Andy Montoya
The human body is an incredible machine, capable of transforming a simple cheese sandwich into fuel for the body. The journey of a cheese sandwich through the digestive system involves several organs and processes aimed at breaking down the food into its constituent nutrients for absorption. The process begins in the mouth, where mechanical digestion starts as teeth chew the sandwich, reducing it to smaller pieces. Saliva, which contains the enzyme amylase, is released to aid in breaking down the carbohydrates in the bread. From the mouth, the chewed sandwich, now called a bolus, travels through the oesophagus to the stomach, where it is mixed with gastric juices containing hydrochloric acid and enzymes like pepsin, which break down the proteins in the cheese. The stomach's churning action turns the food into a semi-liquid substance called chyme, which then moves into the small intestine, where the majority of digestion and nutrient absorption occur. The pancreas releases digestive enzymes, such as amylase, lipase, and proteases, into the small intestine to further break down carbohydrates, fats, and proteins. Bile, produced by the liver and stored in the gallbladder, is also released to aid in fat digestion.
| Characteristics | Values |
|---|---|
| How digestion begins | Anticipation of food stimulates the secretion of saliva in the mouth and gastric juices in the stomach. |
| Mouth | Mechanical digestion occurs as teeth chew the sandwich, breaking it down into smaller pieces. Saliva contains an enzyme called amylase, which breaks down carbohydrates in the bread. |
| Pharynx | Food is swallowed and moves into the pharynx, a common passageway for food and air. |
| Esophagus | Food travels down the esophagus through peristalsis, a series of wave-like muscle contractions. |
| Stomach | The bolus is mixed with gastric juices containing hydrochloric acid and enzymes like pepsin, breaking down proteins in the cheese. The stomach's churning action turns the food into a semi-liquid substance called chyme. |
| Small Intestine | Chyme moves into the small intestine, where most digestion and nutrient absorption occur. The pancreas releases digestive enzymes like amylase, lipase, and proteases, breaking down carbohydrates, fats, and proteins. Bile from the liver aids in fat digestion. |
| Absorption | Nutrients from digested food, including sugars, amino acids, fatty acids, vitamins, and minerals, are absorbed through the walls of the small intestine into the bloodstream. |
| Large Intestine | The remaining indigestible parts, such as fiber and water, enter the large intestine, where water is absorbed. |
| Rectum | The walls of the rectum vibrate, and the food exits as faeces. |
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What You'll Learn
How does digestion begin?
The digestion of a cheese sandwich begins even before the first bite is taken. The sight and smell of food trigger the brain to ready the digestive tract for nourishment. This is the cephalic phase of digestion, which is stimulated by the parasympathetic nervous system. The brain then stimulates the secretion of saliva in the mouth and gastric juices in the stomach.
Once a bite of the sandwich is taken, mechanical digestion begins. The teeth chew the sandwich, breaking it into smaller pieces and mixing it with saliva, which contains an enzyme called amylase that starts to break down the carbohydrates in the bread. This chewed mass of food is called a bolus.
The bolus is then swallowed and moves into the pharynx, a common passageway for both food and air. From the pharynx, the bolus travels down the esophagus through a series of wave-like muscle contractions called peristalsis. As the bolus approaches the stomach, a ring-like muscle called the lower esophageal sphincter relaxes, allowing the chewed food to enter.
In the stomach, the bolus is mixed with powerful gastric juices containing hydrochloric acid and enzymes like pepsin, which break down the proteins in the cheese. The stomach's churning action further mixes and turns the food into a semi-liquid substance called chyme.
Thus, the digestion of a cheese sandwich begins with the stimulation of the brain and the secretion of saliva, followed by mechanical digestion in the mouth, and then the breakdown of food in the stomach with the help of gastric juices and enzymes.
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What happens in the mouth?
The journey of a cheese sandwich through the digestive system begins when you bite into it. In the mouth, mechanical digestion occurs as the teeth chew the sandwich, breaking it down into smaller pieces. This process is called mastication and helps to reduce the size of food particles, making them easier to swallow. The action of chewing also mixes the food with saliva, which is produced by the salivary glands. Saliva contains an enzyme called amylase, which initiates the chemical breakdown of starch and carbohydrates present in the bread. This chemical digestion in the mouth is an important step in the overall process of breaking down food into smaller, absorbable nutrients.
The sight, smell, and even the thought of food can stimulate the secretion of saliva in the mouth. This is part of the cephalic phase of digestion, which is controlled by the parasympathetic nervous system. The hungrier you are, the more you salivate, as your brain prepares the digestive tract for incoming nourishment. This is why you may find yourself salivating at the sight of a delicious cheese sandwich!
Once the sandwich has been chewed and mixed with saliva, it forms a bolus, which is then ready to be swallowed. This marks the end of the mouth's role in the digestive process, as the bolus passes through the pharynx and into the oesophagus, beginning its journey towards the stomach. The passage of the bolus through the oesophagus is facilitated by peristalsis, a series of wave-like muscle contractions that propel the food downwards.
