The Digestive System

Chapter 2

The Digestive System

Introduction

The cell requires materials for growth, repair of worn-out structures and source of energy for these activities. The food material we eat, serves these purposes. Thus, regular supply of food is the need of the body. The food we eat contains complex substances and it is necessary to convert these complex substances into simpler forms; so that they can be absorbed into blood and transported to the cells for utilization. This conversion of complex food into a simple form, making it suitable for absorption is described as digestion of food. This conversion is both mechanical and chemical. The organs that cause mechanical splitting of food include teeth, tongue and muscles that help in mastication and peristaltic movement in the gastrointestinal tract. The chemical conversion is caused by the secretory action of alimentary canal and the secretions produced and poured into it by the glands, i.e. salivary glands, liver and pancreas.
The Alimentary Canal.
Thus, the organs involved in and constituting the digestive system are placed into two groups:
1. The alimentary canal; a long tube starting from mouth extending up to the anus and
2. Accessory organs, i.e. teeth, tongue, salivary glands, liver and pancreas.

The Alimentary Canal

It is 09 meters long tube extending from mouth to the anus; through which the food passes. It serves the following functions:

1. Receiving the food (Ingestion).

Peristalsis).

3. Mechanical splitting of food.

4. Chemical splitting of food.

5. Absorption of food and

6. Elimination of unabsorbed food residue (Excretion). On the basis of structural peculiarities, the alimentary canal comprises:

7. Mouth cavity

8. Pharynx

9. Esophagus

10. Stomach

11. Small intestine

12. Large intestine

(1) Mouth Cavity

The alimentary canal begins at mouth. The mouth or oral cavity extends from lips to oropharynx. The roof of the mouth cavity is formed by a hard anterior bone palate; it is soft on the posterior. From the posterior margin of soft palate a small muscular flap hangs down called as uvula. The soft palate is attached to the tongue by the gloss palatine arches and to the wall of oropharynx by the pharyngoplasties arches. The palatine tonsils are composed of lymphoid tissue and are located in the fosse between the two arches. The floor of mouth cavity is occupied by tongue. The mouth cavity consists of two parts, the outer vestibule and inner mouth cavity proper. The vestibule is the space outside the gum and teeth and inside the lips and cheeks. The lips, cheeks and tongue help in moving the food between upper and lower teeth during mastication (chewing).

Teeth and Mastication

Teeth develop within sockets of the alveolar processes of maxillae and mandible. Dense connective tissue covered by smooth mucous membrane, the gums, covers these processes and extends a little way into each socket. The sockets are lined with a fibrous membrane called the periodontal membrane. A tooth possesses crown, neck and root. The crown projects above the gum, the neck is surrounded by the gum and the root is beneath it. A tooth is made up of a very hard material called dentin. The dentin of the crown is covered by enamel, which is harder than dentin. A bone like substance called cementum covers the dentin of the root and anchors the tooth to the periodontal membrane that lines the sockets. The central part of the tooth contains the pulp cavity that shows a connective tissue called pulp, the blood vessels and nerves.

The Salivary Glands

The three pairs of salivary glands are the parotid, submandibular and sublingual glands. The parotid glands are the largest. They lay one on each side below and slightly in front of the ear. The submandibular glands are present on each side, beneath the Jaw-bone and the smallest sublingual glands lie beneath the tongue on each side. The function of the salivary glands is secretion of saliva which is stimulated by the sight, smell and the presence of the food in the mouth. Saliva moistens the mouth, cleanses the tongue and makes speech easier. The enzyme ptyalin acts on cooked starches in alkaline medium and converts them to a soluble form called maltose. The action of the enzyme continues till the food is rendered acidic by the action of gastric fluid.

1. Adventitia, the outer covering consisting of Peritoneum in the abdomen.

2. Muscle layer consisting of both longitudinal and circular muscles.

3. Submucous layer of loose connective tissue. . Mucous membrane lining consisting of epithelial cells.

(a) Adventitia The outermost layer of digestive tract is primarily composed of a connective tissue. In the thorax region (i.e. in case of esophagus) this connective tissue merges into the connective tissue of surrounding structures. In the abdomen these covering forms peritoneum. The peritoneum consists of two layers the parietal and the visceral. In this case, the outer layer is called the serosal.

(b) Muscle Layer It consists of double layers of muscles, the inner layer in which the muscles are arranged circularly around the tube while in the outer layer the muscles are arranged longitudinally along the long axis of the tube. At different points along the tract, the fibers of circular layer are thickened to form sphincters that guard the movement of food from one region of the tract to the other. The rhythmic contractions of these muscles produce peristalsis that propels the food onwards.

(c) Submucous Layer 2.6 The Digestive System It is composed of thick layer of areolar connective tissue and contains blood vessels, lymphatic, and nerves.

(d) Mucous Membrane This innermost lining of the tract has three distinct layers:

(a) The epithelial layer that borders on the lumen. It is a simple columnar epithelium throughout the tract except in the esophagus where it is a stratified squamous epithelium.

