The digestive system

What are the six basic processes that the digestive system performs?

• Ingestion- the process of taking foods and liquids into the mouth.
• Secretion- each day cells in the walls of the GI tract and accessory organs secret 7 litres of water, acid, bufffers and enzymes into the lumen of the tract.
• Miing and propulsion- Contraction and relaxtion of smooth muscle in the walls of the GI tract mix food and secretions and move them towards the anus. 
• Digestion- mechanical and chemical processes break down ingested food into small molecules. 
• Absorption- the products of digestion being introduced into the epithlial cell lining the lumen of the GI tract.
• Defecation- Waste, indigestible substances, bacteria, cells from the GI tract and unabsorbed digested materials leave the body through the anus.

What are the four layers of the GI tract?

• The mucosa
• The submucosa
• The muscularis 
• The serosa and peitonum

What is the mucosa?

A mucous membrane which lines the inner part of the GI tract. Composed of a layer of epithelium, a layer of areolar connective tissue (called the lamina propria) and a layer of smooth msucle (called the muscularis mucosae). The muscularis mucosae creates folds in the muscosa, incresing the surface area for digestion and absorption. The mucosa also contains lymphatic nodules which protect against the entry of pathogens through the GI tract.

What is the submucosa?

The submucosa consists of areolar connective tissue that binds the mucosa to the muscularis. It contains blood and lymphatic vessels that receive absorbed food molecules. The ENS is also located in the submucosa, this controls the secretions of organs in the GI tract.

What is the muscularis?

The muscularis a thick layer of muscle in the mouth, pharynx and upper esophagus, it consists in a part of the skeletal muscle that produces voluntary swallowing. Skeletal muscle also forms the external anal sphincter, which permits voluntary control of defecation. In the rest of the tract the muscularis consists of smooth muscle usually arranged as an inner sheet of circular fibres and an outer sheet of longitudal fibres. Involuntary contractions of these smooth muscles help to break down food physically, mix it with digestive secretion and propel it along the tract.

What is the serosa and peritoneum?

The outermost layer around the organs of the GI tract below the diaphragm is a membrane composed of a simple squamous epithelium and areolar connective tissue. The serosa secretes a slippery, watery fluid that allows the tract to glide easily against other organs.

What parts inside the mouth aid digestion?

• Tongue
• Salivary glands
• Teeth

How does the tongue aid digestion?

The tongue forms the floor of the oral cavity. It is composed of skeletal muscle covered with mucous membrane. The superior surface and lateral areas of the tongue are covered with papillae. Some papillae contain taste buds. Glands in the tongue secrete lingual lipase which digests triglycerides once in the acid environment of the stomach.

How do salivary glands aid digestion?

Most saliva is secreted by the salivary glands, which lie outside the mouth and release their secretions into ducts that empty into the oral cavity. There are three pairs of salivary glands: parotid, submandibular, and sublingual. Saliva lubricates food and starts the chemical digestion of carbohydrates. Salivation is controlled by the autonomic nervous system.

How do the teeth aid digestion?

The teeth, or dentes, project into the mouth and are adapted for mechanical digestion. A typical tooth consists of three principal portions: crown, root, and neck. Teeth are composed primarily of dentin and are covered by enamel, the hardest substance in the body. Humans have two sets of teeth: deciduous and permanent. Mechanical digestion in the mouth results from chewing- mastication, food is mixed with saliva and shaped into a bolus.

What is the strucuture and function of the pharynx?

• The pharynx is a funnel-shaped tube that is composed of skeletal muscle and lined by mucous membrane. 
• The nasopharynx is involved in respiration.
• Muscular contractions of the oropharynx and laryngopharynx help propel food in the oesophagus. 

What is the structure and function of the oesophagus?

• The oesophagus is a muscular tube lined with stratfied squamous epithelium that lies posterior to the trachea. It begins at the end of the laryngopharynx, passes through the mediastinum and diagphram and connects to the superior aspect of the stomach.  
• The esophagus transports food to the stomach and secretes mucus. 

How does a bolus pass from the mouth into the stomach?

Swallowing is divided into three stages the voluntary, pharyngeal and esophageal stages, each of these stages help the bolus pass from the mouth to the stomach. In the voluntary stage of of swallowing, the blous is forced to the back of the mouth cavity and into the oropharynx by the movement of the tongue upward and backward against the palate. With the passage of the bolus into the oropharynx the involuntary pharyngeal stage of swallowing begins. Breathing is temoprarily interrupted when the soft palate and uvula move upward to close off the naspharynx, the epiglottis seals off the larynx and the vocal chords come together. After the bolus passes though the oropharynx, the respiratory passageways reopen and breathing resumes. Once the upper oesophageal sphincter relaxes, the blous moves into the oesophagus. 

Food is then passed through the oesophagus by peristalsis. The circular muscle fibres in the section of oesophagus above the bolus contract, constriting the wall of the oeophagus and squeezing the bolus downwards. Longitudinal muscle fibres around the bottom of the bolus contract, shortening the section of oesphagus below the bolus and pushing its walls outwards. After the bolus moves into a new section of the oesphagus the cycle repeats. As the bolus approaches the end of the oesphagus, the lower oesophageal sphincter relaxes and the bolus moves down towards the stomach.