While the mouth is primarily responsible for the initial mechanical breakdown of food, it also plays a crucial role in the chemical breakdown of certain nutrients. The enzyme amylase in saliva is specifically designed to target and break down starch and carbohydrates, which are abundant in bread. This early stage of chemical digestion in the mouth ensures that by the time the food reaches the stomach, some nutrients have already been partially broken down, making it easier for the body to complete the digestion process and absorb the necessary nutrients.
The mouth, therefore, serves as the crucial starting point for both mechanical and chemical digestion. It is the first step in the journey of a cheese sandwich through the digestive system, setting the stage for the subsequent processes that will eventually break down the food into its constituent nutrients and allow the body to utilise its energy and nutritional content.
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What happens in the stomach?
Once the chewed sandwich is swallowed, it passes through the oesophagus and into the stomach. The ring-like muscle at the top of the stomach, the lower oesophageal sphincter, relaxes to allow food to enter.
The stomach then mixes the chewed food with gastric juices, including hydrochloric acid and enzymes like pepsin, which break down the proteins in the cheese and meat. This process is called peristalsis. Peristalsis turns the food into a thick semi-liquid substance called chyme. Hydrochloric acid is strong enough to dissolve metal, but mucus secreted by the cells of the stomach wall protects it from the acid. The acid also kills microorganisms that may be present in the food.
The digestive enzyme pepsin breaks down the proteins in the cheese and meat. The acid and enzymes work together to break down the molecules and particles. This process takes 2-3 hours.
The chyme then moves into the small intestine, where the majority of digestion and nutrient absorption occur.
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What happens in the small intestine?
The small intestine is the primary site for digestion. It is where the majority of digestion and nutrient absorption occurs. The small intestine has a remarkable and expansive absorptive area, which allows us to harness the nutrients in our food.
The presence of chyme in the upper portion of the small intestine triggers the pancreas to secrete bicarbonate to neutralise the acid, lipase to digest fats, amylase to digest starches, and proteases to digest proteins. The pancreas releases these digestive enzymes into the small intestine to break down carbohydrates, fats, and proteins. The liver produces bile, which is stored in the gallbladder and released into the small intestine to aid in fat digestion.
The gallbladder contracts when fat is present in the chyme (as it is from the cheese and mayonnaise in a sandwich). The bile acts like a detergent, emulsifying the fat and breaking it into small globules, which aids fat absorption. The cells of the small intestine secrete additional enzymes to complete digestion.
Once starches and sugars have been digested into monosaccharides (glucose, galactose, or fructose), they are ready for absorption. They pass through the lining of the small intestine and into the bloodstream via the portal vein to the liver. The walls of the intestinal lumen are wrinkled and folded, increasing the absorptive area threefold. However, this is still not enough, so the folds are covered with tiny finger-like projections called villi, which are in turn covered by microscopic projections called microvilli. This combination of folds and projections increases the absorptive area of the small intestine 600-fold, to the size of a tennis court.
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What happens in the large intestine?
The large intestine is part of the digestive tract and is approximately 5 feet long, making up one-fifth of the length of the gastrointestinal (GI) tract. It includes the appendix, cecum, colon, and rectum. The cecum is the first part of the large intestine, followed by the colon, which is made up of the ascending colon, transverse colon, descending colon, and sigmoid colon.
The large intestine has three primary functions: absorbing water and electrolytes, producing and absorbing vitamins, and forming and propelling feces toward the rectum for elimination. It absorbs water, vitamins, and electrolytes from waste material. By the time indigestible materials have reached the colon, most nutrients and up to 90% of the water have already been absorbed by the small intestine. The ascending colon absorbs the remaining water and other key nutrients from the indigestible material, solidifying it to form stool. The descending colon stores feces that will eventually be emptied into the rectum. The sigmoid colon contracts to increase the pressure inside the colon, causing the stool to move into the rectum. The rectum holds the feces awaiting elimination by defecation.
The large intestine also contains bacteria, also called gut flora or the microbiome, that help with digestion. They help break down remaining nutrients and make vitamin K.
The intestinal wall is made up of multiple layers. The four layers of the large intestine from the lumen outward are the mucosa, submucosa, muscular layer, and serosa. The muscular layer is made up of two layers of smooth muscle, the inner, circular layer, and the outer, longitudinal layer. These layers contribute to the motility of the large intestine.
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Frequently asked questions
Digestion begins in the mouth when we bite into our food. We chew to break it into smaller pieces, and mix it with saliva, which contains an enzyme called amylase to help break down carbohydrates.
In the stomach, the chewed sandwich, now called a bolus, is mixed with gastric juices containing hydrochloric acid and enzymes like pepsin, which break down the proteins in the cheese. The stomach's churning action turns the food into a liquid called chyme.
The chyme moves into the small intestine, where the majority of digestion and nutrient absorption occur. The pancreas releases digestive enzymes, such as amylase, lipase, and proteases, to further digest fats, carbohydrates, and proteins. Bile from the liver is also released to aid in fat digestion.
The broken-down nutrients, including sugars, amino acids, and fatty acids, are absorbed through the walls of the small intestine into the bloodstream, providing energy and essential components to the body's cells.
The remaining indigestible parts, such as fibre and water, enter the large intestine, where water is absorbed from the food. The waste then moves into the rectum and is eliminated from the body through the anus as faeces.
