What are the components of the human digestive system?

What are the components of the human digestive system?

Humans have a complete digestive system. Swallowing forces food and water from the mouth into the pharynx. Food continues through an esophagus to the stomach. Food processing starts in the mouth. Most digestion and absorption occurs in the small intestine. The colon absorbs most of the remaining water and ions, which causes the wastes to compact. The rectum briefly stores the wastes before they are expelled through the anus.The liver and pancreas have an accessory role in digestion. They produce substances that are secreted into the small intestine.

Human Digestive system

 

The human alimentary canal: The alimentary canal begins at the mouth and the pharynx.

From here, food travels down a muscular tube called the esophagus and into the stomach. The stomach connects to the small intestine, which in turn connects to the large intestine. Some form of mechanical and chemical digestion breaks down food at every step along this tube and Nutrients are absorbed across the walls of both intestines. All products remaining after food travels through the large intestine are waste, which exit the body through the anus.

Saliva: Contains water, salt and an enzyme called salivary amylase. Salivary amylase breaks down starch into smaller sugars. Contains buffers, substances that neutralize acidic foods. Antibacterial substances that kill bacteria in the food. Also eases the passage of food through the pharynx and esophagus.

The Esophagus The esophagus shuttles food from the pharynx (part of mouth) to the stomach. The muscles surrounding the esophagus perform peristalsis, the rhythmic and stepwise contraction of muscle that forces food to move along the esophageal passage. Sphincter muscles at each end prevent backflow.

The Stomach A saclike organ. Inner surface is highly convoluted, allowing it to fold up when empty and expand when full. Digestion occurs in the stomach as gastric juice, is secreted by gastric glands located in the stomach lining. Gastric juice is an acidic mixture of enzymes and mucus. Gastric glands are composed of two types of cells: 

Parietal cells: secrete hydrochloric acid (HCl)

Chief cells: secrete pepsinogen, the precursor to the enzyme pepsin.

Protein digestion is initiated by the acidic environment of the stomach. The HCl, lowers the acidity of the gastric juice to a pH level of 2. Low pH levels are necessary for pepsinogen to be converted into the active digestive enzyme pepsin, which further breaks down food proteins. If chief cells were to secrete pepsin directly into the stomach, the pepsin would digest the cells themselves. Additionally, mucus lines the stomach walls and prevents it from being digested. A mixture of gastric acid and partially digested food called chyme passes out of the stomach.

The Upper Digestive Tract

What happens to food in the mouth?

Teeth mechanically break food into smaller particles. Enzymes in saliva begin the chemical digestion of carbohydrates and fats.

What are the functions of the stomach?

The stomach receives food from the esophagus and stretches to store it.Stomach contractions break up food and mix it with gastric fluid. They also move the resulting mixture (the chyme) into the small intestine.Chemical digestion of proteins begins in the stomach.

The oral cavity (Digestion in Mouth):  Mouth, the teeth. the tongue, Salivary Glands.

 The teeth: break food into smaller pieces to expose more of the food’s surface area to digestive enzymes.

 The tongue: a muscle used to manipulate food and form it into a bolus, a round, easy-to swallow ball.

Salivary glands: secrete mucous saliva.

Most Chemical Digestion Occurs in the Small Intestine

How does the structure of the small intestine affect its function?

 The surface of the small intestine is highly folded and each fold has many projections (villi). Brush border cells at the surface of a villus have tiny projections (microvilli) at their surface. The many folds and projections greatly increase the surface area for the two functions of the small intestine—digestion and absorption.

Dudenum :It’s a tube of about 30 cm long :

–        Receives bile via bile duct from the liver

–        Receives pancreatic juice from the pancreas, through the pancreatic duct

–        Releases digestive juice from its walls. This intestinal juice contains protease and the enzyme lipase for fat digestion

The Small Intestine

Duodenum: Controls the release of food into the small intestine

Most of the chemical breakdown (Digestion) and absorption of Food takes place here. Bile salts break down fats in the chyme while pancreatic fluid, composed of bicarbonate, neutralizes the acid. The small intestine also contains a host of enzymes that help digest various food molecules. The enzymes present in the small intestine and their functions are outline in the table below.

