Overview of the GI tract Flashcards
What is the GI tract made up of?
- Mouth
- Esophagus
- Tongue
- Salivary glands
- Liver
- Gallbladder
- Pharynx
- Stomach
- Pancreas
- Small intestine
- Large intestine
- Anus
Why is the GI lumen considered to be external to the body?
The lumen of the gut is a very hostile environment.
There are conditions for digestion that are tolerated in the gut but NOT in the body such as:
- pH of stomach:
Which is as low as 2.0 and the rest of the body pH is 6.8 - 8.0. If the acid of the gut got through to the body it would destroy our tissues. - Digestive enzymes:
They are capable of destroying the body’s own tissue - Microorganisms:
They exist in the large intestine but destructive or lethal if it entered the body - Food are foreign particles:
They would be attacked by the immune system if entered body as eaten (hence not broken down).
After absorption through the wall, food components are considered to enter the body.
What are the 4 basic processes of the digestive system?
- Motility
- Secretion
- Digestion
- Absorption
What is motility in the digestive system?
Motility in the gut is under voluntary and involuntary control.
Voluntary control is under the control of skeletal muscle. Skeletal muscle is responsible for chewing, swallowing and defaection.
Involuntary control - Smooth muscle is involved in mixing the food as it goes through the gut and moving the gut contents forward.
Smooth muscle contractions can either be:
1. Phasic - Action potential induced bursts of contraction
- Propulsive movements
- Mixing movements
- Tonic - constant low level of contraction
- Tone maintains a steady pressure on contents of gut and prevents wall from being permanently stretched (damaged - muscles would become tired and wouldn’t be able to move forward contents of the gut)
What is secretion in the digestive system?
- Main type of secretion in the gut is digestive juices and these are secreted by exocrine (outside and into the lumen) glands:
- These contain water, electrolytes and specific products for digestion or absorption e.g. mucus, enzymes and bile salts.
- Secretory cells extract large volumes of water and raw materials for secretion
- Secretion requires energy:
1. Active transport of raw material into cell
2. Synthesis of secretory products by ER - Neural or hormonal stimulation releases secretion (regulated)
- Secretions normally reabsorbed into blood
- GI hormones are also secreted. They are secreted by the endocrine (into the blood stream) glands:
- Secreted into blood by endocrine cells along tract wall
- Regulate motility and exocrine gland secretion
What is digestion?
Digestion is the biochemical breakdown of complex proteins, carbohydrates and fats by enzymes.
- This is done by many different enzymes in the gut. Example includes maltose - maltose is broken down by the enzyme maltase to glucose molecules.
- These enzymes digest by hydrolysis. Hence they require the presence of water to break down.
- These enzymes are very specific in the bonds they hydrolyse (they only break down one type of food etc)
- Large molecules are broken down to smaller units in a stepwise manner through the gut that eventually are absorbed.
- We also have water, electrolytes, vitamins etc that are also absorbed.
What happens during protein digestion?
- Protein digestion begins in the stomach:
- Pepsinogen in gastric juice is converted to pepsin at low pH
- Pepsin breaks down proteins to peptides
- Continues in small intestine:
- Trypsinogen in pancreatic juice is converted to trypsin at basic pH
- Trypsin breaks down proteins to smaller peptides
- Peptidases are produced by the small intestine and breaks down peptides to amino acids
Amino acids are actively transported into epithelial cells of villi and from there to the blood.
How are lipids digested?
Lipids are only broken down in the duodenum and small intestine.
- Dietary fat, which is usually exists as triglycerides, will travel through the mouth, oesophagus etc until it gets to the duodenum.
- In the duodenum (where large globules of fat are broken down), lipids combine with bile salts, emulsifying them and forming fat droplets.
- Lipase, from the pancreas, can access the surface area of the droplets and begin to break them down to much smaller molecules of monoglycerides and fatty acids.
- Monoglycerides and fatty acids get absorbed through to the small intestinal epithelial cell where they’re incorporated into chylomicrons and then into the lymphatics and travel to the liver and other tissues where they’re required.
How are carbohydrates digested?
- Polysaccharides:
- Starch is broken down by salivary amylase in the mouth and then pancreatic amylase to maltose
- Glycogen is broken down to glucose
- Cellulose and other indigestible CHO are not broken down by gut enzymes
- Disaccharides:
Enzymes in the small intestine breaks down simpler sugars
- Maltase breaks down maltose to glucose
- Sucrase breaks down sucrose to glucose and fructose
- Lactase breaks down lactose to glucose and galactose
Glucose Is actively transported into epithelial cells of villi and then into the blood.
What are the 4 layers of the gut wall?
- Mucosa - next to the lumen of the small intestine.
- Submucosa - underneath the mucosa
- Muscularis - consists of circular and longitudinal muscle
- Serosa - outer end lining
What is the mucosa layer of the gut wall?
- It lines the lumen
- The surface is highly folded - it varies in different parts of the tract
The mucosa is made up of 3 layers:
- Mucous membrane
- Lamina propria
- Muscular mucosa
What is the submucosa layer of the gut wall?
- Made up of connective tissue, allowing the tract to distend and be elastic
- Contains larger blood and lymph vessels, sends branches to mucosa and muscular externa
- Contains submucosa nerve plexus
What is the muscularis externa layer of the gut wall?
- Major smooth muscle, usually made up of two layers:
1. Inner circular layer
2. Outer longitudinal layer - Responsible for mixing and propulsive movements
- Myenteric nerve plexus lies between two layers
What is the Serosa layer of the gut wall?
- It is the outmost layer, connective tissue
- Secretes serous fluid which lubricates and prevents friction between tract and surrounding tissues and organs
- Continuous with the mesentery - suspends digestive organs from inner wall of abdominal cavity like sling
What 4 factors regulate digestive function?
- Autonomic smooth muscle function
- Intrinsic nerve plexuses
- Extrinsic nerves
- GI hormones
