Digestive System I Lecture Notes PDF

ANATOMY & PHYSIOLOGY II Eng Razaz Salih DIGESTIVE SYSTEM I Lecture (4)  Introduction Digestion is defined as the process by which food is broken down into simple chemical substances that can be absorbed an...

ANATOMY & PHYSIOLOGY II Eng Razaz Salih DIGESTIVE SYSTEM I Lecture (4)  Introduction Digestion is defined as the process by which food is broken down into simple chemical substances that can be absorbed and used as nutrients by the body. Most of the substances in the diet cannot be utilized as such. These substances must be broken into smaller particles, so that they can be absorbed into blood and distributed to various parts of the body for utilization. Digestive system is responsible for these functions.  Introduction Digestive process is accomplished by mechanical and enzymatic breakdown of food into simpler chemical compounds. A normal young healthy adult consumes about 1 kg of solid diet and about 1 to 2 L of liquid diet every day. All these food materials are subjected to digestive process, before being absorbed into blood and distributed to the tissues of the body. Digestive system plays the major role in the digestion and absorption of food substances.  FUNCTIONS OF DIGESTIVE SYSTEM 1. Ingestion or consumption of food substances. 2. Breaking them into small particles. 3. Transport of small particles to different areas of the digestive tract. 4. Secretion of necessary enzymes and other substances for digestion. 5. Digestion of the food particles. 6. Absorption of the digestive products (nutrients) 7. Removal of unwanted substances from the body  Functional Anatomy of Digestive System Digestive system is made up of gastrointestinal tract (GI tract) or alimentary canal and accessory organs, which help in the process of digestion and absorption. GI tract is a tubular structure extending from the mouth up to anus, with a length of about 30 feet. It opens to the external environment on both ends. GI tract is formed by two types of organs: 1. Primary digestive organs. 2. Accessory digestive organs.  1. Primary Digestive Organs Primary digestive organs are the organs where actual digestion takes place. Primary digestive organs are: ✓ Mouth ✓ Pharynx ✓ Esophagus ✓ Stomach ✓ Duodenum ✓ Small intestine ✓ Large intestine ✓ Rectum and Anus  2. Accessory Digestive Organs Accessory digestive organs are those which help primary digestive organs in the process of digestion. Accessory digestive organs are: ✓ Teeth ✓ Tongue ✓ Salivary glands ✓ Exocrine part of pancreas ✓ Liver ✓ Gallbladder. MOUTH Oral Cavity  MOUTH The mouth is the first part of the upper gastrointestinal tract and is equipped with several structures that begin the first processes of digestion. The mouth is made up of the teeth, tongue, hard palate, and soft palate.  MOUTH Mouth is otherwise known as oral cavity or buccal cavity. Mouth opens anteriorly to the exterior through lips and posteriorly through faces into the pharynx. Digestive juice present in the mouth is saliva, which is secreted by the salivary glands.  Functions Of Mouth Primary function of mouth is eating, and it has few other important functions also, that include: 1. Ingestion of food materials 2. Chewing the food and mixing it with saliva 3. Appreciation of taste of the food 4. Transfer of food (bolus) to the esophagus by swallowing 5. Role in speech 6. Social functions such as smiling and other expressions.  Salivary Glands In humans, the saliva is secreted by three pairs of: Major (larger) salivary glands Minor (small) salivary glands.  Major SALIVARY glands Major glands are: Parotid glands Submaxillary or submandibular glands Sublingual glands.  Minor salivary glands 1. Lingual Mucus Glands 2. Lingual Serous Glands. 3. Buccal Glands. 4. Labial Glands 5. Palatal Glands.  Functions Of Saliva Saliva is a very essential digestive juice. Since it has many functions, its absence leads to many inconveniences. Preparation of food for swallowing. Appreciation of taste. Digestive function Role in speech Excretory function. Regulation of body temperature. Regulation of water balance  Properties Of Saliva 1. Volume: 1000 mL to 1500 mL of saliva is secreted per day and it is approximately about 1 mL/minute. 2. Reaction: Mixed saliva from all the glands is slightly acidic with pH of 6.35 to 6.85. 3. Specific gravity: It ranges between 1.002 and 1.012 4. Tonicity: Saliva is hypotonic to plasma  TONGUE The tongue is a muscular organ in the mouth, that manipulates food for mastication, and is used in the act of swallowing. It is of importance in the digestive system and is the primary organ of taste in the gustatory system. The tongue's upper surface (dorsum) is covered by taste buds housed in numerous lingual papillae. The human tongue is divided into two parts, an oral part at the front and a pharyngeal part at the back.  Functions Of The Tongue Mastication (chewing) Deglutition (swallowing) Speech Taste  Mechanical Digestion Mechanical Digestion Begins When the Teeth Break Down Ingested Food The movement of the jaw enables the teeth to grind food into small fragments. The mandible, or jawbone, is the only bone in the head that moves The official name for chewing is mastication. This is the first step in mechanical digestion.  