Lecture 4- Tubular Processing PDF

Summary

This lecture covers renal function and tubular processing, focusing on tubular reabsorption and secretion.  It details the different modes of transport across the tubular epithelium and the reabsorption and secretion of various substances, such as sodium, water, glucose, urea, and chloride. The document also explains transport maximum (Tm), renal plasma threshold, and their role in kidney function.

Full Transcript

Renal function: Tubular processing of the glomerular filtrate: tubular reabsorption and tubular secretion. Dr. Safa Almaghrabi MBBS, MBiomedSc, PhD Assistant Professor in Physiology Department Email: [email protected] Learning Objectives 1. Define the renal pr...

Renal function: Tubular processing of the glomerular filtrate: tubular reabsorption and tubular secretion. Dr. Safa Almaghrabi MBBS, MBiomedSc, PhD Assistant Professor in Physiology Department Email: [email protected] Learning Objectives 1. Define the renal processes: Tubular reabsorption & tubular secretion. 2. Describe the mode of reabsorption of different substances (e.g. Na+, K+, Cl-, glucose, urea, and water). 3. Describe the mode of secretion of different substances (e.g. K+, H+ and organic ions). 4. Define transport maximum (Tm), renal plasma threshold and splay. Basic Renal Proces ses Tubular reabsorption Definition It is highly selective and quantitatively large. Out of 180 L of plasma filtered/day 178.5 L is reabsorbed. Nutritional substances are completely reabsorbed. Many plasma electrolytes are reabsorbed according to body needs. Waste products are poorly reabsorbed. Tubular Reabsorp tion Tubule is one cell- layer thick It in a close proximity to a peritubular capillary Tubule cells are joint by tight- junctions Cell Transport 1. Passive No energy Occur down electrochemical or osmotic gradients 2. Active Needs energy Occur against electrochemical or osmotic gradients Primary Secondary Cotransport Countertransport Tubular Reabsorption Urinary Module-2009 8 Na+ Reabsorption By 1ry active transport (Na+-K+-ATPase) Varies in different tubular regions 65% in PT followed by water and other solutes No reabsorption in the DLOH In ALOH 25% In distal portions 8% subject to hormonal control - Aldosterone  - Atrial natriuretic peptide (ANP)  9 Na+ reabsorption Passive (by osmosis) coupled to Na+ reabsorption in the PT 65% in PT (highly permeable to Water water), water channels (Aquaporins) are always open in PT Reabsor 15 % in DLOH ption 20% in distal portions under ADH control, depending on the hydration of body Glucose and Amino Acid Reabsorption 100 % reabsorbed in PT By Na+ dependent secondary active transport across luminal membrane of PT (Co-transport). 13 Glucose and Amino Acid Reabsorption 14 Chloride and Urea Reabsorption Urea molecule is the smallest compared to other waste products and PT moderately permeable. 50% of urea reabsorbed in PT. Water is reabsorbed especially in PT, the urea concentration in tubular fluid increases, creating a driving force for passive urea reabsorption CL Reabsorbed passively, paracellular. The amount of Cl- reabsorption is directly related to the Na+ reabsorption. Urea Reabsorpti on Chloride and Urea Reabsorption Sodium, Chloride, and Potassium Transport in the TALH 65% reabsorbed in proximal tubule (paracellular). Potassiu 20% is reabsorbed by ALOH. m Reabsorp tion The distal tubule and the collecting duct either reabsorb or secrete potassium according to body needs. Tubular Secretion Potassium Secretion K+ is almost completely reabsorbed in PT by paracellular pathway.. K+ is secreted in late distal and collecting tubule (Principal cells). K+ in plasma >>  Aldosterone >> K+ secretion and excretion in urine. 21 Potassium Secretion H secretion + H+ secretion is extremely important in the regulation of the acid-base balance. In PT via Na+-H+ countertransport (secondary active transport). In DT and CD by H+-ATPase (primary active transport). H+ Secretion in The Proximal Tubule 2ndUrinary year MedModule-2009 2007- lecture 6 24 H+ Secretion in The Late Distal and Collecting Tubule 2ndUrinary year MedModule-2009 2007- lecture 6 25 Organic Ions Secretion Two types of secretory carriers in PT: – for the secretion of organic anions – for the secretion of organic cations. Non-selective (not specific). Many drugs are eliminated from the body through these secretory mechanisms. 26 Filtration, reabsorption and excretion of different substances by the kidneys Substance Amount Amount Amount % reabsorbed filtered reabsorbed excreted Glucose 180 180 0 100 (gm/day) Sodium 25,560 25,410 150 99.4 (mEq/day) Chloride 19,440 19,260 180 99.1 (mEq/day) Bicarbonate 4,320 4,318 2 >99.9 (mEq/day) Urea (gm/day) 46.8 23.4 23.4 50 Creatinine 1.8 0 1.8 0 (gm/day) Filtration Load The amount of a substance filtered into Bowman's space per unit time (minutes) is called the filtered load. Filtration load = plasma concentration of the substance X GFR Transport maximum Tm- limited transport mechanisms Tm is the max amount of a substance that the tubular cells can actively transport within a given time period. There is a limit to the rate which the solute can be transported. This is due to saturation of the specific transport system. Tm for glucose average 375 mg/min Renal Threshold Normal glucose level in plasma is 100mg of glucose/100ml of plasma. Glucose often starts spilling into the urine at glucose concentrations of 180 mg/100 mL (Renal Threshold). FL= 180mg/100ml X 125ml/min= 225mg/min. Glucose is often excreted before the Average renal threshold of 300 mg/100 mL is reached because: Not all nephrons have the same Tm Tm400 Tm300 Tm250 Tm225 Transport Maximum Reabsorption of Glucose splay 1 minute Quiz When the concentration of glucose in the plasma exceeds the renal threshold: a. Glucose reabsorption will increase. b. More glucose will be secreted. c. Glucose will appear in urine. d. The filtered load for glucose will be decreased. Different Kinds of Diuretics Summary Define tubular reabsorption & secretion. What are the different modes of transport across the tubular epithelium? Describe the mode of reabsorption of different substances (e.g. Na+, H2O, Cl-, glucose, urea, potassium). What are the most important substances that are secreted by the renal tubules? Define transport maximum, renal plasma threshold and splay

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