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.

Tubular reabsorption involves transepithelial transport Reabsorbed substance must cross: tubule wall interstitial fluid, wall of the peritubular capillaries thus entering the blood. The single layer of epithelial cells of the nephron tubule has: luminal membrane (facing tubule lumen) basolateral membrane (facing the interstitial spaces between the tubule and peritubular capillaries)

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 Na-influx. B. Secondary active : Co-transport, counter- transport . 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.

Aldosterone (stimulated via renin system) increases Na+ reabsorption 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

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