The Urinary System PDF

The Urinary System  The urinary system  The urinary system consists of two kidneys, two ureters, one urinary bladder, and one urethra.  Urine flows from each kidney, down its ureter to the bladder and to the outside via the urethra  Filter the blood and re...

The Urinary System  The urinary system  The urinary system consists of two kidneys, two ureters, one urinary bladder, and one urethra.  Urine flows from each kidney, down its ureter to the bladder and to the outside via the urethra  Filter the blood and return most of water and solutes to the bloodstream  receive 25% of resting cardiac output via renal arteries  Kidney Functions  Regulation of blood ionic composition  Na+, K+, Ca+2, Cl- and phosphate ions  Regulation of blood pH, osmolarity & glucose  Regulation of blood volume  conserving or eliminating water  Regulation of blood pressure  secreting the enzyme renin  adjusting renal resistance  Release of erythropoietin & calcitriol  Excretion of wastes & foreign substances Nephrons  A nephron consists of a renal corpuscle (Bowman’s capsule ) where fluid is filtered, and a renal tubule into which the filtered fluid passes.  A renal tubule consists of  a proximal convoluted tubule (PCT)  loop of Henle (nephron loop)  distal convoluted tubule (DCT).  Distal convoluted tubules of several nephrons drain into to a single collecting duct and many collecting ducts drain into a small number of papillary ducts.  Structure of Renal Corpuscle  Bowman’s capsule surrounds capsular space  podocytes cover capillaries to form visceral layer  simple squamous cells form parietal layer of capsule  Juxtaglomerular Apparatus  Structure where afferent arteriole makes contact with ascending limb of loop of Henle  macula densa is thickened part of ascending limb  juxtaglomerular cells are modified muscle cells in arteriole  Nephrons  The loop of Henle consists of a descending limb, and a thin and a thick ascending limb.  There are two types of nephrons that have differing structure and function.  A cortical nephron usually has its glomerulus in the outer portion of the cortex and a short loop of Henle that penetrates only into the outer region of the medulla.  A juxtamedullary nephron usually has its glomerulus deep in the cortex close to the medulla; its long loop of Henle stretches through the medulla and almost reaches the renal papilla.  Renal Physiology  Nephrons and collecting ducts perform 3 basic processes  glomerular filtration  a portion of the blood plasma is filtered into the kidney  tubular reabsorption  water & useful substances are reabsorbed into the blood  tubular secretion  wastes are removed from the blood & secreted into urine  Rate of excretion of any substance is its rate of filtration, plus its rate of secretion, minus its rate of reabsorption  Glomerular Filtration Rate  Glomerular filtration depends on  glomerular blood hydrostatic pressure (GBHP)  capsular hydrostatic pressure (CHP) -  blood colloid osmotic pressure (BCOP) -  The net filtration pressure is about 10 mm Hg.  Amount of filtrate formed in all renal corpuscles of both kidneys ~ 125 ml/min in average adult male.  Homeostasis requires GFR that is constant  too high: useful substances are lost due to the speed of fluid passage through nephron  too low: sufficient waste products may not be removed from the body  Changes in net filtration pressure affects GFR  filtration stops if GBHP drops to 45mm Hg  functions normally with mean arterial pressures 80-180  Net Filtration Pressure GFR= the amount of Plasma filtrate per time = 125 ml/min  NFP = total pressure that promotes filtration  NFP = GBHP - (CHP + BCOP) = 10mm Hg  Renal Autoregulation of GFR Two mechanisms  myogenic mechanism  Renal Autoregulation of GFR  tubuloglomerular feedback elevated systemic BP raises the GFR so that fluid flows too rapidly through the renal tubule so that Na+, Cl- and water are not reabsorbed, macula densa detects that there is difference in the concentration of these substances, therefore they release vasoconstrictor substance from the juxtaglomerular apparatus which in turn constricts the afferent arterioles & reduce GFR Neural Regulation of GFR an increase in sympathetic activity to the efferent arteriole would cause vasoconstriction and hence increase the filtration pressure and filtration rate. an increase in sympathetic activity to the afferent arteriole would cause vasoconstriction and hence a decrease in filtration pressure and rate. Renal corpuscle. The entire structure is the renal corpuscle.The blue structure (A) is the Bowman's capsule (2 and 3).The pink structure is the glomerulus with its capillaries.At the left, blood flows from the afferent areteriole (9), through the capillaries (10) and out the efferent arteriole (11). The mesangium is the pink structure inside the glomerulus between the capillaries (5a) and extending outside the glomerulus (5b).  