Digestion and Absorption Flashcards
Define Digestion
An orderly process to breakdown proteins, fats and complex carbohydrates, catalysed by digestive enzymes.
Define Absorption
Digestive products, vitamins, minerals & water taken across mucosa into blood and lymph.
What are the 3 pairs of Salivary Glands
- sublingual glands (under tongue)
- parotid gland (front ear)
- submandibular glands (under jaw)
What is saliva composed of:
Saliva is composed of 2 major constituents: serous and mucinous components.
What is the Serous component of saliva?
Serous component carries out digestive function and has: water, electrolytes, enzymes, IgA
What is the Mucinous component of Saliva?
Mucinous lubricates; principally contains lubricating glycoproteins mucin.
The function of Saliva?
Saliva for Oral Hygiene
- Helps wash away pathogenic bacteria as well as food particles that provide their metabolic support;
- Saliva contains factors that destroy bacteria: thiocyanate ions and proteolytic enzymes;
- Lysozyme: attack the bacteria, aid the thiocyanate ions in entering the bacteria & digest food particles.
- IgA in saliva can destroy oral bacteria, including some that cause dental caries.
What is salivary secretion controlled by?
Autonomic nervous system
- parasympathetic nervous system is the primary physiological control of salivary glands
- ACh is released at parasympathetic nerve endings which stimulate muscarinic receptors & trigger saliva secretion.
- interruption of parasympathetic supply impairs salivation severely.
What are the 2 cell types involved in gastric secretions?
Parietal cells – HCL, Intrinsic factor
Chief cells – Pepsinogens and gastric lipase
What stimuli cause the parietal cells to secrete stomach acid?
- ACh: released by parasympathetic fibres in vagus nerve efferents which synapse with parietal cells
- Gastrin: released by G Cells present in the pyloric glands of the stomach
- Histamine: released by enterochromaffin cells which lie adjacent to parietal cells in the oxyntic gland
The regulation of pepsinogen secretion:
- Via the chief cells
- occurs in response to two signals:
- stimulation of the chief cells by ACh released from the vagus nerves or from the gastric enteric nervous plexus
- stimulation of chief cell secretion in response to acid in the stomach. stimuli directly induce stomach acid secretion by parietal cells
The phases of gastric secretion:
- Cephalic Phase- vagus; pepsin and acid production
- Gastric Phase- Gastrin-histamine production
- Intestinal phase- nervous and hormonal mechanisms
What is gastric secretion made up of?
- HCl
- Mucus
- Pepsin
- Chymosin
- Triacylglycerol lipase
- Intrinsic factor
What does the glands of the small intestine secrete into the bowel?
Digestive enzymes such as.,
- Aminopeptidase
- Dipeptidases
- α-Glucosidase
- β-Galactosidase
- Sucrose α-glucosidase
- Phospholipases
What is the role of the Pancreas in digestion?
The pancreas produces pancreatic juice for digestion:
- pancreatic acini secrete digestive enzymes
- ductile cells secrete a sodium bicarbonate solution
What controls the secretion of pancreatic juice:
Hormones:
- CCK
- Secretin
- ACh
What is the function of ACh, CCK and secretin?
ACh and CCK: stimulate the acinar cells to produce large quantities of pancreatic digestive enzymes but relatively small quantities of water and electrolytes
Secretin: stimulates secretion of large quantities of water solution of sodium bicarbonate by the pancreatic ductal epithelium
The regulation of Pancreatic exocrine secretions:
CEPHALIC PHASE:
- Stimulated by smell, taste, chewing & swallowing
- Mediated by Ach through vagus nerve
- 20% of pancreatic enzymes
GASTRIC PHASE
- Stimulated by proteins & gastric distension
- Mediated by vago-vagal reflex
- 5-10% of pancreatic enzymes
INTESTINAL PHASE:
- Stimulated by acid in chyme & fatty acids
- Mediated by secretin, CCK and vago-vagal reflex
- 70-75% of pancreatic enzymes & fluid
What causes the release of CCK and secretin?
Acid from the stomach releases secretin from the walls of the duodenum
Fats and amino acids cause the release of cholecystokinin
What are the 3 major components of the pancreatic exocrine solution:
- water
- digestive enzymes
- sodium bicarbonate
What are the 4 types of pancreatic digestive enzymes and their functions?
PANCREATIC AMYLASE: starch –> oligosaccharides
PANCREATIC NUCLEASES: RNA and DNA
PANCREATIC LIPASES: major triglyceride enzyme
PANCREATIC PROTEASES: trypsin, chymotrypsin, elastase
What is bile secreted by:
Liver
What are the 2 stages of bile secretion:
1st: secretion by hepatocytes into bile canaliculi; this contains bile acids, cholesterol, bilirubin & phospholipids.
