4/7/08 Urinary System Chapter 24 – Day 2

4/7/08 Review Nephron Structure  Network with blood vessels  Two types of nephrons ♦Cortical Nephrons – loop of Henle does not extend below cortex ♦Juxtamedullary nephrons – loop of Henle extends into medulla ♦Vasa recta – feature of jextamedullary nephrons ♦85% are cortical nephrons Majority of process is done here ♦15% are juxtamedullary Important role in concentrating urine Conserving water in the body

4/7/08 Renal corpuscle  Renal arteries branch into afferent arterioles in the cortex  One afferent arteriole enters the renal corpuscle  Wall of the renal corpuscle is the Bowman’s capsule  Afferent arteriole branches into capillary network ♦Glomerulus, inside the corpuscle within the bowman’s capsule  A second efferent arteriole connects to the glomerulus – brings blood out of the renal corpuscle ♦(Efferent arteriole is thinner than afferent)  Efferent arteriole branches around nephron as peritubular capillaries and vasa recta  Eventually the efferent arteriole connects to venules – carries blood to renal veins and out of the kidney

4/7/08 Nephrons & blood supply Fig. 24.7

4/7/08 Kidneys – Blood supply Fig. 24.5

4/7/08 Glomerulus  Surrounded by 2 layers ♦Bowman’s capsule: Visceral epithelium Parietal epithelium ♦Separated by capsular space  Blood in the glomerulus is bounded by 2 walls ♦Endothelial cells of the blood vessels ♦Visceral layer of the bowman’s capsule Fig. 24.8

4/7/08 Glomerulus  Endothelial cells of blood vessels ♦Have large diameter pores: Fenestrations Basement membrane  Visceral Layer of the Bowman’s capsule ♦Special basement membrane = lamina densa ♦Special cells = podocytes These have special feet like structures called pedicels Spaces between pedicels = filtration slit/slit pore  Special structures of the endothelial cells and visceral layer restrict the passage of particles according to size (analogous to the filter paper)  Capsular space = place where filtrate is released

4/7/08 Glomerulus - barriers Fig. 24.8

4/7/08 Glomerulus blood boundaries Fig. 24.8c

4/7/08 Filtration in the Glomerulus  Blood rushes into glomerulus via the afferent arteriole  Difference in diameter between afferent & efferent arterioles results in High Blood Pressure inside the Glomerulus ♦= Effective Filtration Pressure (EFP)  This pushes blood against the endothelial cells ♦Filtration takes place ♦Fenestrated capillaries – stops blood cells from passing through, but all other contents go through ♦Basement membrane – stops large proteins ♦Pedicels/filtration slits – stops medium-sized proteins from leaving blood

4/7/08 Glomerular Filtration Fig

4/7/08 Filtrate  Remaining contents end up in the capsular space and are called filtrate  Contents of filtrate: ♦Water ♦Small proteins ♦Glucose ♦Amino acids ♦Vitamins ♦Citric acid ♦Ions: Na +, Cl -, K +, HCO 3 ♦ ammonia ♦Urea ♦Bilirubin ♦Neurotransmitter ♦Hormones

4/7/08 Filtrate - PCT  Filtrate passes to proximal convoluted tubule (PCT) ♦PCT walls = simple cuboidal epithelium ♦Outside fluid = peritubular fluid ♦Then peritubular capillaries  PCT – reabsorption begins ♦99% of the reabsorption is of organic materials Glucose, amino acids, vitamins, citric acid ♦Facilitated diffusion ♦Transport of Ions = active transport & passive ♦Water reabsorption = osmosis  Enters peritubular fluid, then to capillaries

4/7/08 Filtrate – Loop of Henle  Tubular fluid with remaining contents flows into Loop of Henle ♦Descending & Ascending Limbs  Juxtamedullary nephrons – in contact with the vasa recta  Reabsorption of water & solutes ♦(specifically Na + & Cl - )  Descending & Ascending limbs have different properties – very important in concentrating urine

