Physiology Flashcards
Function of mouth and oropharynx?
Chops and lubricates food, starts carbohydrate digestion, propels food to oesophagus
Function of oesophagus?
Propels food towards the stomach
Function of stomach?
Stores/churns food, continues carbohydrate, initiates protein digestion, regulates delivery of chyme (fluid that is passed from stomach to intestine, acidic and uneven) to duodenum
Function of small intestine?
Principal site of digestion and absorption of nutrients
Function of the large intestine?
Colon reabsorbs fluids and electrolytes, stores faecal matter before delivery to rectum. No absorption occurs here.
Function of rectum and anus?
Regulated expulsion of faeces
4 layer generalised structure of the GI wall?
- Mucosa
- Submucosa
- Muscularis Externa
- Serosa
What layer is the submucous (Meissner’s) plexus contained within?
The submucosa
What layer is the myenteric (Auerbach’s) plexus contained within?
Muscularis externa
GI motility is mostly due to activity of _____1______
Circular muscle contraction results in _____2_______
Longitudinal muscle contraction results in ______3______
Muscularis mucosae contraction results in _______4_________
1 smooth muscle
2 lumen becomes narrower and longer (it is squeezed against pressure so by narrowing it, it becomes longer)
3 intestine becomes shorter and fatter (by shortening it, it becomes fatter)
4 change in absorptive and secretory area of mucosa (folding), mixing activity. All contents are evenly mixed and exposed to epithelium.
In the stomach, small, and large intestine spontaneous electrical activity occurs as __1___ waves - rhythmic patterns of membrane depolarization and repolarization that spread from cell to cell via __2___ junctions
1 slow
2 gap
Spontaneous activity across the coupled GI cells is driven by specialised pacemaker cells called ________ which are largely located __________
interstitial cells of Cajal (ICCs)
between the circular and longitudinal muscle layers
Smooth muscle in the GI tract is _______ which means _______
single unit smooth muscle
the electrical excitation spreads from cell to cell (not all are directly innervated though) and the contraction happens at the same time as a wave.
Slow wave activity happens all the time- fed or starving state however _______________________________
Depolarising slow waves do not necessarily result in contraction
Contraction in the intestines occurs only if ___________________1______________________
____2______ of slow wave is ultimately what influences whether contraction will happen.
1) the slow wave amplitude is sufficient to reach a threshold to trigger smooth muscle cell calcium action potentials (spikes)
2) Starting point
3 things that can influence whether slow wave aptitude reaches threshold?
Neuronal stimuli
Hormonal stimuli
Mechanical stimuli
Parasympathetic nerve supply of the GI system is by _____1______ and _______2_______
Preganglionic fibres releasing Ach synapse with ganglion cells within ______3_____
Excitatory influences result in increased __________________4____________________________
Inhibitory influences result in relaxation ____________________5______________________
1) Vagus nerves
2) Sacral nerves S2-4
3) the enteric nervous system
4) gastric, pancreatic and small intestinal secretion, blood flow and smooth muscle contraction
5) of some sphincters, receptive relaxation of stomach
Sympathetic nerve supply of the GI system is by _______1_________
Preganglionic fibres (releasing ACh) synapse in the _____2_______
Postganglionic fibres (releasing NA) innervate mainly _____3_____
Excitatory influences ____4_____
Inhibitory influences ______5_________
1) thoracolumbar region
2) directly onto cells
3) enteric neurons
4) Increase sphincter tone
5) Decrease motility, secretion and blood flow
The enteric nervous system is
a division of the autonomic nervous system that governs the GI tract
Explain what happens in peristalsis
Behind the food bollus the circular muscle contracts and the longitudinal muscle relaxes
In front of the food bollus the circular muscle relaxes and the longitudinal muscle contracts
What is segmentation?
rhythmic contractions of the circular muscle layer that mix and divide luminal contents. The position of the contractions varies so food is propelled forward and backwards with no overall direction of movement. This finely chops up the chyme and delays any forward movement allowing time for both digestion and absorption. This occurs in the small intestine in the fed state and in the large intestine where it is called haustration.
What is colonic mass movement?
A powerful sweeping contraction that forces faeces into the rectum – occurs a few times a day often after a meal.
