Digestion

Digestion It is the process by which food is converted into simpler substances that can be absorbed and utilized by the body cells. It takes place in the lumen of the digestive tract either mechanically and chemically by enzymes.

The parts of food that the body cannot use (with other wastes) are eliminated in the form of feces.

Alimentary canal

Accessory organs

Mouth

Mechanical breakdown of food. Begins chemical digestion of carbohydrates.

Pharynx

Connects mouth with oesophagus

Salivary glands

Oesophagus

Secrete saliva, which contains enzymes that initiate breakdown of carbohydrates.

Peristalsis pushes food to stomach

Stomach

Secretes acid & enzymes. Mixes food with secretions to begin enzymatic digestion of proteins.

Liver

Produces bile which emulsifies fat.

Small intestine

Gall bladder ??!!

Mixes food with bile, pancreatic & intestinal juice. Final enzymatic breakdown of food. Main site of nutrient absorption.

Stores bile and introduces it into small intestine.

Pancreas

Produces and secretes pancreatic juice, containing enzymes & bicarbonate ions, into smalll intestine.

Large intestine

Absorbs water & electrolytes to form feces

Rectum

Digestive system

Regulates elimination of feces

Anus

Digestive system

Mouth (oral cavity) (buccal cavity) It is the first part of the digestive tract. It contains accessory digestive organ such as teeth, salivary glands & tongue which forms bolus (+ sense organs, speech).

The lips, cheeks and palate form the boundaries.

Break down of food into small particles mechanically by mastication {using teeth) and chemically by mixing it with saliva (explain later). Pharynx The pharynx is a fibromuscular passageway that connects the oral and nasal cavities to the oesophagus and larynx. So, it serves both the respiratory and digestive systems.

It is divided into 3 regions which are: 1) The upper region, the nasopharynx, is posterior to the nasal cavity. It contains the pharyngeal tonsils, or adenoids. It acts as a passageway for air only. Important note: It has no function in the digestive system.

2) The middle region is the oropharynx. It is the first region for food to enter when it is swallowed. The opening from the oral cavity into the oropharynx is called the fauces. Masses of lymphoid tissue, the palatine tonsils, are near the fauces.

3) The lower region is the hypopharynx (laryngopharynx). It opens into both the oesophagus and the larynx. Food is forced into the pharynx by the tongue. When food reaches the fauces, sensory receptors around the fauces respond and initiate an involuntary swallowing reflex.

This reflex action has several parts where: ♣♣The uvula is elevated to prevent food from entering the nasopharynx. ♣♣ The epiglottis drops downward to prevent food from entering the larynx and trachea in order to direct the food into the oesophagus.

Peristaltic movements propel the food from the pharynx into the oesophagus.

Oesophagus (esophagus) It is a collapsible passageway muscular tube (20 cm).

It passes through the diaphragm and ends at the cardiac orifice (opening) of the stomach.

It is found between the trachea and the vertebral column. Its mucosa has glands that secrete mucus (mucous) to: * Keep the lining moist.

** Well lubricated to ease the passage of food. The chewed food (bolus) is pushed down to the stomach through peristaltic contraction of the oesophageal muscles.

Stomach It is formed from: 1) Cardia. {with the cardiac (oesophageal) sphincter}

2) Fundus. 3) Body (corpus). lesser curvature greater curvature

4) Pylorus (antrum).

Gastric secretions The mucosa is simple columnar epithelium with numerous tubular gastric glands. Four different types of cells make up the gastric glands: 1- Goblet cells: which secrete mucous (mucus). 2- Parietal (oxyntic) cells: which secrete gastric acid (HCl) and intrinsic factor (glycoprotein). (They found in the neck of the gastric gland). 3- Chief (peptic) (zymogenic) cells: which release pepsinogen (which activated to pepsin) and rennin ??!!. (They found in the base of the gastric gland).

4- Endocrine cells (G cells): which produce hormones such as gastrin (polypeptides) (14, 17 & 34 amino acids). Note: Also, it is secreted by G cell of duodenum. The secretions of the exocrine gastric glands (goblet, parietal and chief cells) make up the gastric juice. The products of the endocrine cells are secreted directly into the bloodstream. Note: In stomach small, molecules such as alcohol are absorbed directly and enter the circulatory system.

Regulation of gastric secretions

The regulation of gastric secretion is achieved by neural and hormonal mechanisms.

Gastric juice is produced all the time but the amount varies according to the regulatory factors. Regulation of gastric secretions may be divided into cephalic, gastric and intestinal phases. ** Thoughts and smells of food start the cephalic phase of gastric secretion; ** The presence of food in the stomach initiates the gastric phase; ** The presence of acid chyme in the small intestine begins the intestinal phase. Note: Chyme is the semifluid mass of partly digested food expelled by the stomach into the duodenum.

Stomach emptying Relaxation of the pyloric sphincter allows chyme to pass from the stomach into the small intestine. The rate of emptying depends on the nature of the chyme and the receptivity of the small intestine.

Small intestine The small intestine extends from the pyloric sphincter to the ileocecal valve, where it empties into the large intestine. It is differentiated into the following regions which are: i) Duodenum (25 cm). ii) Jejunum (2.5 m). iii) Ileum (3.6 m). Its diameter is about 2.5 – 3 cm.

The small intestine finishes the process of digestion, absorbs the nutrients and passes the residue on to the large intestine.

Here chyme is further mixed with three different liquids: 1) Bile: It emulsifies fats It neutralizes the chyme It is used to excrete waste products. 2) Pancreatic juice: Which is made by pancreas to digest food. 3) Intestineal juice: Which is made by small intestine to complete the digestion. Regulation The most important factor for regulating secretions in the small intestine is the presence of chyme. This is largely a local reflex action in response to chemical and mechanical irritation from the chyme and in response to distention of the intestinal wall. This is a direct reflex action. Thus the greater the amount of chyme, the greater the secretion.

Large Intestine It extends from ileocecal valve to the anus. Its length is about 1.5 m & diameter is about 6 – 7 cm.

Its subdivisions are:

i) Cecum. ii) Appendix.

iii) Colon. a) Ascending colon. b) Transverse colon. c) Descending colon. d) Sigmoid colon. iv) Rectum. v) Anus.

Its function: Absorb the remaining water from indigestible food matter. Eliminates the wastes from the body.

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