Proper digestion is essential for our health and wellbeing. There are several important digestive juices produced by the body that help break down food and absorb nutrients. In this article, we will discuss 3 key digestive juices – saliva, gastric juice, and pancreatic juice – that play vital roles in the digestion process.
Digestion begins in the mouth, where the mechanical process of chewing breaks down food into smaller pieces. The salivary glands produce saliva, which mixes with the food to form a soft bolus that can be swallowed easily. Saliva contains enzymes that begin breaking down carbohydrates while the food is still in the mouth.
After swallowing, the bolus travels down the esophagus into the stomach. The stomach produces gastric juice, which contains hydrochloric acid, proteases like pepsin, and other substances. The acid helps kill bacteria, while the enzymes begin breaking down proteins. The stomach churns the food to further mix it with gastric juices.
As the partially digested food moves from the stomach into the small intestine, it mixes with pancreatic juice. Produced by the pancreas, pancreatic juice neutralizes stomach acid and contains more digestive enzymes that act on fats, proteins, and carbohydrates, essentially completing the chemical breakdown of food.
Let’s look at each of these 3 key digestive juices in more detail.
Saliva is produced by 3 pairs of major salivary glands – the parotid, submandibular, and sublingual glands. Minor salivary glands throughout the mucous membranes in the mouth also secrete saliva.
On average, the salivary glands produce 1-1.5 liters of saliva per day. The volume of saliva increases when you eat, smell, think about, or even talk about food. Saliva production is controlled by signals from the autonomic nervous system.
Here are the key contents of saliva:
- Water – to moisten food for chewing and swallowing
- Mucus – lubricates food and protects the oral cavity lining
- Electrolytes – like sodium, potassium, calcium, magnesium, and bicarbonate
- Antibacterial compounds – like lysozyme and peroxidase
- Digestive enzymes – mainly salivary amylase that starts breaking down starch
The enzymatic breakdown of starch begins in the mouth. Salivary amylase starts cleaving polysaccharide starch into smaller disaccharide units like maltose. Only about 5% of starch digestion occurs in the mouth, but this prepares the carbohydrates for further digestion.
Saliva also helps form the food into a bolus for swallowing. It lubricates the bolus and contains mucus that helps adhere the food particles together. The slippery mucus helps the bolus easily slide down the esophagus.
Additionally, saliva helps cleanse the mouth and contains compounds like lysozyme and peroxidase that have antimicrobial properties to fight bacteria and germs.
Key Points on Saliva
- Produced by major and minor salivary glands in the mouth
- Contains water, electrolytes, mucus, and digestive enzyme salivary amylase
- Begins chemical digestion of carbohydrates
- Moistens food, helps form bolus for swallowing
- Cleanses the oral cavity and has antibacterial properties
Gastric juice is produced in the stomach by specialized cells called gastric glands. There are millions of gastric glands spread throughout the lining of the stomach (known as the gastric mucosa).
The gastric glands contain different types of cells that secrete a variety of substances that make up gastric juice:
- Parietal cells – secrete hydrochloric acid and intrinsic factor
- Chief cells – secrete pepsinogen, the inactive precursor of pepsin
- Mucous neck cells – secrete bicarbonate and mucus
- G cells – secrete gastrin hormone
The main components of gastric juice include:
- Hydrochloric acid (HCl) – 0.5% concentrated solution, pH of 1.5-3.5
- Pepsinogen – inactive form that is activated into the proteolytic enzyme pepsin in the acidic environment
- Mucus – protects stomach lining from acid and self-digestion
- Intrinsic factor – required for vitamin B12 absorption
- Gastrin – regulates gastric juice production
- Other substances like electrolytes and bicarbonate
Here are the key functions of gastric juice in digestion:
- Acid environment – the highly acidic pH helps kill bacteria and provides optimum environment for pepsin activity
- Protein breakdown – pepsin digests proteins into smaller peptides
- Mucus forms a protective coating on the stomach lining to prevent self-digestion by the acid and enzymes
- Gastrin regulates gastric juice production by a negative feedback mechanism
Due to the acid, the stomach can essentially begin digestion of proteins without needing to wait for the food to reach the small intestines. However, gastric juice only begins protein digestion – full protein breakdown occurs later by pancreatic enzymes.
Key Points on Gastric Juice
- Secreted by gastric glands in the stomach mucosa
- Contains HCl, pepsinogen, mucus, intrinsic factor
- Creates highly acidic environment optimal for enzymatic digestion and killing bacteria
- Pepsin begins protein digestion into smaller peptides
- Mucus protects stomach lining from acid and enzymes
Pancreatic juice is secreted from both the exocrine and endocrine portions of the pancreas. The pancreatic acini are specialized secretory units in the exocrine pancreas that produce inactive digestive enzyme precursors.
The main components of pancreatic juice include:
- Bicarbonate – to neutralize stomach acid in chyme entering duodenum
- Pancreatic amylase – digests carbohydrates
- Trypsinogen and chymotrypsinogen – inactive forms activated in duodenum to digest proteins into peptides
- Pancreatic lipase – digests lipids
- Nucleases – digest nucleic acids
- Other electrolytes and fluids
Pancreatic juice is secreted in an inactive form to prevent premature digestion. Once it enters the small intestine, the enzymes are activated to continue the digestion of all major food components:
- Proteins – trypsin, chymotrypsin, carboxypeptidase split proteins into peptides and amino acids
- Carbohydrates – pancreatic amylase breaks down starch, glycogen, disaccharides
- Fats – pancreatic lipase digests triglycerides into fatty acids and glycerol
- Nucleic acids – nucleases break down DNA and RNA
Pancreatic juice also contains bicarbonate to neutralize the acidic chyme entering from the stomach, regulating the pH for optimal activity of the pancreatic enzymes.
