Obligatory Reabsorption and Secretion in PCT Flashcards
Describe the transport of urea
- filtered out in glomerulus
- 50-60% reabsorbed in PCT
- rest is recycled through more distal tubule segments
Describe the transport of lipid soluble substances
simple diffusion through membrane
Describe the transport of phosphate/sulphate
- sodium-linked transport in proximal tubule
- regulated by parathyroid hormone
Describe the transport of proteins/peptides
- almost all is reabsorbed and degraded in proximal tubule
- rest digested to amino acids within tubule and taken up by tubular epithelial cells
How much of each solute is obligatorily reabsorbed in the proximal tubule?
- glucose, amino acids, lactate = 100%
- water = 65%
- HCO3- = 90%
- Cl- = 50%
- K+ = 55%
What are the different pathways ions can travel to go across proximal tubule epithelium?
- transcellular (through cell body)
- paracellular (through leaky tight junctions between cell bodies)
What are the forces involved in the obligatory reabsorption from the proximal tubule?
- electrochemical gradient across apical and basolateral membranes
- sodium gradient across apical membrane
- early proximal tubule has transepithelial potential (-3mV)
- osmotic gradient from pumping sodium into interstitial space and glucose/amino acids/carboxylic acids moving down concentration gradient
- solvent drag
- chemical gradients for other solutes generated by chemical concentration of solutes left behind after water leaves
What is solvent drag?
when water moves along osmotic gradient along paracellular path and drags solutes along with it
How is the electrochemical gradient established?
- active transport through Ma/K-ATPase
- K+ efflux across basolateral membrane
- creates membrane potential
What is the sodium gradient used to power?
active uptake of filtered glucose, amino acids, phosphate, sulphate and carboxylic acids (electrogenic)
Describe the cells found in the proximal tubule (S1)
- columnar cells joined by leaky tight junctions
- brush border of microvilli on apical membrane
- infoldings on basolateral membrane
^ both increase surface area for absorption and secretion - asymmetrical distribution of proteins between apical and basolateral membrane
Describe the movement of solutes into and out of the cell at apical membrane
- sodium pump decreases intracellular Na
- Na/H exhanger uses sodium concentration gradient to transport H+ out of cell
- H+ binds with filtered bicarbonate to produce carbonic acid which breaks down into H20 and CO2
- both diffuse into cell and dissociate into H+ and bicarbonate
- H+ leaves cell into tubule lumen
- Ca2+ enter through Ca2+ channel
Describe the movement of solutes into and out of cell at basolateral membrane
- chloride, bicarbonate and potassium leave down concentration gradients
- Ca2+ is exchanged for Na+
- Ca2+ leaves cell against concentration gradient but driven by electrochemical gradient
How are additional solutes transported into and out of the cell?
Entry:
- at apical membrane: coupled to Na+ entry
- at basolateral membrane: facilitated diffusion
- their own transporters and channels
What is transport maximum?
- the point where increasing the concentration of a substance does not increase movement of it across the cell membrane
- measured in mg/min or mmol/min
When can Tmax be exceeded?
- in blood concentration is high
- eg. glucose in diabetes
What is renal threshold and what happens when a substance exceeds its threshold?
- threshold is the point a which the amount filtered is close to Tmax
- above it the substance appears in urine
- above it concentration of substance is proportional to plasma concentration
how is amount filtered calculated and what happens if any factor increases?
- plasma concentration (mg/ml) x GFR (ml/min)
- if either increases, Tmax may be exceeded as reabsorption cannot cope
What is clearance and its range?
- virtual volume of plasma entering the kidneys that has been totally cleared of the substance per unit time (ml/min)
(theoretical volume as can’t be completely cleared) - ranges from zero to renal plasma flow
- zero = fully reabsorbed/never filtered
- renal plasma flow: all of substance delivered to kidney in blood ends up in urine
What 3 renal processes determine and modify composition of urine?
- glomerular filtration
- tubular reabsorption
- tubular secretion
What are clearance ratios used for and explain what the different results can mean
- used to determine whether renal transport mechanism is net reabsorption or net secretion
- done by comparing proportions in urine and plasma to that of inulin
- if = 1: neither secreted or reabsorbed
- if >1: substance secreted
- if <1: substance reabsorbed
How do you calculate filtration fraction?
GFR/RPF
