TUBULAR REABSORPTION & SECRETION Dr. Eman El Eter Juxtamedullary nephrons vs Cortical nephrons Urine Formation Preview Introduction Urinary excretion = Glomerular Filtration Tubular reabsorption + Tubular secretion. Tubular secretion means: the net movement of solutes from peritubular capillaries into the tubules. Introduction, cont.. What are the routes through which a substance enter the tubules? 1- Glomerular filtration. 2- Secretion from the peritubular capillaries which occurs in two steps: a. simple diffusion of the substance from peritubular capillaries into renal inerstitium. b. movement across the tubular epithelium into the lumen through active or passive transport. Mechanisms of tubular transport: Active transport: i. Primary active transport: e.g. Na-K-pump, H+-pump ii. Secondary active transport : e.g. Na-K-2Cl co-transport, glucose-sodium co-transport, amino acid-sodium co-transport. Passive transport: i. Simple diffusion e.g. Cl, HCO3-, urea. ii. Facilitated diffusion glucose at the basal border. Osmosis. Thus the molecules moves through ion channels, transporters, pumps & exchangers. Pinocytosis/ exocytosis. Primary Active transport Examples: Sodium potassium ATPase, Hydrogen ATPase, Hydrogen potassium ATPase, and Calcium ATPase. Na transport through PCT High conc Carrier, direct source of Energy ATP, against electrochemical gradient Secondary active transport One of the substances (e.g., sodium) diffuses down its electrochemical gradient,the energy released is used to drive another substance (e.g. glucose) against its electrochemical gradient. Both bind to one carrier protein to be transported together. Thus, secondary active transport does not require energy directly from ATP. The direct source of the energy is that liberated by the simultaneous facilitated diffusion of another transported substance down its own electrochemical gradient. The electrochemical gradient for Na is maintained by Na/K pump.. Secondary active transport, cont Na-K-2Cl transport In thick ascending limb of Henle loop diuretics as furosemide, inhibits the action of the sodium 2- chloride, potassium cotransporter. Simple diffusion Reabsorption of Chloride, Urea, and Other Solutes by Passive Diffusion Negative ions such as chloride are transported along with sodium because of electrical potentials. Urea: Inner modulary collecting duct, passive urea reabsorption is facilitated by specific urea transporters Osmosis Transport of solutes out of the tubules by either primary or secondary active transport, makes their concentration inside the cells to decrease while increase in the renal interstitium. This creates a concentration difference that causes osmosis of water in the same direction that the solutes are transported, from the tubular lumen to the renal interstitium. Water reabsorption is mainly coupled to sodium. Permeability differs through the nephron. Route: Paracellular via tight junctions Effect of ADH (Distal & collecting tubules). Transport through PCT Criteria of PCT: Highly metabolic cells, rich in mitochondria. Transporter proteins & channels. Wide surface area of brush border, basolateral membrane Renal tubular reabsorption through PCT Solute reaborption in the proximal tubule is isosmotic (water follows solute somatically and tubular fluid osmolality remains similar to that of plasma= equal amount of solute and water are reabsorbed). 65%-70% of water and sodium reabsorption occurs in the proximal tubule 90% of bicarbonate, calcium, K+ 100% of glucose & amino acids. Secretion: organic acids and bases such as bile salts, oxalate, urate, and catecholamines, certain drugs, such as penicillin and salicylates, Proximal tubules: coarse adjustment. Distal tubules: fine adjustment (hormonal control). Reabsorption through PCT Loop of Henle Responsible for producing a concentrated urine by forming a concentration gradient within the medulla of kidney. When ADH is present, water is reabsorbed and urine is concentrated. Counter-current multiplier. 16 Absorption through loop of Henle: Descending limb: is water permeable and allow absorption of 25% of filtered H2O. It is impermeable to Na-CL. Thin ascending limb: is impermeable to H2O, but permeable to Na-Cl, where they are absorbed passively in this part. Thick ascending limb: is impermeable to H2O. Na-K-2Cl co-transport occur in this part. By the end of ascending limb of loop, the tubular fluid becomes hypo-osmolar to plasma. Na+-2Cl-K+ co transport: Thick ascending limb of Henle Distal convoluted tubule and collecting ducts What happens here depends on hormonal control: Aldosterone affects Na+ and K+ ADH facultative water reabsorption Parathyroid hormone increases Ca++ reabsorption. Fine adjustment of tubular filtrate takes place here according to body needs. The first portion of DCT forms part of JGA, that provides feedback control of GFR and RBF of the same nephron. The next early portion has the same characteristics as ascending limb of Henle that is impermeable to water but absorbs solutes. So it is called the diluting segment & the osmotic pressure of the fluid ~ 100 mOsm/L. 20 DCT Late distal tubule& collecting tubules Have similar functional characteristics. Composed of two types of cells: a. Principal cells: absorb Na+& H2O and secrete K+ b. Intercalated cells: absorb K+ & secrete H+ Impermeable to Urea. water permeability under ADH Secretion of K+ and reabsorption of Na+ controlled by aldosterone. Cell types of the nephron Medullary collecting ducts Reabsorb