Propels faeces in to the rectum (normally nearly empty) triggering the defaecation reflex in response to rectal stretch
What is migrating motor complex?
A powerful sweeping contraction from stomach to terminal ileum
Explain tonic contractions
These are sustained contractions:
- low pressure - organs with a major storage function (e.g. stomach)
- high pressure - sphincters
How many sphincters are there? What is there order from mouth to anus?
6 Upper oesophageal sphincter Lower oesophageal sphincter Pyloric sphincter Ileocaecal valve Internal (smooth muscle) and External (skeletal muscle) anal sphincters where internal sits within external bu both are at same level.
Function of upper oesophageal sphincter?
This is skeletal muscle and it relaxes to allow swallowing and closes during inspiration
Function of the lower oesophageal sphincter?
Relaxes to permit entry of food to the stomach and closes to prevent reflux of gastric contents to the oesophagus
Function of Pyloric sphincter?
Regulates gastric emptying, usually prevents duodenal gastric reflux
Function of Ileocaecal valve?
Regulates flow from ileum to caecum- distension of ileum opens, distension of proximal colon closes
Why is it difficult to lose weight once it is gained?
Increased body fat alters brain function. Long term obesity induces brain re-programming. Your brain views the extra weight as normal and dieting as threat to body survival.
The site of integration of feeding behaviour is the ____1_____
The neural centre responsible is the _______2______
The three basic concepts that underly this control mechanism are _________3___________
1 brain
2 hypothalamus
3 satiety signalling, adiposity negative feedback signalling and food reward
Definition of satiation
Sensation of fullness generated during a meal
Definition of satiety
Period of time between termination of one meal and the initiation of next
Definition of adiposity
The state of being obese
Five examples of satiation signals are…
Cholecystokinin (CCK) Peptide YY (PYY3-36) Glucagon-like peptide 1 (GLP-1) Oxyntomodulin (OXM) Obestatin
What is the hunger signal?
Ghrelin
Long term control of weight is by two hormones, describe these.
Leptin - made & released from fat cells
Insulin - made & released from pancreatic cells
Levels in blood increase as more fat is stored
In the brain they tell the brain to alter energy balance and eat less and increase energy burn.
In the obese state this malfunctions
If you have reduced leptin what happens?
The body mimics starvation- you have no appetite control- you are constantly hungry
Does injecting individuals with leptin work to reduce their weight?
Injecting lectin works in individuals who lack leptin
However, the vast majority of obese individuals have high levels of leptin and they have increased leptin resistanc.
The stomach is a ____1__ shaped bag that relaxes receptively driven _____2___ to accommodate food from the oesophagus
1) J
2) vagus
What are the 2 mechanical regions of the stomach?
Orad- fundas and proximal body
Caudad- distal and antrum
In the orad region of the stomach there is no ____1____ activity instead contractions are due to the _____2_____
The result is intermittent propulsion of products to the _____3_____ and stomach size _____4______
1) slow wave
2) thin musculature
3) caudad region
4) decreases as it empties
What does the minimal mixing of products in the orad region of the stomach allow?
Carbohydrate partial digestion by salivary amylase
In the caudad region ____1___ occur continuously but not all reach threshold. Velocity of contraction ___2____ towards the junction overtaking the movement of ____3___ which results in ______4_____ of chyme allowing _______5_________
1) slow waves
2) increases
3) chyme
4) retropulsion
5) mixing of gastric products
Strength of antral wave determines escape of chyme through pyloric sphincter this is governed by ?
Gastric and duodenal factors
The rate of emptying of the stomach is proportional to ….
the volume of chyme in the stomach
The duodenum must be ready to receive chyme, how can it delay stomach emptying?
Neuronal response: the enterogastric reflex – decreases antral activity by signals from intrinsic nerve plexuses and the ANS
Hormonal response – release of enterogastrones [e.g cholecystokinin CCK)] from duodenum inhibits stomach contraction
What are the 2 secretory regions of the stomach?
the oxyntic gland area (proximal stomach including the fundus and body)
the pylorlic gland area (distal stomach, designated the antrum)
What do D cells secrete?
What is its purpose?
Somatostatin
inhibits HCl secretion
What do Enterochromaffin-like cells secrete?
What is its purpose?
