Digestive System

General description

The primary function of the digestive system is to prepare food that is eaten so that it may be transported and used by body cells. The digestive system consists of the alimentary canal, associated organs and  digestive processes. The alimentary canal is a long tube extending from  mouth to anus through which food passes. When food is ingested, the digestive system breaks it down into nutrients that can be used by the body. In the mouth, food is chewed  and swallowed, then it is propelled downward to the stomach. Here, gastric fluids begin to change the nature of food from solid to semiliquid. Most digestion and absorption occurs in the small  intestine through capillaries and lymph vessels in the intestinal lining. Undigested food is moved through the digestive tract and eliminated from the body through the anus. The digestive system is also referred to as the gastrointestinal tract.

 

Upper digestive system

 

Upper digestive tract

The following section covers the alimentary canal and associated organs from the mouth to the small intestine.

 

Mouth

Food taken into the mouth is broken up by chewing. The teeth and tongue start the physical digestive process, breaking down food particles to a smaller size and smoother consistency. Salivary glands supply digestive enzymes and moisture to start the chemical digestive  process and to aid mastication and swallowing.

 

Pharynx

Food passes through the pharynx on the way from the mouth to the oesophagus. The pharynx lies behind the nose and larynx and is  common to both the digestive and respiratory systems. It is involved in  respiration as air passes through it from the nose to the larynx. A small  “lid”, the epiglottis, closes the trachea during the act of swallowing to prevent food from reaching the lungs.

 

Oesophagus

The oesophagus connects the pharynx with the stomach. Food is moved
down the oesophagus by peristalsis (waves of contraction).

 

Stomach

The stomach is an enlarged portion of the alimentary canal between  the oesophagus and duodenum. It enlarges when a meal is eaten  and acts as a reservoir for the food while digestive enzymes and  hydrochloric acid act on it. At frequent intervals, the stomach releases  small amounts of this semiliquid (chyme) into the small intestine.

 

Pylorus

The stomach joins the duodenum at the pylorus. The pyloric sphincter prevents
food passing from the small intestine back into the stomach.

 

Small intestine

The small intestine comprises three continuous parts; the duodenum, jejunum and the ileum. It is about 7 metres long. The small intestine  continues the digestive process that started higher up the alimentary  canal. Digestive juices and bile are emptied into the duodenum from  the pancreas and gall bladder at the sphincter of Oddi, whilst glands  in the intestinal mucosa add other juices. Nutrients are absorbed into the bloodstream and lymphatic system through capillaries and lymph vessels.

 

Sphincter of Oddi

Controls the flow of fluids from the ampulla of Vater into the duodenum.

 

Bile ducts

The cystic and hepatic ducts and the common bile duct, passes bile from the liver to the gall bladder and the duodenum.

 

Cystic duct

The cystic duct links the gall bladder to the hepatic duct to form the common bile duct.

 

Hepatic ducts

The hepatic duct leads from the liver to the common bile duct.

 

Gall bladder

The gall bladder is a reservoir for bile from the liver. Here, mucus is added and water removed to form concentrated bile.

 

Liver

The liver is the largest gland in the body and has many functions connected with digestion. It stores certain vitamins and iron, and is the main heat producing organ of the body.

 

Pancreas

The pancreas is a gland that secretes pancreatic juice into the duodenum through the pancreatic duct as part of the digestive process.It also has an endocrine function.

 

Islets of Langerhans

There are irregularly distributed clusters of cells in the pancreas called the Islets of
Langerhans that are not a part of the digestive system, but of the endocrine system.
(See section on Endocrine System).

 

Ileum

The ileum is the last part of the small intestine leading to the large intestine. It is about 3 metres long.

 

Upper Digestive System

 

Mouth

 

Pancreas

 

Lower digestive system

The following section covers the alimentary canal and associated organs from the large intestine to the anus.

 

Large intestine

The large intestine is short (approximately 1½ metres), but larger in circumference than the small intestine. It is a continuation of the small  bowel, terminating at the rectum. The large intestine absorbs water  and some chemicals from the digested food, creating faeces.

 

Colon

The colon is continuous but has clearly identifiable parts. These are referred to as the caecum,the ascending colon, transverse colon, descending colon and sigmoid colon

Caecum

The first portion of the large intestine is called the caecum.

 

Appendix

Leading from the caecum is the appendix.

 

Rectum

Leading from the sigmoid colon to the anal canal is the rectum.

 

Anal canal

The last part of the alimentary canal from the rectum to the exterior is the anal canal.

 

Anal sphincter

The anal sphincter is a circular muscle that controls the opening of the anus.

 

Anus

The anus is the opening of the alimentary canal to the exterior at the end of the anal canal.

 

Lower Digestive System

 

Glossary of Terms Digestive System

Accessory organs

(ak-ses’o-re) Structures which aid in the digestive process – tongue,teeth, salivary glands, pancreas, liver and gall bladder.

 

Alimentary canal

(al-e-men’tar-e) The continuous tract from mouth to anus through which food moves during the process of digestion.

 

Anus

(a’nus) Terminal portion of the intestinal tract, about 25 to 35 mm long.

 

Appendix

(ah-pen’diks) Blind, worm-like pouch found at the caecum. Has no known role in digestion.

Bile

Fluid secreted by the liver, stored in the gall bladder, and emptied into the small intestine where it assists in absorption of fats.

 

Bowel

Term covering the large and small intestines.

 

Caecum

(se’kum) First portion of the large intestine. The ileum joins the caecum at a right angle, and the appendix is attached to the caecum.

 

Chyme

Semiliquid material resulting from action of digestive juices on food in the stomach.

