Week 10 Digestive system Flashcards
Digestive processes within the body (6)
- Ingestion • Propulsion • Mechanical breakdown • Digestion (enzymatic) • Absorption • Defecation
The 2 main parts of the digestive system
The gastrointestinal tract/ alimentary canal • Tube which food travels through from mouth to anus
The accessory organs • Helps with digestion but is not part of the tract and food does not pass through them.
Where does digestion occur?
- Mouth: Salivary amylase: Carbohydrates • Stomach: Pepsin: Proteins • Small intestine: Bile and pancreatic juice: Lipids
Two groups of the digestive system organs:
-Organs of the
Gastrointestinal (GI) tract
• Accessory digestive
organs
What is peristalsis
- Propels food along GIT
- Involuntary
- Unidirectional
- Note: Adjacent segments
What is segmentation
- Mixes content in the small intestines
- Nonadjacent segments
- Increases absorption
Digestion
- Ca________
- Pr_________
- Li_______
Involves Digestive Enzymes
Carbohydrases (eg. disaccharidases)
• breakdown complex carbohydrates/disaccharides
• example - salivary amylase in saliva in mouth
Proteases
• breakdown proteins to peptides amino acids
• example - pepsin in the stomach
Lipases
• breakdown fats to fatty acids + monoglycerides
• example - pancreatic lipases in pancreatic secretions
Absorption
Small Intestine • Carbohydrates • Amino acids • Lipids • Nucleic Acids
Large Intestine
• Vitamins/minerals
• Electrolytes
• Water
Absorption
Small Intestine • Carbohydrates • Amino acids • Lipids • Nucleic Acids
Large Intestine
• Vitamins/minerals
• Electrolytes
• Water
Defecation (poo)
Excretion mechanism for solid waste
• Expels indigestible substances
• Example - cellulose
Major layers of the GIT
The Mucosa of the GIT • innermost layer of GI tract • secretes mucus, digestive enzymes and hormones • absorbs nutrients • protects against infection via MALT (mucosa-associated lymphoid tissue) • possesses its own thin layer of smooth muscle cells – muscularis mucosae – twitch constantly to dislodge adhering material – folds into the villi of small intestine
Mucosal Epithelium
Stratified squamous epithelium: • mouth, oesophagus and anus (for protection against wear and tear) Simple columnar epithelium: • everywhere else (for nutrient absorption)
Mouth Anatomy & Digestive Processes
Mouth (buccal (oral) cavity)
• Continuous with the oropharynx
• Tongue
• Interlacing bundles of skeletal muscle fibres.
• Mixes food with saliva.
• Forming a bolus by pushing food against the
hard palate.
• Initiates swallowing by pushing bolus into
pharynx.
• Teeth
• Enamel, dentin and cement all calcified but
avascular
• Enamel lacks collagen
• Salivary glands
Salivary Glands (3)
Three major sets of salivary glands in the mouth
- Sublingual Gland
- Parotid gland
- Submandibular gland
Composition of saliva
Salivary Glands Secrete 1.5L/24h •Largely water •Slightly acidic •Electrolytes (Na+, K+ Cl- , PO43 - and HCO3-) •Proteins: mucin, lysozyme, IgA •Metabolic wastes (urea and uric acid
- Moistens food to aid in swallowing
- Cleanses the mouth
- Prevents overgrowth and infection of harmful bacteria
- Dissolves food particles so they can be tasted – Contains salivary amylase – breaks down amylose (starch)
GIT Structures: The Pharynx and Oesophagus
Oropharynx and below: • below nasopharynx • contains tonsils and epiglottis Oesophagus: • muscular tube (25cm) •Through mediastinum of the thorax • passes through the diaphragm • joins stomach at the gastro-oesophageal sphincter
Lesser and Greater Omentum
Two mesenteries tether the stomach to other digestive organs
and cavity wall.
• Greater Omentum also contains a lot of fat deposits
• Appearance of a lacy apron.
• Contains lymph nodes
Emptying the stomach
Food mixed with gastric juice becomes Chyme.
