Digestive Physiology Flashcards
What is the function of the digestive system
To transfer nutrients (including water, salts or ions, and vitamins) to ECF (the internal environment)
Organs of the tract
Mouth, pharynx, esophagus, stomach, small intestine, large intestine
Accessory digestive organs
Salivary glands, liver, gall bladder, and pancreas
What absorbable units are carbohydrates, proteins, and fats broken down to
Carbohydrates-monosaccharides (mostly glucose, also galactose, and fructose)
Proteins-amino acids
Fats(triglycerides)-one molecule is broken down to 1 mono glycerine and 2 fatty acids
What enzymes are involved in digesting carbs, proteins, and fats
The enzymes that catalyze above reactions cause hydrolysis (add H20 at bond; breaks subunits)
Enzymes are biological catalysts specific for certain reactions
Main enzymes
Carbohydrate-amylase
Protein-pepsin
Fat-lipase
What are the 4 layers of the GI tract
Serosa-the outer layers of organ; it’s a membrane that secretes fluid. A layer covers the abdominal organs and is called the serosa aka visceral peritoneum.
Another layer of peritoneum lines the abdominal wall called parietal peritoneum
Muscle layer (muscularis externa)- it’s smooth muscle It has an outer longitudinal layer (which can contract and shorten the tube) and an inner circular layer (which can contract and constrict the tube) causes mixing and propelling movements of tract
Submucosa- connective tissue nerve cells, blood and lymph vessels and glands
Mucosa- the innermost layer of mucous membrane= membrane that secretes mucus which lines the entire tract
What are the two nerve plexus more within GI tract and what makes it special
Submucous (submucosal) and myenteric plexus
make up the enteric (intestine) nervous system
GI tract has own local nervous system; no other organ system has this
Short reflexes
From receptor to nerve plexus to effector
Long reflexes
From receptor to CNS to ANS fibers to nerve plexus To effector cells
(Sight or smell of food can cause GI responses)
These reflexes are through a CNS effect on autonomic neurons that go to GI tract
What does saliva include in it?
Contains mucus (protein) which lubricates the food
Saliva contains enzyme amylase
Saliva contains lysozyme an antibacterial enzyme-kills some bacteria
How does swallowing occur?
Boris of food is pushed by tongue into the pharynx which stimulates pressure receptors and their afferents t. Euro s that go to the swallowing center in the medulla oblongata
- The soft palate and uvula close off the nasal cavity
- The trachea is closed off which keeps food out of respiratory tract
- epiglottis (flap at the top of larynx) covers larynx
- glottis=the laryngeal opening closes because the vocal cords or folds come together
- respiration’s are inhibited - The bolus passes the upper esophageal (or pharyngoesophageal) sphincter which then closes.
Steps 1-3 are known as oropharyngeal phase abs takes about 1 second
Esophageal phase
Primary Peristaltic wave moves the bolus of food into the stomach
If the primary peristaltic wave can’t get the bolus to stomach the distention of the esophagus triggers secondary peristaltic waves
Heart burn
Occurs when acidic gastric contents move into esophagus and irritates it.
Can occur with large meal and a full stomach or during pregnancy
How can pregnancy trigger heartburn
Enlarged uterus can push the lower esophageal sphincter up from abdominal cavity into thoracic cavity (where pressure is lower)
Increased abdominal pressure can force stomach contents into esophagus
What are the regions of the stomach
The stomach is located in the abdominal cavity
Regions=fungus (upper part, above the esophagus opening), body (middle part), antrum (lower part, with thick muscle)
Pyloric sphincter
Is at junction of the stomach and the first part of small intestine (duodenum); it is a sphincter of strong circular muscle
Chyme
A sludge (thick liquid); the stomach makes what’s been described as a smoothie which includes partly digested food solution, with enzymes too- salivary amylase and (will see soon) another enzyme is added
What are the functions af HCl?
Secreted by cells of the stomach called parietal cells
Functions: Kills microorganisms (remember lysozyme in saliva does too) -kills most microorganisms that enter food, but some make it to large intestine and divide there
Uncoils proteins- remember H+ affects protein shape; decreases food particle size by affecting connective tissue proteins
Activates pepsinogen
Creates an acid environment for pepsin (needs acid to work/be active
Pepsinogen
Secreted by chief cells
Pepsin (from pepsinogen) is a protein digesting enzyme (starts doesn’t finish breakdown)
Pepsin secreted in inactive forms (called pepsinogen) which protects the secreting cells from self digestion (a lot of cell structure is made of protein)
HCl helps activate pepsinogen- the low pH affects the enzyme shape allowing it to become active
Pepsin can then activate pepsinogen!