2nd: secretion is made of water, bicarbonate, NaCl when by ductal epithelial cells are stimulated by Ach & Secretin
What are the functions of bile:
- bile salts & acids reduce surface tension and are responsible for the emulsification of fat for digestion and absorption.
- eliminates waste products from blood in particular bilirubin & cholesterol (500mg of cholesterol converted to bile acids daily)
- bile + pancreatic juice neutralises gastric juice as it enters the small intestine.
Digestion of carbohydrates from the mouth:
IN MOUTH
– ingestion of food; chewing & swallowing
- salivary amylase starts to digest starch.
IN STOMACH
- Digestion continues for 1 hour via salivary amylase which converts starch to disaccharide (e.g. maltose).
IN DUODENUM:
- Secretin is released when low pH chyme (the partially digested food) enters duodenum
- this stimulates secretion of pancreatic fluid rich in HCO3- to neutralizes the chyme.
Pancreatic amylase hydrolyses most starch to disaccharides
Intestinal brush border enzymes breakdown disaccharides to monosaccharides
What can a deficiency of one or more of the brush border cause:
- Diarrhoea
- Bloating
- Flatulence after ingestion of sugar
What is lactose intolerance
DEFINE: Inability to digest lactose effectively results in bloating, gas and diarrhoea.
- Intestinal lactase activity is high at birth but decrease in adulthood.
- Low level of lactase associated with intolerance to milk
- Incidence is common among blacks, Asians and Mediterranean
What is the 4 types of lactase deficiency:
- Primary lactase deficiency
- Secondary lactase deficiency
- Developmental lactase deficiency
- Congenital lactase deficiency
Where are the hexoses (oligosaccharides) absorbed?
- small intestinal villi and microvilli
- hexoses usually removed by the time the food reaches the ileum.
What does the absorption of hexoses depend on?
Sodium ions in the intestinal lumen.
Digestion of proteins:
IN STOMACH:
- Digestion begins in stomach by gastric juice:
- HCl activates pepsinogen, makes environment
suitable (pH2-3) for pepsin.
- Pepsin hydrolyses proteins to proteoses,
peptones and few polypeptides.
- pepsin has ability to digest collagen, a major
constituent of intercellular connective tissue of
meat.
What are the 2 groups of proteolytic enzymes;
ENDOPEPTIDASES
EXOPEPTIDASES
- The carboxypeptidases of the pancreas are exopeptidases that hydrolyze the amino acids at the carboxyl ends of the polypeptides
- Trypsin, chymotrypsin, and elastase
- Attack the peptide bonds on the inside of a protein or peptide molecule, whereas the exopeptidases hydrolyze the terminal peptide bonds.
- Elastase digests elastin fibres that partially hold meats together.
Absorption of proteins:
- Small amount (2-5%) of the protein in the small intestine escapes digestion and absorption. Some are eventually digested by bacterial action in colon.
- Peptides are absorbed together with a proton supplied by an Na+/ H+ exchanger by the peptide transporter 1.
- Absorbed peptides are digested by cytosolic proteases, and any amino acids that are surplus to the needs of the epithelial cell are transported into the bloodstream by a series of basolateral transport proteins
- A wide variety of dipeptides and tripeptides are taken up across the brush border membrane by the proton-coupled symporter known as PepT1. The proton gradient is created by the action of NA/H exchangers in membrane.
- Cytosolic peptidases breakdown remaining linkage between amino acids in dipeptides and tripeptides
Absorption of amino acids is rapid in the duodenum and jejunum but slow in the ileum.
Digestion of lipids:
IN STOMACH:
- Up to 10% of fats (triglycerides) digested in stomach by lingual and gastric lipases.
IN SMALL INTESTINE
- First step is emulsification of large fat globules to very small sizes (in small intestine).
- Most fats digested in small intestine by pancreatic lipases.
IN DUODENUM:
- GB empties stored bile into the duodenum in response to CCK stimulus that itself is initiated by fatty food.
- Bile acts as an emulsifier, to allow water soluble pancreatic lipases act on them.
- bile salts, and especially lecithin are important in this role; making the fat globules easily fragmentable by agitation.
- Agitation produced by intestinal segmentation breaks fats into small droplets, 1µm in diameter
- Lecithin and bile salts coat small fat droplets to form micelles, exposing more surfaces for enzymatic action.
- Hydrophobic region of bile attracted to surface of layer of fat globules.
- Hydrophilic region projects into surrounding watery fluid, decreasing interfacial tension.
Digestion by lipolytic enzymes:
Lipolytic enzymes in pancreatic juice include:
- Glycerol ester hydrolase
- cleaves 1st and 3rd fatty acids from triglyceride to
give 2 free fatty acids and 2-monoglyceride.
