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MBBS lecture by Dr Raghuveer Choudhary
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Tubular Functions
Renal Physiology 2
Dr Raghuveer Choudhary
Q. List different functions of renal tubules - Reabsorption: Transport of substance from lumen of tubule to blood. - Secretion: addition of substance to the glomerular filtrate coming from blood . - Synthesis: addition of new substance to glomerular filtrate e.g ammonia.
3
TUBULAR FUNCTIONThe glomerular filtrate is
formed at a rate of 125 ml/min. or 180 L/day. It passes to the renal tubules.
In the tubules, the tubular fluid is subjected to the 2 main tubular functions, reabsorption & secretion.
It is finally excreted as urine at a rate of about 1-2 ml/min. or ca. 1.5 L/day.
Reabsorption & Secretion Secretion:- refers to the transport of solutes
from pritubular capillaries in to the tubular lumen,i.e. it is the addition of substance to the filterate.
Reabsorption- denotes the active transport of solutes & passive movement of water from tubular lumen in to the peritubular capillaries,,i.e. the removal of a substance from the filterate
Filtered Load
Filtered load (mg/min) =GFR(ml/min) x Plasma Conc. of that solute(mg/ml)
Is the amount of solute transported across the glomerular membrane per unit time
Renal Tubular Transport Maximum (Tm)
Tm=it referes to maximum amount of a given solute that can be transported (reabsobed or secreted) per minutes by the renal tubules.
Tm – pertains to solutes that are actively transported.
Substances that are passively transported (urea) do not exhibit a Transport Maximum(Tm)
Tubular Reabsorption is a Function of the Epithelial Cells Making up the Tubule
Lumen
Plasma
Cells
substance to be reabsorbed must be transported
across the tubular epithelial membranes into the renal interstitial fluid
through the peritubular capillary membrane back into the blood
The transporting pathways of substance through the renal tubular
epithelial cells Transcellular pathway:
through the cell membranes
Paracellular pathway:
through the junctional spaces
Movement of substances in and out of cell
Types of carrier proteins – A uniport carrier: transport one substance. – A symport carrier: transport two substances in the same
direction. – An antiport carrier: transport two substances in the opposite
directions.
Mechanisms of Reabsorption
1. Passive transport
1). Down electrochemical gradient;
2). not require energy;
3). Mode:Diffusion,Osmosis,facilitated diffusion
4). Example:H2O
2. Active transport1). Against an electrochemical gradient;
2). require energy;
3). Depend on carrier proteins that penetrate through the membrane
4). divided into two types:– Primary active transport: coupled directly to an
energy source(hydrolysis of ATP)– Secondary active transport :coupled indirectly to
an energy source(an ion gradient)
Primary active transport is linked to hydrolysis of ATP
Importance: move solutes against an electrochemical gradient
energy source: hydrolysis of ATP Example: sodium-potassium ATPase pump
Inside Outside
Na+
Na+
Na+
K+
K+K+K+
Na+
Na+
Na+
Inside Outside
Na+ Na+Na+
K+
K+
K+
K+
Na+-K+ ATPase hydrolysis ATP release energy
Transport Na+ out of the cell into the interstitiumTransport K+ from the interstitium into the cell
The intracellular concentration of sodium is lower (chemical difference)The cell interior is electrically negative than the outside (electrical difference)
Favor Na+ to diffuse from the tubular lumen into the cell through the brush border
Secondary active transport
Co – transport:
glucose-sodium transport
amino acids -sodium transport
phosphate -sodium transport Counter- transport:
H+-Na+ transport
Co – transport ofGlucose (or amino Acids) along withSodium ions throughThe brush border ofThe tubular epithelialcells
Tubular Reabsorption
A) Active transport; against electrochemical gradient.(1) Primary active transport
Requires energy directly from ATP.Example; Na+ reabsorption in PCT
(2) Secondary active transport-It does not require energy direct from ATP.
a) Co-transport two substances bind to a specific carrier are cotransported in one direction.
b) Counter-transport two substances bind to a specific carrier are
transported in two directions.B) Passive reabsorption;
(1) Simple diffusionPassive reabsorption of chloride & Osmosis of water
(2) Facilitated diffusionNeed carrier.