Where are the chief cells and parietal cells located
Are in gastric glands-are below gastric pits
Function of mucus secreted in stomach and through your tract
Forms protective coating
Is alkaline, has bicarbonate ion which helps neutralize acid next to stomach mucosa and protects stomach from damage
What secretes intrinsic factor
Parietal cells secrete it abs it’s needed for vitamin B12 absorption
Intrinsic factor is essential. Need B12 for DNA and hemoglobin synthesis therefore RBC production
Gastrin
a hormone
Different kind of secretion
Effects are not in stomachs lumen it goes into the blood to cause effects
Secreted by cells in Antrum of the stomach (inferior end)
Travels to other areas of the body through the blood including the body of the stomach
Stimulates acid secretion by parietal cells
Controls HCl secretion
Stimulates chief cells to secrete pepsinogen
Gastric emptying
Peristaltic waves are stronger in the antrum (thicker muscle) 
These waves mix the contents and close the pyloric sphincter
Before it closes the sphincter small amount of chyme moves into duodenum
Most chyme is forced backward
Mixing of chyme therefore happens mostly in Antrum where contractions are strongest
Pacemaker cells
Create slow waves-oscillations in membrane potential which cause a basic electrical rhythm
Won’t cause much contraction but if depolarizer further could increase strength of contraction
Nerves and hormones can depolarize further- can prevent contraction or cause stronger constrictions
What factors in the stomach affect stomach emptying.
Increased fluidity will cause increased emptying
Increased chyme in the stomach (increased distention, increased gastric motility, increased emptying) stronger contraction
What factors in the stomach affect stomach emptying
Increased distention of duodenum
Increased fats, hypertonicity (or increased osmolarity), acid in duodenum-these all decrease stomach emptying
What is the enterogastric reflex
- Stimulus in duodenum (distention, fats, hypertonicity, acid)
- Activation of receptors
- Afferents impulses to CNS
- Efferent impulses (via ANS neurons) to the stomach, decreases stomach peristalsis and emptying
(Long reflex)
Can also occur via short reflex or within the tract reflex
Stimuli in duodenum (increased fats, increased acid, increased osmolarity (or hypertonicity))
Receptor->enteric nervous system nerve plexus(es) -> stomach smooth muscle-decreases contraction
What hormones are released from small intestine to decrease antral contractions?
Hormones called enterogastrones are released.
Fats, hypertonicity, acid, distention can cause the release of this hormone
Pathway
- Stimuli in duodenum (increased fats, increased acid, etc)
- Small intestine endocrine cells release enterogastrones
- Increased enterogastrones go into blood and travel to the stomach and decrease stomach emptying
This is because the stomach needs time to churn and digest so it slows when a lot of food etc
Increased acid in duodenum
Bicarbonate from the pancreas normally neutralizes acid from the stomach-need to slow down emptying to neutralize what’s there (need to wait for more bicarbonate from the pancreas
Increased fat in duodenum
Fats take longer to digest and absorb than other foods- stop stomach emptying till can take care of fat that’s there
Increased hypertonicity
(increased osmolarity occurs when break one large molecule into many small molecules, for example, one protein molecule becomes hundreds of amino acids with digestion in small intestine)
Water moves toward increased osmolarity would move from plasma and decrease volume so reduce amount entering the stomach decrease stomach emptying until absorption of food molecules can occur
Increased distention of duodenum
Slow down stomach emptying to give time to deal with what’s already there
The intestine can tell the stomach to stop
It’s how the stomach is emptied at a rate that is best for digestion plus absorption
What are released by small intestine to inhibit the stomachs activity
Enterogastrones (hormones) Including cholecystokinin (CCK), secretin, and others are released from duodenum to inhibit the stomachs activity- inhibit motility (movement)
ALSO INHIBIT THE STOMACHS SECRETION
What is emptied into the small intestine
Gastric contents empty into it and exocrine (secrete through ducts) pancreas and liver add their secretions to the small intestines lumen
