Digestion is the process in which large molecules of food are converted into smaller and diffusible molecules that can cross the membranes.
The digestive system consists of the alimentary canal and the other abdominal organs that play an important role in digestion. The alimentary canal also known as the digestive tract is the long tube of organs-including the esophagus, stomach, and intestines, that runs from the mouth to the anus. An adult's digestive tract is about 30 feet (about 9 meters) long.
Steps of Digestion
Following are the steps of digestion
Ingestion: Intake of food
Propulsion: It is the rhythmic waves of muscle contractions and relaxations that occur in the digestive organs, helping to push food along from one part of the digestive system to the next, like a squeezing motion
Mechanical Digestion: Physical preparation of food for digestion
Segmentation: Mixing of food in the intestines with the digestive juices
Chemical Digestion: Enzymes breakdown carbohydrates, fat, and proteins
Absorption: Digestive nutrients are transferred from the digestive canal into the blood
Egestion: It is the removal of waste products from the body, also known as defecation
Digestion starts in the mouth. When we see, smell, or think about food, our salivary glands produce saliva. This prepares our mouth for eating and helps break down the food. The oral cavity, located behind the mouth between the upper and lower jaw, has important functions in this process
Food selection: When food enters the oral cavity it is tasted and felt. The food is selected or rejected due to the taste.
Grinding of food: Grinding is the second function of the oral cavity, also known as chewing or mastication. It helps to break down the food into smaller pieces, making it easier to swallow and digest.
Lubrication of food: The third function of the oral cavity is ****to moisten the food by mixing it with saliva, as saliva adds water and mucus to the food.
Chemical digestion: Salivary amylase (an enzyme present in the food) helps in the digestion of starch present in the food. The small pieces of food are then rolled up by the tongue into a small, slippery, spherical mass called a bolus.
Swallowing of the bolus: In swallowing the food moves from the mouth down to the esophagus, swallowing is accomplished by the tongue and mouth muscles.
The pharynx, a passageway for food and air, is about 5 inches long and serves as a pathway for both food and air to pass through.
When we swallow, a flexible flap of tissue called the epiglottis automatically closes over the windpipe to prevent choking.
The bolus, which is the chewed food, then travels down a muscular tube called the esophagus.
Peristalsis forces down the food from the esophagus to the stomach.
At the end of the esophagus, is a muscular ring called a sphincter that allows food to enter the stomach, after the food enters the stomach, the sphincter closes tightly to keep the food from flowing back to the esophagus.
The stomach is a thick-walled expandable bag, located just beneath the diaphragm. The stomach has three regions:
cardiac (located just after the esophagus)
fundus (the largest part of the stomach)
pylorus (located at the other end of the stomach and opens into small intestine)
Following are the functions of the stomach
The muscles of the stomach mix and churn the food, the acids and enzymes present in the stomach break the food into much smaller pieces.
When food enters the stomach the gastric juice is secreted by the gastric glands, it is composed of
mucus, hydrochloric acid, and protein-digesting enzyme- pepsinogen
Hydrochloric acid performs two functions
kills the microorganisms present in the food
converts enzyme pepsinogen into pepsin, which is an active form
When food enters the stomach, the enzyme pepsin breaks down proteins into smaller chains.
Churning action in the stomach generates heat, melting lipids in food.
Food becomes chyme, a thick paste-like liquid.
The pylorus holds chyme until it reaches the right consistency.
Chyme is then released into the small intestine for further digestion.
The small intestine has three parts:
Duodenum- the first part
Jejunum- the coiled midsection
Ileum - the final section
Duodenum is a part of the alimentary canal where most digestion occurs, duodenum receives chyme from the stomach. Ducts from the pancreas and liver deliver pancreatic juice and bile into the duodenum.
Bile salts have the ability to break down or emulsify large fat globules into tiny droplets, they act as detergents on the food particles.
Pancreatic juice contains a variety of enzymes, which digest protein, lipids, and carbohydrates.
The juices in the small intestine, including enzymes and pancreatic juice, break down all the different types of food molecules like sugars, proteins, fats, and nucleic acids.
The small intestine has tiny finger-like projections called villi that increase the surface area for absorption. These villi have a single layer of cells and are connected to blood capillaries and lymphatic vessels. They help absorb nutrients into the body, such as sugars, amino acids, and nucleosides, which are carried to the liver through the bloodstream. The lymphatic system absorbs glycerol, fatty acids, and vitamins.
Food that hasn't been digested and water moves from the small intestine to the large intestine. The main job of the large intestine is to remove water from undigested food and create solid waste for elimination.
The large intestine has three parts
The caecum is a pouch connecting the small intestine and the large intestine. It widens to let food pass from the small intestine to the large intestine.
The colon is a long tube that extends from the caecum to the rectum. It has three sections: the ascending colon, transverse colon, and descending colon. The colon absorbs fluids and salts, while the descending colon holds waste called faeces. Feces are made up of undigested material, bacteria, gastrointestinal cells, bile pigments, and water. The bacteria in the colon aid in the digestion of leftover food.
The rectum is where feces are stored until they leave the digestive system through the anus as a bowel movement.
The liver is a metabolically active organ responsible for many vital life functions.
It makes bile to help in digesting and absorbing fat. Bile is stored in the gallbladder and then released into the small intestine through a special channel called the bile duct.
The liver produces a substance that helps neutralize stomach acid.
It plays a key role in processing and managing nutrients that are delivered to it from the small intestine through the bloodstream.