Hormonal Regulation  Hormones that affect Na+, Cl- & water reabsorption and K+ secretion in the tubules  angiotensin II and aldosterone  decreases GFR by vasoconstricting afferent arteriole  promotes aldosterone production which causes principal cells to reabsorb more Na+ and Cl- and water  increases blood volume by increasing water reabsorption  atrial natriuretic peptide  relaxes glomerular mesangial cells increasing capillary surface area and increasing GFR  inhibits reabsorption of Na+ and water in PCT & suppresses secretion of aldosterone & ADH  increase excretion of Na+ which increases urine output and decreases blood volume  Antidiuretic Hormone  Increases water permeability of principal cells so regulates facultative water reabsorption  Stimulates the insertion of aquaporin-2 channels into the membrane  Reabsorption Routes  Paracellular reabsorption  50% of reabsorbed material moves between cells by diffusion in some parts of tubule  Transcellular reabsorption  material moves through both the apical and basal membranes of the tubule cell  Transport across membranes can be either  active (primary or secondary)  passive  Na+ reabsorption  Normally 99.5% of Na+ filtered is reabsorbed and only 0.5% is excreted outside in the urine.  67% of Na+ is reabsorbed in the proximal tubule.  25% of Na+ is reabsorbed in the loop of Henle.  8% of Na+ reabsorbed in the distal and collecting tubules  Importance of Na+ reabsorption  Na+ symporters help reabsorb materials from the tubular filtrate  Glucose, amino acids, lactic acid, water-soluble vitamins and other nutrients are completely reabsorbed in the first half of the proximal convoluted tubule  Intracellular sodium levels are kept low due to Na+/K+ pump  Importance of Na+ reabsorption  Thick ascending limb of Henle loop has Na+ K- Cl- symporters that reabsorb these ions.  This part of Henle loop is not permeable to water.  K+ leaks through K+ channels back into the tubular fluid leaving the interstitial fluid and blood with a negative charge  Cations passively move to the vasa recta.  Formation of diluted or concentrated urine  Importance of Na+ reabsorption  Na+ enters principal cells through leakage channels  Na+ pumps keep the concentration of Na+ in the cytosol low  Cells secrete variable amounts of K+, to adjust for dietary changes in K+ intake  Aldosterone increases Na+ and water reabsorption & K+ secretion by principal cells by stimulating the synthesis of new pumps and channels.  Chloride reabsorption  65% of Cl- in filtrate is absorbed from proximal convoluted tubule with Na+ (Na+ Cl- symporters).  Absorption of Na+ and other cations create an electrical gradient which stimulates Cl- reabsorption  Water absorption  Water is only reabsorbed by osmosis  obligatory water reabsorption occurs when water is “obliged” to follow the solutes being reabsorbed, this process is performed by aquaporin-1  65% of obligatory water reabsorption occurs in PCT.  15% of obligatory water reabsorption occurs in descending loops of Henle.  facultative water reabsorption occurs in collecting duct under the control of antidiuretic hormone, this process is performed by aquaporin-2 Reabsorption of Bicarbonate, Na+ & secretion of H+ Ions  Na+ antiporters reabsorb Na+ and secrete H+  PCT cells produce the H+ & release bicarbonate ion to the peritubular capillaries  important buffering system  For every H+ secreted into the tubular fluid, one filtered bicarbonate eventually returns to the blood  Secretion of H+ and Absorption of Bicarbonate by Intercalated Cells  Proton pumps (H+ATPases) secrete H+ into tubular fluid  can secrete against a concentration gradient so urine can be 1000 times more acidic than blood  Cl-/HCO3- antiporters move bicarbonate ions into the blood  intercalated cells help regulate pH of body fluids  Urine is buffered by HPO4 2- and ammonia, both of which combine irreversibly with H+ and are excreted  Reabsorption in the Distal Convoluted Tubule  As fluid flows along the DCT, reabsorption of Na + and Cl - continues due to Na+-Cl - symporters.  Na+ and Cl- then reabsorbed into peritubular capillaries  The DCT serves as the major site where parathyroid hormone stimulates reabsorption of Ca +2.  DCT is not very permeable to water so the solutes are reabsorbed with little accompanying water. Summary