Histamine
stimulates HCl secretion
What do G cells secrete?
What is its purpose?
Gastrin
stimulates HCl secretion
What do chief cells secrete?
What is its purpose?
Pepsinogen
inactive precursor of the peptidase, pepsin. Note: pepsin once formed activates pepsinogen (autocatalytic)
What do parietal cells secrete?
What is the purpose of these secretions?
HCl
activates pepsinogen to pepsin
denatures protein
kills most (not all) micro-organisms ingested with food
Intrinsic factor and Gastroferrin
bind vitamin B12 and Fe2+ respectively, facilitating subsequent absorption
What is the only absolutely essential function of the stomach?
produce intrinsic factor- it aids the absorption of vitamin B12. If there is no intrinsic factor is vitamin B12 cannot be absorbed, will get a type of anaemia.
What are the three phases of gastric acid secretion and inhibition?
Cephalic phase
Gastric phase
Intestinal phase
Describe the cephalic phase that results in gastric acid secretion
(‘in the head’) – before food reaches the stomach preparing it stomach to receive food. This is driven directly and indirectly by the CNS and vagus nerves (CN X).
Describe the gastric phase that results in gastric acid secretion
Occurs when food is in stomach. It involves both physical and chemical mechanisms. Distension of the stomach due to the food activates reflexes that cause acid secretion.
Describe the cephalic phase that results in gastric acid inhibition
Vagal nerve activity decreases upon cessation of eating and following stomach emptying
Describe the gastric phase that results in gastric acid inhibition
antral pH falls when food exits stomach (due to decreased buffering of gastric HCl) – release of somatostatin from D cells recommences, decreasing gastrin secretion prostaglandin E2 (PGE2) continually secreted by the gastric mucosa acts locally to reduce histamine- and gastrin-mediated HCl secretion
Why doesn’t the stomach digest itself?
It has a sticky protective mucus layer that prevents protons from reaching the surface but even if they do they are neutralised by bicarbonate contained within the layer.
How does perastalsis in the interdigestive or fasting state occur?
Migrating motor complex
Describe the migrating motor complex
occurs between meals every 90 – 120 min
strong peristaltic contraction slowly passing length of the intestine
clears small intestine of debris, mucus and sloughed epithelial cells between meals – ‘housekeeper function’
What is the migrating motor complex inhibited by?
feeding and vagal activity
Seven hormonal secretions of the small intestine?
gastrin secretin cholecystokinin GIP glucagon like peptide 1 motlilin ghrelin
What does secretin from the small intestine promote and what is it released from?
from S cells of duodenum, released in response to H+ and fatty acids in lumen
promotes secretion of pancreatic and biliary HCO3-
What does motilin from the small intestine promote and what is it released from?
from M cells of duodenum and jejunum, secreted during fasting state
initiates the migrating motor complex
Succus (juice) of the small intestine varies throughout what things does the secretion usually contain?
mucus – for protection/lubrication (from goblet cells)
aqueous salt - for enzymatic digestion (mostly from the crypts of Lieberkühn)
Pancreatic endocrine secretions?
insulin and glucagon into the blood
Pancreatic exocrine secretions?
digestive enzymes (acinar cells), aqueous NaHCO3- solution (duct cells) – secreted to the duodenum collectively as pancreatic juice
Duct cells secrete 1 – 2 litre of _____1______into the duodenum per day
This neutralises acidic ___2___ entering the duodenum
provides optimum ___3__ for pancreatic enzyme function
protects the mucosa from ____4_____
1) alkaline (HCO3- - rich) fluid
2) chyme
3) pH
4) erosion by acid
What must all carbohydrate be converted to for absorption?
Monosaccharides glucose, galactose or fructose
Describe the role of alpha amylase in carbohydrate absorption
breaks down linear internal α-1,4 linkages but not terminal α-1,4 linkages. Hence, no production of glucose
cannot cleave α-1,6 linkages at branch points (in amylopectin) or α-1,4 linkages adjacent to branch points
products are thus linear glucose oligomers (maltotriose, maltose) and α-limit dextrins.
What do Oligosaccharidases do?
Break down oligosaccharides into monosacharrides
Four examples of Oligosaccharidases?
