10/28/2010
Objectives Steps of Urine Formation
To describe the function of the nephron in I. Glomerular filtration – Renal corpuscle detail. (Glomerulus + Capsule) To identify the steps of urine formation. II. Tubular reabsorption – PCT* To examine the movements of substances *Functions in both reabsorption and secretion per their respective locations in the nephron – focusing on the glomerulus. To understand the forces responsible for the III.Tubular secretion – DCT (mostly) initial formation of filtrate.
Efferent Glomerular capsule Step 1: Filtration arteriole Glomerulus Afferent arteriole Parietal layer Glomerulus: Capillary network located of glomerular capsule within the Bowman’s/ Glomerular Capsular space Red blood cell Capsule. Efferent Proximal arteriole tubule cell
Role is filtration of blood producing: Juxtaglomerular apparatus Filtrate = Blood minus cells and • Macula densa cells of the ascending limb excluding large proteins. Similar to of loop of Henle plasma.
Operates on hydrostatic pressure. Afferent arteriole
Juxtaglomerular Renal corpuscle apparatus
Figure 25.8
Nephron Capillary Beds Cortical nephron • Efferent arteriole supplies peritubular capillaries Efferent arteriole Renal Glomerular capillaries Step 1: Glomerulus corpuscle (glomerulus) Afferent arteriole Glomerular (Bowman’s) capsule Proximal Afferent arteriole Specialized for filtration: convoluted tubule Efferent arteriole Via fenestrated glomerular Peritubular capillaries Ascending or thick limb endothelium (capillaries) of the loop of Henle Blood Flow - Afferent arteriole > glomerulus > efferent art. Vasa recta - Blood pressure is high because Juxtamedullary afferent art. are smaller in diameter than nephron efferent art. (causes inc in B.P.) - Arterioles are high-resistance vessels.
(a)
Figure 25.7a
1 10/28/2010
Nephron Capillary Beds Nephron Capillary Beds
2. Peritubular capillaries 3. Vasa recta Low-pressure, porous capillaries adapted for Long vessels parallel to long loops of Henle absorption Arise from efferent arterioles of Arise from efferent arterioles juxtamedullary nephrons Cling to adjacent renal tubules in cortex Function information of concentrated urine Empty into venules.
Net Filtration Filtration Pressure Pressure Glomerular blood pressure (HPg) – main force The pressure responsible pushing water and solutes out of the blood. (GBHP) for filtrate formation
Opposing forces: NFP =GBHP – [CHP + BOP] > Capsular Hydrostatic Pressure (CHP) – Pressure exerted by fluids in the glomerular/renal capsule. NFP = 10 mm Hg
> Colloid Osmotic Pressure of Glomerular blood (OPg) – osmotic pressure in the fluid/blood Glomerular Filtration Rate (GFR) – Volume of of the glomerular capsule due to dissolved filtrate formed each minute by the combined substances in blood. (BOP) activity of all 2 million glomeruli in the kidneys. GFR = 125 ml/min
Filtrate Step 2: Tubular Reabsorption
Contains: Occurs throughout the nephron after the - Water renal corpuscle, but most significant in the - Urea Proximal Convoluted Tubule . - Glucose - Amino acids Reclamation Process - Ions: Na+, K+, Mg++, Ca++, Cl-, SO4-, PO4-
2 10/28/2010
Materials Reabsorbed
Include: I. Ions: Calcium, Magnesium, Potassium, Chloride, Some Na+ (most abundant cation), & HCO3 - II. Large Molecules/Metabolites: Amino acids, Glucose, Urea III. Water
Scanning electron micrograph of cut renal tubules (430x)
Process of Reabsorption Reabsorption
Fluid is moved from the filtrate in the Reabsorption of solutes back into the tubule peritubular capillaries makes the blood to the peritubular capillaries. hypertonic > attracts water via osmosis . Movement of materials is AGAINST their concentration gradient. Tubular maximum – max amt of a substance that can be reabsorbed. Reabsorption involves active transport – therefore ENERGY is required.
Step 3: Tubular Secretion Afferent arteriole Glomerular capillaries Reverse of reabsoprtion: selective addition to urine. Efferent arteriole Cortical radiate Movement of substances from blood directly artery Glomerular capsule into the filtrate via active transport. Rest of renal tubule containing filtrate
+, + + Substances Secreted: Drugs, K H , NH 4 Peritubular capillary Secretion very important in acid/base homeostasis. Three major renal processes: Glomerular filtration To cortical radiate vein Tubular reabsorption Tubular secretion Urine
Figure 25.10
3 10/28/2010
Role of Kidney in Urine Composition Acid/Base Regulation <1% of total filtrate During Respiratory Acidosis (CO2 high) Mostly water Kidneys reabsorb bicarbonate and Contains metabolic wastes and unneeded secrete H + (and NH4 + ions). substances e.g. excess ions. Certain disease conditions may reveal solutes that should not be there e.g. proteins During Respiratory Alkalosis (CO2 low) Kidneys secrete bicarbonate and retain H+ and NH4 +
1 At the basolateral membrane, Na + is pumped into the interstitial space by the Na +-K+ ATPase. Active Na + transport creates concentration gradients that drive: 2 “Downhill” Na + entry at the Nucleus Filtrate Interstitial Peri- luminal membrane. in tubule fluid Tubule cell tubular 3 Reabsorption of organic lumen capillary nutrients and certain ions by Na + 2 cotransport at the luminal 3Na + 3Na + membrane. 1 Glucose 4 2K + 2K + Reabsorption of water by Amino 3 acids osmosis. Water reabsorption Some K+ increases the concentration of ions Vitamins the solutes that are left 4 behind. These solutes can
H2O then be reabsorbed as they move down their 5 Lipid-soluble concentration gradients: substances – 2+ + 6 5 Lipid-soluble Cl , Ca , K Cl – and other substances diffuse by the Tight junction Paracellular ions, urea route transcellular route. Primary active transport Transport protein 6 Cl – (and other anions), Secondary active transport Ion channel or aquaporin K+, and urea diffuse by the Passive transport (diffusion) paracellular route.
4