Human Biology (2nd Lecture) PDF

2. Digestive system, digestion and nutrition 2.1. Digestive system: 2.1.1. Components:  Digestive system consists of digestive tract and associated organs like salivary glands, liver and pancreas.  The digestive tract starts with mouth and ends in anus.  The major parts of the digestive...

2. Digestive system, digestion and nutrition 2.1. Digestive system: 2.1.1. Components:  Digestive system consists of digestive tract and associated organs like salivary glands, liver and pancreas.  The digestive tract starts with mouth and ends in anus.  The major parts of the digestive tract are the mouth, the pharynx, the esophagus, the stomach, the small intestine and the large intestine. 3.1.2. Functions:  The function of the digestive system is digestion 1 2. Digestive system, digestion and nutrition 2.1. Digestive system: 2.1.1. Components (cont.): 2 2. Digestive system, digestion and nutrition 2.2. Digestion:.2.1. Definition:  Digestion is a process of conversion of the highly complex insoluble, i.e. indiffusible, constituents of food (namely carbohydrates, fats and proteins) into soluble absorbable substances, i.e. can ready diffuse through the intestinal epithelium, into the blood or lymph. 2.2.2. Processes of digestion:  Digestion involves the following processes:-  a- Mechanical process: Which includes mastication (Chewing), swallowing, gastro-intestinal motility and defecation.  b- Secretory process: Which is the activity of the digestive glands to secrete their secretions.  c- Chemical process: Which includes both the action of enzymes and that of certain non-enzymic substances.  d- Microbiological process: this includes the activities of bacteria 3 and some protozoa in lumen of the digestive tract. 2. Digestive system, digestion and nutrition 2.2. Digestion: 2.2.3. Secretion of the digestive system:  There are 2 kinds of secretions:-  a- Enzymes.  b- Non-enzymic secretions: Which includes HCl, mucus (mucin) and bile. 4  2. Digestive system, digestion and nutrition 2.2. Digestion: 2.2.4. Digestion in mouth:.2.4.1. Types:  There are two prcesses, one mechanical or mastication, and the other chemical by the action of saliva.  In mastication, the teeth cut and grind the solid material (chewing) to small pieces which are mixed with the saliva to form a round mass called bolus ‫مضغة‬. 2.2.4.2. Teeth: 2.2.4.2.1. Functions:  The teeth are the organs of mastication.  They are designed to cut, tear and grind ingested food so it can be mixed with saliva and swallowed. 5 2. Digestive system, digestion and nutrition 2.2. Digestion: 2.2.4. Digestion in mouth: 2.2.4.2. Teeth: 2.2.4.2.2. Types:  Twenty deciduous ‫ متساقطة‬teeth, or so-called baby teeth, appear early in life and are later replaced by 32 permanent teeth.  The first deciduous tooth usually erupts at about 6 months of age.  The remainder follow at the rate of 1 or more a month until all 20 have appeared.  Deciduous teeth are shed generally between the age of 6 and 13 years. 6 2. Digestive system, digestion and nutrition 2.2. Digestion: 2.2.4. Digestion in mouth: 2.2.4.2. Teeth: 2.2.4.2.2. Types: 7 2. Digestive system, digestion and nutrition 2.2. Digestion: 2.2.4. Digestion in mouth: 2.2.4.2. Teeth: 2.2.4.2.3. Structure:  A typical tooth can be divided into three main parts: crown, neck and root.  The crown is the exposed portion of a tooth.  It is covered by enamel, the hardest and chemically most stale tissue in the body.  The neck is the narrow portion that is surrounded by the gingivae, or gums.  It joins the crown of the tooth to the root.  The root fits into the socket of the alveolar process of either upper or lower jaw. 8  2. Digestive system, digestion and nutrition 2.2. Digestion: 2.2.4. Digestion in mouth: 2.2.4.2. Teeth: 2.2.4.2.3. Structure (cont.): ‫د‬+‫ال نوج‬ ‫بالمذكرة‬ 9ttps://www.google.com/search? h q=teeth+structure&source=lnms&tbm=isch&sa=X&ved=2ahUKEwjEjqTtuPfnAhUjA2MBHQ9dCGwQ_AUoAXoECBAQAw&biw=1094&bih=506#imgrc=_kton_TF 2. Digestive system, digestion and nutrition 2.2. Digestion: 2.2.4. Digestion in mouth: 2.3.4.3. Saliva: 2.3.4.3.1. Definition and sources:  Saliva is a mixed secretion secreted by 3 pairs of salivary glands: parotid‫نكفيه‬, submaxillary and sublingual. 2.3.4.3.2. Nature:  It is colorless, turbid and viscid slightly alkaline fluid. 2.3.4.3.3. Composition:  It consists mainly of water and contains about 1 % solid material formed of organic and inorganic substances.  The two most important organic constituents of saliva in relation to digestion are mucin (a glycoprotein) and the digestive enzymes which are salivary amylase (ptyalin), maltase and lysozyme.  The inorganic substances are chiefly chloride, phosphates and bicarbonates of sodium, potassium and calcium. 10 2. Digestive system, digestion and nutrition 2.2. Digestion: 2.2.4. Digestion in mouth: 2.3.4.3. Saliva: 2.3.4.3.4. Secretion control:  The secretion of the saliva is mainly controlled by the autonomic nervous system.  It is secreted continually, but it is increased as a result of a reflex brought about by the smell, sight, or even the thought of food. 2.3.4.3.5. Functions of saliva:  1. Saliva moistens and lubricates the food to be easily swallowed.  2. It dissolves solid food stuffs due to its high water content and thus sub serves the sense of taste.  