Digestive system Flashcards
Digestive system: Components
● Digestive system: mouth, pharynx, oesophagus, stomach, small intestines, large intestines and anus
● Includes accessory organs salivary glands, teeth, liver, gallbladder and pancreas
● Complemented by hormones, enzymes, and bacteria
Digestive system: Function
● Physical and chemical breakdown of food
● Absorption of nutrients
● Elimination of waste
The Alimentary Canal
- Continuous hollow tube
- Approximately 30 ft long in cadavers
- Food is always considered external
- Tissues similar along length of canal with slight modifications to aid digestion (mechanical/chemical)
The Alimentary Canal: Structure
Within the structure of the alimentary canal are layers of muscle.
Two layers of mostly smooth (involuntary) muscle:
- Inner circular layer and Outer longitudinal layer
Separated by neurovascular and lymphatic vessels
Waves of contraction cause peristalsis to mix and push contents along
Sphincters of thickened circular muscle at points to act as valves
What do the Mouth and pharynx do?
Purpose: starts the process of chemical and mechanical digestion. Mixing food with enzymes and making food swallowable.
Mouth/ oral cavity boundaries:
- Anteriorly – lips, Posteriorly – oropharyngeal isthmus, Laterally – cheeks / buccal surfaces (buccinators muscle), Superiorly – palate (hard/soft), Inferiorly - tongue and floor
Floor;
- Formed by mylohyoid and geniohyoid muscles (mandible to hyoid). U-shaped diaphragm. Supports tongue
Hard palate:
- Anterior ⅔ . Made of Maxilla and palatine bones.
Soft palate:
- Posterior ⅓. Mobile muscular fold. Uvula – fold of muscle; saliva, speech, gag reflex
Tongue
- Accessory organ. Functions in mastication, swallowing, speech and taste. Base attached to hyoid bone and to floor by lingual frenulum. Skeletal muscle fibres (intrinsic and extrinsic).
Teeth
- Accessory organ. Functions in mastication and speech
Salivary glands
- Accessory organs. Three bilateral pairs of glands which release saliva into mouth through ducts. Parotid, submandibular and sublingual
Pharynx
- Nasopharynx, oropharynx and laryngopharynx.
Oesophagus
- Purpose - transports food/chyme from mouth to stomach. Resistant to damage
- Muscular tube
- Begins distally to mouth (oropharynx) around C6
- Approximately 25cm long and 2cm in diameter
- Lies in midline posteriorly to trachea and anteriorly to spine
- Passes through diaphragm posterior to central tendon at level of T10 (oesophageal hiatus)
Stomach: structure and function
- Purpose: mix/ pummel food into chyme. Supports further mechanical and chemical digestion
- Described as C or J shape
- 15-25cm long with maximum capacity of 4L. Collapses when empty.
- Position, Below diaphragm, left upper quadrant or left hypochondriac region and epigastrum.
Sections: - Greater (lateral, convex) and lesser (medial, concave) curvature.
- Superiorly, cardia connecting oesophagus to stomach. Cardiac notch. Fundus often demonstrated with gas in.
- Main body - largest section
- Pylorus, inferior, pyloric antrum (superior) and pyloric canal (inferior). Pylorus continuous with duodenum.
Small intestines
- Purpose: prepare food so that it can be absorbed through the GI Tract.
- Muscular tube stretching from pyloric sphincter to ileocaecal sphincter at large intestine
- Longest section of digestive tract ~ 7m. Varies greatly depending on age sex etc.
Region:
3 divisions:
Duodenum (5%)
Jejunum (40%)
Ileum (55%)
Small intestine- Duodenum
- C shaped and medial aspect concave.
- In 4 sections: superior descending horizontal ascending.
- Receives: Gastric chyme from stomach, Digestive juices from pancreas and Bile from liver/ stored in gallbladder.
- Duodenums Function – breakdown of food using enzymes.
Small intestine- Jejunum
- Villi: Finger-like projections. Give velvety appearance.
- Each has capillary bed and lacteal (lymphatic vessels of the small intestine which absorb digested fats).
- Digested food absorbed into capillary or lacteal.
- Microvilli Creates the Brush border. Abrasion brushes off cells in chyme. Contains Brush border enzymes.
- Most absorption happens in jejunum hence Mucosal membrane has multiple ways of increasing surface area, villi and microvilli and Plicae circulares (circular muscles).
- Function- Absorption of previously digested food from duodenum
Small intestine- Ileum
- Junction between jejunum and ileum indistinct.
