The Digestive System
Digestion is the breakdown of carbohydrates, proteins, and lipids by the body into molecules that cells can absorb and utilise. Mechanical and chemical digestion are the two types of digestion. Chewing, crushing, and breaking food into tiny pieces is known as mechanical digestion. Chemical digestion is the process of breaking down food into simpler compounds, mostly with the help of enzymes. Enzymes are proteins that help complicated compounds break down chemically faster. The digestive system is depicted in Figure 10-1. The alimentary canal is the most important component of the digestive system.
Food travels via the alimentary canal, which is made up of organs. The mouth, oesophagus, stomach, small intestine, and large intestine are among these organs. See Plate Four for a more thorough representation of the digestive system. Digestion is aided by other organs in the digestive system. The salivary glands are one of these organs. the liver, the gallbladder, and the pancreatic Keep in mind where these organs are in respect to the alimentary canal. Food that has not been digested does not move through these organs. Instead, by releasing digestive fluids into the alimentary canal, they aid digestion.
Figure 10-1 The structure of the digestive system
Digestion in the Mouth
When you take your first mouthful of food, your digestion of a meal like Thanksgiving dinner with turkey and all the fixings begins. Mechanical digestion occurs as soon as food enters the mouth. Food is torn, ground, and chopped into smaller bits by the teeth. Soft food is mashed and mixed with saliva by the tongue. Saliva is a tasteless, watery liquid that moistens chewed food and initiates chemical digestion.
The organs that generate saliva are known as salivary glands. Figure 10-1 shows the three main salivary glands. They can be found on the mouth's floor and upper back. Saliva is discharged into the mouth by these glands. Amylase is found in saliva. Amylase [AM uh lays] is an enzyme that aids in the chemical digestion of complex carbohydrates like sweet potatoes and stuffing.
The tongue rolls the food into a bolus after it has been eaten and softened in the mouth. The bolus is pushed to the back of the throat by the tongue, where it is swallowed. Figure 10-2 depicts the swallowing process. During swallowing, the epiglottis, a tiny flap of tissue, stops food from entering the windpipe. The meal is subsequently swallowed and travels through the oesophagus. The oesophagus is a muscular tube that connects the mouth to the stomach [ih SOF uh gus].
The process of peristalsis transports food from the oesophagus to the stomach. The wavelike contraction of muscles that propels food through the digestive system is known as peristalsis. Your turkey meal will take a long time to digest. A circular muscle called a sphincter [SFINGK tur] is located at the stomach's entrance and opens and shuts to prevent food from backing up into the oesophagus.
Figure 10-2 During swallowing, the epiglottis prevents food from entering the windpipe.
The stomach, a sac-like digestive organ located between the oesophagus and the small intestine, is located between the oesophagus and the small intestine. Three layers of muscle make up the stomach walls, each placed at a distinct angle. Muscle contractions begin to twist, spin, and churn the food as it reaches the stomach. Mechanical digestion includes the twisting, spinning, and churning of food in the stomach. The meal is combined with the contents of the stomach. To assist liquefy food and break it down into simpler forms, gastric fluids are released into the stomach. Water, acid, and enzymes are all present in gastric juices.
In the stomach, the chemical breakdown of complex carbohydrates that began in the mouth slows down. Proteins and lipids, on the other hand, are broken down by the stomach fluids. Proteins, such as those in the turkey from Thanksgiving dinner, are broken down into amino acids by one enzyme in the stomach secretions. The digestion of the protein in milk is started by another enzyme.
The majority of foods last three to four hours in the stomach. The partially digested meal is converted into a viscous liquid called chyme while it is in the stomach. A sphincter at the stomach's end permits chyme to enter the small intestine in tiny amounts at a time.
