017 Physiology of the GI tract: absorption 1 Flashcards
Where does most of our nutrient absorption occur in small intestine?
Jejunum
What absorption takes place in the ileum?
Bile salts and vitamins
What are changes in villi structure along the SI?
In the duodenum, villi are much shorter and wider.
In the jejunum, villi are long fingerlike projections that increase SA by 10 fold.
In the ileum, villi are shortened again.
What is the microvilli?
Cell that form the brush border membrane increasing surface area by 20 fold.
What is the estimated surface area for absorption?
Approximately 200m2
What are the 3 pathological mechanism for malabsorption syndrome?
Intraluminal mechanisms
Intestinal mechanisms
Post epithelial mechanisms
What is coeliac disease?
Intolerance to gluten. Gluten causes complexes that bind to epithelium and activate T lymphocytes, which produces toxins that cause lysis in the enterocytes. As you move up the villi, enterocytes become damaged. Hence the decrease in villi length, normal absorption cannot occur.
What does the duodenum absorb?
Iron -10%
Calcium -30%
Where is phosphate absorbed?
Jejunum
In which regions is water absorbed?
All regions of the intestine.
Describe the differences in Na+ concentration from lumen to blood plasma
Na+ concentration is both high in the lumen (-140mM) and blood, whilst low in the enterocyte (-15mM).
What is the significance of SGLT1?
Mutations in this gene causes glucose galactose malapsorption. This is also responsible for absorbing 260 molecules of H2O per glucose (5L per day).
How much fluid is normally absorbed in the small intestine?
8.5L
How much fluid is secreted in the intestines per day?
1.5L
Describe intestinal fluid secretion
Chloride secretion occurs from the cells lining the crypts of lieberkuhn in all intestinal segments. In the basal lateral membrane, the NKCC symporter transports Cl into the enterocyte. This is moved out of the cell into the lumen by CFTR carrier protein, pulling NA+ and H2O along with it due to osmotic gradient.
What stimulates the intestinal fluid secretion?
Secretagoges e.g. VIP and parasympathetic nervous system
How does Cholera toxin cause secretory diarrhoea?
Cholera bacteria secretes a toxin. There is a receptor on the apical part of the enterocyte in the small intestine. This stimulates the enzyme adenylyl cyclase, which catalyzes ATP to form cAMP. The increase in cAMP will enhance expression of CFTR protein. This drives Cl-, hence Na+ and H2O secretion. Patients with this disease may secrete 12L per day.
What is the difference between secretory diarrhea and osmotic diarrhea?
Osmotic diarrhea - inability to absorb carbohydrate, resulting in delivery of carbohydrates to the colon and causing water to be drawn into the colon due to osmotic gradient. e.g. SGLT1 malfunction.
Secretory diarrhea - the excessive stimulation of the CFTR protein causing Cl- transportation out into the intestinal lumen, causing water to be drawn into the intestine due to osmotic gradient.
How can we treat patients with secretory diarrhoea?
Give patients fluid containing sources of glucose and NaCl. Secretory diarrhoea affects secretion from the crypts. SGLT1 is not affected, so by giving fluid, we are essentially replenishing lost fluids and electrolytes.
Where does fluid absorption take place in relation to fluid secretion?
Fluid absorption takes place on the villi. Fluid secretion takes place in the crypts of lieberkuhn.
Which protein digestion enzymes are found in the villi enterocytes?
Endopeptidase
Aminopeptidase
Which enzymes are used for protein digestion in the stomach?
Pepsin
Which protein digestion enzymes are found in pancreas secretions?
Trypsin, Chymotrypsin, Carboxypeptidase
Which enzymes are used digest peptides into single amino acids?
Dipeptidase
Tripeptidase
Explain protein absorption from gut lumen into the blood.
Na+/H+ antiporters transport Na+ down the concentration gradient into the enterocyte, whilst transporting H+ into the lumen. The PepT1 symporter then uses energy of H+ concentration gradient to drive small peptides into the cell, where they are hydrolized and enter through basal lateral membrane through amino acid specific channels.
What does Amylase break down?
Starch/glycogen into maltose
What does Maltase break down?
Maltose into glucose
What does sucrase break down?
Sucrose into glucose and fructose
What does lactase break down?
Lactose into glucose and galactose
Discuss the presence of intestinal GLUT2 during digestive phase and its significance in diabetes and obesity.
Between meals, SGLT1 is the dominant transporter of glucose at the brush border membrane. Its high affinity allows it to absorb low levels of glucose in the lumen. During the digestive phase, GLUT2 is the dominant transporter at the brush border membrane. This provides a high capacity route for glucose absorption. It seems that GLUT2 is up-regulated in recent models of obese and diabetic human patients.
Who qualifies for bariatric surgery?
Those with BMI >40 or BMI >35
How many bariatric surgery procedures are performed annually in England?
8000
Where does protein absorption mainly occur
Jejunem and upper ileum
Name some luminal digestive enzymes
Trypsin, chemo trypsin, aminopeptidase, carbixylopeptidase
Name some brush border protein digestion enzymes
Endopeptidase, aminopeptidase
Where is Maltese located
Brush border digestion
What are the 3 common types of bariatric surgery
Adjustable gastric band: a band that can adjust the cardiac sphincter of the stomach
Route en Y gastric bypass: cutting out a portion of the stomach and the duodenum, so that a small pouch of the stomach is just directly connected to the jejunem
Vertical sleeve gastrectomy: just cutting out a huge chunk of the stomach
