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MCAT Digestive and Excretory Systems

Terms in this set (122)

Which enzyme contained within saliva initiates the chemical breakdown of food?
alpha-amylase: catalyzes alpha-1,4 glycosidic linkages in starch

chewing increases surface area of food, which enables more enzymes to act
______ is a clump of chewed food in the mouth that gets pushed into esophagus by swallowing
bolus
Esophagus
A muscular tube that connects the mouth to the stomach.
peristalsis
Involuntary waves of muscle contraction that keep food moving along in one direction through the digestive system.
lower esophageal sphincter
collection of smooth muscle that helps to seal off the stomach from esophagus

sphincter: ring of muscle normally closed
stomach structure
fundus, body, pylorus
stomach structure: fundus
top portion

collects excess gas produced by digestion; can expand to store ingested food during a large meal prior to digestion
stomach structure: body
middle section

primary site of digestion in stomach
stomach structure: pylorus
bottom part of stomach

prevents passage of undigested food into small intestine
Which enzyme in the stomach catalyzes the breakdown of proteins?
pepsin
chyme
Partially digested, semiliquid food mixed with digestive enzymes and acids in the stomach.

semiliquid food is created from layers of smooth muscle that churn the contents of stomach
gastric juices
-maintain pH of 2 in stomach
-combination of acid, enzymes, and hormones from exocrine glands
-break down chyme
stomach exocrine glands
gastric pits secrete gastric juices to maintain pH and aid in digestion
4 major cells types in stomach:
Mucus/Goblet Cells
Parietal (oxyntic) Cells
Chief (peptic) Cells
G Cells
Mucousa/Goblet Cells
-line the stomach
-secrete mucus: sticky glycoprotein and electrolytes; is bicarbonate rich

*have a lot of rough ER f golgi to make mucus
Chief Cells
-gastric exocrine cells
-secrete pepsinogen, which is the zymogen of pepsin
-pepsinogen made on rough ER
-pepsinogen becomes pepsin by low pH conditions
pepsin
-released by chief cells of stomach
-enzyme that digests dietary proteins
parietal (oxyntic) cells
-gastric exocrine cells
-secrete HCl into stomach lumen through active transport
-bicarbonate expelled to interstitial fluid side
-also secretes instrinsic factor

result: lowers stomach pH, raises blood pH
intrinsic factor
secreted by parietal cells that helps the ileum absorb vitamin B12
G cells
-gastric exocrine cells
-secrete gastrin, which increases HCl secretion by parietal cells
-activated by input from parasympathetic nervous system through vagus nerve
Gastrin
-peptide hormone released by G cells
-stimulates parietal cells to secrete HCl (and gastric juices)
-[Gastrin] increases with more food in the stomach
what causes the stomach to release gastrin?
stomach expansion, presence of polypeptides in the stomach, and/or acetylcholine release from vagus nerve
how does food move from stomach to small intestine?
food is chyme and leave stomach through pyloric sphincter and into small intestine
What are the major hormones which stimulate gastric exocrine cells for increased secretion?
Acetylcholine: increased secretion of all cells

