S1: Alimentary Transport 2 Flashcards
Functions of the alimentary tract
- Absorption of nutrients, salts and water
- Colon absorbs 90% of water from the contents that arrive at it, to produce a semi-solid material
How is water absorbed and how can malfunction lead to problems?
- Fluids secreted into small intestine are absorbed back into blood
- Absorption is achieved by transport of ions e.g. Na+ and water follows by osmosis
- Malfunction can lead to nutritional, salt and water imbalance.
- Diarrhoea is caused by abnormal water absorption in the intestine
What are the two internal neuronal plexuses in the gut that form the enteric nervous system?
- Submucosal Meissner plexus which regulates the digestive glands
- Myenteric Auerbach plexus which is connected with gut motility
Compare the intrinsic and extrinsic control of the gut
INTRINSIC CONTROLS
- Nerve plexuses near the GI tract initiate short reflexes
- Short reflexes are mediated by local enteric plexuses (gut brain)
EXTRINSIC CONTROLS
- Long reflexes arising within or outside the GI tract
- Involve CNS centres and extrinsic autonomic nerves
- Parasympathetic reflexes
What is the outer layer of the gut called?
The outer layer of the gut is covered by the serosa which is continuous with the mesentery containing blood vessels,lymph vessels and nerve fibres.
What detects different chemical substances in the gut lumen?
Sensory neurones connected to mucosal chemoreceptors detect different chemical substances in the gut lumen
What do stretch receptors do?
They respond to tension in the gut wall caused by the food and chyme.
What are paravertebral ganglia?
Symptathetic trunk/ganglia
What is absorbed in the upper small intestine?
All dietary nutrients, water and electrolytes
What gives the small intestine a large surface area?
Epithelial folds
Villi
Microvilli
What are villus cells and crypt cells?
- Villus cells are involved with absorption
- Crypt cells are involved with secretion and sit at the base between fingers
What is crypt of lieberkuhn?
The epithelia of the villi extend down into the lamina propria where they form crypts. Many important cells reside in the crypts, including those involved in host defence and signalling.
The crypt cells also contain stem cells that replenish the epithelial cells further up the villi
What form are carbohydrates digested in?
Carbohydrates can only absorbed in the form of monosaccharides
How is complex CHO reduced to disaccharides?
It starts in the mouth by amylase
If it enters the duodenum then pancreatic amylase will break it down
What do specific brush border enzymes do in carbohydrate digestion?
It converts disaccharides to monosaccharides (galactose and glucose)
Name some carbohydrase
Sucrase acts on sucrose –> Glucose and fructose
Lactase acts on lactose –> Galactose and glucose
Glucoamylase acts on glucose oligomers –> Glucose
Maltase acts on maltose –> Glucose and glucose
What does insufficient lactase?
Lactose intolerance, bacteria ferment the unbroken lactose leading to excess gas generation and intestinal discomfort
Describe protein digestion
Proteins arrive in the small intestine as polypeptides and these are denatured by gastric acid and broken down by pepsin in the stomach.
Proteins are broken down into oligopeptides and amino acids by pepsins (pancreatic peptidases)
Oligopeptides (consist 2-20 amino acids) are broken down into amino acids and di/tripeptides (done by Di-peptidases is brush border peptidases)
Single amino acids and tri/bipeptides can be absorbed by the enterocyte and be broken down by cytoplasmic peptidases into AA.
AA, di/tripeptides can pass into the blood.
What are the 3 sites of protein digestion?
- Gut lumen e.g. pancreatic proteases, Pepsin in gastric juice
- Brush border peptidases e.g. Di-pepitdases
- In enterocytes e.g. cytoplasmic peptidases
What membrane of the enterocyte is more permeable to amino acids?
The basolateral membrane is more permeable to amino acids than is the brush border membrane. Therefore diffusion is more important for the basolateral transport, especially for amino acids with hydrophobic side chains
Name some dietary lipids
- Tryglycerides are the major dietary lipids
- Phospholipids
- Cholesterol
- Fat soluble vitamins (A, D, E, K)
What is steatorrhoea?
Excess fat in faeces
Gallstones, pancreatitis, Crohn’s disease and liver disease can lead to fat malabsorption leading to steatorrhoea.
Explain fat digestion
- Salivary lipase digests a small fraction of the TGs
- Gastric lipase breaks down approx 10-30% of fats
- The remainder is digested by pancreatic lipase which binds to the surface of small emulsion particles
Bile salts from the gall bladder and liver emulsify the fat into droplets allowing them to enter suspensions. As well as micelle formation (allow it to be absorbed)
How is fat emulsified?
The large fat droplet is broken down by muscular actions
Bile salts prevent small droplets from combining
The surface area where lipase can act is increased
Explain absorption of nutrients
Almost all nutrients enter the blood stream by crossing enterocytes (absorptive cells of small intestine)
There are 2 routes by which molecules enter the blood stream from the small intestine:
- Transcellular route: Across plasma membrane of enterocyte (through cell)
- Paracellular route: Across tight junctions between the enterocytes (between cells)
Large organic molecules eg. AA, glucose cannot enter the blood stream via tight junctions of the paracellular route so they take the transcellular route
Nutrients e.g. CL- enter via paracellular transport
Explain carbohydrate absorption
- Glucose and galactose are rapidly absorbed into enterocyte by a secondary active transport process
- Fructose and mannose are slowly absorbed into enterocyte with passive transport on a GLUT 5 carrier
Monosaccharides such as glucose can be actively transported with sodium across the epithelium. Sodium binds to the transport protein and this allows glucose or galactose to bind - this is a co-transporter called SGLT-1. The glucose/galactose and Na+ enter the enterocyte together. SGLT-1 only transports a combination of sodium and either glucose or galactose, it won’t transport one alone.
Inside the enteroyte, GLUT-2 transports glucose, fructose and galactose into the capillaries within the villus.
Fructose and mannose are slowly absorbed by passive transport on a facilative GLUT-5 carrier into enterocyte. GLUT5 is specific for fructose with no ability to transport glucose or galactose.
So GLUT-5 and GLUT-2 work in series to promote the absorption of fructose.
Explain protein absorption
There are 2 mechanisms:
- Amino acids are transported on a sodium coupled carrier system (co-transport). There are seperate carriers for different types of amino acids
- Some di/tri peptides are transported on a carrier system using inwardly directed H+ gradient
The amino acids are then actively transported into the capillary
Explain fat absorption
Fats are diffuse into lacteals which are lymphatic vessels present in each villus (rather than capillaries). They drain from the intestines and flow into the blood stream via the lymphatic system
- Fats have to broken down into fatty acids and glycerol (monoglycerides) before being absorbed in entercytes.
In the enterocyte, triglycerides are formed from fatty acids and monoglycerides. The triglycerides are then packaged with cholesterol, phospholipids and proteins into chylomicrons which are transported into the lacteals.
Short chain free fatty acids will enter the portal vein and combine with bile in the liver to form micelle, through action of lipases.
