Lecture 3 : Role of the Tubules in Urine Formation Flashcards
Glomerular filtration
dumps cell-free and protein-free blood filtrate into the container (glomerular capsule or Bowman’s capsule)
The kidneys will process ____ of filtrate a day
180 L (47 gallons)
Tubular reabsorption
reclaims what the body needs to keep (from tubule to blood) – which is ALMOST everything
Tubular secretion
selectively adds to the waste container (from blood to tubule)
Anything remaining in kidney tubule at the end is EXCRETED as ____
urine
The ____ and the _____ are low pressure capillary beds adjacent to the renal tubule
peritubular capillaries
vasa recta
Peritubular Capillaries:
Efferent arteriole gives rise to peritubular capillary bed
Surround PCT and DCT in cortex - will reclaim (reabsorb) most of the solutes and water from the filtrate
Vasa recta:
Efferent arteriole gives rise to vasa recta – long straight capillaries
Follow nephron loop deep into medulla – maintain osmotic gradient that makes it possible to concentrate urine
Processes we need to understand in the renal tubule:
Filtrate to blood
Blood to filtrate
Countercurrent mechanisms
Role of blood flow in vasa recta of medulla
In PCT:
most reabsorption occurs here and is unregulated
maintains osmotic equilibrium
Nephron loop:
only water leaves in descending limb
only salt leaves in ascending limb
(Exceptions to what you think about osmosis)
In DCT and collecting duct:
hormones control reabsorption
Aldosterone:
Na+ reabsorption
ADH:
water reabsorption
The renal tubule has a single layer of ___
but each region has a unique histology
epithelium
The filtrate is modified through the processes of ___ and ____ which occur by transport mechanisms in the epithelial cells of the renal tubule.
reabsorption
secretion
Proximal tubule histology:
simple cuboidal epithelium with microvilli
Descending limb of nephron loop histology:
simple squamous epithelium
Distal tubule histology:
simple cuboidal epithelium with few microvilli
Thick ascending limb of nephron loop histology:
simple cuboidal epithelium with no microvilli
Proximal Tubule Cells are Specialized for ___
Transport
Microvilli
Increase surface area for absorption
Mitochondrion
Provide the energy needed for the active transport processes
The proximal convoluted tubule is the primary site for tubular reabsorption:
nearly ALL glucose, amino acids, and most electrolytes are reabsorbed in PCT, and about 65-70% of Na+ and water.
Mechanisms of Tubular Reabsorption:
Transcellular Route and Paracellular Route
Transcellular Route:
1) transport across the apical membrane
2) diffusion through the cytosol
3) transport across the basolateral membrane
4) movement through the interstitial fluid and into the capillary
Paracellular Route:
1) movement through leaky tight junctions, particularly in the proximal convoluted tubule
2) movement through the interstitial fluid and into the capillary
Transcellular transport mechanisms :
review of passive transport processes
1) simple diffusion = fat-soluble molecules directly through phospholipid bilayer
2) osmosis = solvent like water through a specific channel protein or lipid bilayer
3) carrier-mediated facilitated diffusion = via a protein carrier (they are specific for a chemical)
Transcellular transport mechanisms :
Primary and secondary active transport
primary active transport = ATP-driven Na+K- pump stores energy by creating a steep concentration gradient for a Na+ entry into the cell
secondary active transport = as Na+ diffuses back across the membrane through a membrane cotransporter protein, it drives glucose against its concentration gradient into the cell
The Na-K ATPase pump moves _____ out of the cell and ____ into the cell for each ATP molecule that is hydrolyzed; this establishes a concentration gradient for Na+.