Gastrin and Histamine: mainly increase HCl secretion

*released from G cells in stomach
Small Intestine function
Digestive organ where most chemical digestion and absorption of food takes place (90%)
Small intestine structure
duodenum, jejunum, ileum
small intestine duodenum
-first and smallest section
-where most digestion occurs
-pH of 6 b/c HCl from stomach gets neutralized from HCO3- in pancreas
small intestine jejunum
-middle section
-has a lining designed to absorb carbs and proteins
small intestine ileum
-final section
-absorbs vitamin B12, bile salts, and products not absorbed by jejunum
enterocytes
-cells of intestinal wall lining
-specialized absorptive cells in the villi of the small intestine
-apical (lumen) side surface has even smaller projections (microvilli)
Villi
-Fingerlike extensions of the small intestinal wall
-increase the surface area for digestion and absorption
microvilli
-Fingerlike extensions of plasma membrane of apical epithelial cells
-increase surface area, aid in absorption
-membrane bound layer called brush border
lacteal
-within each villus
-capillary network and lymph vessles that absorb fats
brush border
-Name referring to the layer of microvilli which cover the enterocytes that comprise the villi of the intestine
-Contains various membrane bound digestive enzymes which break apart small macromolecules into their smallest constituent parts.
Crypts of Lieberkuhn
-exocrine glands within the small intestine's villi
-secrete a juice w/ pH 7.6 and lysozyme to weaken the bacterial cell wall
Where does most digestion occur?
duodenum of small intestine
Where does most absorption occur?
jejunum of small intestine
What is pancreatic juice made up of?
1) water
2) Bicarbonate
3) Enzymes: trypsin, amylase, lipases
how does the pancreas aid in digestion?
-secretes bicarbonate ions and other digestive enzymes into the duodenum
-the bicarbonate neutralizes the acidic gastric acid
how does the pancreas function as an endocrine gland?
secretes insulin and glucagon
how does the pancreas function as an exocrine gland?
creates enzymes that aid in digestion in small intestine
pancreas function
Assists in the digestion of foods by secreting enzymes that help to break down many foods
all pancreatic enzymes are released as ________
zymogens
acinar cells
-in pancreas
-release digestive enzymes into main pancreatic duct
pancreatic duct
carries enzymes released by acinar cells to duodenum
trypsin
-pancreatic enzyme
-activates other enzymes
-degrades proteins into small polypeptides (basic)
-zymogen: trypsinogen
enteropeptidase (enterokinase)
converts trypsinogen to trypsin
chymotrypsin
-pancreatic enzyme
-degrades proteins (aromatic)
pancreatic amylase
-pancreatic enzyme
-hydrolyzes polysaccharides in small intestine
-brush border finishes degrading these carbs before they're absorbed
lipase
-pancreatic enzyme
-degrades fats into fatty acids
what problem does bile solve?
-the intestinal fluid is aqueous, so fat clumps together and reduces the surface area for lipase
-bile emulsifies (breaks) the fat in the small intestine
-then the products are shuttled to brush border in bile micelles and are absorbed by enterocytes
bile
-fluid consisting of bile salts, bile pigments, cholesterol and lecithin
-is amphipathic
-produced by liver
-stored in gall bladder
-gall bladder releases bile through cystic duct, which empties into common bile duct shared with the liver
-common bile duct empties into pancreatic duct
-synthesized from cholesterol
Deoxyribonuclease and ribonuclease
-Nucleases released by pancreas
-Break phosphodiester bond between nucleotides of DNA and RNA
what happens once digestion and absorption occurs in small intestine?
chyme exitst ileum through cecum and into the ascending colon of the large intestine
cecum
portion of intestines which connects small and large intestines
bacteria flora
-in large intestine
-bacteria are in a symbiotic relationship with the human
-they produce vitamin K, B12, thiamin, riboflavin
large intestine function
-tube-like structure
-absorbs water from the undigested food from the small intestine
-functions: water and electrolyte absorption