11 3. It plays a role in regulating the water balance of the body. 2. Digestive system, digestion and nutrition 2.3.4.3.5. Functions of saliva (cont.):  4. It has some cleaning action on the mouth and teeth.  5. Certain substances, such as mercury, lead and iodine salts, are excreted from he body through the saliva.  6. Bicarbonates, phosphates and proteins in saliva have an important buffering action, protecting the mouth against acids and alkalies.  7. It initiates the digestion of carbohydrates only in the mouth by the following enzymes:  A) Salivary amylase (ptyalin): Which acts on polysacearides converting them to the disaccharide maltose.  B) Salivary maltase: is present in traces in the saliva. It acts on maltose converting it into its simplest form, the monosaccharide glucose.  C) Lysozyme enzyme: is specialized to digest polysaccharides of the buccal bacteria. 12 2. Digestive system, digestion and nutrition 2.2. Digestion: 2.2.4. Digestion in mouth: 2.3.4.3. Saliva: 2.3.4.3.6. Reasons of the little significance of salivary amylase:  The salivary amylase has a little significance in the body because of the following:  1. The short time the food remains in the mouth.  2. Its action ceases soon after the food has been swallowed.  3. The hydrochloric acid of the stomach juice penetrates the bolus in about 20 minutes after which time the pH is too low for the action of amylase. 13 2. Digestive system, digestion and nutrition 2.2. Digestion: 2.2.1. Definition: 2.2.2. Processes of digestion: 2.2.3. Secretion of the digestive system: 2.2.4. Digestion in mouth: 2.2.4.1. Types: 2.2.4.2. Teeth: 2.2.4.2.1. Functions: 2.2.4.2.2. Types: 2.2.4.2.3. Structure: 2.3.4.3. Saliva: 2.3.4.3.1. Definition and sources: 2.3.4.3.2. Nature: 2.3.4.3.3. Composition: 2.3.4.3.4. Secretion control: 2.3.4.3.5. Functions of saliva: 2.3.4.3.6. Reasons of the little significance of salivary amylase: 14 2. Digestive system, digestion and nutrition 2.2. Digestion: 2.2.5. Digestion in the stomach: 2.2.5.1. Stimulation of the gastric juice:  When food reaches the stomach, the gastric juice is secreted in response to a nervous as well as a hormonal stimulation.  The nervous stimulation is initiated by the taste, smell and sight of food or even by thinking of food.  Upon contact of food with the pyloric mucosa it releases a hormone called gastrin which passes into the blood stream, circulates with the blood and when is returns to the wall of the stomach it stimulates the gastric cells to 15 produce their secretions. 2. Digestive system, digestion and nutrition 2.2. Digestion: 2.2.5. Digestion in the stomach: 2.2.5.2. Nature and sources of gastric juice:  The gastric secretion is a watery solution containing HCl, secreted from parietal (oxyntic) cells and gastric enzymes, (pepsin, rennin and lipase) secreted from chief (peptic) cells.  In addition, the gastric juice contains mucin and inorganic salts. 16 2. Digestive system, digestion and nutrition 2.2. Digestion: 2.2.5. Digestion in the stomach: 2.2.5.3. How the gastric cell secretes HCl ?  I. CO2 combines with water to form carbonic acid (H2 CO3) by the action of carbonic anhydrase enzyme.  II. Carbonic acid becomes ionized into bicarbonate (HCO3) and (H +) ions.  III. Bicarbonate ions unite with Na+ ions (from the ionization of NaCl into Na+ & Cl ) to form NaHCO3, and H+ ions with Cl ions giving HCl which diffuses to the gastric cavity.  The lack of chloride ions from food for long periods decreases and even stops the secretion of HCl. 17 2. Digestive system, digestion and nutrition 2.2. Digestion: 2.2.5. Digestion in the stomach: 2.2.5.4. Functions of HCl:  1. It kills the majority of the bacteria taken in with the food, thus preventing putrefaction ‫ تعفن‬in the stomach.  2. It converts the inactive pepsinogen into active pepsin and provides a favorable medium for its action.  3. It causes partial hydrolysis of some sugars.  4. It changes proteins into acid metaproteins, a preparatory step necessary for protein digestion.  5. It produces curdling ‫تخثر‬of milk in the stomach which helps to keep it exposed for a longer period to the action of pepsin.  6. It regulates the evacuation of the stomach by stimulation of the mucosa of the pyloric sphincter to relax.  7. When HCI leaves the stomach to duodenum it stimulates the mucosa of the latter to release secretine and pancreozymin hormones which stimulate liver and pancreas to secrete their secretions. 18 2. Digestive system, digestion and nutrition 2.2. Digestion: 2.2.5. Digestion in the stomach: 2.2.5.5. Enzymes of the gastric juice:  The gastric juice contains two enzymes, pepsin and rennin.  Pepsin is secreted in an inactive form called pepsinogen, which is converted by HCl into active pepsin (pH 1-2).  Pepsin digests proteins to peptones.  Peptones are stages in the hydrolysis of proteins.  They are not absorbed in the stomach but require further digestion in the small intestine.  Rennin (optimum pH 5-6) acts on the soluble protein of milk and makes it clot to form a curd ‫ الرائب‬.  The curd must be further digested by pepsin or other protease enzymes.  Rennin acts only in the stomach of young mammals because of 19 the strong acidic medium in stomach of older mammals.