How does a bolus pass from the mouth into the stomach?

Swallowing is divided into three stages the voluntary, pharyngeal and esophageal stages, each of these stages help the bolus pass from the mouth to the stomach. In the voluntary stage of of swallowing, the blous is forced to the back of the mouth cavity and into the oropharynx by the movement of the tongue upward and backward against the palate. With the passage of the bolus into the oropharynx the involuntary pharyngeal stage of swallowing begins. Breathing is temoprarily interrupted when the soft palate and uvula move upward to close off the naspharynx, the epiglottis seals off the larynx and the vocal chords come together. After the bolus passes though the oropharynx, the respiratory passageways reopen and breathing resumes. Once the upper oesophageal sphincter relaxes, the blous moves into the oesophagus. 

Food is then passed through the oesophagus by peristalsis. The circular muscle fibres in the section of oesophagus above the bolus contract, constriting the wall of the oeophagus and squeezing the bolus downwards. Longitudinal muscle fibres around the bottom of the bolus contract, shortening the section of oesphagus below the bolus and pushing its walls outwards. After the bolus moves into a new section of the oesphagus the cycle repeats. As the bolus approaches the end of the oesphagus, the lower oesophageal sphincter relaxes and the bolus moves down towards the stomach.

What are the four main regions of the stomach?

• The cardia- which surrounds the superior opening of the stomach. 
• The fundus- the portion superior and to the left of the cardia.
• The body- the large central portion of the stomach.
• The pyloric part- the pyloric part consists of the pyloric cannal which connects to the body, the pyloric antrum which connects to the pyloric cannal and pylorus which connects to the duodenum. 

What four layers is the stomach wall composed of?

• The mucosa.
• The submucosa.
• The muscularis.
• The serosa. 

What happens during mechnical digestion in the stomach?

Once food reaches the stomach, the stomach wall is stretched, and the Ph of the stomach contents increases because proteins in food have buffered some of the stomach acids. These changes in the stomach trigger nerve impulses that stimulates the flow of gastric juice and initiates mixing waves. These waves macerate food and mixes it with the secretions of the gastric glands producing chyme. Each mixing wave forces a small amount of chyme through the partially closed pyloric sphincter into the duodenum, a process called gastric emptying. Most of the chyme is forced back into the body of the stomach, the next mixing wave pushes chyme forwards again and forces more into the duodenum. After the stomach has emptied some of its contents into the duodenum, reflexes begin to slow thee exit of chyme from the stomach. This prevents overloading of the duodenum. 

What happens during chemical digestion?

In chemical digestion the enzyme pepsin begins protein digestion, which breaks down the peptide bonds between amino acids of proteins. Therefore, as a result the proteins become fragmented into peptides, which are smaller strings of amino acids. Pepsin is effective in the most acidic part of the stomach with a Ph of 2. The chief cells secrete pepsin in an inactive form, it is not active pepsin until it contacts hydrochloric acid in the gastric acid. Mucus secreted by mucous cells coats the mucosa, forming a thick barrier between the cells of the stomach lining and the gastric juice. The stomach wall is impermeable to most substances. Among the substances the stomach can absorb are water, ions, short-chain fatty acids, some drugs, and alcohol.

What is the structure of the pancreas?

• The pancreas lies behing the stomach.
• Secretionspass from the pancreas to the duodenum via the pancreatic duct, which unites with the bile duct from the liver and gallbladder, forming the hepatopancatic duct, which enters the duodenum.
• The pancreas is made up of small clusters of glandular epithelial cells, arranged in clusters called acini. The acini constitute the exocrine portion of the pancreas. The cells within acini secrete pancreatic juice. The remaining cells are organised into pancreatic islets, which constitutes the endocrine portion of the pancreas.

What is pancreatic juice?

• Pancreatic juice is a clear, colourless liquid which contains enzymes that digest starch (pancreatic amylase); proteins (trypsin, chymotrypsin, and carboxypeptidase); triglycerides (pancreatic lipase); and nucleic acids (ribonuclease and deoxyribonuclease).

What is the structure of the liver and gallbladder?

The liver has right and left lobes. The gallbladder is a sac located in a depression under the liver, which stores and concentrates bile produced by the liver.

What are the several components that make up the structure of the liver?

• Hepatocytes- the major functional cells of the liver which perform metabolic, secretory and endocrine functions.
• Bile canaliculi- small ducts between hepatocytes that collect the bile produced.
• Hepatic sinusoids- highly permeable blood capillaries between rows of hepatocytes that receive oxygenated blood from branches of the hepatic artery and nutrient-rich deoxygenated blood from branches of the hepatic portal vein. Hepatic sinusoids converge and deliver blood into the central vein. Also present in the hepatic sinusoids are fixe phagocytes called reticuloendothelial (Kupffer) cells which destroy worn out white and red blood cells, bacteria and other foreign matter in the venous blood draining from the gastrointestinal tract. 

How does bile contricute to digestion?

