K4 - The Proximal Tubule and Loop of Henle Flashcards
how much of the plasma that enters the glomerulus is filtered?
20%
what is glomerular filtrate?
modified filtrate of the blood containing ions and solutes at plasma concentrations but lacks RBCs, large plasma proteins
how much of the filtered fluid is reabsorbed in the proximal tubule?
80ml/min
what is the relationship between the the fluid reabsorbed in te proximal tubule and the filtrate?
iso-osmotic - same osmolarity
what does the proximal tubule reabsorb?
- Sugars
- Amino acids
- Phosphate
- Sulphate
- Lactate
what is secreted in the proximal tubule?
- H+
- Hippurates
- Neurotransmitters
- Bile pigments
- Uric acid
- Drugs
- Toxins
what is trasncellular reabsorption?
reabsorption across cell layer
what is paracellular reabsorption?
reabsorption through tight junctions between epithelial cells
what is primary active transport?
Energy is directly required to operate the carrier and move the substrate against its concentration gradient
what is secondary active transport?
The carrier molecule is transported coupled to the concentration gradient of an ion (usually Na+)
what is facilitated diffusion?
Passive carrier-mediated transport of a substance down its concentration gradient
where is the Na+/K+ ATPase found?
basolateral membarne
why is the Na+/K+ ATPase at the basolateral membrane important?
it is essential for Na+ reabsorption into the peritubular capillary
how is O2 and CO2 transported across the membrane?
Diffusion through the lipid bilayer
how is Na+ trasnported across membrane?
Diffusion through channels
How is glucose transported across the epithelial cell?
Facilitated diffusion
How is Na+ and K+ transported across the epithelial cell?
Primary active transport
How is Na+ and glucose transported across the epithelial cell?
Secondary active transport
Isoosmotic fluid reabsorption across “leaky” proximal tubule epithelium due to?
(1) Standing Osmotic Gradient
(2) Oncotic Pressure Gradient
how does Na+ move across the tubular epithelium to be reabsorbed?
- Na enters the cell via a symport with glucose, amino acids and a countertarasnporter with H+.
- Na then moves across the tubular cell to the basolateral membrane
- Exits the cell to be reabsorbed into the blood via the Na/K ATPase
How is glucose and amino acids reabsorbed?
move across luminal membrane with sodium in symport
glucose then leaves the cell via facilitated diffusion
how is water reabosrbed across tubular epithelium?
Osmotic gradient between Na and Cl will be set up.
water moves down this gradient via paracellular transport
absorbed into peritubular capillary by osmosis
what is the transport maximum?
max amount of substrate that can be reabsorbed before the transporter proteins become saturated
what is the renal threshold?
concentration of a substance dissolved in the blood above which the kidneys begin to remove it into the urine
why would glucose not be absorbed and when would this happen?
- when transport maximum has been reached
- then renal threshold has been reached (around 10-12 mmol/l
why does the osmolarty of tubular fluid not change along the proximal tubule
salt and water are absorbed in equal amounts
what % of salt and water are reabsorbed in the proximal tubule?
67%
what % of glucose is reabsorbed in the proximal tubule?
100%
What is tubular fluid when it leaves the proximal tubule?
iso-osmotic
what is the function of the loop of henle?
- Generates a cortico-medullary solute concentration gradient
which enables the formation of hypertonic urine
what is the Opposing flow in the two limbs of the loop of henle?
countercurrent flow
what establish a hyper-osmotic medullary interstitial fluid?
loop of henle and the vasa recta
Describe what happens on the ascending limb of the loop of henle
- along the entire length of the ascending limb Na+ and Cl- are being reabsorbed (thick upper AL is achieved by active transport, in the thin lower AL this is passive)
- the ascending limb is relatively impermeable to water
- little or no water follows salt reabsorption
Describe what happens on the decending limb of the loop of henle
does not reabsorb NaCl and is highly permeable to water
describe the process of the triple co-transporter?
- Na, Cl and K ions all pas through a triple transporter on the luminal end
- Na and Cl are transported though the basolateral membrane where they are absorbed into the interstitial fluid as aCl
- K ions are recycled
Describe the 5 stages that occur in the loop of henle
- Solute removed from lumen of ascending limb (water cannot follow)
- Tubular fluid is diluted and osmolality of interstitial fluid is raised
- Interstitial solute cannot enter the descending limb
- Water leaves the descending limb by osmosis
- Fluid in the descending limb is concentrated
what type of fluid leaves the proximal tubule?
iso-osmotic
what type of fluid enter sthe distal tubule?
hypo-osmotic
what is the purpose of countercurrent multiplication?
To concentrate the medullary interstitial fluid to enable the kidney to produce urine of different volume and concentration according to the amounts of circulating antidiuretic hormone
What runs alongside the long loop of Henle of juxtamedullary nephrons and acts as a countercurrent exchanger?
vasa recta
What features of the vasa recta reduce NaCl and urea being washed away?
- vasa recta capillaries follow hairpin loops
- vasa recta capillaries are freely permeable to NaCl and water
- blood flow to vasa recta is low
what happens to blood osmolarity as it dips down into the medulla?
rises
what happens to blood osmolarity as it rises up into the cortex?
falls
