Alimentary System


Layers of GI Wall

Beginning in the lumen:- 

1. Mucosa - contains the epithelium, the lamina propria (loose connective tissue) and the smooth muscle muscularis mucosae 

2. Submucosa - thick, irregular connective tissue, glands, nervous tissue, provides support to mucosa. Submucosal (Meissner's) plexus directly underneath. 

3. Muscularis externa - two concentric thick layers of smooth muscle. Inner circular layer of muscle constricts lumen. Outer longitudinal layer of muscle shortens tube. Responsible for gut motility. In between the muscular layers is Myenteric (Auerbach's) plexus. Additional layer of muscle in the stomach, innermost oblique muscle between the inner circular layer and the submucosa. 

4. Serosa/adventitia - connective tissue outer layer of canal. Adventitia when outside the peritoneal cavity (oesophagus and rectum), serosa when inside the peritoneal cavity (stomach, small intestine, large intestine). 

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Nervous Control of Alimentary Function

Autonomic control: long (parasympathetic) and short (ENS) reflex. 

Parasympathetic: vagus nerve (CNX), except for salivation which occurs via the facial (CNVII) and glossopharyngeal (CNIX). 
Stimulatory - increases secretion, increases motility 

Sympathetic: splanchnic nerve, inhibitory except for in salivation
Decreases secretion, decreases motility 

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Arterial Supply to GI Tract

Descending abdominal aorta branches: 

- Celiac trunk supplies:- 
          Top end of small intestine
- Superior mesenteric artery supplies:- 
          Lower end of small intestine
          Ascending colon
          Transverse colon
- Inferior mesenteric artery supplies:- 
          Descending colon
          Sigmoid colon

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Venous Drainage from GI Tract

Stomach drains into gastric veins
Pancreas drains into splenic vein
Small intestine, caecum, ascending colon, transverse colon drain into superior mesenteric vein
Descending colon, sigmoid colon, rectum drain into inferior mesenteric vein

Gastric veins, splenic vein, superior mesenteric vein and inferior mesenteric vein drain into hepatic portal vein

Hepatic portal vein drains into hepatic vein in the liver

Hepatic vein drains into the inferior vena cava 

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Carbohydrates in the Diet

Monosaccharides (6C): glucose, galactose, fructose, absorbed by small intestine 

Disaccharides:- two monosaccharides linked by glycosidic bonds. Lactose (glucose + galactose, broken down by lactase), sucrose (glucose + fructose, broken down by sucrase), maltose (glucose + glucose, broken down by maltase) 

Polysaccharides:- starch plant storage form of glucose, glucose monomers linked by alpha-1,4-glycosidic bonds which can be hydrolysed by amylases in saliva and pancreas. 
  - alpha-amylose: glucose linked in straight chains
  - amylopectin: glucose chains highly branched 

Cellulose:- constituent of plant cell walls. Unbranched, linear chains of glucose monomers linked by beta-1,4-glycosidic bonds, dietary fibre (no enzymatic digestion in vertebrates) 

Glycogen:- animal storage form of glucose, glucose monomers linked by alpha-1,4-glycosidic bonds 

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Absorption of Carbohydrates

Na+/K+ ATPase on basolateral membrane of small intestinal epithelial cells pumps 3 Na+ ions out of the cell for every 2 K+ ions into the cell. This produces a large electrochemical Na+ gradient into the cell. 

Glucose and galactose transporter SGLT1 on apical membrane requires simultaneous binding of glucose and Na+, causing a conformational change, Na+ and glucose enter the cell. Glucose can accumulate in the cell until the conc. in the cell exceeds the conc. in the bloodstream. 

Fructose is transported across the apical membrane by GLUT-5. 

Glucose, galactose and fructose exit cell into blood on facilitated transporter GLUT-2. 

Because there has been movement of Na+ in glucose and galactose transport, an osmotic gradient is generated and water enters the blood through the tight junctions. 

Fructose transport requires no Na+ movement, so no water is uptaken through tight junctions. 

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Digestion and Absorption of Proteins

Polymers of amino acids linked together by peptide bonds (C-N), large variations in chain length. Small proteins 3-10 amino acids in length are known as peptides. 

Proteases/peptidases hydrolyse peptide bonds and reduce proteins/peptides to amino acids. Endopeptidase acts on interior amino acids, exopeptidase acts on terminal amino acids. 

At apical membrane, Na+ and amino acid bind protein causing conformational change allowing entry to the cell. Amino acid leaves through facilitated transporter on basolateral membrane. Osmotic force generated due to Na+ movement, water enters through gap junctions. 

PepT1 receptor on surface of small intestine transports dipeptides and tripeptides, and is H+ dependent. Na+/K+ ATPase on basolateral membrane pumps out Na+ resulting in an electrochemical gradient pulling Na+ into the cell. Apical NHE3 transporter exchanges Na+ for H+, resulting in production of acid microclimate on brush border of small intestine as H+ gets trapped in mucus. Apical PepT1 then uses the H+ ions to drive di- and tripeptide uptake into the epithelial cell. 

We are not yet sure how di- and tripeptides leave the cell through the basolateral membrane, but we know that they do,. 

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