Digestive System Flashcards
Digestion
Process by which the body extracts nutrition, allows macromolecules to enter the body by passing through the membranes lining the digestive tract Three main processes: 1. Macromolecule breakdown 2. Absorption 3. Excretion
What organizing principles can help to keep in mind when studying digestion?
Location, pH, and surface area
Each location of digestive tract has specific function
Environments of different locations have different pH values
Surface area is critical for system of exchange between external and internal environments of body
What are the main components of the digestive tract?
Mouth, esophagus, stomach, small intestine, large intestine, and anus
Entire digestive tract is considered to be outside of the body, open to environment at the mouth and anus
Nutrients don’t enter into the body until they pass through membrane of one of the digestive organs and absorbed into bloodstream
Ingestion
Process of taking in food through mouth
Digestion in the Mouth
- Ingestion of food into mouth
- Physical and chemical digestion
Physical: breakdown of food through chewing to increase surface area of food, enabling more enzymes to act on food
Chemical: alpha-amylase primarily initiates digestion which is contained in saliva
Cleaves alpha-1,4 glycosidic bonds
Chewed food forms a clump in the mouth called a bolus
Bolus pushed into esophagus by swallowing and then moved down esophagus
What is the major carbohydrate in the human diet?
Starch
Composed of many polymers of glucose and other sugar molecules with alpha-1,4 glycosidic bonds
Peristalsis
Contraction of the smooth muscle in the digestive tract
Creates a wave of motion that pushes along the partially digested food
Occurs in esophagus, small, and large intestines
What role does saliva play in digestion?
Contains alpha-amylase to start breaking down starch in the mouth (cleaving alpha-1,4 glycosidic bonds)
Also acts as lubrication for food, helping it move down the esophagus
How does food enter the stomach?
From the esophagus, through the esophageal sphincter (collection of smooth muscle that helps seal off the stomach from esophagus) and into the stomach for digestion
Stomach
Rounded, flexible pouch that serves to both mix and store food
Outer surface: smooth
Inner surface: has many folds that allow the stomach to expand as it fills
Includes the fundus, body, and pylorus
Fundus
Collects excess gas produced by digestion
Can expand to store food ingested during a large meal prior to digestion
Stomach body
Primary site of digestion
Pylorus
Lower part of the stomach that prevents the passage of undigested food into the small intestine
Sphincter
Ring of muscle that is normally contracted so that there is no opening at the center
How does the stomach perform physical digestion?
Composed of several layers of smooth muscle that powerfully churn the contents of the stomach and reduce food to a semi-fluid mass called chyme
What is the lining of the stomach called?
Mucosa
Gastric Juice
Combination of acid, enzymes, and hormones released by cells in lining of the stomach
Full stomach: pH of 2
Low pH of stomach denatures proteins and kills ingested bacteria
Gastric pits
Recesses in the stomach that contain exocrine glands (secrete molecules onto epithelial surfaces instead of the blood stream)
Use ducts to deliver secretions to specific locations in external environment (lumen of digestive tract)
Exocrine cells in lining of stomach maintain acidity of environment and aid in digestion
Exocrine Glands
Secrete molecules onto epithelial surfaces instead of the blood stream
Some produce stomach enzymes, sweat, saliva, milk, and earwax
Four major types of cells in the stomach
- Mucous cells
- Chief (peptic) cells
- Parietal (oxyntic) cells
- G cells
Mucous cells of the stomach
All perform same basic function of secreting mucus (some secrete pepsinogen)
Line stomach wall and necks of exocrine glands
Mucus: composed mainly of sticky glycoprotein and electrolytes
-functions to lubricate stomach wall so food can slide without causing damage
- protects epithelial lining of stomach from low pH
Chief cells
Located deep in exocrine glands in stomach
Secrete pepsinogen (zymogen precursor to pepsin)
- inactive precursor to enzyme that can be activated and become a functional enzyme
- Low pH activates pepsinogen to become pepsin
- Digests proteins
Parietal Cells
Found in exocrine glands of stomach
Secrete hydrochloric acid (HCl) into lumen of stomach through active transport (large energy usage)
Produces carbonic acid in cell from CO2, H+ expelled to lumen and bicarbonate ion expelled to interstitial fluid (lowers pH of stomach, raises pH of blood)
Also secrete intrinsic factor, helps ileum absorb vitamin B12
G cells
Secrete gastrin
- large peptide hormone that stimulates parietal cells to secrete HCl
Communicate with both outside (stomach lumen) and inside (bloodstream) of body
Activated through variety of methods: presence of polypeptides in stomach, stomach distension, input from parasympathetic nervous system through vagus nerve
What are the major hormones that affect secretion of stomach juices?