(b) The areolar connective tissue to which the epithelial cells are attached and

(2) Pharynx A muscular cone shaped tube, situated behind the nasal cavity, oral cavity and larynx. The base of the cone is above while the apex below. It communicates with nasal cavity, mouth cavity and larynx on the anterior side. The part of pharynx behind nasal cavity and into which the nasal cavity opens is known as nasopharynx. Like nasal cavity nasopharynx is linked with pseudostratified columnar epithelium. On the lateral sides the auditory tubes open into the nasopharynx. The part of pharynx located behind mouth cavity and into which it opens is known as oropharynx. The mucous membrane of oropharynx is composed of stratified squamous epithelium. The laryngopharynx extends from oropharynx above to the larynx and esophagus below. It is lined with stratified squamous epithelium and like oropharynx it serves as a passageway for both food and air. The laryngopharynx continues downwards as esophagus.

(3) Esophagus It is a muscular tube that continues from pharynx above and opens into the stomach below. It is situated behind trachea and passes through mediastinum of the thorax. It pierces the diaphragm and enters into the abdomen, where it opens into the stomach. The peristaltic movement of the esophagus moves the food towards the stomach.

(4) Stomach It is the most dilated part of the alimentary canal lying just below the diaphragm; obliquely in the epigastric, umbilical and left hypochondriac regions of the abdominal cavity. It is a collapsible bag like structure serving as a temporary reservoir for food. The oesophagus opens into the stomach at the cardiac orifice and duodenum continues from it at the pyloric orifice. It is l' shaped and shows two curvatures termed as the greater curvature and lesser curvature. The part of the stomach above the cardiac orifice is called the fundus, the main middle part is the body and lower part is the pyloric antrum.

Small Intestine

It is a long tube extending from pyloric sphincter of stomach up to ileocecal valve. It is the most convoluted portion of the digestive tract. It is about 05 meres in length and lies in the abdominal cavity, surrounded by the large intestine. It has three regions which are continuous with each other. The three regions are duodenum (25 cm) followed by jejunum (02 metros) and ileum (03 metros). The duodenum is curved around the head of pancreas and is tightly attached to posterior abdominal wall. The common bile ducts from liver and pancreatic duct from pancreas join together and open into the duodenum at about its midpoint. The opening of these ducts is guarded by the sphincter of odd. The jejunum is the middle region and is suspended in the abdominal cavity by the mesentery. The ileum is the terminal 03 meters long region and opens into the caecum of large intestine.

Large Intestine (colon)

It is about 01.5 metro long, starting from caecum in the right iliac fosse and terminating into rectum deep in the pelvis. Its lumen is larger than the small intestine. It surrounds the coiled up small intestine and is divided into different parts, i.e. caecum, ascending colon, transverse colon, descending colon and sigmoid colon.

Rectum and Anus

Rectum is a small, dilated tube continuous with sigmoid colon above and to the anal canal below. It shows four layers of tissues as described in the general plan of alimentary canal. The mucous membrane shows large number of goblet cells which secrete mucous. The anal canal is about 03.8 cm long, and leads from rectum to the exterior. There are two sphincters that control the opening of anus, the internal anal sphincter consisting of involuntary muscles and the external anal sphincter of voluntary muscles.

Pancreas

It is a yellowish gray colored gland situated in the epigastric and left hypochondriac regions of abdominal cavity at the level of first and second lumbar vertebrae. It is about 12–15 cm long with a broad head situated in the curve of duodenum, the middle part of the body situated behind the stomach and a tapering tail reaching to the spleen on the left side. It is a compound gland composed of large number of lobules. Each lobule is composed of many alveoli consisting of secretory cells. The lobule pours its secretions through a tiny duct into the pancreatic ducts; that extends the whole length of the gland and finally pours the secretions into the duodenum. The pancreatic duct before pouring its contents into duodenum; joins with common bile duct coming from liver. The union of these ducts form a dilated portion called ampulla of water. Opening of this ampulla in duodenum is controlled by the sphincter of Oddi. Sometimes common bile duct and pancreatic duct open separately into the duodenum. The lobules are separated by a delicate connective tissue network. In this connective tissue, between the alveoli are found small groups of specialized cells. These are described as the Islets of Langerhans.

Functions of Pancreas :

The Pancreas (b) TS of Pancreas Pancreas is both exocrine and the endocrine gland. The exocrine secretion of gland is pancreatic juice that is poured into the duodenum and contains enzymes that digest carbohydrates, proteins, and fats. The composition of Pancreatic juice is as follows: Water Mineral salts Trypsinogen About 600 - 800 ml of Pancreatic juice is produced per day. Chymotrypsin Ogen The pH of this juice is 8 - 8.4. Amylase Lipase The entry of acidic chime in the duodenum stimulates intestinal cells to secrete hormones; secreting, cholecystokinin, pancreozymin which in turn stimulates the secretion of pancreatic juice. The chemical digestion of food caused by the enzymes of pancreatic juice is described in the table at the end of this chapter

Liver

It is the largest gland in the body situated in the uppermost part of the abdominal cavity on the right side beneath the diaphragm. It weighs about 1500 grams and is irregular in shape. The upper surface is convex and lies beneath the diaphragm, while the under surface is irregular and is in contact with stomach, duodenum, right colic flexure of the colon and right kidney. It is enclosed in a thin capsule and partially covered by a layer of peritoneum. It is divided mainly into two lobes the right and left, however, the under surface of liver shows two more lobes; the quadrate lobe is the quadrilateral area between gall bladder and the fosse for the round ligament, and caudate lobe is another quadrilateral area between inferior vena cava and the fosse for ligament venosum.