Pancreatic amylase, maltase, sucrase, lactase ------Starch

Trypsin, chymotrypsin, aminopeptidase,carboxypeptidase ------Protein 

Nucleases------ Nucleic acids 

Bile salts, lipase ------Fats

What are the roles of the small intestine?

Chemical digestion is completed in the small intestine. Enzymes from the pancreas and enzymes embedded in the membrane of brush border cells break large molecules into smaller absorbable subunits. Small subunits (monosaccharides, amino acids, fatty acids, and monoglycerides) enter the internal environment when they are absorbed into the intersitial fluid in a villus. Most fluid that enters the gut is also absorbed across the wall of the small intestine.

The Small Intestine

• The small intestine is split into three sections: the duodenum (30 Cm), jejunum (2.5m), and the ileum(2-4m).

• Chemical digestion takes place in the duodenum and the jejunum.

• The ileum, the final section of the small intestine, is where most absorption of the nutrients takes place.

• Small projections called villi cover the ileum walls.

• Cells lining the villi are covered in folds of plasma membrane that form even smaller projections called microvilli.

• Villi and microvilli increase the ileum’s surface area, providing more surface across which nutrients can be absorbed.

• Nutrients pass through capillaries in the lining of the villi and into the bloodstream, where they circulate first to the liver, then throughout the rest of the body.

• Villi and microvilli projections also contain digestive enzymes to further digest food. 

Carbohydrate Digestion and Absorption

Figure description

Enzymes break polysaccharides down to simple sugars, or monosaccharides. Monosaccharides are actively transported into brush border cells, then out into interstitial fluid. Proteins are broken into polypeptides, then amino acids. Amino acids are actively transported into brush border cells, then out into interstitial fluid.

1.      In the small intestine, carbohydrates are broken down into monosaccharides, or simple sugars .

2.      This process began in the mouth, where salivary amylase broke polysaccharides into disaccharides (two-unit sugars). A pancreatic amylase carries out the same reaction in the small intestine. Disaccharides are substrates for enzymes embedded in the plasma membrane of the brush border cells. The enzymes split disaccharides into monosaccharides. For example, sucrase breaks sucrose into glucose and fructose subunits. Lactase splits lactose into glucose and galactose. Monosaccharides are actively transported into a brush border cell, then out into the interstitial fluid inside a villus. From here they enter the blood.

Protein Digestion and Absorption

3.      Protein digestion began in the stomach, where pepsin broke proteins into polypeptides. It is completed in the small intestine.

4.      The pancreas secretes proteases such as trypsin and chymotrypsin that break polypeptides into peptide fragments. Enzymes at the surface of the brush border cell break these fragments into amino acids. Like monosaccharides, amino acids are actively transported into brush border cells, then out into the interstitial fluid. From here they enter the blood.

5.       Movements of the intestinal wall break up fat globules into small droplets. Bile salts coat the droplets, so that globules cannot form again.
 

6.       Pancreatic enzymes digest the droplets to fatty acids and monoglycerides.

7.      Monoglycerides and fatty acids diffuse across the plasma membrane's lipid bilayer, into brush border cells.
8.      A In a brush border cell, the products of fat digestion form triglycerides, which associate with proteins.  I he resulting lipoproteins are then expelled by exocytosis into the interstitial fluid inside the villus.

The Large Intestine

Food from the small intestine empties into the large intestine, or colon. No digestion and only a small percentage of absorption take place in the large intestine. The large intestine primarily functions to concentrate waste material into a form called feces. Movements of the large intestine compact feces and move it into the rectum, where it is exited from the body through either the anus. Feces is expelled from the body through the voluntary movements of muscles surrounding the rectum.

Rectum: Muscular storage chamber where the undigested food (feces) is held and molded before being pushed through anus during expulsion.

Anus: The exit of alimentary canal, it is closed by a ring of muscle (the anal sphincter). The ring is relaxed during expulsion of feces.

What are the functions of the large intestine?

By absorbing water and mineral ions, the colon compacts undigested residues and other wastes as feces, which are siored briefly in the rectum before expulsion.