Acetylcholine: increases secretion of all cell types
Gastrin: mainly increases HCl secretion
Histamine: mainly increases HCl secretion
When and how does food leave the stomach?
Food leaves stomach through pyloric sphincter (to small intestine) once it is adequately broken down into chyme
Why is it important that pepsin is secreted as a zymogen?
The body is made out of proteins, so want to try and avoid digestion of self proteins
Pepsinogen is precursor that is inactive, so doesn’t harm the cells of the stomach as it is released
Becomes active in response to acidic environment of the stomach
Small Intestine
90% of digestion and absorption occur here
Small intestine is 3 meters in human
Three parts: duodenum, jejunum, and ileum
Most digestion in duodenum, most absorption in jejunum and ileum
Villi
Finger-like projections on the wall of the small intestine
Increase surface area of intestinal wall, allowing for greater digestion and absorption
Within each villus is a capillary network and a lymph vessel (lacteal)
Lacteal
Lymph vessel located in each villus of the small intestine
Responsible for absorbing fats
Microvilli
Smaller finger-like projections on apical (lumen side) surface of the cells of each villus
These cells are known as enterocytes
Increase surface area of intestinal wall further
Ample surface area is essential to facilitate absorption of nutrients through the wall of digestive tract and into body
Brush Border
Fuzzy covering on the villi of the small intestine formed by the microvilli
Contains membrane bound digestive enzymes, such as carbohydrate digesting enzymes (dextrinase, maltase, sucrase, and lactase), protein-digesting enzymes called peptidases, and nucleotide-digesting enzymes called nucleosidases
Goblet cells
Epithelial cells that secrete mucus to lubricate the intestine and help protect the brush border from mechanical and chemical damage
Goblet cells secrete a sticky mix of glycoproteins and electrolytes
How do nutrients get digested in the small intestine?
Breakdown of macromolecules into smaller pieces take place within the lumen of the small intestine
Pieces are broken down further within the brush border (micro-digestion)
Microvilli are adjacent to channels through which nutrients are transported into the body
Crypts of Lieherkuhn
Located deep between the villi are the intestinal exocrine glands
Glands secrete an intestinal juice with a pH of 7.6 and a lysozyme
Lysozyme contributes to regulation of bacteria within the intestines by weakening the bacterial cell wall
Enterocytes
Highly specialized cells which line the small intestine on the microvilli
Specialized for absorption
Not well suited to the secretion of digestive enzymes, which are secreted by the pancreas to continue to break down macromolecules and facilitate their absorption
Pancreas
Endocrine gland that secretes insulin and glucagon, regulators of carbohydrate and fat metabolism
Exocrine gland that creates enzymes which aid the digestive process in the small intestine
- Acinar cells release digestive enzymes into main pancreatic duct (carries enzymes to duodenum)
- Major enzymes are trypsin, chymotrypsin, pancreatic amylase, lipase, ribonuclease, and deoxyribonuclease (all released as zymogens)
What is the pH of the duodenum
Fluid inside duodenum has pH of 6 because hydrochloric acid from stomach is neutralized by bicarbonate ion secreted by the pancreas
Trypsin
Enzyme secreted by pancreas as a zymogen, which is activated by enzyme enterokinase (located in the brush border)
When trypsin is activated it then activates other enzymes
Degrades proteins into small polypeptides
Chymotrypsin
Enzyme that degrades proteins into small polypeptides
Released by the pancreas as a zymogen
Carboxypolypeptidase
Enzyme released by the pancreas
Cleaves amino acids from the sides of peptides which are broken down from polypeptides by trypsin and chymotrypsin
Polypeptides reach brush border as small polypeptides and then are broken down into single amino acids, dipeptides, or tripeptides before they are absorbed into enterocytes.
Enzymes in enterocytes reduce tri and di peptides to amino acids
Pancreatic amylase
Enzyme that hydrolyzes polysaccharides to disaccharides and trisaccharides
Similar to salivary amylase, but much more powerful
Degrades nearly all of carbohydrates from the chyme into oligosaccharides
Brush border enzymes finish degrading these polymers to respective monosaccharides before absorbed
Lipase
Enzyme that degrades fat, specifically triglycerides
Why is bile released into the duodenum?
