Small Intestine Flashcards
What is segmentation?
The MAJOR movement inthe small intestine.
- Oscillating, ring-‐like contractions of the circular smooth muscle lasting a few seconds.
- Areas (few cm) alternatively contract and relax.
- Mixes chyme with digestive juices
- Exposed to absorptive epithelial surface
SLOWLY propels chyme forward!
-Allows ample time for digestion and absorption
What are the major forms of motility in the small intestine?
SEGMENTATION Weak PERISTALSIS (compared to stomach)
How is segmentation controlled?
Initiated by pacemaker cells which produce the BASIC ELECTRICAL RHYTHM (slow wave potential).
If S.I. slow wave brings the circular smooth m. layer to threshold - CONTRACTION and thus SEGMENTATION OCCURS!
What is the gastroileal reflex?
The duodenum and ileum start to segment at the same time!
-Duodenum starts to segment in response to distension from chyme
-Ileum starts to segment while it is empty!
Responds to gastrin secreted from stomach (a hormonal response)
TRUE of FALSE:
Segmentation decreases in frequency as it moves along the S.I.?
TRUE
What is the Migrating Motility Complex (MCC)?
[THE SWEEPER]
After meal has been ABSORBED;
slow, weak, repetitive, peristaltic waves begin!
- -Waves start at stomach and move a short distance down S.I. before dying out
- -Following wave starts a little bit before where the previous wave died out
~1.5 hrs for the peristaltic waves sweep the remnants of preceding meal + bact. to colon
Once wave reaches end, cycle begins again
Repeat until next meal
What neural control is present at the Illeocaecal juncture?
INTRINSIC PLEXUSES!
- DISTENSION of the ILEAL side causes the sphincter to relax
- P on the CAECAL side causes the sphincter to contract more forcefully
When do small intestine secretions increase?
After a meal in response to local stimulation by the presence of chyme!
What is absorbed in the small intestine?
All CARBS., FATS, PROTEINS and MOST ELECTROLYTES, WATER and VITAMINS are absorbed by the S.I. INDISCRIMINATELY (excluding Ca and Fe).
If the circular smooth m. layer is brought to threshold, segmentation occurs. How is the muscle responsiveness altered?
Moving the starting potential closer/further from threshold by:
- distension
- gastrin
- extrinsic nerve activity (↑ by parasympathetic and ↓ by sympathetic stimulation)
Why does segmentation decrease in frequency as it moves along the S.I.?
What is the outcome of this?
Faster pacemaker depolarisation in duodenum!
- Duodenum (12/min)
- Ileum (9/min)
3-‐5hrs to move through!
OUTCOME:
More chyme on average is pushed forward than pushed back, THUS moving the chyme very slowly forward.
What hormonal control is present at the Illeocaecal juncture?
GASTRIN enhances RELAXATION of the sphincter when it’s released at the onset of a meal
Why do we need the barrier between the small and large intestine?
To prevent bacteria from large intestine infiltrating the small intestine (and all it’s ample nutrients)!
What does the small intestine secrete?
NO DIGESTIVE ENZYMES SECRETED!
Exocrine gland cells secrete aqueous salt & mucus secretion (succus entericus)!
- Mucus for PROTECTION against pancreatic enzymes, and LUBRICATION
- H20 in secretion helps digestion (hydrolysis reactions)
What environment do pancreatic enzymes need to function optimally?
An alkaline and liquid environment!
HCO3- secreted into lumen in exchange for Cl-
–this increases the pH
Cl- is then actively secreted back into lumen,
THUS, H20 and Na+ passively follow through TJs)
[H20 maintains liquid chyme]
Where does the majority of absorption occur in the small intestine?
The duodenum and jejunum (50% reserve capacity)
[Ileum has specialised transport mechanisms for absorption of bile salts and vitamin B12]
What mucosal adaptions does the S.I. have for absorption?
- Large SA
2. Epithelial cells posses a variety of SPECIALISED TRANSPORT MECHANISMS
What contributes to the S.I.’s large SA?
Circular folds
Lined by villi
Lined by microvilli
What is the structure of villi?
Epithelial cells cover surface
- -(connected by TJs)
- -(have luminal brush boarder [w. carriers and digestive enzymes])
CT core (lamina propria)
Capillary network
Lacteal
What do Crypts secrete + what is there function?
SECRETE WATER AND ELECTROLYTES
Mitotic activity of STEM cells allows the entire mucosal lining to be replaced in 3 days!
–As cell migrates up villus, it becomes more mature
THUS, the conc. of brush boarder enzymes increases,
THUS, the capacity of absorption increases
THEREFORE, by the time cell reaches the tip it has maximal digestive and absorptive capacity!
–Paneth cells present
What is the function of paneth cells?
ANTIMICROBIAL
Produce lysozymes and defensins
What is the function of brush boarder enzymes?
(Integral proteins that function as enzymes)
Complete digestion of CARBS & PROTEIN (fat digestion is completed)!
What are some examples of brush boarder enzymes?
