tubularfunction.pdf

SESSION OUTCOME
At the end of the lecture, the group should be able to:
1. describe the basic principles of solute transport in a renal tubule
and;
2. understand the concepts on reabsorption and secretion along
different parts of the nephrons.

ESSENTIAL
QUESTIONS:
HOW IS URINE PROCESSED?
HOW SOLUTES ARE
TRANSPORTED ACROSS RENAL
TUBULES?

Nephron:
functional unit of the kidney
 The nephron consists of:
 Vascular components
 Afferent & efferent
arterioles
 Glomerulus
 Peritubular capillaries
 Vasa recta
 Tubular components
 Proximal convoluted
tubule
 Distal convoluted tubule
 Nephron loop (loop of
Henle)
 Collecting duct
 Tubovascular component
 Juxtaglomerular

Concentration and
Dilution of the Urine
• Maximal urine concentration
= 1200 - 1400 mOsm / L
(specific gravity ~ 1.030)
• Minimal urine concentration
= 50 - 70 mOsm / L
(specific gravity ~ 1.003)

8
CELL MEMBRANE
Polar heads
Hydrophilic
Non-polar tails
hydrophobic.

Membrane
transport
Passive
Simple diffusion
Facilitated
Diffusion
Active
Primary
active
Secondary
active

PASSIVE
TRANSPORT
With concentration
gradient (downhill)
No need for energy
10

ACTIVE TRANSPORT
• Against concentration gradient (UP hill)
• need energy (ATP, Creatine
Phosphate)
12

Tubular reabsorption
Both requires passive and active
transport mechanism
Solute can be transported through
paracellular pathway or transcellular
Water moves through osmosis

Primary Active Transport of Na+
Sodium diffuses across the luminal
membrane into the cell down an
electrochemical gradient
Sodium is transported across the
basolateral membrane against an
electrochemical gradient by the
sodium-potassium ATPase pump.
 Sodium, water, and other substances
are reabsorbed from the interstitial
fluid into the peritubular capillaries
by ultrafiltration

Secondary Active Transport mechanism
No energy required
Includes Sodium glucose
transporter(SGLT2 and SGLT1)
Sodium-hydrogen exchanger(NHE)
Carrier molecule is needed

“PUNONG PUNO NA
AKO SA INYONG
LAHAT!, DI KO NA
KAYA ah!!!”
PETMALU KA
BATO!!!!
GLUCOSE

TUBULAR TRANSPORT
MAXIMUM
It is the maximal amount of a substance (in mg) which
can be transported (reabsorbed or secreted) by tubular
cells/min.
 Carrier proteins have a maximum transport
capacity (Tm) which is due to saturation of the
carriers.

Glucose transport maximum
Transport maximum: 375 mg/min
Filtered load : 125 mg/min
Plasma concentration: 100mg/100 ml

Substance Measure Filtered* Excreted Reabsorbed % Filtered Load Reabsorbed
Water L/day 180 1.5 178.5 99.2
Na
+
mEq/day 25,200 150 25,050 99.4
K
+
mEq/day 720 100 620 86.1
Ca
++
mEq/day 540 10 530 98.2
HCO3
-
mEq/day 4320 2 4318 99.9+
Cl
-
mEq/day 18,000 150 17,850 99.2
Glucose mmol/day 800 0 800 100.0
Urea g/day 56 28 28 50.0

Interstitial
Fluid
Tubular
Lumen
Tubular
Cells
Reabsorption of Water and Solutes is Coupled
to Na+ Reabsorption
Na +
K+
ATP
Na +
K+
ATP Na +
- 3 mV
0 mv
H20
Cl-
Urea
Na +
H +
glucose, amino
acids
Na +
- 70 mV
Copyright © 2006 by Elsevier, Inc.

Figure 27-5;
Guyton and Hall
Mechanisms by which Water, Chloride, and Urea
Reabsorption are Coupled with Sodium Reabsorption

TRANSPORT OF SOLUTE ACROSS THE
TUBULES

Cellular Ultrastructure and Primary Transport
Characteristics of Proximal Convoluted Tubule
65-67% filtered load is reabsorb
Contains extensive brush border
structure
Site for the secretion of toxin, organic
acids and bases
Reabsorption is highly passive

TRANSPORT IN EARLY PCT
Na ion uptake primarily coupled by
hydrogen ion(antiporter) ,glucose(co-
transport),amino acid and lactate
Transtubular osmotic gradient drive
water movement across cell
membrane

TRANSPORT IN LATE PCT
Na and Cl is transported through
parallel operation of Na/H exchanger
and Cl/base
Cl is passively reabsorb in the
basolateral membrane

Cellular Ultrastructure and Transport Characteristics of
Thin and Thick Loop of Henle
 not permeable to H2O
 very permeable to H2O)
 No brush border structure (few
mitochondria) and minimal level of
metabolic activity.
 No brush border structure (few
mitochondria) and minimal level of
metabolic activity.
 20% of filtered load of Na, Cl and K are
reabsorbed

TRANSPORT IN THICK
ASCENDING LIMB
 Movement of sodium across the
luminal membrane is mediated primarily
by a 1-sodium, 2-chloride, 1-potassium co-
transporter
 Tubular fluid is very diluted
 Site of action of some diuretics

Cortical and Juxtamedullary nephron segments

Countercurrent Multiplier System
in the Loop of Henle
 Active transport of sodium ions and co-
transport of potassium, chloride, and other
ions.
 Active transport of ions from the
collecting ducts into the medullary
interstitium.
 Facilitated diffusion of large amounts
of urea from the inner medullary collecting
ducts into the medullary interstitium.

Animal Max. Urine Conc. (mOsm /L)
Beaver 500
Pig 1,100
Human 1,400
Dog 2,400
White Rat 3,000
Kangaroo Mouse 6,000
Australian Hopping Mouse 10,000

• The vasa recta
serve as
countercurrent
exchangers
• Vasa recta blood
flow is low
(only 1-2 % of
total renal
blood flow)

Sodium Chloride Transport in Early Distal Tubule

• not permeable to H2O
• not very permeable to urea
• permeability to H2O depends on ADH
• not very permeable to urea

Figure 27-12;
Guyton and Hall
Sodium Chloride Reabsorption and Potassium
Secretion in Collecting Tubule Principal Cells

Cellular Ultrastructure and Transport Characteristics of
Medullary Collecting Tubules
 The permeability of the medullary collecting
duct to water is controlled by the level of
ADH
 Unlike the cortical collecting tubule, the
medullary collecting duct is permeable to
urea,

HORMONES THAT REGULATE TUBULAR SECRETION

EFFECT OF ANG II IN RENAL TUBULES

Summary of Water Reabsorption and
Osmolarity in Different Parts of the Tubule
• Proximal Tubule: 65% reabsorption, isosmotic
• Desc. loop: 15-20% reabsorption, osmolarity increases
• Asc. loop: 0% reabsorption, osmolarity decreases
• Early distal: 0% reabsorption, osmolarity decreases
• Late distal and coll. tubules: ADH dependent
water reabsorption and tubular osmolarity
• Medullary coll. ducts: ADH dependent water
reabsorption and tubular osmolarity

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