Intestinal fluid is aqueous, so fat clumps together and is not soluble, meaning reduced surface area for digestion
Bile emulsifies the fat, breaking it up into small particles without changing it chemically, increasing surface area of fat so lipase can degrade into fatty acids and monoglycerides
Products are shuttled to brush border in bile micelles and then absorbed by enterocytes
Bile
Substance that is produced in the liver and stored in the gall bladder which emulsifies fat in small intestine
Released by gall bladder through cystic duct, emptying into common bile duct (shared with liver), emptying into the pancreatic duct before reaching duodenum
After bile transports fatty acids and monoglycerides to brush border, bile is reabsorbed by small intestine and transported back to liver
Where does chyme exit the small intestine and enter the large intestine?
Exits ileum through the cecum into ascending colon of large intestine
What are the five parts of the large intestine?
- Ascending Colon
- Transverse Colon
- Descending Colon
- Sigmoid Colon
- Rectum
What are the functions of the large intestine?
The large intestine is involved in excretion, absorption, and some digestion
Major functions are water absorption and electrolyte absorption
Dysfunction leads to diarrhea or constipation
What is the function of the bacterial flora in the large intestine?
Bacteria are in a symbiotic relationship within the human body
Live off of partially digested food within the digestive tract and produce vitamin K, B12, thiamin, and riboflavin that can be used by the body
What is the composition of water and solid materials in healthy feces?
75% water
Remaining solid mass is 30% dead bacteria, 10-20% fat (bacteria and sloughed enterocytes), 10-20% inorganic matter, 2-3% protein, and 30% roughage (i.e. cellulose) and undigested matter
Rectum
Storage for feces until waste is eliminated through the anus
What system are the hormones involved in digestion a part of?
Endocrine system (not exocrine) Circulate within the bloodstream, so communication between organs of digestive systems takes place within the body
In what order does hormone regulation usually occur for the digestive system?
Tends to function according to orderly chain of command
Brain stimulates stomach to begin digestive process, stomach signals small intestine, small intestine releases hormones that act on the pancreas
All these processes provide feedback to brain on how digestion is progressing in the body
Enteric Nervous System
Location where most of nervous input to digestive system takes place
Large network of neurons surrounding the digestive organs, helping to regulate processes such as smooth muscle contraction (peristalsis), fluid exchange, blood flow to digestive organs, and hormone release
Nervous input from the brain also helps start the production of hormones
How is the nervous system involved in digestion?
Enteric nervous system regulates processes like peristalsis, fluid exchange, blood flow, and hormone release
Nervous input from brain starts production of hormones
Parasympathetic input via vagus nerve helps prepare stomach and small intestine for digestion
- Causes stomach to produce gastrin in anticipation of digestion
Gastrin
Hormone which is released by G cells of stomach and stimulated by Acetylcholine release from the vagus nerve
Also stimualted when G cells sense that stomach is full
Target: stomach
Effects: Stimulates production of HCl
What signals the duodenum to start digestion?
The entrance of chyme into the duodenum
Both the message and the messenger telling duodenum to start digestion
What are some digestive hormones that are released from the duodenum?
Secretin, cholecystokinin, and Gastric inhibitory polypeptide
Local peptide hormones that are secreted by the small intestine after a meal
Each increases blood insulin levels, especially in presence of glucose
Secretin
Hormone secreted from duodenum
Acts as a messenger from small intestine to pancreas that indicates the presence of chyme and that pancreatic enzymes are needed
HCl in duodenum causes secretin release
Secretin stimulates release of sodium bicarbonate and enzymes in pancreas (control pH)
Gastric inhibitory polypeptide
Hormone released by duodenum in response to fat and protein digestates in duodenum, and to lesser extent in response to carbohydrates
Mild effect in decreasing motor activity of stomach
Cholecystokinin
AKA CCK
Hormone released by duodenum when food in upper duodenum, particularly fat digestates
Causes gallbladder contraction and pancreatic enzyme secretion
Decreases motor activity of stomach
Fats are more difficult to digest than carbohydrates, hydrophobic in aqueous env.
Decreased motility of stomach causes stomach to release chyme at slower pace, giving pancreatic enzymes more time to emulsify fats
What enzymes do the salivary glands release?
Salivary amylase: starch to maltose
What enzymes does the stomach release?
Pepsin as pepsinogen: proteins to peptides (autocatalysis)
What enzymes does the pancreas release?
Pancreatic amylase: starch to maltose
Lipase: fats to fatty acids and glycerol
Nuclease: nucleic acids to nucleotides
Trypsin: proteins to peptides, zymogen activation
Chymotrypsin: proteins to peptides
Carboxypeptidase: peptides to shorter peptides and amino acids
What enzymes does the small intestine release?
Aminopeptidase: peptides to shorter peptides and amino acids
Dipeptidase: Dipeptides to amino acids
Enterokinase: Trypsinogen to Trypsin
Nuclease: Nucleic acids to Nucleotides
Maltase: Maltose to glucose
Lactase: lactose to galactose and glucose
Sucrase: sucrose to fructose and glucose
What form of transmembrane transport drives a majority of macromolecule absorption?
