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