- Ileum has More mesenteric fat than jejunum for protection and insulation,
- Ileum lumen smaller and thinner walls,
- Circular folds less distinct and absent at terminal end.
- Peyer’s patches increase in number towards the end, areas of large numbers of bacteria.
- Function of ileum - Absorption Vitamin B12 and bile salts and remaining products not absorbed in Jejunum
Large intestine
- Large refers to its larger diameter
- 1.5m (5ft) long, 6.5cm diameter
- Runs from ileocecal valve to anus
- Frames small intestine on 3 sides
- No villi on mucus membrane as no chemical digestion therefore no structural alterations to increase surface area
- Function to move matter along, therefore goblet cells produce alkaline mucus that is a lubricant that eases passage of faeces
- Longitudinal muscle is reduced in muscularis externa into 2 bands of muscle formed by Teniae coli
- Creating Pocket like sacs called haustra
- Teniae coli and haustra absent in rectum and anus
Large intestine, Rectum and Anal canal
Caecum - 6cm long Receives chyme from ileum via ileocaecal sphincter. Appendix hangs from it, Worm-like and usually twisted
Ascending colon - Right side of abdomen, 20cm long. Turns at right colic/hepatic flexure. More posterior.
Transverse colon - Turns at left colic/splenic flexure. More anterior.
Descending colon - Left side of abdomen. More posterior.
Sigmoid colon - S shaped, begins near iliac crest. Projects medially. More anterior
Rectum - 13cm long and anterior to sacrum and coccyx.
Anal canal - Internal sphincter - Smooth muscle (involuntary) and External sphincter - Skeletal muscle (voluntary).
Liver
- Irregular wedge shaped organ
- Largely covered by costal cartilages
- Anatomically divided into 4 lobes
- 2 main lobes – right and left – divided by falciform ligament
- Larger right further divided into caudate and quadrate lobes:
- Falciform ligament
- Round ligament
- Coronary ligament
- Covered by fibroelastic capsule
- The posterior cranial aspect of the liver not completely covered by peritoneum. “bare area” (area nuda).
- Many metabolic and regulatory function
- Main Digestive role to produce bile
Other roles:
Synthesis
Breakdown
Storage
Gallbladder
- Sac like organ
- Inferior surface of liver
- Stores and concentrates bile – 90ml volume
- Bile from liver to gallbladder via right and left hepatic ducts into common hepatic duct
- Resistance at sphincter of Oddi - controls flow into duodenum and prevents reflux.
- Bile to gallbladder via cystic duct
Pancreas
- 20cm long
- Head, neck, body and tail
- Exocrine, directly related to GI tract – function
Produces 1200 – 1500 mL pancreatic juices per day. Clear colourless liquid consisting of water, salts, sodium bicarbonate making it alkaline and enzymes which breakdown fats carbohydrates and nucleic acids. - Endocrine related to blood glucose
- Secretion of: Insulin from ß cells - lowers blood glucose and Glucagon – α cells - raises blood glucose
- Homeostasis of blood glucose level. Feedback loop between receptors from insulin releasing cells and glucagon releasing cells, liver and pancreas to regulate and balance
Peritoneal cavity
- Peritoneal cavity is innermost layer of abdominal wall
- Closed sac for men, unclosed in women (opening for uterine tubes)
- Two layers named by its role/location:
Parietal: lines abdominal wall/peritoneal cavity
Visceral (serosal): covering organs - Folds of peritoneum (mesenteries) are connected to the abdominal wall to suspend/hold the GI tract
- Is a potential space- a space between two adjacent structures that are normally pressed together. Contains peritoneal serous fluid 5-20ml.
Processes
- Ingestion
- Propulsion
- Digestion (mechanical and chemical)
- Absorption
- Egestion / Elimination
Peristalsis and segmentation
- Chyme is Moved by Peristalsis and mixed by segmentation
- Segmentation: Localised mixing contractions
- Process of segmentation:
Contraction of circular muscle
Constricts intestine -> segments
Muscle fibres in middle of each segment contract – dividing segment further
First fibres relax -> large segment again
Repeats – chyme sloshed back and forth - After most of food absorbed segmentation stops and peristalsis begins
- Peristalsis migration reaches end of small intestine/ileum in 90 – 120 minutes
- Chyme remains in small intestines 3 – 5 hours
Mechanical digestion
- Movement through
- Breakdown and increasing surface area for enzyme action and later absorption
- Mixing with chemicals / secretions
Chemical digestion
- Carbohydrates; amylase (salivary glands and pancreas)
- Proteins; proteases (pancreas) and hydrochloric acid (stomach)
- Fats; bile (liver) and lipases (pancreas)