C) PinocytosisIt is an active transport mechanism for reabsorption of proteins and
peptides in the proximal convoluted tubules.
Q. List different Characteristic features of PCT - PCT is about 15 mm long and 55 μm in diameter. - PCT wall is lined by single layer of epithelial cells that are connected by tight junctions at their luminal edges, but there is a space between the cells along the rest of their lateral borders (lateral intercellular spaces) which contains interstitial fluid. - The luminal borders of cells have brush border due to presence of large number of microvilli which increase surface area for reabsorption. - The PCT cells have large numbers of mitochondria (energy supply).
Proximal Convoluted Tubule
65% of the nephron function occurs in PCT.
The PCT has a single layer of cuboidal cells with millions of microvilli.
– Increased surface area for reabsorption.
PCT's main function is reabsorption.
The PCT is full of mitochondria
Reabsorption in Proximal Tubule
100% Glucose, protein and Amino Acids 60% Sodium, Cl, and H2O. 80% PH, HCO3, K. 60% Ca. 50% of Filtered Urea.
Reabsorption of glucose: Position: proximal tubule. All the filtrated glucose is reabsorbed under
normal condition. Secondary active transport, accompanied by
the primary active transport of sodium .
Renal threshold for glucose:
the maximal blood sugar concentration which can not result in glucosuria.
Reasons: there is a limit to the amount of transporter proteins and binding site.
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GLUCOSE:
At normal blood glucose levels (~100 mg%), glucose is freely filtered at a rate of 125 mg/min. (= plasma conc. X GFR = 100 mg% x 125 ml/min.). The amount filtered is completely reabsorbed from the upper half of PCT by Na+-glucose cotransport (mechanism: see before).
There is, however, a limited number of Na+-glucose carriers:a- At a blood glucose level of less than 180 mg%, all the filtered glucose can be reabsorbed because plenty of carriers are available. b- At a blood glucose level of 180 mg%, glucose starts to appear in urine. This level of blood glucose is called the renal threshold for glucose. It corresponds to a renal tubular load of 220 mg/min.
c- At a renal tubular load of glucose of 320 mg/min, all the carriers are saturated, i.e., the transport maximum for glucose, TmG, is reached.
Any further in filtered glucose is not reabsorbed & is excreted in urine.
Glucose reabsorption The transporter for glucose on the basolateral
membrane has a limited capacity to carry glucose back into the blood. If blood glucose rises above 180 mg/dl, some of the glucose fails to be reabsorbed and remains in the urine glucosuria.
Tubular maximum for glucose (TmG):
The maximum amount of glucose (in mg ) that can be reabsorbed per min.
It equals the sum of TmG of all nephrons. Value; 300 mg/min in ♀ , 375 mg/ min in ♂.
Renal Threshold for Glucose • Is approximately 180 mg/dl• If plasma glucose is greater than 180 mg/dl:
– Tm of tubular cells is exceeded
– glucose appears in urine
Plasma Glucose (mg%)
Glucose filtered (mg/min)PG x GFR/100
100 125
200 250
300 375
400 500
500 625
600 750
180
180
180
GLUCOSE REABSORPTION HAS A TUBULAR MAXIMUM
Plasma Concentration of Glucose
GlucoseReabsorbedmg/min
Filtered Excreted
Reabsorbed
375mg/min
Glucose titration curve
Plasma Glucose (mg%)
Glucose filtered (mg/min)PG x GFR/100
100 125200 250300 375400 500500 625600 750
180
Glucosuriapresence of glucose in urine
1. Diabetes mellitus –blood glucose level > renal threshold.
2. Renal glucosuria –It is caused by the defect in the glucose transport mechanism.
3. Phlorhizin–A plant glucoside which competes with glucose for the carrier and results in glucosuria (phloridzin diabetes).
