Renal 4 Flashcards
Tubular Reabsorption (3)
High capacity & variable; very selective Many electrolytes and nutrients almost completely reabsorbed Most waste products poorly reabsorbed.
Tubular Secretion (2)
Variable
Important for certain
electrolytes (K+, H+), drugs,
toxins.
Once fluid is reabsorbed across the tubule epithelium
into the interstitial fluid, it enters
peritubular capillaries
via Bulk Flow
Reabsorption =
Kf (Net Reabsorptive force)
= Kf (Reabsorptive Force)-(Filtration force)
=Kf ((πC + Pif) – (PC + πif))
Normal rate of Peritubular Capillary Reabsorption =
124 ml/min (GFR?)
Normal rate of Peritubular Capillary Reabsorption = 124 ml/min (GFR?)
Large: (2)
Large Kf (high surface area and permeability)
Large πC (due to volume lost during filtration)
> —% of filtered sodium
reabsorbed
99
> 99% of filtered sodium
reabsorbed
All along —
nephron
Sodium Reabsorption can
occur via both (2)
Transcellular
and Paracellular
Transcellular pathways (2)
- Passive across apical and active across basolateral membranes - Apical Na+ transport may be secondary active and tied to reabsorption of other substance (ex. Glucose)
Transport maximum
Maximum rate some substances
can be transported across the
epithelium (absorption or
secretion)
transport maximum is due to
saturation of transport
proteins
Renal Threshold is the
plasma
concentration that saturates the
carrier (tubular load)
Once the transport maximum is
reached for all nephrons,
further
increases in tubular load are not
reabsorbed and are excreted.
examples of transport max (6)
glucose, amino acids,
phosphate, sulfate, urate,
(creatinine, PAH)
As [Glucose] plasma increases, the [Glucose] filtrate —, Glucose tubular load —.
increases
increases
Reabsorption of Water: Passive But Influences Many Solutes
Strictly —
passive
As Na+ reabsorbed, creates
gradient for — reabsorption via
osmosis
H2O
Proximal tubule highly permeable
to —
H2O
As H2O moves via osmosis: (2)
- Carries other solvents along = Solvent drag (size restricted) - Creates gradients for the passive reabsorption of other solute (ex. Cl- and Urea)
H2O Permeability varies as move through nephron (2)
- Ex. Ascending limb of Loop of
Henle (not permeable to H2O - Collecting duct (regulated; ADH)
PT Reabsorption of Na+, Cl-, Urea
and H2O exhibit
Gradient-Time
Transport
The concentrations of solutes in different parts of the tubule depend on relative reabsorption of the solutes compared to water: (2)
if water is reabsorbed to a greater extent than the solute, the solute will become more concentrated
in the tubule (e.g., creatinine, inulin)
if solute is reabsorbed to a greater extent than
water, the solute will become less concentrated in the tubule (e.g., glucose, amino acids)
Regional Tubular Transport (5)
- Early Proximal Tubule (early PT or PCT) and
Late Proximal Tubule (late PT or PST) - Loop of Henle
- Early Distal Tubule (early DT)
- Late Distal Tubule and Cortical Collecting
Duct (Late DT and CCD) - Medullary Collecting Duct (MCD)
Loop of Henle (3)
Thin descending limb
Thin ascending limb
Thick ascending limb
Proximal Tubule
PT =
PCT + PST
Cells have High capacity for Active
and Passive Reabsorption (4)
- Large # mitochondria
- Large surface area on both apical
and basolateral membranes - Large number of membrane
proteins - High permeability for H2O.
PT Reabsorbs (2)
– ~ 65% of filtered Na+, Cl-, HCO3-,
and K+
– All filtered glucose and amino
acids.
PT Secretes (3)
- metabolic waste products such as H+ ions ,
organic acids, and bases such as: bile salts,
oxalate, urate, and catecholamines - Harmful drugs or toxins
- Para-Aminohippuric Acid (PAH)
Early PT (3)
Paracellulary
Passive down ΔEC
Solvent drag
Late PT (4)
Transcellular
Apical 2° active Formate/Cl- antiporter
Basolateral facilitated diffusion
Paracellularly
*SGLT2 inhibitors used in
treatment of
T2DM (ex.
Glifozins; Invokana).
Δ in [Solute] as Filtrate Passes through Proximal Tubule Tx = [solute] in filtrate Px = [solute] in plasma (4)
Na+ and H2O - Isotonic Solution Reabsorbed Proximal portions reabsorb all Glucose, Amino Acids and 65% HCO3- Distal Portion reabsorbs more Cl- and urea Creatinine not actively reabsorbed
- Thin Descending Limb (tDL) (3)
High H2O permeability (Contain AQP-1 channels): major role in concentration/dilution of urine No active solute transport Urea secretion via facilitated diffusion
- Thin Ascending limb (tAL) (3)
does not reabsorb significant amounts of any
solutes
impermeable to water
Urea secretion via facilitated diffusion
- Thick Ascending Limb (TAL) (4)
Impermeable to H2O and Urea
Major site of Na+, K+, Cl- reabsorption, H+ secretion
By end of Loop of Henle, more solute reabsorbed
than H2O
Distal End of TAL forms part of JG apparatus
Early Distal Tubule (6)
Parallels TAL in that it absorbs solute w/out H2O Impermeable to H2O (and urea). Reabsorbs ions Na+-Cl- co-transporter in apical membrane moves Na+ and Cl- into cells from tubule lumen Na+-K+-ATPase transports Na+ into interstitium. Cl- diffuses into the interstitium through channels in basolateral membrane.
Reabsorbs ions (3)
- 5% of Na+ & Cl-
– Further dilutes filtrate
– Diluting Segment
Late Distal/Cortical Collecting Duct (CCD)
Two Cell Types
- Principal Cells
2. Intercalated Cells
- Principal Cells (4)
60-70% of cells Site of Aldosterone and ADH action –Reabsorb Na+ and H2O –Secrete K+
Late Distal/Cortical Collecting Duct (CCD) These segments are completely impermeable to ---
Urea
2. Intercalated Cells cell types (2)
Alpha Intercalated Cells
Beta Intercalated Cells
Alpha Intercalated Cells (4)
Function important during acidosis Secrete H+ Reabsorb K+ and HCO3- 1° Active Transport of H+ across apical membrane can secrete H+ against a great ΔC (1000:1)
Beta Intercalated Cells (3)
Function important during alkalosis
Secrete K+ and HCO3-
Reabsorb H+
Medullary Collecting Duct (MCD) (4)
Processes < 10% of Na+ and H2O but very important (10% of 180 L). Site of Aldosterone and ADH action Urea reabsorbed via facilitated diffusion Secretes H+ using same mechanism as α-intercalated cells.
Regional Tubular Transport
Tx/Px) (5
1. Proximal Convoluted Tubules (PCT) 2. Loop of Henle 3. Early Distal Convoluted Tubules (DCT) 4. Late Distal Tubule and Cortical Collecting Duct 5. Medullary Collecting Duct (CT)
