Chapter 17

What is digestion?

The mechanical and chemical breakdown of foods into forms that cell membranes can absorb

What is chemical digestion?

Uses enzymes to break down food particles, by changing them into simpler chemicals

What is mechanical digestion?

Breaks down large pieces of food into smaller ones, but does not change chemical composition

What is the digestive system?

• Organs of the digestive system carry out ingestion, propulsion, absorption, and defecation

• Consists of the alimentary canal and accessory organs

What is the Alimentary canal?

• The food passageway​

• Consists of organs that extend from the mouth to the anus ​

• Mouth, pharynx, esophagus, stomach, small intestine, large intestine, and anal canal​

• Muscular tube, about 8 m long​

What are the Accessory organs?

• Consists of organs that empty secretions into alimentary canal ​

• Food does not pass through them​

• Salivary glands, liver, gallbladder, pancreas​

What are the 4 layers of the Alimentary canal?

• Mucosa

• Submucosa

• Muscularis (externa)

• Serosa

What is the Mucosa?

• Innermost layer, mucous membrane​

• Folded in some areas, to increase surface area​

• Absorbs dietary nutrients, secretes mucus and enzymes​

What is the Submucosa?

• Connective tissue layer​

• Nourishes cells, transports absorbed food molecules​

What is the Muscularis (externa)?

• Muscle tissue; contains circular and longitudinal layers ​

• Moves tube and food materials​

What is the Serosa?

• Outermost layer; serous fluid eliminates friction​

• Visceral peritoneum of organs within abdominal cavity​

What ate the 2 types of movement of the alimentary canal?

• Mixing movements

• Propelling movements

What are Mixing movements?

• Muscle in small sections contracts rhythmically​

• Does not move materials in one direction​

• Example: segmentation in small intestine, churning in stomach​

What are Propelling movements?

• Moves materials in one direction​

• Peristalsis: Ring of contraction progresses down tube; propels food particles down the tract in wavelike motion​

What innervate the alimentary canal?

Branches of the sympathetic and parasympathetic divisions of the autonomic nervous system

What is the Submucosal plexus?

Controls secretions​

What is the Myenteric plexus?

Controls gastrointestinal motility​

What are Parasympathetic impulses?

Increase activities of digestive system (secretion and motility)​

What are Sympathetic impulses?

Inhibit digestive actions (secretion and motility)​

What are Enteroendocrine cells?

• Found in stomach and small intestine​

• Secrete hormones to regulate GI organs and processes​

What is the Mouth?

• First part of alimentary canal​

• Ingests food​

• Organ of speech and sensory reception​

• Surrounded by lips, cheeks, tongue, palate​

• Includes oral cavity and vestibule​

What is Mastication?

Mechanical breakdown of solid particles, mixes them with saliva​

What are the Cheeks?

• Form the lateral walls of the mouth​

• Contain muscles for facial expression and chewing​

• Have an inner lining of stratified squamous epithelium (moist)​

What are the Lips?

• Highly mobile structures that surround the mouth opening​

• Sensory receptors judge temperature and texture of food​

• Boundary between skin and mucous membrane inside mouth​

What is the Tongue?

Thick, muscular organ that occupies the floor of the mouth, and nearly fills the oral cavity when the mouth is closed​

• Lingual frenulum​

• Papillae​

• Lingual tonsils

What is the Lingual frenulum?

Connects tongue to floor of mouth

What is the Papillae?

Projections that move food, contain taste buds​

What is the Lingual tonsils?

Lymphatic tissue masses on root of tongue​

What is the Palate?

• Forms the roof of the oral cavity​

• Consists of an anterior portion, the hard palate, and a posterior portion, the soft palate:​

• Hard palate consists of palatine processes of maxillae and palatine bones​

• Soft palate consists of muscular arch, ends in uvula​

What are the Palatine tonsils?

Lymphatic masses on sides of tongue

What are the Pharyngeal tonsils (adenoids)?

Masses of lymphatic tissue in posterior wall of pharynx​

What are the Teeth?

