Digestion

There are three phases of digestion, the cephalic (head), the gastric (stomach) and intestinal (err… dur?). They work together to absorb nutrients from food before metabolism by the liver.

Cephalic

The 'head' phase. Thoughts of food, smell, taste – all these things stimulate the vagus nerve to secrete acetylcholine, which acts on M2 receptors on the basolateral surface of the parietal cell.

Gastric phase

Initiated by the distension of the stomach when food enters it:

• Enterochromaffin-like cells secrete histamine, which acts of the H2 receptors on the basolateral surface of the parietal cell.
• G cells – secrete gastrin, which act of the basolateral surface of the parietal cell (Again)

Because of these three things, the CO2 + H2O goes to HCO3- + H+ in the parietal cell, and apical K+/H+ ATPase pump pumps hydrogen ions into the stomach lumen. And apical chloride channels pumps chloride ions into the lumen, and Hydrochloric acid forms.

Parietal cells also secrete intrinsic factor (which binds to vitamin B12 so its not digested and can be absorbed in the terminal ileum). And gastroferrin binds to iron, so it can be absorbed in the duodenum).

1. Prostaglandins stimulate goblet cells (NSAIDs inhibit prostaglandins).
2. Due to this goblet cells are secreting mucus (contains mucin) which protects the mucosal lining from acid.
3. D cells secrete somatostatin which inhibits gastric blood supply
4. Chief cells – secrete Pepsinogen
5. Acid kills bacteria and and activates pepsinogen (which then autocatylsises itself)

Intestinal

Carbohydrate metabolism

Digestion

Polysaccharides are digested by amylases. Small amount in saliva, but most in pancreatic juices. Produces monosaccharides (glucose), disaccharides (maltase) and a few others, but enterocytes can only absorb a mono. Oligosaccharidases, in the brush border of enterocytes, complete the final digestion of the disaccs.

Absorbtion

At the apical (top) surface of enterocytes, the cytoplasm is constantly depleted of sodium by a Na+/K+ pump. This pump takes in 2 K+ in exchange for losing 3 Na+ using energy derived from the hydrolysis of ATP using ATPase. This maintains a small –ve energy gradient in the cell. The Na+ gradient (both in volume and electrogenically) is used to transport monosaccharides, amino acids and bile salts into the cytoplasm using different Na+ cotransporters – such as sodium-glucose co-transporter (SGCT-1).

This is used in oral rehydration solution – which combines glucose and salt, so that Na+ that gets depleted in stuff like gastroenteritis, is replaced when enterocytes absorb Na+ together with glucose.

Transportation

Absorbed monosaccharides leave (such as glucose and galactose) the enterocyte by facilitated diffusion through selective channels (e.g. GLUT2 – which is sodium independent) in the basolateral (anything that's not the top) surface. They then enter the circulation via the rich villus capillary network.

Fructose uses GLUT5 sodium independent co-transporter on both the apical and basolateral membranes.

Proteins

Digestion

Digestion begins in stomach with action of pepsin (released by chief cells), then the pancreatic juices contain loads more proteases. Different types do different things:

Endopeptidases

Split up longer chains of peptides by breaking the chain at a specific point

• Trypsin
• Chymotrypsin
• Proelastase

Exopeptidases

Remove single amino acids from the ends of chains, leaving short oligopeptides.

• Carboxypeptidase A
• Carboxypeptidase B
• Enterokinase (this is the one that activates trypsinogen by removing one amino acid. Trypsin then autocatalyses the same reaction.)

Peptidases

Found in the enterocyte brus343h border, these complete digestion of the oligopeptides, producing single aminos, and di- and tri-peptides that are absorbed.

Absorption

Amino acids enter along with Na+ ions, again using co transporters, five different ones that are selective for:

• Neutral
• Aromatic
• IMINO
• Positively charged aminos
• Negatively charged

Transportation

Amino acids move from the cytoplasm to the circulation via facilitated diffusion, via selective channels in the basolateral membrane.

Lipids

Emulsion

Unlike carbs and proteins, lipids are not water soluble, so they need to be introduced into a AMPHIPATHIC environment. Churning and mixing and a high pH promotes formation of an emulsion. Bile salts, phospholipids and cholesterol esters (which are amphipathic), help to form mixed micelles.

These are basically macromolecular complexes, where the ampipathic components make a hydrophobic core and a hydrophilic surface – the lipids end up in the core, and the surface allows them to be carried to the enterocyte surface.

Digestion

The main types are triglycerides, phospholipids and cholesterol esters. Lipases, phospholipases and cholesterol esterases break them down into fatty acids, monoacyl glycerol, lysophospholipids and cholesterol. These are found on the surface of the brush border of the enterocytes.

Absorption

The digested lipids are then absorbed across the cell membrane into the cytosol where they are re-esterified and joined to proteins called apolipoproteins to form lipid rich lipoprotein particles called chylomicrons

Transportation

Chylomicrons are actively secreted into the basolateral space, and carried through lymphatic channels to the villus lacteals, which carries them to the circulation via the thoracic duct.

After this the process moves on to the liver, where all the stuff that just got absorbed get metabolised