large intestine structure
cecum, appendix, colon (ascending, transverse, descending, sigmoid), rectum, anus
chain of communication between brain and digestive system
brain --> stomach --> small intestine --> pancreas
enteric nervous system
-subdivision of ANS
-directly controls GI system and operates independently of brain and spinal cord
-embedded in lining of GI system as a large network of neurons
what prepares the stomach for digestion?
parasympathetic input from vagus nerve; causes stomach to release gastrin
secretin
-hormone secreted by small intestine
-Trigger: released in response to HCl in chyme
-Function: stimulates secretion of sodium bicarbonate and enzymes in pancreas (pancreatic juices)
Gastric inhibitory polypeptide
-hormone secreted by small intestine
-Trigger: released in response to fat and proteins in chyme
-Function: stimulates Insulin release by pancreas, decreases motor activity of stomach
Cholecystokinin (CCK)
-hormone secreted by small intestine (duodenum wall)
-Trigger: released in response to fat in chyme
-Function: stimulates enzyme secretion in pancreas (pancreatic juices) and decreases motor activity of stomach
-also causes gallbladder contraction
How is glucose absorbed?
-Secondary active transport by co-transport with Na+
-glucose gets brought into enterocyte and then into portal vein that goes to liver
where do absorbed monosaccharides go from the intestinal epithelia?
monosacchardies are transported to the liver via the haptic portal vein
what are the main sources of dietary carbs?
sucrose, lactose, starch
How are proteins digested and absorbed?
-They are broken into various mono, di, and tri peptides by digestive enzymes (Pepsin)
-transported into the enterocyte via sodium symporter or facilitated diffusion
-broken into single amino acids by intracellular proteases,
-moved into the local capillaries/ go to liver
How are lipids digested and absorbed?
-Fat aggregates are broken apart via lipase into FA and Glycerol
-transported into enterocytes via micelles
-converted back into triglycerides by the smooth ER
-removed via exocytosis as chylomicrons into lymphatic vessels and eventually the thoracic duct.
Chylomicrons
the class of lipoproteins that transport lipids from the intestinal cells to the rest of the body
Peaks at 1-2 hrs after meal
albumin
plasma protein which carries free fatty acids throughout blood
also transports drugs and other molecules
lipoproteins
clusters of lipids associated with proteins that serve as transport vehicles for lipids in the lymph and blood
very low density lipoproteins (VLDL)
lipoproteins that transport triglycerides and other lipids from the liver to various tissues in the body (bad)
High-density lipoprotein (HDL)
lipoproteins that carry fat and cholesterol away from the tissues back to the liver for breakdown and removal from the body (good)
liver
has metabolic, storage, and immune functions
liver hepatic portal vein
-carries nutrient-rich blood from digestive system to liver, so it can be processed and recirculated
-connects capillary beds of intestines with capillary beds of liver for amino acids and glucose to be absorbed
liver hepatic artery
-supplies liver with oxygenated blood
liver hepatic vein
-carries deoxygenated blood from the liver back to the right atrium of the heart via the inferior vena cava
hepatocytes
cells that make up the liver; contain a lot of rough ER and free ribosomes
metabolic functions of liver
-carb metabolism: maintain blood glucose through gluconeogenesis and glycogenesis
-fat metabolism: beta-oxidation, ketosis
-protein metabolism: amino acid deamination (forms urea)
-detoxification: secretes detoxified chemicals for secretion from kidney
storage functions of liver
-blood storage: can expand to act as a blood reservoir
-glycogen storage
-vitamin storage
immune functions of liver
-blood filtration: kupffer cells phagocytize bacteria picked up from intestines
-erythrocyte destruction: kupffer cells destroy irregular erythrocytes that aren't destroyed by spleen
kupffer cells
liver macrophages which phagocytize bacteria and irregular erythrocytes

