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Functions of Digestive System
Ingestion – active, conscious process of taking materials into mouth
Mechanical Processing – crushing process that increases the surface area of the food, exposing it to more enzymatic activity (occurs in mouth and stomach)
Digestion – break down of food into small fragments suitable for absorption
Functions of Digestive System (Continued) Secretion – release
of water, acids, enzymes and salts by epithelium of digestive tract
Absorption – movement of ions, vitamins, and water across digestive epithelium into interstitial fluid of digestive tract
Excretion – removal of waste from body fluids
Digestive Pathway
Oral cavity (teeth and tongue)
Pharynx Esophagus Stomach Small Intestine
– Duodenum, Jejunum, Ileum
Large Intestine– Cecum or
(ascending Colon), Colon or (Transverse Colon), Rectum or (Descending Colon)
Digestive Pathway CONT…..
Don’t forget to label the 3 parts of each type of intestine, the pancreas and the appendix
Histology of Digestive Tract
Two major types of tissue that constantly produce a lubricating liquid, allowing all components of digestive tract to slide without friction
Visceral peritoneum – covers organs
Parietal peritoneum – lines inner surfaces of body wall
Oral Cavity Also referred to as the
buccal cavity Functions to:
– Perform sensory analysis of material before swallowing
– Mechanically process material with teeth, tongue, and palatal surfaces
– Lubricate material by mixing it with mucus and saliva
– Perform limited digestion of carbohydrates and lipids
Anatomy of Oral Cavity (Continued)
The tongue functions to:
Mechanically process food
Manipulate food to assist in chewing
Sensory analysis through touch, temp, and taste
Secretion substances including the enzyme lingual lipase (which initiates lipid digestion)
Saliva and Salivary Glands 3 pairs of salivary glands:
– Parotid salivary gland – produce saliva that has high concentration of salivary amylase (begins to break down starches, though no absorption occurs in oral cavity)
• 25% of Saliva
– Sublingual salivary gland –produce saliva that acts as buffer and lubricant
• 5%
– Submandibular salivary gland – produce saliva rich in buffer, libricating proteins called mucins, and salivary amylase
• 70%
Bad Breath
Clinically termed Halitosis
Typically caused by breakdown of epithelial cells, blood cells, and bacteria combined with saliva on tongue
Low Salivary flow – dehydration and morning breath due to breathing through mouth during sleep
Garlic and Onion Breath – compounds taken in blood to lungs
Teeth
Perform mastication (chewing) Mastication breaks down tough
connective tissues in meat and plant fibers and helps to saturate food with saliva
Pharynx and Epiglottis
Pharynx: common passageway for food, liquid, and air
Epiglottis: prevents food from going into the trachea and instead directs it to the esophagus
Esophagus A hollow muscle tube
approximately 25 cm long and 2 cm in diameter
Empties into the stomach through the cardiac sphincter, which prevents backflow from the stomach into the esophagus
Peristalsis= muscle movement of the esophagus. Food at this area is called bolus
Stomach
Performs 4 major functions:– Stores ingested food– Mechanically breaks
down ingested food– Chemically breaks
down ingested food Ingested substances
combine with digestive juices of stomach to form acidic soupy substance called chyme
Anatomy of Stomach Shaped like a J Lesser curvature (1st
part) Greater curvature (2nd
part) Can be divided into four
regions:– Cardia– Fundus– Body– Pylorus
• Pyloric sphincter regulates the release of chyme into the small intestine
Stomach’s Volume
Volume of stomach increases while you eat and decreases as it releases chyme
When full it can contain between 1-1.5 liters of material
Digestion—breaking food down both physically & chemically Food breakdown as mechanical digestion
Mixing food in the mouth by tongue & teeth, churning food in stomach & segmentation in small intestine
Mechanical digestion prepares food for further degradation by enzymes
Digestive System FunctionsMechanical & Chemical Digestion
Digestion—breaking food down both physically & chemically Food breakdown as chemical digestion
Enzymes break down food molecules into their building blocks
Each major food group uses different enzymes
Carbohydrates are broken to simple sugars
Mouth & small intestines
Proteins are broken to amino acids
Stomach & small intestines
Fats are broken to fatty acids & alcohols Small intestines
Digestive System Functions
The Small Intestine Responsible for 90% of
nutrient absorption (most of rest occurs in large intestine)
Averages 6 m (19.7 ft) in length
Averages diameter of 2.5-4.0 cm
Duodenum – “mixing bowl” – combines chyme with digestive secretions from pancreas and liver
Jejunum – bulk of chemical digestion and nutrient absorption occurs here
Ileum – the longest segment which ends in ileocecal valve (separating ileum from cecum of large intestine)
Histology of Small Intestine Plicae circulares
are folds that line the small intestine, greatly increasing surface area for absorption
Plicae are lined by tiny fingerlike projections called intestinal villi, which are covered by simple columnar epithelium
Pancreas Lies posterior to the stomach
and is bound posteriorly to abdominal cavity
Produces 1000 ml of digestive enzymes/day (pancreatic juice) and delivers them to the duodenum through pancreatic duct
When chyme enters the small intestine, it triggers the duodenum to release the hormone secretin, which in turn stimulates the pancreas to release these enzymes
Liver The largest visceral
organ, weighing about 1.5 kg
Primary function is to regulate the composition of circulating blood
Blood from absorptive surfaces of digestive tract enters liver
Liver cells extract nutrients or toxins from blood and then allows filtered blood to reenter systemic circulation
It stores excess nutrients, and releases these to correct nutrient deficiencies
Gallbladder Stores and
concentrate bile immediately before it is released in the the small intestine
It only releases bile when stimulated by the intestinal hormone cholecytstokinin CCK, which is released when chyme enters the duodenum
The Large Intestine Begins where it
meets the Ilium and ends at the anus
Major functions: Reabsorption of
water and the compaction of intenstinal contents into feces
Absorption of vitamins liberated by bacterial action
The storage of fecal material prior to defecation
More About the Large Intestine
Approx. 1.5 m long 7.5 cm wide Divided into three
parts: Cecum Colon (ascending,
transverse and descending)
Rectum
The Rectum Expandable Temporarily stores feces, as movement of feces
into rectum triggers urge to defecate Anal canal is last portion of rectum Internal anal sphincter – involuntary (smooth) External anal sphincter – voluntary (skeletal)
Constipation Usually results due to an
infrequent urge to defecate Infrequent bowel movements
(fewer than 3 per week) Difficulty in at least 25% of
the movement Feeling of incomplete
evacuation Type 1 and 2 qualify as
constipated Potential Causes
– Incomplete chewing of food– Insufficient fiber in diet– Dehydration– Quitting smoking (nicotine is a
laxative)