O reabsorption in PCT Stimulates posterior pituitary to secrete ADH H 2 O reabsorption by collecting duct Stimulates thirst in addition, ADH aka vasopressin is a vasoconsrictor 23-38
23-39 Glomerular filtrate moves down through tubular lumen. Composition is altered as inorganic and organic ions and macromolecules and water are transported out of or into the fluid. tubular lumen - primarily at the PCT and DCT. Mechanisms involved include: Tubular reabsorption Tubular secretion Reabsorption of Water Urine Formation II: Tubular Reabsorption and Secretion Filter 125 mL H 2 O /min or 180 L/day Filter 16,800 mmol Na each day Urinate only 1-2 L of fluid and only ~150 mmol Na each day
23-40 Tubular Reabsorption at the Proximal Tubule Na+ is key - Na + , most abundant cation in filtrate; transport of nearly all other ions/solutes is linked to Na + transport Glucose cotransported with Na + by Na + -glucose transport (SGLT) proteins, At normal blood glucose levels all glucose is reabsorbed
23-41 Transport Maximum The amount of solute renal tubules can reabsorb is limited by the number of transporter proteins in the cell membrane If all transporters are occupied, any excess solute cannot be transport and remains in tubular fluid and excreted in urine Transport maximum: ALL transporters are saturated Each solute has its own transport maximum, e.g., Glucose - blood glucose level above 220 mg/dL glycosuria, i.e., excretion of glucose in urine
Summary Tubular Reabsorption and Tubular Secretion 23-42 Nitrogenous Wastes: Net Secretion of Ammonia, Urea, and Uric Acid – *Creatinine filtered but not reabsorbed and not secreted What renal process does the urine concentration of creatinine reflect - what is its clinical value?
23-43 Urine Formation III Water Conservation Filtered each day: 180 L H 2 O and 16,800 mmol Na Each day Urinate only 1-2 L of fluid and ~150 mmol Na Kidney: eliminates metabolic wastes from body without excessive water loss How? As fluid flows through tubule, it becomes concentrated due to H 2 O reabsorption AND return of that H 2 O to bloodstream and body. 3 Key Structures Mediate Conservation of H 2 O: loop of Henle, generates medullary osmotic gradient; Collecting ducts, can reabsorb H 2 O but not Na, Cl and Vasa Recta, absorbs H 2 O reclaimed by CD & returns it body without washing out medullary osmotic gradient Vasa Recta
23-44 Renal Clearance Renal clearance = volume of blood plasma from which a particular waste is completely removed in one minute Represents the net effect of all three processes Glomerular filtration of the waste + Amount added by tubular secretion – Amount removed by tubular reabsorption Renal clearance
Chapter 24 - Fluid-Electrolyte and Acid-Base Balance - Biol 2402, Fall 2015 The real reason why the kidney is so important - cellular function demands the composition of the intracellular and extracellular fluid compartments be narrowly controlled - volume which ions and compounds are present individual and collective concentration, i.e., osomolarity Three types of homeostatic balance
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