 

Colon

(ko’lon) Second portion of the large intestine which is subdivided into four sections;
ascending colon, transverse colon, descending colon and sigmoid colon.

 

Convoluted

(kon’vo-lut-ed) Tortuously structured with multiple folds.

 

Digestion

(di-jest’yun) Process by which ingested food is converted for absorption and use as nutrients for body cells.

 

Digestive system

(di-jest’iv) The group of structures concerned with the process of digestion (alimentary canal and accessory organs).

Duodenum

(du-o-de’num) First portion of small intestine, about 250 mm long,extending from the stomach to the jejunum.

Elimination

(e-lim-i-na’shun) Removal of waste materials from the body.

 

Epiglottis

(ep-i-glot’is) “Lid” which covers and closes the larynx during swallowing to prevent entry of food into lungs.

Gall bladder

(gawl’blad-der) Small sac in which bile made by the liver is stored until needed in the duodenum for fat digestion. 

 

Gastric

(gas’trik) Pertaining to the stomach.

 

Gastric fluid

Digestive juices produced by glands in the stomach wall to convert solid food to a semiliquid state.

 

Gastrointestinal

(gas-tro-in-tes-ti’nal) Pertaining to the stomach and intestines.

 

Ileum

(il’e-um) The third portion of the small intestine which joins the caecum at a right angle.

Ingestion

(in-jes’chun) Act of taking food into the body via the mouth.

 

Intestine, large

(in-tes’tin) Lower portion of the intestinal tract; extends from the caecum to the anus and is about 1.5 m long.

 

Intestine,small

Upper portion of the intestinal tract; extends from the stomach to the caecum and is about 7 m long. Most of the process of digestion and absorption takes place in the small intestine.

Jejunum

(je-joo’num) Second portion of the small intestine extending from the duode- num to the ileum.

Liver

(liv’er) Large organ located in the upper right portion of the abdominal cavity. The liver produces bile for fat digestion and plays other important roles in digestion.

 

Lymphatic system

Network which assists in distributing nutrients to cells and removing wastes.

 

Mastication

(mas-ti-ka’shun) The act of chewing.

 

Nutrient

(nut’tre-ent) A substance which provides nourishment to body cells.

 

Oesophagus

(e-sof’ah-gus) Tube-like passageway which connects pharynx to stomach.

 

Orifice

(or’i-fis) An opening.

 

Pancreas

(pan’kre-as) A gland which is both endocrine and exocrine. Located behind the stomach,the pancreas produces secretions concerned with digestion.

Peritoneum

A large protective membrane that surrounds many of the organs in the abdomen and pelvic regions.

Pharynx

(far’inks) Cavity common to the digestive system and the respiratory system by which food passes from mouth to oesophagus.

Rectum

(rek’tum) Portion of the intestinal tract which connects the sigmoid colon to the anus. The rectum is about 130 mm long.

 

Salivary glands

(sal’i-ver-e) Oral glands which secrete saliva to start chemical digestion and to moisten food to facilitate swallowing.

Exercises on the Digestive System

 

A. Write in the table below the medical terms for the numbered items in the diagram. Then check your answers against the fully labelled diagram in this section.

 

B. Write in the table below the medical terms for the numbered items in the diagram. Then check your answers against the fully labelled diagram in this section.

 

C. Write in the table below the medical terms for the numbered items in the diagram. Then check your answers against the fully labelled diagram in this section.

 

D. Write the medical terms in the table below for the numbered items in the diagram. Then check your answers against the fully labelled diagram in this section.

 

E. Read the definitions of terms below. Each describes an organ, a part or function of the digestive system. You may refer to the previous text to fill in the appropriate word or phrase.

Definition

1. More commonly known as the digestive system.
2. The whole digestive tract extending from the  mouth to the anus.
3. Where the physical digestive process starts.
4. The three continuous parts comprising the  small intestine.
5. Glands which supply digestive enzymes and
6. A small “lid” which closes the trachea during the  act of swallowing.
7. A tube connecting pharynx with the stomach.
8. Waves of contractions, which move the food down the oesophagus to the stomach.
9. A portion of the alimentary canal, where hydrochloric acid and digestive enzymes act on food.
10. Comprises three continuous parts and is about 7 metres long.
11. The sphincter which controls the flow of fluids  from the ampulla of Vater into the duodenum.
12. A reservoir for bile, which adds mucus and  removes water.
13. The largest gland in the body which has many  functions connected with digestion.
14. A gland which secretes juice into the duodenum  and also has an endocrine function.
15. A continuation of the small bowel which absorbs  water and some chemicals from the digested food, creating faeces.

F. Use a medical dictionary to look up the meaning of these terms which are commonly used in describing conditions related to the digestive system.

 appendicitis

 ascites

 borborygmus

 cholangiogram

 cholangitis

 cholecystectomy

 cholecystitis

 cholelithiasis

 cirrhosis

 cleft palate

 colostomy

 dental caries

 diverticulosis

 diverticulum

 dysentery

 dyspepsia

 dysphagia

 gallstone

 gastritis

 gastroenteritis

 gastrostomy

 gingivitis

 Haematemesis

 haemorrhoids

 hemicolectomy

 hepatitis

 hiatus hernia

 ileostomy

 ileus

 intussusception

 irritable bowel syndrome

 jaundice

 malabsorption syndrome

 melaena

 pancreatitis

 peritonitis

 pyloric stenosis

 pyloroplasty

 reflux oesophagitis

 sialolithiasis

 steatorrhoea

 strangulated hernia

 tonsillectomy

 ulcer

 volvulus