4/7/08 Loop of Henle  Descending limb = thin ♦Permeable to H 2 O ♦Mostly impermeable to salts (Na +,Cl -)  Ascending limb = thick ♦Impermeable to H 2 O and solutes ♦Prevents diffusion by concentration gradient ♦Allows selective permeability ♦Only Na + & Cl - can leave via active transport while wastes stay in ♦Salts (Na +,Cl - ) are pumped out by active transport = reabsorption of Na + & Cl - into blood Fig

4/7/08 Loop of Henle  As tubular fluid moves up, salts enter peritubular fluid via active transport into vasa recta  Results in a high solute concentration of peritubular fluid  Know: ♦Direction of H 2 O? Direction of salts? ♦From which limb? ♦What is the significance/importance? ♦How does the countercurrent work? Fig

4/7/08 Loop of Henle  Countercurrent flow drives reabsorption ♦Osmotic concentration is constantly maintained ♦Allows reabsorption of H 2 O into blood b/c osmotic concentration is high in the peritubular fluid (due to high salts) ♦= contercurrent multiplication (exchange increases)  Loss of H 2 O from descending limb concentrates solutes in urine  Tubular fluid containing mostly wastes & some ions enters distal convoluted tubule (DCT)

4/7/08 Filtrate – Distal Convoluted Tubule  Further reabsorption of salts (Na + & Cl - ) ♦Active and Passive  Presence of aldosterone alters rate of Na + reabsorption  Ca 2+ may be reabsorbed depending on calcitriol  Secretion also occurs in DCT  Wastes are not reabsorbed – moves forward to collecting tube Fig

4/7/08 Filtrate – Distal Convoluted Tubule  Further reabsorption of salts (Na + & Cl - ) ♦Active and Passive  Presence of aldosterone alters rate of Na + reabsorption  Ca 2+ may be reabsorbed depending on calcitriol  Secretion also occurs in DCT  Wastes are not reabsorbed – moves forward to collecting tube Fig

4/7/08 Nephron - Secretion  Reabsorption & secretion take place in the DCT  Secretion: ♦Other wastes that did not enter the nephron during filtration ♦Transported later by secretion ♦Peritubular capillaries → nephron ♦PCT & DCT are involved in secretion ♦K + and H + ions – depends on blood levels/blood conditions Example: H + ions increase from CO 2 conversion, they are secreted to maintain balance Diabetes: ketones are released = acidic, so more H + is secreted ♦NH 4 + ions (comes from removal of NH 3 group from amino acids) ♦Other secreted compounds = drugs, creatinine, neurotransmitters

4/7/08 Filtrate – Collecting Duct  Urine moves into the collecting tubule (urine) ♦Some reabsorption: Na + ions HCO 3 - ions Some urea reabsorption (helps to maintain high salt concentration in peritubular fluid)  Concentration gradient is very important to keep the flow of things moving properly ♦Ions & H 2 O enter peritubular fluid, then go to vasa recta Keeps a constant concentration gradient of high salt levels in the peritubular fluid  H 2 O reabsorption in kidney = very important process ♦Helps to keep H 2 O balanced in the body ♦Depends on solute concentration

4/7/08 Urine - hydration  If the body is well hydrated H 2 O absorption occurs only in the descending limb of Loop of Henle  Solute concentration gradient in Loop of Henle & Collecting Tubule Fig

4/7/08 Urine - hydration  Dilute urine = large volume because high amounts of water are retained  Only ions are reabsorbed heavily  If the body is dehydrated – activates pituitary gland to produce antidiuretic hormone (ADH) ♦Body conserves H 2 O in the kidney & thirst response for more H 2 O intake ♦Special H 2 O channels open in collecting tubule ♦H 2 O is reabsorbed from collecting tubule ♦Driven by diffusion because salt concentration is already high outside ♦Yields concentrated urine – small volume (conc. urea)  Diuretics like caffeine: ♦Prevent H 2 O reabsorption in collecting tubule, force H 2 O out of cells ♦Leads to dilute urine but reduces blood volume ♦Are also used to alter blood pressure

4/7/08 Fig. 24.6

4/7/08 Kidneys – Blood supply Fig. 24.7

4/7/08 Kidneys – Blood supply Fig. 24.8

4/7/08 Kidneys – Blood supply Fig. 24.5

4/7/08 Kidneys – Blood supply Fig. 24.5