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Digestive system: what is it?

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DIGESTION is important for breaking down food into nutrients, which the body uses for energy, growth, and cell repair. Food and drink must be changed into smaller molecules of nutrients before the blood absorbs them and carries them to cells throughout the body. The body breaks down nutrients from food and drink into carbohydrates, protein, fats, and vitamins. Many different organs play essential role in this process of food, from mechanical disruption by the teeth, to the creation of bile by the liver. Bile production of the liver play an important role in digestion. Mechanical digestion is breaking down of food into smaller pieces while being prepared for chemical digestion. Mechanical process start from the mouth. Chemical digestion start in mouth and continues into the intestines. Several different enzymes break down macromolecules into small molecules that can be absorbed


The digestive process starts in your mouth when you chew. Your salivary glands make saliva, a digestive juice, which moistens food so it moves more easily through your esophagus into your stomach. Saliva also has an enzyme that begins to break down starches in your food.


When you swallow, food enters your esophagus, which is connected to your stomach. Once the food is in your esophagus, waves of involuntary muscular contractions, called peristalsis, move the food toward your stomach.

Stomach Digestion

The stomach is a thick wall organ that lies between the esophagus and the part of the small intestine. Mucous membrane lines the stomach which contains glands that secrets gastric juice. The gastric juice is highly acidic. Gastric gland start secreting before food enters the stomach Food enters your stomach through a muscular ring that closes to keep the food in your stomach and stomach acid out of your esophagus. As you continue eating, your food’s mixed with gastric acid and other digestive juices in your stomach. Then the stomach empties this mixture into the small intestine.

Small Intestine Digestion

Your food is digested more thoroughly in your small intestine, also known as the duodenum. The small intestine, as well as the liver and pancreas, produces digestive juices and enzymes that separate out the nutrients in food. Among those enzymes are lipase and amylase from the pancreas. Muscular contractions keep the food moving along toward the large intestine. Your pancreas makes a digestive juice that has enzymes that break down carbohydrates, fats, and proteins. The pancreas delivers the digestive juice to the small intestine through small tubes called ducts. Your liver makes a digestive juice called bile that helps digest fats and some vitamins. Bile ducts carry bile from your liver to your gallbladder for storage, or to the small intestine for use.


The digested food continues its journey into the large intestine. The nutrients fats, carbohydrates and proteins, for example – have been broken down and are ready to be absorbed through the intestinal walls into your bloodstream for transport throughout your body. In your large intestine, more water moves from your GI tract into your bloodstream. Bacteria in your large intestine help break down remaining nutrients and make vitamin K. Waste products of digestion, including parts of food that are still too large, become stool.

Waste Elimination

Waste products from digestion are not absorbed through the intestinal walls but continue moving through your digestive tract into the colon. Waste products include dietary fiber. Waste products leave your body via bowel movements.

TASK 2 (b) Lipid (Fat) Calories Not all lipids (fats) contain the same amount of energy, as determined through calorimetry. One gram of beef or pork fat yields about 9.5kcal, and this is the average for one gram lipids from meat, fish, or eggs. One gram of butter fat yields about 9.27, and one gram of dairy fat gives about 9.25. Lipids from vegetables and fruits averages 9.30 kcal. The average heat of combustion (bomb calorimeter) for lipid is generally given as 9.4 kcal per gram. The net energy (average calories) for humans is the same, at 9.4 kcal per gram of fat. This is usually rounded to 9 calories per gram. Carbohydrate Calories Gross energy from glucose is 3.74 kcal per gram, and 4.20 for starch. The average for carbohydrate is given as 4.2 kcal per gram gross energy, and the net energy for humans is the same. The actual amount of energy from any carbohydrate varies depending on the shape of the molecule. The calories from carbohydrate is usually rounded to 4 calories per gram. Protein Calories Energy from protein can depend on the amount of nitrogen the protein contains, as well as the digestibility of the food. The higher the nitrogen content, the lower the amount of energy that can be derived through human metabolism. Proteins from meat, eggs, beans, and corn have about 16% nitrogen. Protein from nuts and sees, and most grains (cereals) have a higher nitrogen content of around 18.9%. Protein from milk has a lower nitrogen content of about 15.7%. In a bomb calorimeter, the average number of calories in one gram of protein is 5.65 kcal per gram. The average net energy for humans is generally given as 4.2 kcal per gram, the same as for carbohydrate. This, again, is usually rounded to 4 calories per gram.

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