Mechanical Digestion Saliva Moistens Food and Begins the Process of Chemical Digestion The Tongue Creates a Bolus so It Can Travel Down the Pharynx and Esophagus PHARYNX  The Pharynx The pharynx is the part of the throat that is behind the mouth and nasal cavity and above the esophagus and the larynx, or the tubes going down to the stomach and the lungs. The pharynx is the portion of the digestive tract that receives the food from your mouth. Branching off the pharynx is the esophagus, which carries food to the stomach, The Pharynx  THE ESOPHAGUS  The Esophagus The esophagus or oesophagus, commonly known as the food pipe or gullet, The esophagus is a muscular tube connecting the throat (pharynx) with the stomach. The esophagus runs behind the windpipe (trachea) and heart, and in front of the spine. Length :25 cm Diameter:2 cm  Structure Of The Esophagus The wall of the esophagus from the lumen outwards consists of: ✓Mucosa ✓submucosa (connective tissue) ✓layers of muscle fibers between layers of fibrous tissue and an outer layer of connective tissue. The mucosa is a stratified squamous epithelium of around three layers of squamous cells, which contrasts to the single layer of columnar cells of the stomach. Most of the muscle is smooth muscle although striated muscle predominates in its upper third.  Structure Of The Esophagus It has two muscular rings or sphincters in its wall, one at the top and one at the bottom. A sphincter is a circular muscle that normally maintains constriction of a natural body passage or orifice, and which relaxes as required by normal physiological functioning. The lower sphincter helps to prevent reflux of acidic stomach content.  Functions Of The Esophagus Formation of a bolus Swallowing Food is ingested through the mouth and when swallowed passes first into the pharynx and then into the esophagus. Reducing gastric reflux Constriction of the upper and lower esophageal sphincters help to prevent reflux (backflow) of gastric contents and acid into the esophagus, protecting the esophageal mucosa. STOMACH  STOMACH The stomach is a J-shaped dilated portion of the alimentary tract situated in the epigastric, umbilical and left hypochondriac regions of the abdominal cavity. Stomach is a hollow organ situated just below the diaphragm on the left side in the abdominal cavity. Volume of empty stomach is 50 mL. Under normal conditions, it can expand to accommodate 1 L to 1.5 L of solids and liquids. However, it is capable of expanding still further up to 4 L.  Parts Of Stomach Stomach has four parts: 1. Cardiac region 2. Fundus 3. Body or corpus 4. Pyloric region.  Parts Of Stomach 1. Cardiac Region: Cardiac region is the upper part of the stomach where esophagus opens. The opening is guarded by a sphincter called cardiac sphincter, which opens only towards stomach. This portion is also known as cardiac end. 2. Fundus: Fundus is a small dome shaped structure. It is elevated above the level of esophageal opening  Parts Of Stomach 3. Body or Corpus Body: is the largest part of stomach forming about 75% to 80% of the whole stomach. It extends from just below the fundus up to the pyloric region. 4. Pyloric Region: Pyloric region has two parts, antrum and pyloric canal. The body of stomach ends in antrum.  Structure Of Stomach Wall Stomach wall is formed by four layers of structures: 1. Outer serous layer: Formed by peritoneum. 2. Muscular layer: Made up of three layers of smooth muscle fibers, namely inner oblique, middle circular and outer longitudinal layers. 3. Submucus layer: Formed by areolar tissue, blood vessels, lymph vessels and Meissner nerve plexus. 4. Inner mucus layer: Lined by mucussecreting columnar epithelial cells. The gastric glands are situated in this layer. Under resting conditions, the mucosa of the stomach is thrown into many folds. These folds are called rugae The rugae disappear when the stomach is distended after meals. Throughout the inner mucus layer, small depressions called gastric pits are present. Glands of the stomach open into these pits. Inner surface of mucus layer is covered by 2 mm thick mucus.  Functions 1. Digestion : The stomach releases proteases (protein-digesting enzymes such as pepsin) and hydrochloric acid, which kills or inhibits bacteria and provides the acidic pH of 2 for the proteases to work. Food is churned by the stomach through muscular contractions of the wall called peristalsis. 2. Absorption: Some absorption of certain small molecules nevertheless does occur in the stomach through its lining.  Gastric Juice Gastric acid, gastric juice or stomach acid, is a digestive fluid formed in the stomach and is composed of: a) Hydrochloric acid (HCl) b) Potassium chloride (KCl) c) Sodium chloride (NaCl) The acid plays a key role in digestion of proteins, by activating digestive enzymes, and making ingested proteins unravel so that digestive enzymes break down the long chains of amino acids.  Gastric Secretory Cells Chief cells: secrete pepsinogen (an inactive enzyme). Parietal cells: secrete hydrochloric acid (HCl) and "intrinsic factor" (which helps absorption of vitamin B12 in the intestines). Mucous cells: secrete mucus and alkaline substances to help neutralize HCl in the gastric juice. G cells: secrete a hormone called gastrin , which stimulates the parietal cells and overall gastric