3 Na+ ions
2 K+ ions
Other substances are co-transported with Na+ in secondary active transport :
glucose, amino acids, some ions, vitamins
Sodium Potassium Pumps are on the ____
basal surface of the cuboidal cells of the PCT
Secondary Active Transport Proteins are on the ____
apical surface of the cuboidal cells of the PCT
The sodium gradient provides a driving force to move other substances
Sodium and glucose cotransport
Sodium and amino acid cotransport
Sodium and hydrogen ion antiport
The majority of tubular reabsorption occurs in the _______
Proximal Convoluted Tubule (PCT)
sodium, glucose, amino acids, ions, urea
“Water follows solutes” =
“obligatory” water reabsorption by osmosis because aquaporins are always present in PCT cells
_______ are specific and limited
Transport proteins
Transport maximum
is # of transport proteins available in tubule cell membrane
If the amount of a substance in the filtrate exceeds the number of transport proteins that are available, the transporters are _____
saturated
When all the transport proteins are saturated for a given substance (ex. glucose), some of the substance _____
stays in the urine.
Diabetes mellitus = ______ if transport maximum exceeded, glucose is excreted in the urine; water stays with glucose so increased urine volume = _____
high blood glucose
diuresis
Proximal Convoluted Tubule is always in osmotic equilibrium (isosmotic) with surrounding interstitial fluid and blood in ____
peritubular capillaries
PCT CANNOT SEPARATELY ADJUST ___ AND WATER!
SALT
Osmolarity at the end of the Proximal Convoluted Tubule
Osmolarity of blood entering kidney = 300
Filtrate is isosmotic with plasma at end of PCT : solutes and water have moved together
Lost water on the way down so filtrate becomes more concentrated
Osmolarity of the filtrate changes as it moves through the ____
nephron loop
The kidneys have an osmotic gradient extending from the cortex through the depths of the _____
medulla
The Renal cortex is ____ with arterial blood (~300 mOsm)
isosmotic
An increasing osmotic gradient extends through the whole renal medulla creating the ______
medullary osmotic gradient
The nephron loop dips into a ___ environment
salty
the deeper it extends into the medulla, the saltier the interstitial space around the tubule
The thick and thin limbs of the ______ function differently
nephron loop
Simple squamous cells
of thin descending limb:
no significant active transport
Simple cuboidal cells
of thick ascending limb:
active transport
The Nephron Loop separates salt movement from _____
water movement
Cells in the thin _____ have aquaporins that allow water to leave tubule by osmosis
descending limb
Cells in the thick ______ have no aquaporins and are impermeable to water; they actively transport salts out of tubule into interstitial space
ascending limb
In PCT-
most reabsorption occurs here and is unregulated
Nephron loop -
only water leaves in descending limb
only salt leaves in ascending limb
______ creates an osmotic gradient through interactions of the two limbs of the nephron loop
COUNTERCURRENT MULTIPLIER
The nephron loop acts as a _____
countercurrent multiplier
Filtrate flows in the opposite direction in the two limbs of the nephron loop,
= ______
countercurrent
Tubule cells that line the descending limb of the nephron loop are (passively) permeable to water, but NOT to ____
salts
Tubule cells that line the ascending limb of the nephron loop actively pump salts out of the tubule lumen into the interstitium, but:
WATER CANNOT LEAVE the ascending limb because these cells have no aquaporins
The countercurrent multiplier depends on three properties of the nephron loop to establish the osmotic gradient:
1) filtrate flows in the opposite direction (countercurrent) through parallel sections of nephron loop
2) the descending limb is permeable to water, but not salt
3) the ascending limb is impermeable to water, but pumps out salts
The countercurrent exchanger ensure the medullary gradient doesn’t get ______
washed away
Vasa recta are very permeable to water and solutes:
So, blood within vasa recta remains nearly isosmotic with surrounding fluid, absorbing water and solutes without undoing osmotic gradient.
Osmolarity at the start of the distal convoluted tubule :
At start of DCT,
~20% of filtered water ~10% of filtered Na+
remains in filtrate
If we do no further adjustments to filtrate, we will make a finished urine that is more dilute than blood, and that carries wastes and some ions away
The remainder of the tubule is where we ___ absorption and secretion to “fine tune” filtrate to the needs of the body
regulate
At the DCT and Collecting Duct:
Make urine more dilute or more concentrated than blood
Change ion concentrations further
Adjust pH
