THE DIGESTIVE SYSTEM AND BODY METABOLISM

• Digestion: Breakdown of ingested food

• Absorption: Passage of nutrients into the blood

• Metabolism: Production of cellular energy (ATP)

• Two main groups of the Digestive System are

  • Alimentary canal:  continuous coiled hollow tube

  • Accessory digestive organs

Organs of the Alimentary Canal

• Mouth

• Pharynx

• Esophagus

• Stomach

• Small intestine

• Large intestine

• Anus

MOUTH (ORAL CAVITY) ANATOMY

• Lips (labia) – protect the anterior opening

• Cheeks – form the lateral walls

• Hard palate – forms the anterior roof

• Soft palate – forms the posterior roof

• Uvula – fleshy projection of the soft palate

• Vestibule – space between lips externally and teeth and gums internally

• Oral cavity – area contained by the teeth

• Tongue – attached at hyoid and styloid processes of the skull, and by the lingual frenulum

• Types of Tonsils are

  • Palatine tonsils

  • Lingual tonsil

PROCESSES OF THE MOUTH ARE

• Mastication (chewing) of food

• Mixing masticated food with saliva

• Initiation of swallowing by the tongue

• Allowing for the sense of taste

PHARYNX ANATOMY

• Nasopharynx – not part of the digestive system

• Oropharynx – posterior to oral cavity

• Laryngopharynx – below the oropharynx and connected to the esophagus

• Functions of the Pharynx–Serves as a passageway for air and food; Food is propelled to the esophagus by two muscle layers

• 2 muscle layers of the pharynx–Longitudinal inner layer Circular outer layer

• Peristalsis: Food movement is by alternating contractions of the muscle layers

ESOPHAGUS

• Esophagus–Runs from pharynx to stomach through the diaphragm; Conducts food by peristalsis
  (slow rhythmic squeezing); Passageway for food only (respiratory system branches off after the pharynx)

LAYERS OF THE ALIMENTARY CANAL ORGANS

• Mucosa– Innermost layer; Moist membrane; Surface epithelium; Small amount of connective tissue
  (lamina propria); Small smooth muscle layer

• Submucosa–Just beneath the mucosa

• Soft connective tissue with blood vessels, nerve endings, and lymphatics

• Muscularis externa – smooth muscle; Inner circular layer; Outer longitudinal layer

• Serosa–Layer of serous fluid-producing cells

• Visceral Peritoneum–outermost layer of serosa

STOMACH ANATOMY

• Location of stomach– on the left side of the abdominal cavity

• cardioesophageal sphincter–where food enters

REGIONS OF THE STOMACH

• Regions of the stomach– cardiac region, fundus, body, and pylorus

• Cardiac region of the stomach–near the heart

• Fundus

• Body

• Phylorus – funnel-shaped terminal end

• Food empties into the small intestine at the pyloric sphincter

• Rugae – internal folds of the mucosa

• External regions–Lesser curvature & Greater curvature

• Layers of peritoneum attached to the stomach–lesser omentum & greater omentum 

• Lesser omentum–attaches the liver to the lesser curvature

• Greater omentum–attaches the greater curvature to the posterior body wall; Contains fat to insulate, cushion, and protect abdominal organs

FUNCTIONS OF THE STOMACH ARE

• Acts as a storage tank for food

• Site of food breakdown

• Chemical breakdown of protein begins

• Delivers chyme (processed food) to the small intestine

SPECIALIZED MUCOSA OF THE STOMACH–Simple columnar epithelium

• Mucous neck cells – produce a sticky alkaline mucus

• Gastric glands – secrete gastric juice

• Chief cells – produce protein-digesting enzymes (pepsinogens)

• Pepsinogens: protein-digesting enzymes

• Parietal cells – produce hydrochloric acid

• Endocrine cells – produce gastrin

• Structure of the Stomach Muscosa–Gastric pits formed by folded mucosa; Glands and specialized cells are in the gastric gland region

SMALL INTESTINE

• Small intestine– The body’s major digestive organ; Site of nutrient absorption into the blood; Muscular tube extending form the pyloric sphincter to the ileocecal valve; Suspended from the posterior abdominal wall by the mesentery

• Subdivisions of the Small Intestine– duodenum, jejunum, ileum

• Duodenum–Attached to the stomach; Curves around the head of the pancreas

• Jejunum–Attaches anteriorly to the duodenum

• Ileum–Extends from jejunum to large intestine

• Chemical Digestion in the Small intestine–Source of enzymes that are mixed with chyme (Intestinal cells Pancreas); Bile enters from the gall bladder

• Villi– Fingerlike structures formed by the mucosa; Give the small intestine more surface area

• Microvilli of the Small Intestine–Small projections of the plasma membrane; Found on absorptive cells

• Structures Involved in Absorption of Nutrients–Absorptive cells, Blood capillaries, lacteals

• Lacteals–specialized lymphatic capillaries

• Folds of the Small Intestine–Called circular folds or plicae circulares; Deep folds of the mucosa and submucosa; Do not disappear when filled with food; The submucosa has Peyer’s patches 

• Peyer’s patches–collections of lymphatic tissue

DIGESTION IN THE SMALL INTESTINE

• Enzymes from the brush border

  • Break double sugars into simple sugars

  • Complete some protein digestion

• Pancreatic enzymes play the major digestive function

  • Pancreatic amylase: Help complete digestion of starch

  • Carry out about half of all protein digestion (trypsin, etc.)