Lactase
Maltase
Sucrase
Isomaltase
Where does digestion of glucose, galactose and fructose take place? ie the products of carbohydrate digestion?
Duodenum and Jejenum
Glucose and galactose are absorbed by ______1____ and fructose is absorbed by _____2_______Exit for all monosaccharides is mediated by facilitated diffusion by __3___
1) secondary active transport (SGLT1)
2) facilitated diffusion (GLUT-5).
3) GLUT-2
Explain the mode of operation for SGLT1
- 2 Na+ binds:
- Affinity for glucose increases, glucose binds:
- Na+ and glucose translocate from extracellular to intracellular:
- 2 Na+ dissociate, affinity for glucose falls:
- Glucose dissociates:
- Cycle is repeated
The transporter is also permeable to water and each cycle thousands of water molecules are moved from the lumen and into the enterocytes and ultimately back into the rest of the body.
Protein must (after the first six months of life) be digested to ___1___ and amino acids for efficient absorption ___2__ major pathways exist
1) oligopeptides
2) four
Initial digestion of proteins in the stomach involves…
denaturation by HCl
cleavage into peptides by pepsin
Digestion of proteins in the duodenum involves…
5 pancreatic enzymes
some are exopeptidases that break peptide bonds from end-pieces of terminal amino acids and some are endopeptidases that break peptide bonds of nonterminal amino acids (i.e. within the molecule).
Where can additional break down of oligopeptides take place after digestion in duodenum and stomach
At the brush border or in cytoplasm of enterocytes
Amino acids are transported across the apical membrane via a variety of amino acid transporters. Oligopeptides are transported across the apical membrane by the _______1________ (IMPORTANT KNOWLEDGE)
Oligopeptides with the cytoplasm are hydrolysed to amino acids by ____2_____ within the enterocyte
Amino acids exit the enterocyte across the basolateral membrane by several, _______3______
1) H+/oligopeptide co-transporter, PepT1
2) peptidases
3) Na+-independent. transporters
For digestion an absorption what must lipids be converted from and too?
Must be converted from solid fat and oil masses into an emulsion of small oil droplets suspended in water
What enzyme aids the breakdown of TAGs?
Lipases
In the stomach what stimulates release of gastric lipase?
secreted in response to gastrin from chief cells
What stimulates secretion of pancreatic lipase?
CCK
Note CCK also stimulates bile flow
Describe the role of bile salts, where are they released from and into, why?
Bile salts released into the duodenum in bile from the gall bladder in response to CCK act as detergents to help emulsify large lipid droplets to small droplets
Where are the final products of lipid digestion stored in and released from?
Mixed Micelles
3 ways different lipids are absorbed?
Short chain and medium fatty acids diffuse through the enterocyte, exit through the basolateral membrane and enter the villus capillaries
Long chain fatty acids and monoglycerides are resynthesized to triglycerides in the endoplasmic reticulum and are subsequently incorporated into chylomicrons
Cholesterol is transported by endocytosis
Ca2+ absorption can occur by two mechanisms- passive and active. With [Ca2+] in chyme < 5 mM absorption is mainly ___1_____
1) active
It is __1___ iron form that can be absorbed and this binds to ___2____ in the stomach
Reduction of Fe3+ is done by ________3_________
1 Fe2+
2 gastroferrin
3 HCl, vitamin C or brush border cytochrome b ferric reductase
Fat soluble vitamins? What is needed for these to be absorbed?
bile secretion
intact intestinal mucosa
In the large intestine electrolyte absorption which drives the absorption of water is mediated by ____1_____
Ion secretion is mediated by _______2______
1) surface epithelial cells (colonocytes)
2) crypt cells
The large intestine is divided into a series of saccule called ______ and these are not static
The process of _____ is a very slow form of segmentation this allows time for _______
haustra
haustration
fluid and electrolyte reabsorption
3 patterns of motility in the large intestine?
Haustration (non-propulsive segmentation)
Peristaltic propulsive movements (mass movement)
Defaecation (periodic egestion)
The internal anal sphincter is controlled by ____1______ fibres which relax involuntarily. The external anal sphincter is controlled by a ____2______ nerve supply
When the rectum is distended activation of rectal stretch receptors occur and the ____3______ reflex is initiated and relaxes the ___4____ sphincter. If defecation is not desired, ________5________ can delay it.