• Hardest structures in the body​

• Not part of skeletal system​

• 20 primary (deciduous) teeth​

• 32 secondary (permanent) teeth​

• Central incisors, lateral incisors, canines (cuspids), first premolars (bicuspids), first molars, second molars, third molars​​

• Chewing mixes food with enzymes​

• Consist of crown and root, which meet at neck​

• Enamel covers crown

What is Dentin?

Makes up most of tooth and surrounds pulp cavity

Incisors are used for?

Biting

Canines are used for?

Grasping and tearing

Molars and premolars are used for?

Grinding

What is the Periodontal ligament?

Holds root in place, along with cementum

What are Dental Caries?

• Cavities within enamel of a tooth​

• Formed when sticky foods lodge between teeth or in crevices of molars​

• Bacteria on teeth metabolize sugars​

• Acidic by-products destroy enamel and dentin​

What are Salivary glands?

• Secrete saliva

• Moistens food particles and binds them together​

• Dissolves food so it can be tasted​

• Contains enzymes (begin chemical digestion of carbohydrates) and bicarbonate ions (keep pH favorable for enzyme activity and protect teeth from acidic foods)​

• 3 pairs of major salivary glands:​

  • Parotid glands: Largest; secrete serous saliva with amylase​

  • Submandibular glands: Floor of mouth; serous + mucous​

  • Sublingual glands: Under tongue; mainly mucous​

What are Serous cells?

• Produce a watery fluid, containing a digestive enzyme called salivary amylase ​

• Amylase splits starch and glycogen into disaccharides​

What are Mucous cells?

• Secrete mucus ​

• Mucus binds food particles and lubricates food while swallowing​

What structures do not help in food digestion?

Pharynx and esophagus

What is the Pharynx?

• Cavity posterior to the mouth ​

• Extends from nasal cavity to esophagus​

• Parts of pharynx:​

  • Nasopharynx

  • Oropharynx

  • Laryngopharynx

What is the Nasopharynx?

Posterior to nasal cavity; air passage; contain openings to auditory tubes​

What is the Oropharynx?

Posterior to oral cavity; air and food passage​

What is the Laryngopharynx?

Posterior to larynx; passageway to esophagus​

What is the Esophagus?

• Tubular muscular organ that extends from the pharynx to the stomach​ (25 cm)

• Penetrates the diaphragm through the esophageal hiatus​

• Contains mucous glands in submucosa, which moisten and lubricate lining of inner wall with mucus​

• In muscularis, superior 1/3 is skeletal, middle 1/3 is skeletal+ smooth, inferior 1/3 is smooth muscle​

• Lower esophageal (cardiac) sphincter regulates food passage into stomach, and closes to prevent regurgitation of food​

What is the 1st stage of Swallowing?

• Voluntary stage​

• Saliva is mixed with chewed food, forming a bolus​

What is the 2nd stage of Swallowing?

Starts as food reaches oropharynx, stimulates sensory receptors, and ​ triggers swallowing reflex:​

• The soft palate and uvula rises, to protect nasal cavity from food​

• The hyoid bone and larynx elevate​

• The epiglottis closes off top of the larynx, to protect trachea​

• The longitudinal muscles of pharynx contract​

• The inferior constrictor muscles relax and the esophagus opens​

• Peristaltic waves force food into esophagus​

What is the 3rd stage of Swallowing?

Peristalsis transports food in the esophagus to the stomach​

What is the structure of the Stomach?

• J-shaped, pouch-like organ, about 25 to 30 centimeters long​

• Inferior to the diaphragm, in the upper-left portion of the abdominal cavity​

• Rugae are folds of mucosa and submucosa that allow for distention​

• Contains the following layers of smooth muscle:​

  • An inner circular layer​

  • An outer longitudinal layer​

  • Some portions have third (innermost) layer of oblique fibers​

What are the functions of the Stomach?

• Receives food from the esophagus​

• Mixes food with gastric juice​

• Initiates protein digestion​

• Has limited absorption​

• Moves food into small intestine ​

What is the Cardia of the stomach?

• Region near opening to esophagus​

• Contains lower esophageal sphincter​

What is the Fundus of the stomach?