• Pyloric part holds 30ml of chyme
• During peristalsis about 3ml of chyme moves into
the duodenum of the small intestine via the Pyloric
valve,
• The rest is propelled backward and remixed.
Digestive Processes of the Liver
- Produces bile (500-1000mls per day)
• pH 7.6 to 8.6 (alkaline)
• yellow-green in color
• Main bile pigment is bilirubin (yellowish)
• Waste product of heme from worn-out RBC’s
• Metabolised by bacteria in small intestine to Stercobilin
(brown) which gives it’s colour to faeces.
• Bile emulsifies fats → aids fat digestion/absorption
• Bile salts keep cholesterol dissolved within bile - Metabolises lipids and other nutrients
- Generates large amounts of ATP
- Detoxifies toxins, alcohol, drugs, waste products
- Primary site for glucose storage for the body
• in the form of glycogen
All liver functions are assisted by the:
Enterohepatic (portal) circulatory
system
• delivers blood directly from the small
intestine to the liver
• Recycles bile salts!
The Gallbladder
Small green sac lined with smooth muscle
• Right, lower surface of liver
• Stores and concentrates bile before release into
small intestine
• Gallbladder contraction is stimulated by intestinal
hormone – cholecystokinin
• Cholecystokinin is a hormone released when acidic, fatty
chyme enters small intestine from the stomach
The Pancreas
Endocrine Gland: • Islets of Langerhans •Insulin, Glucagon, Somatostatin, Grehlin •Exocrine Gland: •Acini: Clusters of Acinar cells. •Produces digestive enzymes (1.2L per day)
Exocrine Region of the Pancreas
Zymogen granules • Contains proenzymes (inactive enzymes) • Ducts • Transport secretions • Duct cell secrete bicarbonate and water (pH8)
Region of the small intestine (3)
Main site for nutrient absorption Three regions - • duodenum (first 25cm) • jejunum (2.5 metres) • ileum (last 3.6 metres)
Structure anatomy of small intestines
Structural modifications for absorption • Circular folds • Slow the passage of food • Villi • The core of which has a dense capillary bed and a wide lymphatic capillary • Microvilli • Cytoplasmic extensions • Fuzzy appearance (brush border) • Plasma membranes contain enzymes
Cell types of the Small Intestine
Enterocytes • Simple columnar cells • Absorb nutrients and electrolytes • In crypts, secretory cells • Intestinal juice • Goblet cells • Secrete mucus • Enteroendocrine cells • Secrete hormones to regulate gastric secretion. • Intestinal crypts only • Paneth cells • Deep in the crypts • Produce defensins and lysozyme • Stem cells • Produce all the cell types
Gross Anatomy of the Large Intestine, 4 main sections
1. The caecum • pouched sac • contents of small intestine enter here • contains appendix (circled)
2. The colon • ascending • transverse • descending • sigmoid (sigmoid = S-shaped)
- The rectum
(rect = straight) - The anal canal
Features of the large intestine
Reclaims remaining water and electrolytes: • Teniae coli (ribbons) • Longitudinal muscle layer is reduced to three bands • Haustra (to draw up) • Tone of the teniae coli causes pocket-like sacs • Epiploic appendages • Small fat-filled pouches (significance unknown)
Microscopy of the large intestine
• No circular folds, villi or brush border • Simple columnar epithelium • Thicker mucosa • Tremendous numbers of goblet cells
Bacterial Microbiota of the Large Intestine
Home of thousands of different types of bacteria • Recover energy from otherwise indigestible foods • Downside 500ml of gas/day • Synthesise some vitamins • B complex vitamins and Vit K • Beneficial bacteria outcompete/suppress harmful bacteria • Evidence indicates bacterial species in gut influence weight, health and mood!
The end of the line:
Rectum and Anal Canal
Rectum (= straight) • Walls stretch when containing faeces: • Defecation reflex. • Rectal valves • Anal canal • Internal anal sphincter • Involuntary • External anal sphincter • Voluntary • Hemorrhoidal veins