Key Points on Pancreatic Juice
- Secreted by pancreatic acini cells into pancreatic duct
- Contains bicarbonate and inactive digestive enzyme precursors like trypsinogen
- Enzymes activated in small intestine to complete digestion of proteins, carbs, lipids, and nucleic acids
- Bicarbonate neutralizes acidic chyme from stomach
|Digestive Juice||Secreted By||Key Components||Major Functions|
|Saliva||Salivary glands in mouth||Water, electrolytes, mucus, salivary amylase enzyme||Moistens food, lubricates bolus, begins starch digestion, antimicrobial properties|
|Gastric juice||Gastric glands in stomach lining||HCl, pepsinogen, mucus, intrinsic factor||Creates acidic environment, begins protein digestion by pepsin, kills bacteria|
|Pancreatic juice||Pancreatic acini cells||Bicarbonate, enzyme precursors like trypsinogen||Neutralizes chyme pH, completes digestion of proteins, carbs, fats, nucleic acids in small intestine|
Digestion in the Mouth
Digestion starts in the mouth with the mechanical breakdown of food by chewing. The teeth cut and grind food into smaller pieces, increasing the surface area for chemical digestion. Saliva mixes with the food and helps form a soft bolus that can be easily swallowed.
The salivary glands secrete 1-1.5 liters of saliva each day. The volume increases when you smell, taste, think about, or eat food. Saliva contains the digestive enzyme amylase which starts breaking down starch into maltose.
Additionally, saliva moistens the food and contains mucus to lubricate the bolus. It has antimicrobial compounds like lysozyme that kill microbes. The mucus adheres food particles together into a slippery bolus that can smoothly slide down the esophagus when swallowed.
Mouth Phase Summary
- Food mechanically broken down by chewing
- Saliva mixes with food and digests some carbohydrates by salivary amylase
- Mucus lubricates bolus allowing it to slide down esophagus when swallowed
- Antimicrobial compounds in saliva kill bacteria and cleanse mouth
Digestion in the Stomach
After swallowing, peristalsis pushes the food bolus through the esophagus into the stomach. Gastric glands in the stomach lining secrete 2-3 liters of gastric juice each day.
Gastric juice contains hydrochloric acid, pepsinogen, mucus, and other compounds. The highly acidic environment kills bacteria and provides optimal pH for pepsin activation.
Pepsin is the protease enzyme in gastric juice that begins digesting proteins. It breaks intact proteins into smaller peptides. Mucus protects the stomach lining from the acidity and prevents self-digestion.
The stomach stores food and mixes it with gastric juice through churning contractions. Only partly digested food called chyme exits the stomach into the small intestine in a controlled manner by the pyloric sphincter.
Stomach Phase Summary
- Gastric glands secrete gastric juice containing hydrochloric acid, pepsinogen, mucus
- Highly acidic pH kills bacteria, activates pepsin to digest proteins
- Mucus protects stomach lining from acid and enzymes
- Stomach mixing churns food and gastric juice to form acidic chyme
- Pyloric sphincter regulates exit of chyme into small intestine
Digestion in the Small Intestine
Chyme from the stomach enters the duodenum of the small intestine. The pancreas and intestinal glands secrete digestive juices that neutralize the acidic chyme and contain enzymes to continue digestion.
Pancreatic juice has a high bicarbonate concentration that neutralizes the acidity. It also contains enzyme precursors like trypsinogen that get activated in the small intestine.
These pancreatic enzymes digest proteins, fats, carbohydrates, and nucleic acids. The intestinal glands secrete mucus and intestinal juice with further enzymes to complete digestion into absorbable molecules.
The enzymatic digestion is facilitated by bile produced in the liver and stored in the gallbladder. Bile emulsifies fats to increase the surface area exposed to lipase. Bile also facilitates lipid absorption in the intestine.
The small intestinal lining has finger-like villi and microvilli to maximize nutrient absorption into the bloodstream. Indigestible matter gets pushed into the large intestine as waste.
Small Intestine Phase Summary
- Pancreatic juice neutralizes chyme and contains digestive enzymes activated to further break down proteins, carbs, fats, nucleic acids
- Intestinal glands secrete mucus and enzymes
- Bile emulsifies fats and aids in lipid digestion and absorption
- Villi and microvilli on intestinal lining maximize nutrient absorption
- Indigestible matter forms feces that is stored in large intestine
In summary, the 3 main digestive juices vital to digestion are:
- Saliva – secreted by salivary glands in the mouth, starts starch digestion by amylase
- Gastric juice – secreted by gastric glands in the stomach lining, contains HCl and pepsin to begin protein breakdown
- Pancreatic juice – secreted by the pancreas, neutralizes chyme and contains enzymes to complete digestion
Proper secretion and functioning of these digestive juices is necessary for optimal digestion and absorption of nutrients from our diet. Deficiencies or reductions in any of these juices can lead to digestive problems.
The mouth, stomach, and small intestine work together with these key juices to mechanically and enzymatically break down complex food into simpler molecules that can be absorbed. Without this complex digestive process, our bodies would not be able to obtain the nutrients we need for energy, growth and repair.