• Rounded area that rises above cardia​

• Temporary food storage, which sometimes contains swallowed air​

What is the Body of the stomach?

• Main portion​

• Lies between fundus and pylorus​

What is the Pylorus of the stomach?

• Distal portion, closest to small intestine​

• Funnel-shaped pyloric antrum narrows to become pyloric canal​

• Pyloric sphincter: Circular smooth muscle, controls gastric emptying​

What are the components of Gastric juice?

• Pepsinogen

• Pepsin​

• Gastric lipase

• Hydrochloric acid

• Mucus

• Intrinsic factor

What is Pepsinogen?

Inactive form of pepsin; secreted by chief cells​

What is Pepsin?

Active enzyme that breaks down proteins into polypeptides; forms from pepsinogen in presence of hydrochloric acid

What is Gastric lipase?

Fat-splitting enzyme, found in small quantities; action inhibited by low pH​

What is Hydrochloric acid?

Produced by parietal cells; converts pepsinogen into pepsin​

What is Mucus?

Secreted by mucous cells; provides lubrication and protects stomach lining​

What is a Intrinsic factor?

Produced by parietal cells; required for absorption of vitamin B12​

What do Sympathetic impulses do to gastric activity​?

Decreases

What do Parasympathetic impulses do to gastric activity​?

Increases; promote release of histamine, which stimulates gastric secretion​

What does the Somatostatin hormone do?

Inhibits hydrochloric acid secretion​

What does the Gastrin hormone do?

Increases gastric juice secretion

What does the Cholecystokinin (CCK) hormone do?

Released by small intestine cells when proteins and fat enter the small intestine; decreases gastric motility​

What is the Cephalic phase?

The sight, taste, smell, or thought of food triggers parasympathetic reflexes. Gastric juice is secreted in response.

What is the Gastric phase?

Food in stomach chemically and mechanically stimulates release of gastrin, which, in turn, stimulates secretion of gastric juice; reflex responses also stimulate gastric juice secretion.​

What is the Intestinal phase?

As food enters the small intestine, it stimulates intestinal cells to release intestinal gastrin, which, in turn, briefly promotes the secretion of gastric juice from the stomach wall. This phase primarily inhibits gastric juice secretion by sympathetic reflex and the hormone cholecystokinin responses.

Where are most nutrients absorbed?

Small intestine

What substances does the Stomach absorb?

• Some water​

• Certain salts​

• Certain lipid-soluble drugs​

• Some alcohol ​

What is Chyme?

The mixing of food in the stomach with gastric juice to produce a semifluid paste

What is the pathway of Chyme?

• Peristaltic waves push chyme toward pylorus of stomach​

• Small amount of chyme is transported through pyloric sphincter at a time​

• When peristaltic wave reaches distal portion of stomach, contents are pushed backward, resulting in mixing action​

• As chyme enters duodenum, accessory organs add secretions to act on chyme​

• When chyme starts to fill the duodenum, stretch receptors initiate the enterogastric reflex; this slows stomach emptying and intestinal filling​

What does the Lower esophageal sphincter do?

Prevents backflow of chyme into esophagus

What is Vomiting?

• Result of reflex that empties stomach in reverse direction​

• Causes include certain drugs, toxins from food, overstretching of stomach, body motion changes, motion sickness associated with inner ears​

• Controlled by vomiting center in medulla oblongata​

• Motor response involves deep breath, raising of sof palate, closing of nasal cavity and glottis, relaxation of lower esophageal sphincter, contraction of diaphragm and abdominal wall muscles; forces food out through mouth​

What is Heartburn?

• Stomachache results from eating a lot of food too quickly​

• Takes up to 20 minutes for hypothalamus to sense full stomach​

• Excess fullness leads to abdominal pain and gastric reflux, as stomach contents enter esophagus​

• Stomach contents in esophagus cause inflammation, called esophagitis; feels like the pain is derived from the heart, so it is called heartburn​

• Antacids can provide some relief​

What is the Pancreas?

