The GI tract has its own nervous system that can sense food and trigger reflexes. The enteric plexus is involved in gastroileal, ileogastric, and intestino-intestinal reflexes which regulate movement and emptying of contents through the intestines. Hormones like secretin and CCK are released in response to food in the intestines and stimulate secretion of enzymes from the pancreas and bile from the liver to aid in digestion. Bile and pancreatic enzymes break down carbohydrates, proteins, and lipids into smaller molecules that can be absorbed into the bloodstream.
1. Regulation of Intestinal Function
The GI has its own nervous system (enteric nervous system), it can
sense and form reflex actions due to the presence of food.
In the intestine the enteric plexus is very extensive and this is involved in
several reflexes:
1- Gastroilial reflex =
increase gastric activity causes increased motility of ileum
and movement of chyme (food mixed with digestive juice)
through the ileocecal valve.
2- Ileogastric reflex =
when ileum is distended this decreases the motility and
emptyingof the stomach.
3- Intestino-intestinal reflex =
over-distension of one part of the intestine causes reflex
relaxation of the rest of the intestine.
2. Regulation of Pancreatic Secretion
HORMONES :
a) Secretin = released from intestinal mucosa in response to presence of acidic
chyme in small intestine.
Acidic Sodium
Chyme Bicarbonate
Intestinal Neutralization Glandular
Mucosal Cells of
Cell Pancreas
Secretin Blood
Disfunction of the above may cause duodenal ulcer.
3. Regulation of Pancreatic Secretion
HORMONES :
b) CCK = released mainly in response to presence of fat, proteins and
carbohydrates in small intestine.
Fatty Digestive
Chyme enzymes
Intestinal Digestion Glandular
Mucosal Cells of
Cell Pancreas
CCK Blood
4. Gallbladder Emptying
The bile is secreted by the liver and stored in the gallbladder. Both secretin and CCK
stimulate secretion of bile by the liver. CCK is major stimulator of gallbladder contraction.
Neural reflexes also help with gallbladder contraction and bile release into duodenum.
Vagus
Liver
ACh +
CCK
Gallbladder
+
Pancreas
CCK
FAT Duodenum
...
CCK
Blood
5. Digestion and Absorption
Food mainly contains:
Carbohydrates – mainly polysaccharides (starches) and disaccharides.
Lipids (fat) – mainly triglycerids
Proteins – polymers of amino acids
Also in small amounts food contains:
- Nucleic acids, DNA and RNA
- Salts and minerals, Na, Cl, Ca, Fe, PO4, Cu, etc.
- Vitamins, C, D, Thiamin (B1), Riboflavin (B2), B12 etc.
6. Digestion
Digestion = breaking down of large compounds in food into smaller and simpler
substances that can be absorbed and used by the cells of the body.
7. = glucose
Carbohydrates in Food = fructose
Most common carbohydrates ingested are:
= galactose
- Starch – long chain of glucose (polysaccharides) with occasional branching.
- Sucrose (table sugar) – a disaccharide consist of glucose and fructose.
- Lactose (milk sugar) – a disaccharide consist of glucose and galactose.
8. Lactose Intolerance
Deficiency in enzyme lactase
Inadequate lactose digestion
(Lactose in milk)
High level of lactose in intestine (Colon)
Osmotic effect (water moves Bacterial action on
from blood to lumen) undigested lactose.
Intraluminal volume and gas
DIARRHEOA & GAS
10. Digestion & Absorption of Carbohydrates
- Short oligosaccharids (α-limit dextrins), maltotrios and maltose are broken down to
single sugars (monosaccharides) by the brush border enzymes.
- The resulted monosaccharides are : Glucose (80%)
Fructose (10%)
Galactose (10%)
-The monosaccharides (glucose & galactose) are transported into epithelial cells by
secondary active transport and fructose is transported by facilitated diffusion.
Fructose Na+ Glucose Intestinal Lumen
Fructose K+ Glucose
Na+ Blood
11. Digestion of Proteins
- Proteins are polymeres of amino acids, they can be formed from less than 100 amino
acids up to thousands.
-There is no digestion of proteins in mouth, the digestion starts in the stomach by the
action of pepsin.
pepsinogen Chief cells
Pepsin in the stomach
Protein Polypeptides + amino acids
- Most protein digestion occurs in duodenum and jejunum.
Pancreatic enzymes
Food from
the stomach
Duodenum (small intestine)
12. - Pancreatic enzymes are
activated in the duodenum
by the action of enterokinase.
Enterokinase
13. - Final Products of
protein digestion are:
Amino acids,
Dipeptides
and Tripeptides.
14. Absorption of Proteins
- There are five different transport systems for protein digestion
products:
Neutral amino acids
Acidic amino acids Secondary active transport
IMINO acids
Basic amino acids Facilitated diffusion
Di- and Tri-peptides Tertiary active transport
15. Digestion of Lipids
- For their digestion, lipids must be first emulsified.
- The bile secreted into duodenum combines with lipids and reduces their
surface tension breaking the lipids into small droplets.