• Biles contribution to digestion is the emulsification of dietary lipids. The breakdown of large lipid globules into the suspension of small lipid globules and in the absorption of lipids following digestion. The principal bile pigment is bilirubin which is derived from heme. When worn out red blood cells are broken down, iron, globin and bilirubin are released. The iron and globin are recycled but some of the bilirubin is excreted in bile.  Bilirubin is eventually broken down in the intestine and one of its breakdown products- stercobilin gives faeces its normal brown colour.

What are the 7 functions of the liver?

• Carbohydrate metabolism.
• Lipid metabolism
• Protein metabolism
• Processing drugs and hormones
• Excretion of bilirubin
• Storage of vitmains and minerals
• Activation of vitamin D

What is the basic structure of the small intestine?

• The small intestine has three portions- the duodenum, the jejunum and the ileum. The duodenum is the first portion which is the shortest part and attaches to the pylorus of the stomach. The next portion is the jejunum which is 1 metre long and is in the left upper quadrant. The ileum measures 2 metres and joins the large intestine at the ileocecal sphincter/ valve, it is in the right lower quadrant.

What is the wall of the small intestine composed of?

The wall of the small intestine is composed of the mucosa, the submucosa, the muscularis and the serosa. 

What is the structure of the mucosa like?

The epithelial layer of the small intestinal mucosa consists of simple columnar epithelium that contains many types of cells. Absorptive cells release enzymes to digest food and contain microvilli to absorb nutrients. There are also goblet cells which secrete mucus. It also contains intestinal glands, which contain three types of endocrine cells that secrete hormones into the bloodstream S cells, CCK cells, and K cells which secrete secretin, cholecystokinin and glucose-dependent insuliotripic peptide.

What is the structure of the submucosa like?

The submucosa contains duodenal glands that secrete an alkaline mucus, it helps to neutralise gastric acid in chyme.

What is the structure of the muscularias like?

The muscularis consists of two layers of smooth muscle the outer longitudinal layer and inner circular layer.

What is the structure of the serosa like?

The muscularis consists of two layers of smooth muscle the outer longitudinal layer and inner circular layer.

How does mechanical digestion occur in the small intestine?

Two types of movements contribute to intestinal motility in the small intestine; segmentations and peristalsis. Segmentations are localised contractions that slosh chyme back and forth mixing it with digestive juices and bringing food particles into contact with the mucosa for absorption. After most of the food is absorbed peristalsis begins in the lower portion of the stomach and pushes chyme forward along a small stretch of the small intestine.

How does chemical digestion occur in the small intestine?

• Starches and dextrin’s not reduced to maltose by the time chyme leaves the stomach are broken down by pancreatic amylase. Maltase splits maltose into two molecules of glucose. Sucrase breaks sucrose into a molecule of glucose and a molecule of fructose. Lactase digests lactose into a molecule of glucose and a molecule of galactose.

How does absorption occur in the small intestine?

• Absorption  occuurs by the passgae of nutrients from digested food in the GI tract, into the blood or the lymph.
• Monosaccharides, amino acids, and short-chain fatty acids pass into the blood capillaries.
• Long-chain fatty acids and monoglycerides are absorbed as part of micelles, resynthesized to triglycerides, and transported in chylomicrons to the lacteal of a villus.
• The small intestine also absorbs water, electrolytes, and vitamins.

What are the four regions that make up the basic structure of the large intestine?

• The large intestine extends fro the ileocecal sphincter to the anus. 
• The cecum is the first segment of the large intestine 
• The cecum merges with the next segment called the colon, which is the longest portion of the large intestine. The colon is divided into the ascending, descending, transverse and sigmoid portions.
• The next segment is the rectum. 
• The final segement is the anal canal.

What four layers are found in the wall of the large intestine?

• The mucosa.
• The submucosa.
• The muscularis.
• The serosa.

How does digestion and absorption occur in the large intestine?

• Mass peristalsis occurs in the large intestine, a strong peristaltic wave that begins in the middle of the colon and drives the colonic contents into the rectum. Food in the stomach initiates mass peristalsis.
• The final stage of digestion occurs in the colon through the activitiy of bacteria, bacteria ferment any remaning carbohydrates and release hydrogen, carbon dioxide and methane gases. 

What is the defecation reflex?

 Faeces consist of water, inorganic salts, epithelial cells, bacteria, and undigested foods. The elimination of faeces from the rectum is called defecation. Defecation is a reflex action aided by voluntary contractions of the diaphragm and abdominal muscles and relaxation of the external anal sphincter.

What are the phases of digestion?

• Digestive activities occur in three overlapping phases: cephalic phase, gastric phase, and intestinal phase.
• During the cephalic phase of digestion, salivary glands secrete saliva and gastric glands secrete gastric juice in order to prepare the mouth and stomach for food that is about to be eaten.
• The presence of food in the stomach causes the gastric phase of digestion, which promotes gastric juice secretion and gastric motility.
• During the intestinal phase of digestion, food is digested in the small intestine. In addition, gastric motility and gastric secretion decrease in order to slow the exit of chyme from the stomach, which prevents the small intestine from being overloaded with more chyme than it can handle.