The physiology of the gastrointestinal tract introduction

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Dr. Hawa Al-Dhahir (M.B.Ch.B., M.Sc., Ph.D.-United Kingdom)

Department of physiology / College of medicine – Baghdad University.



The main function of the gastrointestinal tract is to provide the cells and tissues of the body with continual supply of water, electrolyte and various nutrients.

The gastrointestinal tract (GI tract) is a coiled hollow tube which passes right through the body and opened to the outside at both ends(mouth and anus), so that the lumen of the tube can be considered as an internal continuation of the external environment. Technically speaking the contents of the digestive tract are not truly inside the body until they are absorbed across the cells that line the digestive tract.

Food and fluids will enter at the top end (the mouth) and during their passage through the tract the food will be subjected to various mechanical and chemical changes. It will be chopped and grounded by the teeth, propelled forward through the tract by contraction of its muscular wall, and it will be broken down by chemical reactions performed by the digestive enzymes, into smaller and simpler compounds.

The digested food along with water and electrolytes will be absorbed by the intestinal mucosa to enter the blood or lymph and will be utilized either to build up the body structure or to provide energy. The undigested food will pass at the lower end (the anal canal).

The GI tract is divided into successive compartments starting with:

  1. Oral cavity.

2- Pharynx.

3- Esophagus.

4- Stomach.

5- Small intestine. - Duodenum

- Jejunum

- Ileum

6- large intestine -Caecum -Ascending colon -Transverse colon -Descending colon

-Sigmoid -Rectum and anal canal

The accessory organs are:

1- Salivary glands. 2- Pancreas. 3- Liver.


There are 3 main structures in the oral cavity these are:

  1. Teeth.

  2. Tongue.

  3. Salivary glands.

TEETH: they are important for the process of mastication (chewing). Mastication is vital because

1-the process of mastication will result in the opening or destruction of the cellulose covering of most fruits and raw vegetables since most fruits and vegetables have indigestible cellulose membranes that must be broken down before the food can be utilized. Cooking and steam also breaks the cellulose covering.

2-chewing aids in the digestion of food because the digestive enzymes act only on the surface of the food particles. Therefore chewing will help in digestion by cutting the large food particles into smaller ones with larger total surface area being exposed to the intestinal secretion.

3-with proper mastication the bolus formed will be reduced into a paste form. This will prevent excoriation or damage of the mucus membrane of the esophagus, stomach and small intestine, and will facilitate the movement of food from one segment of the GI tract to the succeeding segment of the gut.

After understanding the importance of chewing one can understand why people without teeth or dentures will have difficulty in swallowing dry food and they often complain of indigestion in addition they may complain of painful contraction of the esophagus upon swallowing because the food bolus hasn’t been turned into a paste.

TONGUE: this is an extremely mobile mass of striated muscle covered with a mucus membrane. It helps greatly in mastication and swallowing.


There are 3 chief paired salivary glands these are:

  1. Parotid.

  2. Submandibular (submaxillary).

  3. Sublingual.

In addition there are many small salivary glands scattered in the lining of the oral cavity and are named according to their position.

Labial -------- lips

Buccal -------- cheeks

Palatal -------- palate

Lingual -------- lingual tonsils

There are 3 types of secretory cells in the salivary glands.

  1. Serous cells: these cells secret the serous secretion that provide the enzyme ptyalin (salivary α- amylase) for the digestion of starch, and the secretion of these cells is thin and watery.

  2. Mucus cells: these cells secret mucus secretion which contains mucin and the secretion is viscid(thick).

  3. Seromucinous cells.


The daily secretion of saliva is about 1-1.5 liter per day. A large proportion of this 24 hours volume is secreted at meal time. Ordinary mixed saliva contains:

  1. Water 99.5%.

  2. Solids 0.5%.

The solid materials are:

  1. Organic

  2. Inorganic

Organic constituents of saliva:

1- Protein mucin.

2- Ptyalin or α-amylase for the digestion of starch.

3- Lingual lipase this enzyme is secreted by Ebner’s gland on the dorsal surface of the tongue. It plays an important role in the hydrolysis of triglycerides. It differs from pancreatic lipase in that it does not need a detergent for its action. It is also very hydrophobic and so has a great ability to bind to dietary triglycerides. The enzyme is active over a wide range of PH and has an acidic PH optimum. And it is very stable; its action starts in the mouth and continues in the stomach and even in the upper intestine. It can digest as much as 30% of dietary triglycerides.

4- Urea, uric acid & creatinine.

5- Kallikrein which is an enzyme that acts on plasma protein to produce a very powerful vasodilator polypeptide called kinin.

6- Specific blood group antigen (ABO system).

Blood groups substances agglutinogens (ABO system) are present in 80% of the people we call them (secretors). The activity of saliva of secretors is several hundred times greater than that of the red blood cells which make them of medico-legal significance because it may make it possible to determine an individual blood group from a recently used drinking vessel or glass or discarded cigarettes.

7- Somatostatin, glucagon, renin and several growth factors.

8- Lysozyme which can destroy the bacteria by lysis.

9- Lactoferrin which binds to iron and deprive organisms of nutrient iron and it is bacteriostatic.

10- Proline-rich protein that protects tooth enamel and binds toxic tannins.

11- Immunoglobulin A which can destroy the bacteria including those that cause the dental caries.

12- Glucose is absent from saliva.

Inorganic constituents of saliva:

Saliva contains different ANIONS such as chloride, phosphate, bicarbonate, Floride and CATIONS such as calcium, sodium and potassium.

Floride is important to prevent dental caries, calcium salts might be the source of tartar deposits on the teeth.

The concentration of sodium and chloride in the saliva is less than that in the plasma, while potassium concentration in the saliva is higher than that in the plasma.


Na⁺ 146 mEq/liter 10

Cl⁻ 110 mEq/liter 10

K⁺ 5 mEq/liter 25

Glucose 80-120 mg/dl zero

The low sodium concentration of the saliva and the absence of glucose in the salivary secretion might be of significance in that it will help in the taste of sweet and salty substances. Aldosterone increases the absorption of Na⁺ and Cl⁻ from the saliva and the secretion of K⁺ to the saliva similar to the kidney. In Addison disease there is a high Na⁺/K⁺ ratio in saliva.

The rate of salivary secretion is determined mainly by the end piece secretion while the composition is determined mainly by the ducts. The ductal epithelium is very impermeable to water. The absorption of sodium and chloride occurs at a faster rate than the secretion of the potassium thus saliva becomes hypotonic. When salivary flow rates are high, there is little time for re-absorption so saliva contains more of the normally reabsorbed and less of the normally secreted ions. The PH of the saliva is between (6-7.4). This PH is quiet favorable for the action of the enzyme ptyalin. At PH 7 the saliva is saturated with calcium so that the teeth do not lose calcium to the saliva. On the other hand at more acidic PH the calcium will be lost from the teeth to the saliva.

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