Liver is made up of lobules which are polyhedral in shape. Each lobule is composed of cubical cells arranged radially, radiating from the central vein. Between two columns of cells there are containing mixture of blood from the tiny branches of the portal vein.

Physiology of Liver

(1) Secretion of bile Hepatic cells secrete an alkaline fluid called bile. It is composed of 90 per cent water, bile salts (sodium glycocholate, sodium taurocholate), bile pigments (bilirubin, biliverdin), cholesterol and mucin. Bile salts are helpful in digestion and absorption of fats. They reduce surface tension of fats and help in their emulsification. RBCs are destroyed in the liver and spleen. The hemoglobin of RBCs is broken down into globin and harem. Haim is again broken down into iron and porphyrins, which form bile pigments later on.

(2) Glycogenic functions The portal vein carries the blood from the digestive tract which contains glucose and amino acids. It pours the blood into liver. The hepatic cells, by the action of enzymes, convert glucose into glycogen and it is then stored in liver. Whenever there is need of energy to body, the hepatic cells convert glycogen to glucose and pass it to the body cells through blood circulation. Thus, this glycogenic function helps to maintain the sugar level constant in the blood.

(3) Formation of urea Portal vein carries blood containing amino acids to the liver. Hepatic cells by the action of their enzymes cause deamination of amino acids, i.e. amino group is set free which forms urea. Urea is carried away to kidneys by the blood and is given out of the body through urine.

(4) Denaturation of fats The digested fats in the form of the fatty acids and glycerol are absorbed in the central lacteal duct. The fatty acids and glycerol again unite to form fats. This fat is carried away by the blood and is stored in the different tissues in saturated form. Whenever the energy is needed, this saturated stored fat is carried to the liver by blood. Then the hepatic cells cause the desaturation of this fat and pass to different tissues for utilisation.

(5) Functions related to blood (a) Formation of RBCs in fontal life. (b) Destruction of RBCs forming bile pigments and iron. (c) Formation of plasma proteins, i.e. albumin and globulin. (d) Formation of prothrombin and fibrinogen responsible for blood clotting. (e) Formation of heparin, a natural anticoagulant in the blood. (f) Storage of iron, vitamin B12.

Functions of the Various Organs and the Physiology of Digestion and Absorption

The food is masticated or chewed by the teeth and moved round the mouth by the tongue and the muscles of the cheeks. It is mixed with saliva and formed into a soft mass or bolus ready for swallowing. The secretions of the salivary glands contain an enzyme 'ptyalin' which converts starches into maltose. The salivary secretion also moistens and lubricates the food. It also cleans the mouth and keeps the structures within the mouth soft and pliable. The bolus of food is pushed back into the pharynx by the upward movement of the tongue. The muscles of the pharynx further propel it down into the esophagus. The presence of bolus in the pharynx stimulates a wave of peristalsis and propels the bolus through the esophagus to the stomach. The walls of the esophagus are lubricated by mucous which assists the passage of the bolus during peristaltic contraction of the muscular wall.

The stomach acts as a temporary reservoir for food thus allowing the digestive juices to act on food. It also produces gastric juice which contains water, mineral salts, mucous, hydrochloric acid, enzymes, i.e., pepsinogen, renin and the intrinsic factor. Water in the gastric juice further liquefies the swallowed food. Hydrochloric acid acidifies the food and stops the action of salivary ptyalin. It also converts pepsinogen to the active enzyme pepsin and kills many microbes which may be harmful to the body. Pepsin converts proteins into peptones, and renin changes soluble caseinogens from the milk to insoluble casein which in turn is converted by pepsin into peptones. Renin is present in gastric juice of infants only. The intrinsic factor is necessary for the absorption of vitamin B12. The mucous in the gastric juice prevents mechanical injury to the stomach wall by lubricating the contents. It also prevents chemical injury by acting as a barrier between the stomach wall and other constituents of gastric juice. Muscular action of the stomach mixes the food with gastric juice and moves it to the small intestine. Absorption takes place in the stomach and to limited extent water, glucose, alcohol and some drugs are absorbed through the walls of the stomach into the venous circulation.

The end products of digestion of carbohydrates, proteins and fats are monosaccharides (mainly glucose), amino acids, fatty acids and glycerol respectively. Glucose is absorbed into the capillaries of the villi and transported in the portal circulation to the liver. Amino acids also follow the same path. Fatty acids and glycerol are absorbed into the lacteals of the villi and are transported via, the thoracic duct to the left subclavian vein. Then they are carried to the liver where they are reorganized and recombined.