Tubular Functions Flashcards
What substance is 100% reabsorbed into the blood?
glucose
What are the different ways that different substances can be reabsorbed?
- active transport
- passive diffusion
- facilitated diffusion
- osmosis
- transcellular and paracellular pathways
Why does active transport require energy?
it involves moving against a concentration gradient
What is osmosis?
the force that drives water transport; it moves to where there is a high concentration of substances down a concentration gradient
What is the transport maximum?
the limit to the rate at which a solute can be transported
When will glucose be excreted in the urine?
when the filtered load exceeds the capability of the tubules to reabsorb glucose e.g. in uncontrolled diabetes cases
What is renal threshold?
the plasma concentration of a substance at which it starts to be excreted in urine
What is the ideal and actual renal threshold of glucose respectively?
- ideal = 300mg/dL
- actual = 180mg/dL
Why can glucose appear in the urine before the transport maximum is reached?
not all nephrons have the same transport maximum for glucose, and some of the nephrons begin to excrete glucose before others have reached their transport maximum
What percentage of water and electrolytes is reabsorbed at the proximal tubules?
> 60%
What does the proximal tubule reabsorb?
- 65% Na+, Cl- and Ca2+
- 90% HCO3
- almost all glucose and AA
What are the 3 segments of the loop of Henle and what is their permeability?
- thin descending segment – highly permeable to water and moderately permeable to solutes
- thin ascending segment – impermeable to water
- thick ascending segment – impermeable to water
What does the thin descending loop of Henle allow?
simple diffusion of substances
Where is the NKCC2?
in the luminal (apical) membrane of the thick ascending limb of the loop of Henle
What does the NKCC2 do?
transport the 3 ions from the tubular lumen into the epithelial cells
Where is the Na+/K+ATPase?
in the basolateral membrane of the thick ascending limb of the loop of Henle
What does the Na+/K+ATPase do?
maintain a low intracellular sodium concentration
Why is the early distal tubule called the diluting segment?
it reabosrbs most of the ion and is impermeable to water
What are the 2 important cell types involved in tubular transport?
- principal (P) cells – potassium secretion mediated by aldosterone
- intercalated (A) cells – H+ secretion and potassium/HCO3- reabsorption
What is the reabsorption of water from the late distal tubule controlled by?
ADH
How much filtered water do the medullary collecting ducts reabsorb?
less than 5%
What 6 factors regulate tubular reabsorption?
- glomerulotublar balance
- hormonal factors
- aldosterone
- angiotensin II
- ADH
- ANP
What does an increase in GFR do?
increase solute reabsorption in the proximal tubule and thick ascending limb which prevents overloading of the distal tubule
What does glomerulotubular balance and tubuloglomerular feedback act together to do?
buffer the effects of spontaneous changes in the GFR on urine output
What is reabsorption of sodium and chloride coupled to?
movement of H +, glucose, amino acids, organic acids, phosphate and other substances across tubular wall, either by cotransport or exchange
Where is glucose reabsorbed?
in the proximal part of the PCT with sodium
What do SGLTs and GLUTs do respectively?
- SGLT = filters glucose into cell
- GLUT = reabsorb glucose back into blood
What are the 3 steps of glucose transport in the PCT?
- glucose and sodium bind to SGLT2 in the apical membrane
- glucose is carried into the cell as sodium moves along the electrochemical gradient
- sodium is pumped out of the cell and glucose exits by facilitated diffusion via GLUT2
How do amino acids enter and exit tubular cells respectively?
- enter with sodium cotransport
- leave basolateral membrane via diffusion
What can high and low potassium cause respectively?
- high = fatal cardiac arrhythmias
- low = cardiac arrhythmias and muscle weakness
What is the response to low potassium levels?
- increased activity of type A intercalated cells
- increased hydrogen ion secretion and K reabsorption
What is the response to high potassium levels?
- increased aldosterone
- increased ENaC in apical membrane and Na+/K+ATPase in basolateral membrane
- increased incorporation of K secreting channels into luminal channels
What is calcium excretion regulated by?
PTH
Where is calcium reabsorbed?
at the thick ascending limb and DCT
What does increased calcium reabsorption at the renal tubules lead to?
reduced calcium excretion which leads to increased serum calcium
What are the effects of increased active vitamin D?
- increased intestinal absorption of calcium and phosphate
- increased calcium reabsorption at renal tubules
When is urea increased?
- renal failure
- dehydration
- GI bleeding
- high protein diet
What can urea accumulation cause?
nausea, vomiting and drowsiness
What is urea important for?
maintaining the solute concentration gradient for the counter-current mechanisms
How does water transport mostly occur at the proximal tubule?
through water channels (aquaporin -1) in the cell membranes, as well as through the tight junctions between the epithelial cells
What is the osmolality of the fluid leaving the proximal tubule?
iso-osmolar to plasma
How does water transport mostly occur at the descending limb of the loop of Henle?
through aquaporin-1 channels
What happens to the osmolality of the fluid flowing through the descending limb of the loop of Henle?
it increases since the limb is highly permeable to water but impermeable to sodium chloride and urea
What happens to the tubular fluid at the thin ascending limb of the loop of Henle?
it becomes more dilute
What happens to the tubular fluid at the thick ascending limb of the loop of Henle?
it becomes very dilute, falling to a concentration of ~140 mOsm/L
What happens to the tubular fluid at the early distal tubule?
it is further diluted to ~100mOsm/L
What does the osmolality of the tubular fluid in the late distal tubule and cortical collecting tubules depend on?
the level of ADH
What does the osmolality of the tubular fluid in the collecting duct depend on?
ADH and the surrounding medullary interstitial osmolarity established by the countercurrent mechanism
What is osmolality?
the number of osmoles per kilogram of solvent determined by the number of solute molecules in a given volume
What is plasma and urine osmolality respectively?
values
- plasma = 280-295 mOsm/kg
- urine = 50-1400 mOsm/kg
What is specific gravity?
a measure of the weight of solutes in a given volume determined by the number and size of the solute molecules (g/ml)
What is the relationship between urine concentration and urine specific gravity?
the more concentrated the urine, the higher the urine specific gravity
Why does urine osmolality vary?
to maintain ECF osmolality
When will urine be more dilute and concentrated respectively?
- dilute = excess body water
- concentrated = water deficit
Why does osmolality remain the same throughout the PCT?
both solutes and water and being reabsorbed
Where is urine the most concentrated?
at the bottom of the loop of Henle
Why does osmolality increase in the descending limb of the loop of Henle?
water is removed and solutes become more concentrated
Why does osmolality decrease in the ascending limb of the loop of Henle?
it is impermeable to water and so there is only solute reabsorption
What does ADH do to the distal convoluted tubule and collecting ducts?
stimulate water reabsorption and production of concentrated urine
What is a countercurrent system?
a system in which the inflow runs parallel to, counter to and in close proximity to the outflow for some distance
What is countercurrent multiplication?
the process of using energy to generate an osmotic gradient that enables you to reabsorb water from the tubular fluid and produce concentrated urine
What is the countercurrent multiplier in the kidneys?
the loop of Henle
What is the countercurrent exchanger in the kidneys?
vasa recta
What are the vasa recta?
the peritubular capillary networks that supply blood to the medulla
What happens as the blood vessel dips into the medulla?
water leaves and then enters again as it goes back into the cortex
What happens as the loop of Henle dips into the medulla?
water leaves and solutes become more concentrated