- Colipase
- required for Glycerol ester hydrolase binding to
surface of emulsion droplets in the presence of
bile acids.
- Cholesterol esterase
- cleaves ester bond in cholesterol ester to give 1
fatty acid and free cholesterol.
- Phospholipase A2
– cleaves ester bond at position 2 of
glycerophospholipid to give 1 fatty acid and 1
lysolecithin.
What is the action of pancreatic lipase and how this affects absorption:
Pancreatic lipase will break down triglycerides to fatty acids and monoglycerides
The monoglycerides and FFA diffuse through the unstirred layer to the mucosal surface; then combined to form triglyceride in SER.
Monoglycerides, FFA, cholesterol and phospholipids diffuse across unstirred layer brush border membrane.
Absorption of Sodium
Main drive for Na+ absorption is by active transport (Na-K-ATPase).
- Na+ leaves cell through the basolateral membranes into paracellular space
- this reduces Na+ concentration inside the cell, but creates electro- positivity in paracellular space.
- Cl- moves along this electrical gradient.
- Na+ moves down into the cell due to electrochemical gradient from the chyme; H2O follows.
Absorption of water
Water:- daily intake of water ≈ 1.5 litres
- 7 litres fluid secreted into GI tract
- About 0.1 litre eliminated in faeces
> 8 litres water absorbed daily
- Net water absorption occurs mainly in small intestine: jejunum more active than ileum
Absorption of bicarbonate:
large amount reabsorbed from upper small intestine indirectly
Absorption of Calcium:
Calcium ions are actively absorbed into the blood especially from the duodenum.
amount absorbed controlled to supply exactly the daily need of the body for calcium.
30-80% of ingested calcium is absorbed.
PTH and vitamin D control calcium absorption
Absorption of Vitamins:
Vitamins absorbed in Jejunum and Ileum:
- water-soluble:- absorbed by carriers Na+cotransporters in jejunum and upper ileum; (except B12 - in terminal ileum by IF)
- fat-soluble in jejunum:
Absorption of Iron:
Iron is actively absorbed from the small intestine (duodenum).
- absorption is regulated by 3 factors:
- dietary intake;
- state of iron stores in the body,
- need for erythropoiesis.
Disorders of iron uptake can lead to anaemia or hemosiderin.
What can cause malabsorption in the small intestine:
Several diseases can cause decreased absorption by mucosa; classified as Sprue
- Non-tropical sprue: this is variously called
- Idiopathic sprue
- Celiac dx (in children)
- Gluten enteropathy
- Tropical Sprue - frequently occurs in tropics
- It is believed to be caused inflammation of
intestinal mucosa as a result of unidentified
infectious agent
Early stages of sprue mainly impairs fat absorption leading steatorrhea
- Severe cases will impair absorption of proteins, carbohydrates, vit K, B12, folic acid calcium.
Removal of more than 50% of SI or a bypass could lead to malabsorption.
- Resection of ileum affects bile acids absorption.
- this leads to deficient fat absorption; then
diarrhoea
- Fat-soluble vitamins not absorbed adequately
What can severe malabsorption lead to?
Severe malabsorption leads to:
(1) severe nutritional deficiency, often developing wasting of the body;
(2) osteomalacia (demineralization of the bones because of lack of calcium);
(3) inadequate blood coagulation caused by lack of vitamin K; and
(4) macrocytic anaemia of the pernicious anaemia type, owing to diminished vitamin B12 and folic acid absorption.
What is absorbed in the large intestine and How:
Water and minerals.
- The large population of bacteria digest small amounts of fiber.
- Undigested residues are fermented by bacteria to SCFA.
- Bacterial activity forms: Vitamin K, Vitamin B12, Thiamin, Riboflavin, Biotin, and gases
How:
- Na+ enters the cell via ENaC channels and is transported to the interstitium through the Na+/K+ ATPase.
- Water and Cl-ions follow passively via the intercellular tight junctions to maintain electrical neutrality.
What are the two common disorders of the large intestine:
- constipation
- megacolon
What is constipation
Constipation: associated with dry, hard faeces in descending colon
- any pathology of LI that obstruct movement of intestinal contents leads to constipation; e.g. tumour, adhesions or ulcers.
- irregular bowel habits developed in lifetime of inhibition of normal defecation leads to constipation.
What is Megacolon?
Megacolon: severe constipation with bowel movement only once in several day, sometimes a week
- large amount of faecal matter accumulate in colon causing distention
- Frequent cause of megacolon is lack or deficiency of ganglion cell in myenteric plexus
Another name for megacolon?
Hirschsprung’s disease