Glucose
filtration rate = 100 mg/min (Pc x GFR) reabsorption rate = 100 mg/min
– site = early portion of the proximal tubule secretion rate = 0 mg/min excretion rate = 0 mg / min Tm = 375 mg/min ideal renal threshold = 300 mg/dL actual renal threshold = 200 mg /dL (arterial) 180 mg/dL (venous)
– “splay”
SGLT 2
PHLORHIZIN
GLUCOSE
100 % REABSORBED
Amino Acids
filtration rate --- small amount reabsorption ---- 100 %
– site -- early portion of the proximal tubule secretion ---- 0 excretion ----- 0
amino acids
amino acids
amino acids
SIMPLE OR FACILITATED DIFFUSION
AMINO ACIDS
100 % REABSORBED
Proteins
peptide hormones, small proteins and small amount of albumin
filtration rate = 7.2 g/day (GFR x protein in the ultrafiltrate)
reabsorption rate = 7.2 g/day– site --- early portion of the proximal tubule
secretion rate = 0 excretion rate = 0
PROTEINS
Endocytosis of Proteins
mediated by apical membrane proteins that specifically bind with luminal proteins and peptides.
they are multiligand endocytic receptors.– Megalin– Cubilin
PROTEINS
100 % REABSORBED
Sodium
filtration rate = 25,560 mEq/day (575-580mg/day)
reabsorption rate = 25,410 mEq/day (98%)– site -- proximal tubule, loop of Henle, distal
tubules and collecting duct. secretion rate = 0 mEq/day excretion rate = 150 mEq/ day
67%
Sodium 25%
5%
1 %
3%
Na reabsorption At basolateral side of the tubular epithelial cell there is an
extensive Na+-K+ ATPase system (= Na+-K+ pump).It pumps 3 Na+ actively out of the cell into the interstitium, and
at the same time carries 2 K+ into the cell.But K+ will diffuse immediately back into the interstitium due
to:(1) high concentration gradient & (2) high permeability of epithelial cells to K+.
As a result of this there is:- intracellular Na+ concentration
At luminal membrane there will therefore be passive diffusion of Na+ into the cell along concentration gradient created by the Na+-K+ pump. This diffusion is facilitated by a protein carrier.
In the early distaltubule, NaCl reabsorption is coupled via a Na-Cl cotransporter. Na+ exit at the basolateral membrane is via the Na/K-ATPase, and Cl− exit is via the Cl− channel.
Late DCT & Cortical CD:(1) Principal Cells:
a. They actively reabsorb Na+ in exchange for K+ secretion. This action is increased by aldosterone.b. Antidiuretic hormone (ADH) causes reabsorption of H2O. In the absence of ADH, the principal cells are impermeable to H2O.
(2) -Intercalated Cells: - These cells secrete H+ .This action is
increased by aldosterone.
DISTAL TUBULE AND CORTICAL COLLECTING DUCT
The distal tubule and the cortical collecting duct are important areas for fi ne regulation of Na+ excretion. The hormone aldosteroneincreases the activity of the Na+ transport proteinsthroughout this area to promote renal Na+ conservation and maintain the extracellular fl uid volume.
DIURETICSDiuresis is defined as an increase in the urine flow rate; diuretics = agents that induce diuresis.
Regulation of sodium excretion
Glomerulotubular balance in PT
Effect of ECF volume on PT Reabsorption
ECFV contraction-increased absorption ECFV expansion-decreased absorption
Factors affecting Na reabsoptionNa reabsorption increased by Na reabsorption decreased by
Peritubular Hydrostatic pressure Peritubular Hydrostatic pressure
Peritubular Colloidal osmotic pressure
Peritubular Colloidal osmotic pressure
ECFV contraction ECFV expansion
Aldosterone Glucocorticoids
GFR decreased GFR
95
Hormones acting on the kidney1. Aldosterone: Stimulus for its secretion:
Blood volume (via renin-angiotentin system). Actions & their site:
It stimulates Na+ reabsorption in DCT & cortical CD through:
1) In principal cells: Na+ reabsorption in exchange with K+.
