ANHB1102 Lecture Notes - Lecture 4: Gastrovascular Cavity, Gastrointestinal Tract, Digestion
24 May 2018
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Digestive System:
Phylogeny (evolution) of the digestive system:
Intracellular digestion – has limitations
Protozoa and sponges
Gastrovascular cavity:
Digestive compartment with a single opening.
Functions in digestion and distribution of nutrients
- extracellular digestion of large food masses
- Specialization of cells lining the lumen of the
cavity for forming digestive secretions and
absorptions.
Alimentary canal:
- Tube between mouth and anus
- Food moves in 1 direction
- Extracellular digestion
- Increasing regional specialization.
Stages of digestion:
- Ingestion: selective intake of food
- Digestion: mechanical and chemical breakdown of food
- Absorption: uptake of nutrient molecules into epithelial cells of digestive tract then to blood
and lymph.
- Compaction: absorption of water and consolidation of the ingestible residue
- Defecation: elimination of waste.
Major regions:
- Mouthparts, pharynx – reception
- Esophagus – conduction
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- Stomach – storage and early digestion
- Gizzard – grinding
- Small intestine – terminal digestion and absorption
- Large intestine – water absorption and concentration of solids
- Rectum and anus – elimination of waste
Regionalization and specialization
Digestion occurs sequentially
Regions of alimentary canal vary
Sphincters and valves control flow
Accessory organs and glands.
Digestive tract (alimentary canal) – Muscular tube extending
from mouth to anus
Gastrointestinal (GI) tract – stomach and intestines
Accessory organs – teeth, tongue, salivary glands, liver, gall
bladder, pancreas.
Digestive functions:
Mechanical digestions
- Physical breakdown of food: cutting and grinding
(teeth)
: churning and mixing
(stomach and SI).
Chemical digestion:
- Enzymes (made by accessory glands associated with
the digestive tract)
- carbs (polysaccharides) → monosaccharides
- Proteins (polypeptides) → amino acids
- Fats → monogylcerides and fatty acids.
- Nucleic acids → nucleotides
- Some nutrients absorbed without digestion
- vitamins, free amino acids, minerals, cholesterol, water.
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Layers of digestive tract wall:
Lumen – central space containing food being
digested
Mucosa – inner mucous membrane layer
modified according to digestive system
- Inner epithelium
- Lamina propria
- Muscularis mucosa
Submucosa – layer of loose CT containing
nerves, blood and lymph vessels
Muscularis externa – 2 layers of smooth muscle
- Inner circular layer
- Outer longitudinal layer.
- serosa – thin layer of CT and
mesothelium
or adventitia – fibrous CT
Motility:
Alimentary tract = 2 opposing layers of smooth muscle
- Inner circular later
- Outer longitudinal layer
2 mechanisms to move food along and within gut: peristalsis and segmentation.
Peristalsis:
- Waves of contraction of circular muscle behind food bolus and relaxation of muscle in front.
Segmentation:
- Alternate constriction of rings of smooth muscle
- Divide and squeeze contents back and forth
- Mixes food
- Does’t oe contents along gut much.
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Document Summary
Functions in digestion and distribution of nutrients extracellular digestion of large food masses. Specialization of cells lining the lumen of the cavity for forming digestive secretions and absorptions. Digestion: mechanical and chemical breakdown of food. Absorption: uptake of nutrient molecules into epithelial cells of digestive tract then to blood and lymph. Compaction: absorption of water and consolidation of the ingestible residue. Large intestine water absorption and concentration of solids. Rectum and anus elimination of waste. Digestive tract (alimentary canal) muscular tube extending from mouth to anus. Accessory organs teeth, tongue, salivary glands, liver, gall bladder, pancreas. Physical breakdown of food: cutting and grinding (teeth) Enzymes (made by accessory glands associated with the digestive tract) Vitamins, free amino acids, minerals, cholesterol, water. Lumen central space containing food being digested. Mucosa inner mucous membrane layer modified according to digestive system. Submucosa layer of loose ct containing nerves, blood and lymph vessels. Muscularis externa 2 layers of smooth muscle.
