Download Tubular reabsorption

Tubular reabsorption
• Tubular Reabsorption Is Selective and Quantitatively Large
• The rate at which each of these substances is filtered is
calculated as:
• Filtration = Glomerular filtration rate \ Plasma
concentration
• A 10 per cent decrease in tubular reabsorption, from 178.5 to
160.7 L/day, would increase urine volume from 1.5 to 19.3
L/day (almost a 13-fold increase) if the glomerular filtration
rate (GFR) remained constant.
Tubular Reabsorption
 Filtered substances are transferred from the
tubular lumen to the peritubular capillaries
 This process is highly selective and variable.
 The return of substances to the blood is needed to maintain the
composition of the ECF. Only excesses of materials are eliminated.
 Reabsorption rates are high: 124 of 125 ml of filtered fluid per minute,
 99% for water
 100% for glucose
 99.5% for salt.
3
Tubular reabsorption involves
transepithelial transport
 Reabsorbed substance must cross:
1. tubule wall
2. interstitial fluid,
3. wall of the peritubular capillaries
thus entering the blood.
 The single layer of epithelial cells of
the nephron tubule has:
i. luminal membrane (facing tubule
lumen)
ii. basolateral membrane (facing the
interstitial spaces between the
tubule and peritubular capillaries)
4
I. Active transport
A. Primary Active: Na-K, Hydrogen, H-K, and
Calcium pumps.
Sodium: on basolateral sides, Na-K pump,
creates negatives inside  increases Nainflux.
B. Secondary active : Co-transport, countertransport . Na -Glcose , Na - a.a , Na - H+
C. Pinocytosis: Proteins .
Basic mechanism for active transport of sodium
through the tubular epithelial cell
 Involves a Na-K ion ATPase
carrier in the tubular cell’s
basolateral membrane
 This pump keeps sodium
concentration low in the tubular
cells and high in the lateral
spaces outside the tubule.
 Water follows reabsorbed sodium
by osmosis. Thus, sodium
reabsorption has a main effect on
blood volume and blood pressure.
 Aldosterone promotes Na+
retention by insertion of additional
Na+ channels into the luminal
membrane, and additional
Na+/K+ ATPase into the
basolateral membrane of the
distal and collecting tubules.
Reabsorption of Na+
 Of the Na+ filtered, 99.5% is normally reabsorbed.
 Sodium is reabsorbed throughout the tubule with the exception of the
descending limb of the loop of Henle
 Of the Na+ reabsorbed, on average
• 67% in the proximal tubule (Plays an important role in the
reabsorption of glucose, amino acids, water, chloride ions, and urea)
• 25% in the loop of Henle (Plays a role in the production of varying
concentrations and volumes of the urine)
• 8% in the distal and collecting tubules (depends on hormonal control,
and plays an important role in regulating ECF volume).
Aldosterone (stimulated via renin system) increases Na+ reabsorption
Atrial Natriuretic Peptide “ANP” (released from heart) decreases Na+ reabsorption
8
Mechanism of Aldosterone Action
Mechanisms of secondary active transport
Transport maximum for substances
that are actively reabsorbed
• Due saturation, when tubular load exceeds
capacity of carrier and specific enzymes.
• Transport maximums for substances actively
reabsorbed:
• Transport Maximums for Substances That Are
Actively Secreted:
– Creatinine 16 mg/min
– Para-aminohippuric acid 80 mg/min
• Tm Na in proximal tubule not exist because it is
referred as (gradient-time transport) the greater
[Na+] the greater reabsorption .
– Because the rate of transport depends on the
electrochemical gradient and the time that the
substance is in the tubule, which in turn depends on
the tubular flow rate.
II. Passive Water reabsorption:
• Osmosis and coupled with sodium
• When the interstitium is hypertonic:
– large part through tight junctions and solvent drag
some solute w/water .
• Ascending and first part of distal tubule
impermeable for water
• ADH  ↑permeability in distal, and coll. tubule
III. Chloride and Urea reabsorption
• Na+ inside cells attracts (-) charge of chloride
(paracellular)
• Also Cl- passively after water influx [Cl-] out
higher
• Also secondary active transport Na+ - Cl-
III. Chloride and Urea reabsorption
• E. Urea : - passively 1/2 of filtered
passively, because its concentration is
increased, as water is reabsorbed from the
tubules.
Urea in inner medull. Coll duct, by facilitated
diffusion.
- Creatinine not reabsorbed at all