caecum and ascending colon. Along with the caecum and colon, the rectum is also present in the large intestine.
• The large intestine is about 1.5 m long. It is smaller but wider than the small intestine.
• The leftover after water and salt absorption enter the rectum. The leftover becomes semi-solid feces.
• The feces ejects from time to time through the anus. This process is egestion.
• A vestigial organ that goes by the name of the vermiform appendix is present on the caecum. The Appendix was helpful in digesting roughage during old times. Now it is not essential.
• The opening of the caecum goes towards the colon. The colon further has three parts – an ascending, a transverse, and a descending part.
• The descending part goes to the rectum which opens out through the anus.
• The ileocaecal valve prevents the backflow of the feces. This valve separates the small and large intestines.

The processes occur in the following manner –
Ingestion -> Digestion -> Absorption -> Assimilation -> Egestion

Digestive Glands

The alimentary canal of the human body has three digestive glands, namely, the salivary glands, the liver, and the pancreas.

Salivary Glands

There are 3 parts of salivary glands

• Parotid Gland – Its secretion passes via the Stenson duct which opens opposite to the second molar tooth in the cheek.
• Submandibular/Submaxillary Gland – Wharton’s Duct opens into the floor of the tongue in the lower jaw.
• Sublingual Gland – Rivinus Duct discharges its secretion into the sublingual part of the mouth below the tongue.

The salivary glands secrets the saliva in the oral cavity. The saliva breaks down the starch into sugar.

Liver

• The liver is the largest gland of the human body weighing about 1.5kgs.
• The liver is present at the upper right side of the abdomen.
• The liver secretes bile. Bile is collected by Hepatic ducts which join with the Cystic duct to form a common Bile Duct.
• Bile is stored and concentrated in the Gallbladder.
• Bile is an essential juice for the digestion of fats.
• Both the bile and pancreatic duct opens up into the duodenum. This common duct is the hepato-pancreatic duct. The Hepato-pancreatic duct is guarded by the sphincter of Oddi.

Secretion and composition of Bile

Daily 500-1000mL of Bile is secreted with a pH of 7.8-8.6 (alkaline)
It consists of water(97%), Bile Salt like sodium glycocholate and sodium taurocholate, Bile pigments –Bilirubin and Biliverdin, Cholesterol, and Electrolytes.

Pancreas

• The pancreas is a gland of the cream color present just below the stomach.
• It consists of both exocrine and endocrine parts. Pancreatic juice secreted by exocrine parts via two Ducts.

1. Duct of Wirsung – The duct joins the common Bile Duct to form Ampulla of Vater which opens into the duodenum.
2. Duct of Santorini – It is an accessory pancreatic duct. This too opens into a duodenum slightly higher than Ampulla of Vater.

• The pancreatic juice transforms the Carbohydrates and proteins into simpler forms.
• It partially digests the food which further reaches the lower part of the small intestine.
• The endocrine part secretes insulin, hormones, and glucagon.
• The exocrine part secretes alkaline pancreatic juice.
• Pancreatic Juice –

1. Pancreas daily secrets1200-1500ml of pancreatic juice with a pH of 7.8-8.4(alkaline).
2. It consists of – cations (Na+, Ca2+, K+, Mg2+, Zn2+), Anions (KCO3-, Cl-), Enzymes(secreted by Acinar cells) such as Pancreatic amylase, Pancreatic lipase, and Proteolytic enzymes. Proteolytic enzymes are Trypsinogen, Chymotrypsinogen, Carboxypeptidase, Ribonuclease, and Deoxyribonuclease.

Trypsinogen(inactive)    [enterokinase]        Trypsin (active)

Chymotrypsinogen          [Trypsin]                    Chymotrypsin

Procarboxypeptidase     [Trypsin]                    Carboxypeptidase

These active forms hydrolyze peptides and help in the digestion of proteins.

Disorders of the Digestive System

Some disorders may occur in the digestive system due to various factors.

• Jaundice: It affects the liver most. Skin and eyes become yellow due to the accumulation of bile pigment.
• Diarrhea: It is the discharge of loose and watery stool. It accompanies the abnormal and frequent bowel movements.
• Vomiting: When ejection of stomach contents occurs through the mouth, and called vomiting. The Medulla controls the reflex action of vomit.
• Constipation: When the feces remain within the rectum, it results in constipation. It may occur due to irregular bowel movements.
• Indigestion: When proper digestion of food doesn’t occur, it results in indigestion. It leads to a feeling of fullness. It may occur due to overeating, lack of enzymes, food poisoning, anxiety, etc.
• Bacterial and viral infections may cause inflammation in the intestinal tract.
• Many parasites may also cause infections. These parasites are tapeworm, roundworm, threadworm, hookworm, pinworm, etc.

Conclusion

In this article, we learned about the human digestive system. We also learned how digestion takes place and what all are the organs involved in the digestion process. We learned about the detailed functions of all the organs and glands.

Try imagining how your body digests food using all these organs when next time you have food. It would be fun. Don’t you agree?