What does the pancreas secrete
Secreted enzymes and a bicarbonate solution-exocrine part
Secreted hormones-endocrine part
Enzymes are secreted by acinar cells
Bicarbonate rich solution secreted by duct cells
Exocrine glands
Secrete into ducts
Endocrine glands
Secrete hormones into the blood
What are the pancreatic enzymes
Amylase
Several protein digesting enzymes (secreted in inactive forms
Lipase
Pancreatic amylase
Starch digested started by salivary amylase continues working in the stomach (in middle of mass of food) until mixed it’s with HCl (which inactivated it)
Pancreatic amylase continues job. It digests carbs to disaccharides and alpha limit dectrins
Still not done with digestion of carbs
Protein digesting enzymes
Secreted in inactive forms (like pepsinogen from the stomach, need to be activated outside the cell- in the linen of digestive organ to protect the secreting cells from self digestion
Pancreas secreted trypsinogen- an inactive form of protein digesting enzyme
Enterokinase (enteropeptidase) is an enzyme on cell membranes of mucosa cells of the duodenum
Enterkinase splits a peptide off trypsinogen which forms trypsin trypsin actives other inactive enzymes then
Also
Chymotrypsinogen to chymotrypsin
Procarboxypeptidase to carboxypeptidase
Which protein digesting enzyme splits bonds of proteins in interior
Trypsin and chymotrypsin
Which protein digesting enzyme splits bonds of proteins on carbonyl end
Carboxypeptidase
Pancreatic lipase
This enzyme breaks a fat molecule into absorbable units - mono glycerine and 2 fatty acids per one fat molecule
Need bile (bile salts) for lipase to work well
Bicarbonate ion- in an alkaline solution
Secreted by the pancreas
Neutralizes acidic chyme from stomach- protects the duodenum from stomach acid
Also created a favorable pH for the enzymes to work (less acidic)
Regulation of pancreatic secretion
Controlled by two hormones 2 enterogastrones released from duodenum-secretin and cholecystokinin (CCK)
Secretin (released from duodenal mucosa)
Released in response to acid in the duodenum
Causes the pancreas (duct cells) to increase release of bicarbonate rich/alkaline solution
Neutralizes acid from the stomach
Cholecystokinin (CCK) (released from duodenal mucosa)
Released in response to fat (main stimulus) and protein products in the duodenum
Causes the pancreas (acinar cells) to release an enzyme rich secretion which digests the food molecules more
Bile
Secreted from duodenum
Made by the liver and stored and concentrated in the gallbladder
Bile contains: bile pigments (bilirubin mostly, from hemoglobins heme), cholesterol, bile salts (derivatives of cholesterol IMPORTANT) and lecithin (a phospholipid
How do bile salts assist in fat digestion
They are NOT enzymes
They act like detergent and keep fats broken up in small droplets (need them since fat isn’t water soluble and want to clump togeth)
What does it mean to emulsify fats
When break a large fat drop or glob into smaller droplets
Emulsion greatly increased the surface area for pancreatic lipase to act
Bile salts maintain the emulsion
The charges on bile salts means that emulsion droplets repel each other
What is the other function of bile salts
Bile salts help in fat absorption by forming micelles
Have inner core that is hydrophobic and outer surface that is hydrophilic
Products of fat digestion can dissolve in hydrophobic core of the micelles/ “ride” in core
Micelles transport products of fat digestion through water in lumen of small intestine to the mucosal lining of small intestine where they are absorbed
Stimulus of bile release
Secretion of bile by the liver is continual
Between meals bile is pushed to the gallbladder; the opening to duodenum is closed
During meal gallbladder contracts and empties into the duodenum
Signal for gallbladder contraction is CCK
CCK also causes relaxation of sphincter of oddi- around duct where it opens into the duodenum
After bile salts help with fat digestion and absorption what happens to them?
They are reabsorbed and returned to the liver and resecreted
Enterohepatic circulation- The recycling path between intestine and liver via the hepatic portal vein
What is secreted into the lumen of small intestine
Pancreatic enzymes, bicarbonate solution, and bile
Why is the small intestine composed of villi and microvilli
This increases the surface area for absorption
Nutrients are absorbed into the capillaries except————-?
