Urinary System Reabsorption & Secretion PDF

Urinary System: Part 2 Reabsorption and Secretion At the Tubule Cells of the Nephron Anatomy & Physiology II BIOL2220 Version 1 1 The Urinary System Reabsorption & Secretion: Mechanisms and Pathways Mechanisms of Transport 1....

Urinary System: Part 2 Reabsorption and Secretion At the Tubule Cells of the Nephron Anatomy & Physiology II BIOL2220 Version 1 1 The Urinary System Reabsorption & Secretion: Mechanisms and Pathways Mechanisms of Transport 1. Primary Active Transport: Two pathways: Is the actual “pump”, usually against the concentration gradient; Needs ATP directly Transcellular 2. Secondary Active Pathway; Transport: Moves with Cells across cells, concentration gradient; good for both Needs ATP indirectly ( a backdoor pump makes the gradient) Lumen Blood Lumen Of 3. Passive Transport: In Diffusion Tubule Capillary Paracellular 4. Pinocytosis: Not a major Transport; player, but useful for between cells, reacquiring larger proteins good for reabsorption 2 The Urinary System Key Point: The mechanisms can be used for both Reabsorption & Secretion: Mechanisms of Transport reabsorption and secretion EXTRACELLULAR/INTERSTITIAL FLUID Na+ Primary Active Transport: via antiport Basal membrane ADP + Pi ATP TUBULE K+ CELL K+ Apical membrane TUBULE FLUID Glucose Na+ Diffusion: via uniport 3 Secondary Active Transport: via symport The Urinary System Reabsorption & Secretion: “The Power of the Pump” EXTRACELLULAR/INTERSTITIAL FLUID Primary Active Transport: The “pump” on Na+ the “back door” (basal membrane) helps drive most of the movement across the “front door” (apical membrane). Basal membrane ADP + Pi ATP TUBULE K+ CELL NH3 NH4+ H+ H+ ADP + Pi K+ Apical membrane ATP TUBULE FLUID Glucose or Na+ Na+ Na+ Amino Acids Na+ Cl- H2O 4 The Urinary System The PCT: A Busy Place for Reabsorption of H2O, Na+, Cl-, Glucose, and AA’s Proximal tubule, including the proximal convoluted tubule and thick descending segment of the loop The sodium-potassium ATPase: major force for reabsorption of sodium, chloride, water, glucose, and AA’s In the first half of the proximal tubule, sodium is reabsorbed by co-transport along with glucose, amino acids, and other solutes. In the second half of the proximal tubule, sodium reabsorbed mainly with chloride ions. Glucose is reabsorbed along with Na+ in the early portion of the proximal tubule. Glucose is typical of substances removed from the forming urine by secondary active transport (also, amino acid reabsorption). Essentially all of the glucose is reabsorbed, and no more than a few milligrams appear in the urine per 24 hours. The renal threshold for glucose is the plasma level at which the glucose first appears in the urine. The TmG is about 375 mg/min in men and 300 mg/min in women. 5 The Urinary System Reabsorption & Secretion: Recapturing Bicarbonate EXTRACELULLAR/INTERSTITIAL FLUID Na+ Bicarbonate is reclaimed for the blood Basal membrane ADP + Pi ATP TUBULE K+ CELL H+ & HCO3- ßH2CO3 ßCO2 & H2O ADP + Pi Apical membrane ATP TUBULE FLUID Na+ H+ H+ H+ & HCO3- àH2CO3 àCO2 & H2O 6 Secreted hydrogen combines with the bicarbonate in the tubule fluid The Urinary System Mechanisms of Hydrogen Secretion and Bicarbonate Reabsorption Although the lungs excrete a large amount of CO2, a volatile acid formed by metabolism, the kidneys are crucial for excreting non-volatile acids. To maintain acid-base balance, the kidneys must not only reabsorb virtually all filtered HCO3-, but must also secrete into the urine the daily production of non-volatile acids (H+). Hydrogen Secretion and Bicarbonate Reabsorption. Hydrogen secretion can occur through Secondary Active Transport. Mainly at the proximal tubules, loop of Henle, and early distal tubule ; More than 90 percent of the bicarbonate is reabsorbed (passively ) in this manner. The Proton Pump Primary Active Transport, beginning in the late distal tubules and continuing through the reminder of the tubular system It occurs at the luminal membrane of the tubular cell Hydrogen ions are transported directly by a specific protein, a hydrogen-transporting ATPase (proton pump). Accounts for only about 5 percent of the total hydrogen ion secreted Important in forming a maximally acidic urine. Hydrogen ion concentration can be increased as much as 900-fold in the collecting tubules. 7 Tubular fluid pH can drop to about 4.5: the lower limit of pH for normal kidneys. The Urinary System Reabsorption & Secretion: “New” Bicarbonate & Buffering with Ammonia EXTRACELLULAR/INTERSTITIAL FLUID Note: Glutamine can be broken down into 2 Ammonium and 2 HCO3- Bicarbonate ions. This is called “new” bicarbonate. Na+ Basal membrane ADP + Pi ATP TUBULE K+ CELL Ammonia can be made Glutamine from the breakdown of various amino acids ADP + Pi Apical membrane ATP TUBULE FLUID Na+ NH4+ H+ NH3 NH4+ 8 Alkaline buffer for acidic urine NH3 picks up the H+ = Buffer! The Urinary System Excretion of Excess Hydrogen Ions and Generation of New Bicarbonate by the Ammonia Buffer System For each molecule of glutamine ( a common amino acid) metabolized in the proximal tubules, two NH4+ ions are secreted into the urine and two HCO3- ions are reabsorbed into the blood. The HCO3- generated by this process constitutes new bicarbonate. Renal ammonium-ammonia buffer system is subject to physiological control. An increase in extracellular fluid hydrogen ion concentration stimulates renal glutamine metabolism and, therefore, increase the formation of NH4+ and new bicarbonate to be used in hydrogen ion buffering; a decrease in hydrogen ion concentration has the opposite effect. with chronic acidosis, the dominant mechanism by which acid is eliminated is via NH4+ generation. 9  The Urinary System Key Point: Use of solutes to create an osmotic gradient that favors The Countercurrent Multiplier reabsorption of Water The Loop of Henle has different “Limbs”. Descending Limb: Permeable to Water, not permeable to salts Ascending Limb: NOT Permeable to Water, but able to Actively Transport Salts out of the Tubule Fluid and into the ECF of the Renal Medulla. The Collecting Tubule can Actively Transport Urea out of the Tubule Fluid and into the ECF of the Medulla. Descending Ascending Limb Limb The Latter Two of these events leads to a Medullary ECF with a Reabsorption low percentage of Water (Hypertonic) …lower even than the Of H2O via Pump Salts from Water percentage found in the descending limb of the Loop of Diffusion: Filtrate Henle. From High H20 To Concentration in Medullary ECF Filtrateà to Low The consequence is that Water from the Tubule Fluid in the H20 Concentration in Medullary ECF Medullary ECF Descending Limb of the Loop will Diffuse back into the body. Becomes Hypertonic= Loop Low H20 Simply Stated: Water flows from High Concentration (the Tubule Concentration Fluid) to Low Concentration (the Medullary ECF). 10