1) parasympathetic fibres
2) somatic
3) rectosphincteric
4) internal
5) voluntary contraction of the external sphincter
Four things commensal bacteria do in the large intestine?
increase intestinal immunity by competition with pathogenic microbes
promote motility and help maintain mucosal integrity
synthesise vitamin K2 and free fatty acids (from carbohydrate) that are absorbed
activate some drugs (e.g. used in treatment of IBD)
Medical word for vomiting?
emesis
Where is vomiting co-ordinated?
Vomiting centre in the medulla oblongata of the brain stem
Explain the overall events in vomiting
Suspension of intestinal slow wave activity
Retrograde contractions from ileum to stomach
Suspension of breathing (closed glottis - prevents aspiration)
Relaxation of LOS- contraction of diaphragm and abdominal muscles compresses stomach
Ejection of gastric contents through open UOS
Repeats of the cycle
What areas of the brain apart from the vomiting centre also receive info in vomiting
CTZ - chemoreceptor trigger zone within the area postrema (AP)
NTS – nucleus tractus solitarius
Role of 5HT3 receptor in vomiting
Systemic toxins or toxic materials in the gut lumen stimulate vomiting by this pathway
Enterochromaffin cells in the mucosa are stimulated by this stimuli and these cause depolarization of sensory afferent terminals in mucosa via the 5Ht3 receptors.
This then allows action potential to be discharged to brain where it can be coordinated by vomiting centre in medulla
Absorbed toxic materials and drugs in blood stimulate vomiting through
CTZ within the AP of brainstem (as it lacks an effective blood brain barrier)
Mechanical stimuli in the pharynx or pathology within in the GI tract stimulates vomiting through
Vagal afferents to brainstem (CTZ and NTS
Motion sickness stimulates vomiting through
Vestibular nuclei
Stimuli within the CNS stimulate vomiting through
Cerebral cortex, limbic system
Water ingested and secreted is normally in balance with the water ____1____
To maintain this balance some water is therefore secreted in the faeces along with ________2_________
1) absorbed
2) bacteria, cellulose and bilirubin
Intestinal fluid movement is always coupled to ___1___ movement.
Reabsorption of __2__ provides a (local) osmotic force for reabsorption of __3___
1) solute
2) Na+
3) water
________1_________are the major mechanisms of postprandial Na+ absorption in the jejunum.
These are both examples of ____2____ active transport and ultimately results in the lumen being more negative which therefore allows parallel absorption of __3__.
1) Na+/glucose and Na+/amino acid cotransport
2) secondary
3) Cl
___________ in the jejunum- stimulated by alkaline environment of the lumen and is mainly involved in keeping the pH constant.
Na+/H+ exchange
___________ exchange in parallel occurs in the ileum and proximal colon and is the primary mechanism of Na+ absorption in the interdigestive period, but does not contribute greatly to postprandial (after eating a meal) absorption
Na+/H+ and CI-/HCO3-
______________ Occurs in the colon (distal particularly) and is regulated by aldosterone
Epithelial Na+ channels (ENaC)
Cl secretion occurs from ____ cells rather than villus.
crypt
What ion secretion is important in many diarrhoeas?
Cl-
_________ is involved in Cl secretion on the apical membrane
Normally there is little secretion of Cl- occurs because apical CFTR is ______________
CFTR
either closed, or not present
CFTR can be indirectly activated by
bacterial enterotoxins
hormones and neurotransmitters
some immune cell products
some laxatives
Activation of CFTR is a result of secondary messenger molecules which are
o cAMP (e.g. cholera toxin, VIP, histamine) o cGMP (e.g. heat stable enterotoxin, guanylin) o Ca2+ (e.g. acetylcholine, bradykinin, 5-HT)
Diarrhoea is defined as….
loss of fluid and solutes from the GI tract in excess of 500 ml per day
Four main causes of diarrhoea
impaired absorption of salts, excessive secretions, non-absorbable solutes or hypermotility.
Only a small fraction (5%) of bile salts entering the duodenum is lost in the faeces; most is reabsorbed by _____1_____ in the _____2______ and undergoes ______3_______
1) active transport
2) terminal ileum
3) enterohepatic recyling