-ENTEROKINASES
activates the proteolytic enzyme trypsinogen to trypsin
-DISACCHARIDASES
(maltase, sucrase, lactase)
digest disaccharides to monosaccharides
-AMINOPEPTIDASES
digest small peptide fragments into a.a’s
What specialised transport mechanisms does the S.I. posses to help in absorption?
TRANSPORT MECHANISM OF Na+
-Absorption of H20, nutrients and electrolytes is linked to Na+ absorption!
- Na+ transport is via:
(a) PASSIVE TRANSPORT
(b) SECONDARY ACTIVE TRANSPORT
***Na+ is actively pumped out of cell and into interstitial fluid by Na+K+ATPase pump in basolateral mem.!
(generates osmotic and electrical gradient for H20 and Cl- to follow).
What are the passive transport mechanisms of Na+ transport?
- Between cells through TJs
2. Alone through cell membrane via Na+ channel
What are the secondary active transport mechanisms of Na+ transport?
- Accompanying another ion - (Cl- [symporter])
- Accompanying a nutrient molecule - (glucose, a.a [symporter])
- By exchange for another ion - (H+ [antiporter])
How is the biochemical balance of the S.I. maintained?
JUICES THAT ENTER S.I. ARE ABSORBED BACK INTO THE PLASMA
THUS, acid-base balance is NOT altered!
Within the S.I., the HCl secreted from parietal cells of the stomach is neutralised by sodium bicarb. secreted by pancreatic duct cells, forming NaCl and carbonic acid!
–Carbonic acid decomposes to CO2 and H20
HENCE, the end products = - Na+ - Cl- - H20 - CO2 (all absorbed by intestinal epithelium and into blood)
When may Diarrhoea occur
-When secretion and absorption are not parallel!
-Loss of fluids
(dehydration, loss of nutrients, metabolic acidosis)
What are the 3 main types of Diarrhoea?
- Motility-related Diarrhoea
- Not enough time to absorb fluids
- Local irritation (e.g. bacterial, viral infection) - Osmotic Diarrhoea
- Fluid retained in lumen
- e.g. lactose intolerance - Secretory Diarrhoea
- Promote secretion of fluid
- e.g. toxins such as cholera
What is lactose intolerance?
INABILITY TO METABOLISE LACTOSE (absence of lactase)
THUS, undigested lactose remains in lumen, creating an osmotic drag for water - diarrhoea
Colonic bact. use lactose as energy source, producing gas
-distension, pain, cramping, flatulence & diarrhoea
What is Vibrio Cholera?
DEHYDRATION FROM FLUID LOSS IN DIARRHOEA
-decreases circulating plasma volume!
Toxin causes Cl- secretion in S.I. - H20 follows
- -can be treated effectively with oral rehydration therapy
- -LEADING CAUSE OF DEATH IN INFANTS IN DEVELOPING COUNTRIES
Discuss CARB. absorption.
–Glucose and galactose are absorbed through the cell mem. by being cotransported with Na+.
The symporter is called the sodium glucose cotransporter (SGCT)
–Fructose is absorbed into the cell by facilitated diffusion through the glucose transporter-5
THE MONOSACCHARIDES THEN LEAVE THE CELL BY FACILITATED DIFFUSION (passive carrier mediated transport) VIA THE GLUT2 TRANSPORTER, BEFORE ENTERING THE CAPILLARIES BY SIMPLE DIFFUSION
Discuss PROTEIN absorption.
Once these small peptides reach the brush boarder enzymes, they are further broken down by aminopeptidases into a.a’s
–A.a’s enter cell via Na+ and energy dependent secondary active transport (symporters)
–Small peptides enter by H+, Na+- and energy dependent tertiary active transport
(inward H+ conc. gradient set up by Na+/H+ antiporter. H+ moves in down its gradient with peptide moving against its gradient)
-Broken down to a.a’s within cell
Exits by facilitated diffusion
Diffuses into capillary
Discuss FAT absorption.
Once within cell, monoglycerides and FFA resynthesised to triglycerides
Triglycerides + coating lipoprotein (ER) form water soluble chylomicrons
These are then exocytosed and absorbed into the lymph!
How are vitamins absorbed?
WATER-SOLUBLE VITAMINS absorbed passively with H20
FAT-SOLUBLE VITAMINS carried in micelles and absorbed with fat-digestion end products
How is vitamin B12 absorbed?
Vit. B12 combined with gastric INTRINSIC FACTOR absorbed by receptor-mediated endocytosis in terminal ileum
[unique]
How is iron absorbed?
INTO cells by active transport OUTof cells (into blood) by transporter ferroportin
REGULATED BY: liver hormone hepcidin:
-blocks ferroportin
(excess stored as ferritin in cells → cells slough off and ∴ lost in faeces)
How is calcium absorbed?
Occurs down electrochemical gradient through Ca+ channel
Exits cell by Ca+/ATPase pump and Na/Ca antiporter
Is enhanced by Vit D
(parathyroid hormone increases vit D activation in liver and kidneys)