Fats. They are absorbed in lacteal of the villus
What happens after the nutrients are absorbed into the capillaries?
Blood from intestines does not go directly into vena cava
First goes to liver for processing of nutrients via the hepatic portal vein
Portal system = capillaries —-hepatic portal vein——capillaries again (liver sinusoids)
Fats go into lymph system
Fats go into ——- system
Lymph
How does Na+ aid in absorption of nutrients
Many nutrients rely on active transport of Na+
Na+ is absorbed and substances are reabsorbed with Na+, then water lots follows
For simple sugars, amino acids it’s secondary active transport. They get free ride in powered by Na+ gradient
Most absorption occurs in the _____
Small intestine
Start carb in mouth, start protein in stomach, all the rest in small intestine
How is fat absorption different than other nutrients?
Absorbed into the lymph in small intestine (bile salts maintain lipid emulsion, pancreatic lipase hydrolyzes fats, are water insulator so carried by micelles to luminal mucosal surface of small intestine)
Passive diffusion of fatty acids and monoglycerides from micelles through intestinal mucosal cell membrane -since are lipid soluble
Once inside intestinal cell, fats are resynthesized then are packaged- fat molecules are enclosed or coated with lipoprotein (makes chylomicrons)
Exocytosis of chylomicrons occurs
Chyclomicrons cant enter blood capillary instead must go through lacteal/lymph vessel of villus
What are the fat soluble vitamins
A,D,E,K
CARRIED IN MICELLES
are absorbed with products of fat digestion
What are water soluble vitamins
B, C
Use diffusion or carrier mediated transport to enter mucosal cells
Exception is B12- needs to be bound with intrinsic factor (from the stomach) and is absorbed by endocytosis in the terminal ileum
Motility (movements) of small intestines
Need to mix chyme with secretions and expose chyme to the absorptive surface and after absorption need to move what’s left into the large intestine
Segmentation
Mixes chyme
Also slowly propels chyme
Divided into segments like sausages
Mixed chyme with enzymes and exposed chyme to absorptive surface/ mucosa
Small intestine also has _____________like the stomach
Basic electrical rhythm (BER)
Slow waves in small intestine that set the frequency for segmentation
Frequency higher in duodenum versus ileum
More activity above moves stuff down
Segmentation is affected by….
Gastric emptying- increased segmentation (with distention of duodenum)
Gastroileal reflex- chyme in the stomach causes gastrin release which increases segmentation in the ileum
Extrinsic nerves ANS- parasympathetic increases segmentation
Sympathetic decreases segmentation
What occurs after segmentation and absorption?
MMC occurs (migrating motility complex)
3 phases- in phase 3 peristaltic waves begin in stomach and move (migrate) down to the end of the ileum (then this repeats) (stomach growling)
Function- moves undigested material, also bacteria, into large intestine
Thought to be controlled by the hormone motilin from small intestine
What is the function of the large intestine
No enzymes are secreted (so function is NOT digestion)
Mucus is secreted
A.) absorption of water and electrolytes from chyme
B.) storage of fecal material- undigested material and bilirubin
Where is out bacterial flora located
The large intestine
Living bacteria produce vitamins including vitamin K
Vitamin K is needed for blood clotting
Bacteria break down substances we can’t (like carbs in beans) produce gas (from their metabolism which is expelled out the anus- called Claris or flatulence
Haustral contractions
Mixing movements of large intestine
Similar to segmentation but much less frequent
Slowly mixed the contents when a sac then becomes a contracted part
Also exposes what’s inside the inside to absorptive mucosa
Mass movements
Typically follow meal occurs with gastroileal reflex
Gastrin also increases colons activity- gastrin initiated mass movements
Distention of rectum with mass movement stimulates stretch receptors and leads to defecation reflex
Defecation is an ______________ reflex
Autonomic
- Internal anal sphincter is smooth muscle (involuntary)
Reflex causes it to relax
External anal sphincter is skeletal muscle (voluntary)
Can contract to delay defecation
(Potty training is learning how to control external anal sphincter)
How long does food stay in stomach
Few hours depending on fluidity and fat content
How long does food stay in small intestine
3-5 hours
How long does food stay in large intestine
More than 10 hours maybe even 24 hours or longer
Where is most alcohol absorbed
Small amount of alcohol absorbed in stomach.
Majority absorbed in small intestine.