Kupffer = Killer
kidney functions
-maintain homeostasis of body fluid volume; blood pressure
-maintain homeostasis of plasma solute composition; control pH
-excrete waste products
what waste products are secreted by kidney?
urea, uric acid, ammonia, phosphate
3 major regions of kidney
-outer cortex: tubules
-inner medulla: loop of henle
-renal pelvis
renal pelvis
-site of entry and exit of renal vein and renal artery
-where urine is emptied
renal vein
carries oxygen-depleted blood away from kidney to heart
renal artery
carries oxygenated blood to kidney
nephron
functional unit of the kidney
glomerulus
-network of capillaries
-recieves blood from afferent arteriole of renal cirulattion
-fluid and solutes are filtered OUT of blood and into Bowman's capsule
Bowman's capsule
-cup-shaped chamber that surrounds glomerulus
-where filtration takes place
hydrostatic pressure
pressure of blood against the walls

promotes filtration because it pushes water and solutes in blood plasma through the glomeral filter
fenestrations
-pores of glomerular endothelium
-hydrostatic pressure forces some plasma through fenestrations and into Bowman
What goes into the filtrate in Bowman's capsule? What remains in blood?
Blood: RBCs, large proteins
Filtrate: Chloride, AAs, Sodium, Water, Glucose, Urea, HCO3-, H+
how does afferent arteriole constriction affect blood flow, hydrostatic pressure, and filtration?
decreases blood blow, decreases hydrostatic pressure, less filtration
how does efferent arteriole constriction affect blood blow, hydrostatic pressure, and filtration?
increases hydrostatic pressure by preventing blood from exiting as quickly, increases filtration
renal tubule
long structure from glomerulus
proximal convoluted tubule
-secretion and absorption
Secretion:
-drugs, toxins, uric acid, bile pigments, other solutes are secreted into filtrate
-H+ secreted through an antiport with Na

Reabsorption:
-Secondary active transport: glucose, NaCl
-Passive transport: HCO3-, K+, H2O
how is glucose reabsorbed in proximal tubule?
sodium/glucose cortansporter
transport maxium
concentration of solute that saturates its transport proteins; at this point, any solute not reabsorbed will end up in the urine
Loop of Henle
increases solute concentration and osmotic pressure of medulla
Descending Loop
-low salt permeability, HIGH water permeability
-as filtrate goes down, water passively diffuses OUT, into medulla
-filtrate osmolarity increases
-has aquaporins
Ascending Loop
-impermeable to water, permeable to salts/ions
-solutes initially diffuse passively, then actively
-increases solute concentration in medulla
vasa recta
The capillaries that surround the tubules of the nephron.

The vasa recta reclaims reabsorbed substances, such as water and sodium ions.
counter current flow
Refers to the loop of Henle, and how the ascending and descending segments flow in separate directions despite being close and parallel.
The vasa recta also has counter current flow but in the opposite direction as the loop of henle.

Interaction between the flow of filtrate through the loop of Henle (countercurrent multiplier) and the flow of blood through the vasa recta blood vessels (countercurrent exchanger)
single effect
active transport of solute by pumps in wall of thick ascending limb creates a concentration gradient

net effect: dilute filtrate in ascending limb; concentrate in medulla; concentrate filtrate in descending limb
counter-current multiplier mechansim
dynamic movement of fluid and solutes

pump slat, equilibriate water, shift filtrate
osmotic gradient
distal convoluted tubule
-receives filtrate from loop of henle
-Reabsorbs: Ca, Na,
-Secretes: K, H, HCO3-

net effect: lower filtrate osmolarity
aldosterone
-hormone released by adrenal cortex
-acts on distal tubule
-increases reabsorption of Na and secretion of K by creating more membrane transport proteins
ADH
-released by posterior pituitary
-acts on collecting ducts of kidney
-promotes water reabsorption
collecting tubule
-segment at end of distal tubule
-responds to ADH
-causes cells to be more permeable to water, so water flows OUT of tubule and into kidney
-makes filtrate more concentrated
collecting duct
-recieves filtrate from distal tuble
-impermable to water, but sensitive to ADH, so water diffuses into medulla

the very concentrated medulla causes water to leave the filtrate, which forms very concentrated urine
urine excretion
urine goes from collecting duct to renal calyx, which empties into renal pelvis, which drains into ureter and then bladder
juxtaglomerular apparatus
-monitors filtrate pressure in distal tubule
-complex of cells from afferent arteriole and distal tubule that regulates blood pressure by secreting renin in response to blood pressure changes
renin-angiotensin-aldosterone system (RAAS)
regulates blood pressure and fluid balance

compensatory process that leads to increased blood pressure and blood volume to ensure perfusion of the kidneys; important in the continual regulation of blood pressure
renin
-hormone secreted by kidney
-initiates a cascade that produces angiotensin I from zymoen angiotensin
angiotensin I
travels to lung through blood; gets converted to angiotensin II
angiotensin II
causes vessels to constrict to increase blood pressure; causes adrenal cortex to secrete aldosterone
what happens when Na is reabsorbed?
water follows

increases blood volume
the vagus nerve in the main nerve in the __________ nervous system. what does the vagus nerve control?
parasympathetic nervous system
body functions: digestion, heart rate, immune system
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