2) In -intercalated cells: Na+ reabsorption in exchange with H+.2. Angiotensin II: It is the most powerful Na+ retaining hormone. Stimulus for its secretion: arterial bl. pressure & blood volume, e.g., hemorrhage (via renin). Actions & their site: 1. It Na+ reabsorption by several mechanisms: a. By stimulating aldosterone secretion. b. In PCT: - By directly stimulating Na+-K+ ATPase at basolateral border.
- By directly stimulating Na+-H+ countertransp. at luminal border.
2. It constricts efferent arterioles.
Hormones affecting Na Reabsorption
Nacl & water reabsorption by Aldosteron,Angiotensin II
water reabsorption- Antidiuretic Hormone ADH
Nacl & water reabsorption- ANP,Dopamine,Urodilatin
Hormones that influence sodium reabsorption in renal tubules
Angiotensin II– stimulates sodium reabsorption in the
proximal tubule, thick ascending limb, distal tubule and collecting duct.
Aldosterone– increases sodium reabsorption and
potassium secretion in the distal tubule and collecting duct
– increases sodium reabsorption in the thick ascending limb
Hormones that influence sodium reabsorption in renal tubules
Atrial Natriuretic Peptide and Brain Natriuretic Peptide– inhibits sodium chloride reabsorption in the
medullary collecting duct– inhibits ADH secretion in the posterior
pituitary gland Catecholamines
– stimulates sodium chloride reabsorption in the PT, thick ascending, DT and CD
Hormones that influence sodium reabsorption in renal tubules
Dopamine– inhibits reabsorption of sodium chloride in
the proximal tubule Adrenomedullin, Urodilatin, Uroguanylin
and guanylin– increases sodium chloride excretion
Potassium
ECF K+ concentration (N = 3.5 - 5.5 meq/L) ICF - 98%, ECF - 2% excretion
– kidneys - 90 - 95%– feces - 5 - 10%
ICF K+ concentration
140 mEq/L X 28 L
3920 mEq
ECF K+ concentration
4.2 mEq/L X 14 L
59 mEq
K+ intake100 mEq/day
K+ outputUrine = 92 mEq/dayFeces = 8 mEq/day
NORMAL K+ INTAKE, DISTRIBUTION OF K+ IN THE BODY FLUIDS AND OUTPUT FROM THE BODYGuyton, Medical Physiology, 2006
filtration rate = 756 mEq/day reabsorption rate = 644 mEq/day (87.8%)
– site -- proximal tubule and ascendong loop of Henle
secretion rate = 31 mEq/day excretion rate = 92 mEq/ day
67%
POTASSIUM20 -25%
4%
12%
Increased ECF potassium concentrationIncreased aldosteroneIncreased tubular flow rate
K+ and acid base disorders
Bicarbonate Reabsorption
Recovery of filtered HCO3− and net acid excretion is usually required to maintain acid-base balance—almost all the filtered HCO3− must be recovered. Most HCO3− reabsorption occurs in the early proximal tubule via the mechanism shown in Figure
Mechanisms of H+ excretion and HCO3− generation.B. Titratable acid excretion as H2PO4 − occurs when secreted H+ combines with fi ltered HPO42−. CA, carbonic anhydrase.
Mechanisms of H+ excretion and HCO3− generation.A. Renal ammoniagenesis results in the formation of NH4+ within cells because NH3 readily combines with H+ at physiologic pH. NH4 +is secreted via the Na/H exchangers
Reabsorption of water: 1. Quantity of reasorption:99% 2. Passive reabsorption: osmotic pressure 3. Position and siginificance: Proximal tubule: 65-70%; Accompanied by the reabsorption of NaCl Has nothing to do with whether the body lack
water or not. Not regulated by hormones;
THANK YOU