• An endocrine gland, secreting insulin and glucagon to regulate blood glucose​

• Also an exocrine gland of the digestive system, secreting digestive fluid called pancreatic juice​

What is the Exocrine pancreas?

• Pancreatic acinar cells make up most of pancreas, and release pancreatic juice into tiny ducts which lead to the pancreatic duct​

• Pancreatic duct (along with the common bile duct from the liver and gallbladder) empties into the duodenum of the small intestine​

Pancreatic duct and Common bile duct join at dilated tube called the?

Hepatopancreatic ampulla (of Vater)​

What does the Hepatopancreatic sphincter (of Oddi) do?

Surrounds ampulla; controls movement of bile and pancreatic juice into duodenum

What is Pancreatic juice?

Contains enzymes that digest carbohydrates, fats, proteins, and nucleic acids:

• Pancreatic amylase​

• Pancreatic lipase

• Trypsin

• Chymotrypsin

• Carboxypeptidase

• Nucleases

• Bicarbonate ions

What is Pancreatic amylase?

Splits starch and glycogen into disaccharides

What is Pancreatic lipase?

Breaks down triglycerides​

What is Trypsin?

Digests proteins; released as inactive trypsinogen, which is activated by enterokinase in small intestine​

What is Chymotrypsin?

Digest proteins; released as inactive, activated by trypsin​

What is Carboxypeptidase?

Digests proteins; released as inactive, activated by trypsin​

What are Bicarbonate ions?

Make pancreatic juice alkaline; buffer stomach acid​

What is the Secretin hormone?

Stimulates pancreas to release pancreatic juicehigh in bicarbonate ions​

What is the CCK hormone?

• Stimulates pancreas to release pancreatic juice high in enzymes​

• Bile release under control of hormone Cholecystokinin, which stimulates contraction of gallbladder

How does the Nervous system regulate pancreatic secretion?

During cephalic and gastric phases, parasympathetic impulses stimulate pancreas to secrete digestive enzymes​

What is the Liver?

• Largest internal organ​

• Located in the upper-right abdominal quadrant, just beneath the diaphragm​

• Reddish-brown organ​

• Well-supplied with blood vessels​

What are the 4 lobes of the Liver?

• Right lobe: Largest lobe​

• Left lobe: Smaller than right lobe​

• Quadrate lobe: Minor lobe, near gallbladder​

• Caudate lobe: Minor lobe, near inferior vena cava​

Lobes consist of lobules, functional units of liver that?

• Contain hepatocytes (liver cells) around central vein​

• Hepatic sinusoids are channels for blood, between plates of cells​

• Hepatic portal vein brings absorbed nutrients to sinusoids​

• Hepatic artery brings oxygen-rich blood, which mixes with oxygen-poor blood from portal vein​

• Blood flows from sinusoids to central veins, then to hepatic veins​

• Bile flows from ductules to hepatic ducts, then to common hepatic duct​

What are the Functions of the liver?

• Removes toxins from the blood​

• Polymerizes glucose to glycogen

• Oxidizes fatty acids; synthesizes lipoproteins, phospholipids, and cholesterol; converts excess portions of carbohydrate molecules into fat molecules​

• Deaminates amino acids; forms urea; synthesizes plasma proteins

• Stores glycogen, vitamins A, D, and B12, iron, and blood​

• Removes damaged red blood cells and foreign substances by phagocytosis​

• Produces and secretes bile​

What is a ELAD?

• Extracorporeal liver assist device

• Can temporarily perform blood-cleansing functions of the liver, until donor liver becomes available​

• “bioartificial,” since it contains artificial portion (cartridges) and biological portion (human liver cells)​

• Patient’s blood plasma is passed through the ELAD, where toxins are removed and liver secretions are added; then plasma is returned to patient​

What is Bile?

A yellowish-green liquid that hepatic cells continuously secrete​

What are the components of Bile?

• Water

• Bile salts

• Bile pigments

• Cholesterol​

• Electrolytes​

What are Bile salts?

• Produced from cholesterol​

• Emulsify fats​

• Only bile component that have a digestive function (act as fat emulsifying agents)​