• Pancreatic enzymes play the major digestive function

  • Lipase–Responsible for fat digestion 

  • Nucleases–Digest nucleic acids 

  • Alkaline content–neutralizes acidic chyme

ABSORPTION IN THE SMALL INTESTINE

• Water is absorbed along the length of the small intestine

• End products of digestion

  • Most substances are absorbed by active transport through cell membranes

  • Lipids are absorbed by diffusion

• Substances are transported to the liver by the hepatic portal vein or lymph

PROPULSION IN THE SMALL INTESTINE

• Peristalsis–the major means of moving food

• Segmental movements–Mix chyme with digestive juices; Aid in propelling food

LARGE INTESTINE

• Large intestine–Larger in diameter, but shorter than the small intestine; Frames the internal abdomen

• Functions of the Large Intestine–1.) Absorption of water; 2.) Eliminates indigestible food from the body as feces; 3.) Does not participate in digestion of food; 4.) lubricant

• Goblet cells–produce mucus to act as a lubricant

STRUCTURES OF THE LARGE INTESTINE

• Structures of the large intestine include: cecum. appendix, colon, rectum

• Cecum– saclike first part of the large intestine

• Appendix–Accumulation of lymphatic tissue that sometimes becomes inflamed; Hangs from the cecum

• Appendicitis–when appendix becomes inflamed

• Colon–Ascending, Transverse, Descending, S-shaped sigmoidal

• Anus–external body opening

FOOD BREAKDOWN AND ABSORPTION IN THE LARGE INTESTINE

• No digestive enzymes are produced

• Resident bacteria digest remaining nutrients

  • Produce some vitamin K and B

  • Release gasses

• Water and vitamins K and B are absorbed

• Remaining materials are eliminated via feces

PROPULSION IN THE LARGE INTESTINE

• Sluggish peristalsis

• Mass movements of the Large Intestine–Slow, powerful movements; Occur three to four times per day

• Defecation reflex: caused by the presence of feces 

• Internal anal sphincter is relaxed

• Defecation occurs– with relaxation of the voluntary (external) anal sphincter

ACCESSORY DIGESTIVE ORGANS

• Accessory digestive organs–salivary glands, teeth, pancreas, liver, gall bladder

• Salivary glands–Saliva-producing glands

• Types of Salivary glands–parotid glands, submandibular glands, sublingual glands

• Parotid glands – located anterior to ears

• Saliva–Mixture of mucus and serous fluids; Helps to form a food bolus; Contains salivary amylase to begin starch digestion; Dissolves chemicals so they can be tasted

• Teeth–The role is to masticate (chew) food

• Humans have two sets of teeth

• Deciduous Teeth–baby or milk) 

• Age 2– when 20 teeth are fully formed 

• Permanent teeth–Replace deciduous teeth beginning between the ages of 6 to 12; A full set is 32 teeth, but some people do not have wisdom teeth

• Classifications of Teeth–Incisors, Canines, Premolars, Molars

REGIONS OF A TOOTH

• Regions of a tooth–crown, neck

• Crown–exposed part; contains Outer enamel, Dentin, Pulp cavity

• Neck–Region in contact with the gum; Connects crown to root

• Root–Periodontal membrane attached to the bone

• Root canal–carrying blood vessels and nerves

PANCREAS

• Pancreas–Produces a wide spectrum of digestive enzymes that break down all categories of food

• Enzymes of the Pancreas– are secreted into the duodenum

• Alkaline fluid– when introduced with enzymes neutralizes acidic chyme

• Endocrine products of pancreas–Insulin & Glucagons

LIVER

• Liver–Largest gland in the body; Consists of four lobes suspended from the diaphragm and abdominal wall by the falciform ligament; Connected to the gall bladder via the common hepatic duct

• Location of Liver–Located on the right side of the body under the diaphragm

• Bile–Produced by cells in the liver

• Composition of Bile–Bile salts, Bile pigment (mostly bilirubin from the breakdown of hemoglobin), Cholesterol, Phospholipids, Electrolytes

• Role of Liver in Metabolism–1) Several roles in digestion; 2)Detoxifies drugs and alcohol; 3) Degrades hormones; 4) Produce cholesterol, blood proteins (albumin and clotting proteins); 5) Plays a central role in metabolism

• Gall bladder sac location–Sac found in hollow fossa of liver

• Gallbladder–Stores bile from the liver by way of the cystic duct

• Bile is introduced into the duodenum in the presence of fatty food

• Gallstones–cause blockages

PROCESSES OF THE DIGESTIVE SYSTEM

• Ingestion– getting food into the mouth

• Propulsion–moving foods from one region of the digestive system to another

• Peristalsis– alternating waves of contraction

• Segmentation– moving materials back and forth to aid in mixing

• Mechanical digestion–Mixing of food in the mouth by the tongue; Churning of food in the stomach; Segmentation in the small intestine

• Absorption–End products of digestion are absorbed in the blood or lymph; Food must enter mucosal cells and then into blood or lymph capillaries

• Defecation–Elimination of indigestible substances as feces

CONTROL OF DIGESTIVE ACTIVITY

• Parasympathetic division–controls most reflexes in digestive activity

• Chemical and mechanical receptors–located in organ walls that trigger reflexes

• Stimuli that are involved in the control of digestive activity–Stretch of the organ; pH of the contents; Presence of breakdown products

• Reflexes of Digestive Activity–Activation or inhibition of glandular secretions; Smooth muscle activity

• Nutrient– substance used by the body for growth, maintenance, and repair

• Categories of nutrients–carbohydrates, lipids, proteins, vitamins, mineral, water

• Carbohydrates–simple sugars, starches, fiber

• Lipids–triglycerides, phospholipids, fatty acids

• Proteins– amino acids

BODY ENERGY BALANCE

• Body energy balance: Energy intake = total energy output (heat + work + energy storage)

• Energy intake–liberated during food oxidation

• Energy output–Heat is usually about 60%; Storage energy is in the form of fat or glycogen