Secondary active transport
Na+/K+ ATPase is important for maintaining sodium and potassium gradients that allow molecules like glucose to be transported into enterocytes
Movement of one ion, i.e. Na+, down concentration gradient is coupled with movement of a second ion, such as glucose up its concentration gradient
What guiding principles dictate what molecules can pass through the plasma membrane?
Size and polarity
Inside of plasma membrane is non-polar
Polar molecules must be broken down into smaller pieces
Non-polar molecules can pass through membrane even if they are large
What are the major carbohydrates in the human diet?
Sucrose, lactose, and starch
Cellulose- polysaccharide making up cell wall of plants cannot be digested by humans and is considered roughage
Disaccharides in human diet
Sucrose: made from glucose and fructose
Lactose: made from glucose and galactose
How is glucose absorbed into enterocytes?
Via secondary active transport down the concentration gradient of sodium
Sodium actively pumped out of the enterocyte on the basolateral side
Creates concentration gradient of high sodium outside, low sodium inside enterocyte
Sodium moves from intestinal lumen through transport protein into enterocyte, bringing glucose with it
Glucose then moves via facilitated transport to basolateral side of cell
Where does the conversion of glucose to fat take place?
Occurs in the liver and adipocytes and is stored in adipocytes
Happens with high glycogen and glucose stores
What does most dietary fat consist of?
Triglycerides
Broken down into monoglycerides and fatty acids in digestive process
How are fats carried to enterocyte membranes?
Carried by bile micelles
After delivering fats, micelles return to chyme to pick up more fat digestates
Micelles carry small amounts of hydrolyzed phospholipids and cholesterol, which diffuse through enterocyte membrane
Fat-soluble vitamins
Hydrophobic vitamins that are co-absorbed with fat
Fat absorption is required for proper uptake
Includes vitamin A, D, E, and K
How are fats modified to travel through aqueous environment of the cell once they have entered enterocytes?
Monoglycerides and fatty acids are converted back into triglycerides at the smooth endoplasmic reticulum
New synthesized triglycerides aggregate within smooth ER lumen w/ cholesterol and phospholipids (who orient charged ends toward aqueous solution of lumen, amphipathic)
Apoproteins attach to outside of globules, globules move to Golgi apparatus and are released from cell via exocytosis
Most globules, now chylomicrons, move into lacteals of lymph system (required due to due large diameter of chylomicron that would not travel well through capillaries)
Enters large veins of neck at thoracic duct after circling through lymphatic system
What types of fats can be absorbed directly into the blood?
More water soluble short chain fatty acids
Absorbed directly into the blood of the villi
When does the concentration of chylomicrons in the blood peak after a meal?
Concentration in blood peaks about 1-2 hours after a meal
Falls rapidly as fat digestates are absorbed into the cells of the body
Most significant absorption of fat occurs in liver and adipose tissue
What is the process of fat absorption
Fat is broken down into fatty acids and monoglycerides by lipase, and then carried to brush border in bile micelles
After diffusing into enterocytes, converted back to triglycerides at smooth ER
Apoproteins, pick up fatty acids and become chylomicrons, diffuse into lacteals, through lymphatic system, into thoracic duct and back into blood
At liver and adipose tissue, chylomicrons stick to side of capillary walls, broken down by lipoprotein lipase, releasing triglycerides, which diffuse into fat and liver cells
Triglycerides then reconstituted at smooth ER
First stop for digested fat is liver
How are fatty acids transported from adipose tissue?
Most fatty acids transported in the form of free fatty acids which combines with protein albumin in the blood
Albumin
Protein that combines with free fatty acids in the protein
Major osmoregulatory protein in the blood
Single albumin molecule typically carries 3 fatty acid molecules
Capable of carrying up to 30
What are the ways in which fats are carried in the blood?
