Urine formation Tubular reabsorption is selective and quantitatively

Urine formation • Tubular reabsorption is selective and quantitatively large : • Filtration rate of y = GFR x [plasma]y

A: Reabsorption : a. across tubular epithalial membrane into renal interstitial fluid. Then b. Through peritubular capillary : 1: Could be transcellular route or paracellular route ( water and solutes, Na by both route) to the interstitial fluid then 2: Only by ultrafiltration (bulk flow) to the blood through peritubular capillaries

ABSORPTION 1 -ACITVE A-Primary active : Na-K ATPase, H-K ATPase and Ca ATPase B-Secondary active reabsorption : Two or more substances , Na-Glucose , a. a. co- transporter (SGLT 1 10% &2 90%), Cl C-Secondary active secretion: Na-H counter-transpoeter D-Pintocytosis : Active transport mechanism for reabsorption of protein

PASSIVE REABSORPTION Water, Chloride, Urea and other solutes

Proximal tubule 1 -Na+ reabsorption : -Active (Na –K Pump) -Passive, (-) inside cells (Chloride and urea) Cl can also absorbed by 2 ndary active transport 2 -Secondary active (specific carrier proteins) Co-transport: Na –Gl (90% by SGLT 2, 10% by SGLT 1) , Na a. a Counter- transport : Na - H 3 - Passive facilitated diffusion gl. (GLUT 2, GLUT 1) on basolateral membrane & peritubular capillaries 4 -Pinocytosis: Proteins (active)

The primary active transport in the kidneys are 1 -Na-K 2 - H 3 - H-K 4 -calcium

B. Transport maximum for substances that actively reabsorbed • Filtered load exceeds carrier capacity. Limit to the rate at which the solute can be transported. • Tm Gl = 375 mg/min • Filtered load = 125 mg/min (GFR x [gl. ]pl. • Threshold gl. = 200 mg/dl =250 mg/min (tub. Load) •

• • • Tm creatinine = 16 mg/min Tm PAH = 80 mg/min Tm aa = 1. 5 m. M/min Tm pl. prot. = 30 mg/min. Tm urate = 15 mg/min Tm Na in proximal tubule not exist because it is referred as (gradient-time transport) the greater [Na]+ the greater reabsorption.

Not all the substances have transport maximum. Transport depends on gradient, permeability and gradient time transport.

SODIUM Na 1 - Active transport in the proximal tubule. 2 -Obeys gradient time. 3 - Co-transporter and counter-transporter. 4 - No maximum transporter in the proximal tub 5 - Has maximum transporter in the distal part and this can increase by aldosterone

C. Water reabsorption: - - Solute transport create concentration difference that cause osmosis of water in the same direction of solute transport, large part through tight junctions Solvent drag some solute w/water ADH ^ permeability in distal, and coll. Tubule Ascending and first part of distal impermeable for water

D. Chloride • Na+ inside cells attracts (-) charge of chloride • Also Cl- passively after water influx [Cl-] out • Also secondary active transport Na+ - Cl • E. Urea : - passively 1/2 of filtered

Reabsorption and secretion along different parts of the nephron 1. Proximal tubule 65% : - 65% of filtered load of H 2 O and Na+ and Clreabsorbed (Active and passive) - Mitochondria, brush border - Extensive labyrinth of intercellular and basal channels. All these provide extensive membrane surface area for rapid transport of Na & other

Concentrations of solutes along prox. Tubule. - Secretion: organic acids and bases (bile salts, oxalate, urate, catecholamines) - Secretion drugs salicylates, penicillin, PAH

2. L. H 20% : a. Descending part of thin segment : High permeable to H 2 O moderate to salts and urea. b. Thick segment : 25% filtered loads of Na. Cl reabsorbed and K+ , Ca++ , HCO 3 - , Mg++ (1 Na+ - 2 Cl- - 1 K+(co-transporter)), Na -H counter transport, Impermeable to H 2 O

Loop of Henle 1. Descending thin limb 2. Ascending thin and thick portion impermeable to water. 3. Thick ascending part is the site of action of loop diuretic

3. Distal tubule : First portion macula densa : a. Diluting segment as thick ascending. 5% of filtered load of Nacl absorbed in early dist tub. b. Late distal &cortical collecting tubule(have similar function) : A-principle cells: Na+ reabs. K+ secre. (K sparing diuretics site) B-Type A Intercalated cells reabs. Hco 3+, H+ secr. Type B Intercalated cells reabs H, secr Hco 3

(Intercalated cells can also reabs or secrete K) (Intercalated cells play a major role in acid base regulation) C- Late distal & cortical tubule also are: -Impermeable to urea -Na+ reabs. by aldosterone and secrete K - A&B intercalated cells involve in regulation of acid base of body fluid -H 2 O permeability controlled by ADH

4. Medullary collecting duct Reabsorb about 10% of filtered water and Na Final site for processing the urine - Sodium reabsorption - H 2 O ADH - Permeable to urea (Can reabs urea) to incr osmolarity to form concentrated urine - H+ secretion Summary of concentration of different soltues in different tubular segments

Summary of concentration of different soltues in different tubular segments