Figure 10-3 The enzymes of the digestive system
|
Organs or Glands |
Secretions and
Enzymes |
Products of Digestion |
|
|
Mouth |
Salivary Glands |
Salivary amylase |
Breaks down starch
into dextrin and maltose |
|
Stomach |
Gastric glands |
Pepsin |
Breaks down proteins
into polypeptides |
|
Rennin |
Digests milk proteins |
||
|
Lipase |
Breaks down emulsified
fats into fatty acids and glycerol |
||
|
Hydrochloric acid |
Stimulates pepsin to
digest protein |
||
|
Small intestine |
Pancreas |
Trypsin |
Digests certain
polypeptides to amino acids |
|
Lipase |
Digest fats to fatty
acids and glycerol |
||
|
Amylase |
Digests starch to
maltose |
||
|
Intestine |
Peptidase |
Digests polypeptides
to amino acids |
|
|
Sucrase |
Digests sucrose to
glucose and fructose |
||
|
Maltase |
Digests maltose to
two molecules glucose |
||
|
Lactase |
Digests lactose to
galactose and glucose |
||
The Small Intestine
The small intestine is where the majority of food is chemically digested and absorbed. The tiny intestine is a lengthy, coiled organ with a diameter of around one inch. The duodenum [doo uh DEE num], the first section of the small intestine, links the stomach and the duodenum. The duodenum is a one-foot-long tube that looks like a C. This is when your Thanksgiving meal's primary digesting begins. The pancreas and intestinal glands are crucial in the last stages of food digestion. The pancreas is a long, squishy gland that secretes digestive enzymes into the duodenum and is located behind the stomach. The intestinal glands are small organs located in the small intestine's lining. They expel digestive enzymes as well as mucus.
The breakdown of proteins is continued by enzymes from the pancreas and intestinal glands. They also break down lipids into fatty acids and convert starch to simple sugars. Figure 10-3 shows a list of many digestive enzymes. Bile, a viscous yellow-green fluid produced by the liver, aids in the digestion and absorption of fat. The gallbladder stores bile before releasing it into the duodenum.
Nutrient absorption takes place throughout the small intestine. The villi [VIL eye] are little fingerlike projections that line the small intestine's walls. Examine Figure 10-4. The small intestine's surface area is increased by the many projections of the villi. The small intestine's greater surface area helps it to absorb the majority of the nutrients that enter your body. Blood veins and vessels carrying lymph fluid are seen within the villi. Vitamins and fatty acids that are fat soluble are absorbed by the lymphatic system. The blood arteries absorb glucose, amino acids, water-soluble vitamins, and minerals. The fully digested meal is subsequently carried throughout the body by blood and lymph.
Figure 10-4 Within the villi are complex networks of blood and lymph vessels. Nutrients pass through the villi into the blood and lymphatic system.
The Role of the Liver
Nutrient-rich blood flows from the small intestine to the liver. The liver further processes nutrients in the bloodstream. The liver is a huge organ that stores carbohydrates, secretes bile, and filters blood. Some digested food is taken from the bloodstream and converted into other forms or stored in the liver. Fats are prepared in a way that allows them to be mixed with proteins. Amino acids are either utilised to create proteins or are stored in the liver until they can be transported to other areas of the body via the bloodstream. Carbohydrates are converted to glucose in the liver. The glucose is subsequently released into the bloodstream, where it is transported to all of the body's cells. Excess glucose is stored as glycogen in the liver. Glycogen is a starchy material that the body uses to keep blood sugar levels stable in between meals.
Elimination of Wastes
When the gut is well-balanced, very few nutrients reach the large intestine. Meals are consumed. The tiny intestine only passes water, fibre, and meals that the body is unable to digest. The fibre from the cranberries and vegetables in your Thanksgiving feast will go undigested.
From the small intestine to the anus, the large intestine is a component of the digestive system. The length of the big intestine is around five feet. The large intestine's primary purpose is to reabsorb the water lost during digestion. The wastes solidify when the water is removed. Bacteria, undigested meals, and cholesterol can all be found in solid waste. The big intestine is lined by a variety of microorganisms. Undigested materials, such as fibre, are broken down by bacteria. Feces, or stools, are the residual solid wastes.
The rectum is the final five to six inches of the big intestine. Solid wastes are stored in the rectum until the body is ready to eliminate them. Solid wastes exit the body through the anus, a muscular hole at the end of the rectum, during the elimination process.
Review of the Lesson
Mechanical and chemical digestion occur as soon as food enters the mouth. Food that has been chewed and softened travels down the oesophagus to the stomach, where it is converted to chyme. The chyme travels to the small intestine, where the majority of chemical digestion occurs. The villi in the small intestine absorb the vast bulk of nutrients. The nutrients in the blood and lymphatic systems are transported to the liver and other organs.
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