Lipoproteins:
Different types: (1) very low-density lipoproteins, (2) intermediate-density lipoproteins, (3) low-density lipoproteins, (4) high-density lipoproteins, (5) chylomicrons (much larger)
95% of lipids in blood plasma are in the form of lipoproteins
Combined with protein albumin
Lipoproteins
Transport vesicles that carry lipids in blood plasma
Made from triglycerides, cholesterol, phospholipids, and protein
As density increases, relative amount of triglycerides decreases, and then relative amount of cholesterol and phospholipids decreases
Very low density have many triglycerides
High-density have very few triglycerides
Most lipoproteins are made in the liver
Very-low density lipoproteins
Lipoprotein made of triglycerides, cholesterol, phospholipids, and protein
Have many triglycerides
Transport triglycerides from liver to adipose tissue
Intermediate-density lipoproteins
Low-density lipoproteins
Lipoproteins made from triglycerides, cholesterol, phospholipids, and protein
Have less triglycerides than VLDLs
Transport cholesterol and phospholipids to the cells
Low-density lipoproteins seem to contribute to hardening of arteries
High-density lipoproteins
Lipoproteins made from triglycerides, cholesterol, phospholipids, and protein
Less triglycerides, less cholesterol and phospholipids to protein ratio
Function less well understood, but possibly protect against hardening of arteries
Liver
Upper right-hand quadrant of abdomen, adjacent to organs of digestive tract
Blood from intestines, stomach, spleen, and pancreas feeds into large hepatic portal vein
Liver functions as a screening mechanism for all of the nutrients, toxins, or other molecules that are absorbed from gut into bloodstream
Removes many ingested toxins from bloodstream
Hepatic portal vein
Large vein that carries all of the blood from the digestive system to the liver before entering systemic circulation
Second blood supply which oxygenates liver, is called the hepatic artery (smaller)
All blood received by the liver collects in the hepatic vein, leads to vena cava for exit
What are the categories of types of functions that the liver serves in the body?
Detoxification, Metabolic functions, storage functions, and immune functions
Metabolic functions of the liver
- Carbohydrate metabolism: maintains normal blood glucose levels through gluconeogenesis, glycogenesis, and release of glucose stores according to the needs of the body
- Fat metabolism: Liver synthesizes bile from cholesterol, converts carbohydrates and proteins into fat, oxidizes fatty acids for energy to form ketone bodies, forms most lipoproteins
- Protein metabolism: Liver deaminates amino acids, forms urea from ammonia in blood, synthesizes plasma proteins such as fibrinogen, prothrombin, albumin, most globulins, and synthesizes nonessential amino acids
- Detoxification: Detoxified chemicals are secreted by liver as part of bile, or modified so that they can be excreted by kidney
Storage functions of the liver
- Blood storage: liver can expand to act as a blood reservoir for body
- Glycogen storage: Liver stores large amounts of glycogen as energy reserve that can be used to regulate blood glucose levels
- Vitamin storage: Liver stores vitamins such as vitamins A, D, B12, and stores iron
Immune functions of the liver
Blood filtration: Kupffer cells phagocytize bacteria picked up from intestines
Erythrocyte destruction: Kupffer cells also destroy irregular erythrocytes, although most destroyed by spleen
Prothrombin and fibrinogen
Important clotting factors in blood that are produced by the liver
What two mechanisms is glucose absorbed in intestinal lumen?
- Secondary active transport down the concentration gradient of sodium
- At high concentrations, glucose raises osmotic pressure around cells, aqueous solution of lumen dragged into space through tight junctions, pulling glucose with it
How are galactose and fructose absorbed in intestinal lumen?
Galactose has similar absorption path to glucose with secondary active transport down sodium conc. Gradient
Fructose absorbed via facilitated diffusion and converted to glucose inside enterocyte
What is the destination of all carbohydrates absorbed in intestines?
Absorbed into bloodstream and carried by portal vein to liver
Where is glucose not transported from high concentration to low concentration through facilitated diffusion?
Enterocytes and cells of renal tubule
What cells are capable of producing and storing some glycogen?
All cells, however muscle cells and liver cells store large amounts
At saturation point of storage, carbohydrates are converted to fatty acids and then triglycerides in a process requiring small amount of energy (fatty acid synthesis)
Protein absorption
Process of breaking polypeptides into amino acids, dipeptides, and tripeptides
Then broken into amino acids in enterocytes
Absorption of products occurs via co-transport mechanism down gradient of sodium like glucose
A few transported via facilitated diffusion depending on polarity / size
Almost all peptides hydrolyzed to amino acids in enterocytes
Amino acids then directly absorbed into blood and taken up by all cells of body, especially liver
How are amino acids transported into cells of the body from the blood?
Never simple diffusion- amino acids are too large and polar
Facilitated or active diffusion
Amino acids upon transport are immediately synthesized into proteins, intracellular AA conc is low (preserve concentration gradient)
What happens to amino acids when cells reach upper limit for storage?
Amino acids can be burned for energy or converted into fat for storage
4 calories of energy can be gained per gram of protein
Carbohydrates produce 4.5 calories per gram
Fats produce 9 calories per gram
Gluconeogenesis can convert to glucose, produces ammonia as a byproduct
Almost all ammonia converted to urea by liver and then excreted in urine
