urinary system kidneys: formation of urine -contains the functional unit for filtration = nephron...
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Urinary System
Kidneys: formation of urine-contains the functional unit for filtration = Nephron-production of urine, absorption of waterand salts
Ureters: transfer of urine from kidneysto bladder Urethra: transfer of urine from bladder to outside - longer into the male (20 cm vs. 4 cm in the female)
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Kidneys• 10-12 cm• retroperitoneal – behind the peritoneum
– not part of the abdominal cavity• surrounded by three layers of tissue
– 1. deepest layer = renal capsule – transparent sheet of dense irregular connective tissue
• continuous with the outer coat of the ureter– 2. middle layer = adipose capsule
• amass of fatty tissue surrounding the renal capsule
– 3. outer layer = renal fascia• thin layer of dense irregular connective tissue
that anchors the kidney to the abdominal wall
• divided internally into an outer cortex and an inner medulla
– medulla consists of 8 to 18 cone-shaped regions called renal pyramids
– the wider base faces towards the cortex, the narrow region (renal papilla) projects down into a cup-like structure called a minor calyx
– renal cortex is divided into an outer cortical zone and a deeper juxtamedullary zone
– the cortex also extends down in between the pyramids to form the renal columns
– renal lobe = renal pyramid + the overlying renal cortex + ½ the adjacent renal colum
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Blood supply
• supplied by a renal artery and drained by a renal vein(s)
• kidney receives 20-25% of the resting cardiac output through the renal arteries (1200mL per minute)
• renal artery divides into segmental arteries – supply segments of the kidney
• the segmental arteries give off branches that pass through the renal columns – interlobar arteries
• at the base on the renal pyramids – between the medulla and cortex – they are called arcuate arteries
• divisions from the arcuate are called the interlobular arteries (pass between the renal lobes)
• the afferent arterioles are derived from the interlobular arteries
• afferent arteriole supplies one nephron and forms the glomerulus (capillary network)
• drainage of the glomerulus is via the efferent arteriole
• efferent arteriole forms the peritubular capillary network which surround the upper portions of the nephron
• an extension of this network covers the lower portion of the nephron (loop of henle) – vasa recta
• the peritubular capillaires form the interlobular veins – arcuate veins – interlobar veins – renal vein
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The Nephron-about one million nephrons-kidneys filter 180 L fluid per day!!!!-each nephron is a renal corpuscle + renal tubules-renal corpuscle: filtering unit consisting of atangled cluster of capillaries -> glomerulus + Bowman’s capsule-tubules: for reabsorption of water and ions leadingto final urine volume and composition -PCT +Loop of Henle + DCT
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Cortical Nephron
• 80-85% of nephrons are cortical nephrons
• Renal corpuscles are in outer cortex and loops of Henle lie mainly in cortex
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Juxtamedullary Nephron
• 15-20% of nephrons are juxtamedullary nephrons
• Renal corpuscles close to medulla and long loops of Henle extend into deepest medulla enabling excretion of dilute or concentrated urine
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Urinary System Function
1. Excretion of Metabolic Wastes: nitrogenous wastes-Urea: by-product of amino acid metabolism -produced when ammonia + carbon dioxide-Creatinine: produced by breakdown of creatine phosphate
(high energy molecule reserve of muscles)-Uric acid: by-product of nucleotide breakdown
-insoluble and ppts in the blood, concentrates in joints
2. Water-Salt balance of blood: reabsorption into blood from the descending Loop of Henle, from collecting duct-reclaim salt from the ascending portion of Loop of Henle-reclaim urea from bottom section of collecting duct-release of anti-diuretic hormone by pituitary: increase reabsorption of water
3. Acid-Base balance of blood: reabsorption of bicarbonate ions from urine in the nephron decreases levels in blood (decreases carbonic acid levels)-movement of hydrogen ions from blood into the nephron, combineswith ammonia to form ammonium (NH4
+)
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4. Secretion of hormones: release of renin by kidneys which leadsto release of aldosterone by adrenal glands (reabsorption ofsalts by kidneys) -release of erythropoietin by kidneys (stimulates RBC production)-activation of vitamin D produced by the skin
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Water Balance
-extracellular fluids: blood plasma, interstitial fluid, CSF, etc….-intracellular fluids: cytosol-unique distribution of ions in ECF and ICF
e.g. -intracellular fluids: higher potassium, phosphate, magnesium - lower sodium, chloride and bicarb ions than in extracellular fluid
-of the 40 liters of water in the average male - 37% is ECF and 63%is ICF
-so the kidney’s ability to modulate the composition of blood plasma can determine the composition of interstitial fluid and therefore ICF
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Water Intake
-average intake - 2.5 L(60% from drinking water, 30% from moist foods, 10% byproduct of metabolism)-regulation of intake - thirst center within the hypothalamus
e.g. as body loses water - osmoreceptors within the thirst centerdetect increase in osmotic pressure within the ECF(increase as little as 1%)
-drinking distends the stomach which inhibits signalling from thethirst center
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Water output
-loses through urine, feces and sweat plus respiration and skin evaporation-2.5 L of water must be lost for water balance
-60% lost in urine, 6% in feces, 6% in sweat, 28% evaporationfrom skin and lungs
-primary means of controlling output is through urine production
-dehydration: ECF becomes concentrated - increase osmotic pressure- pressure increase detected by osmoreceptors in hypothalamus-posterior pituitary gland releases anti-diuretic hormone (ADH)-ADH causes distal convoluted tubule and collecting duct toincrease water reabsorption
-excess water intake: ECF less concentrated - decrease in O.P-osmoreceptors signal to the post. pituitary-P.P decreases ADH release-kidney/nephrons decrease water reabsorption
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Renal physiology
• comprised of filtration at the capsule (1)• reabsorption through the tubules (2)• direct secretion by the cells lining these tubules (3)
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• glomerulus: capillary tangle derived from afferent arterioles (into) and lead into efferent arterioles (out)
• surrounded by a glomerular capsule (Bowman’s capsule) – single layer of epithelial cells
• glomerular capsule: site of initial filtration and the first step in the formation of urine– consists of visceral and parietal layers– visceral layer consists of modified epithelial cells = podocytes– the podocytes wrap around the endothelial cells of the glomerular capillaries and forms the
filtration membrane together with the endothelial cell wall• slits are covered with a slit membrane that permits the passage of small molecules such as water, vitamins, amino acids,
wastes and small plasma proteins
– space between the visceral and parietal layers = glomerular capsule– between the union of the afferent and efferent arterioles are mesangial cells that help regulate
the rate of glomerular filtration
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Renal Physiology• 1. Glomerular filtration
– depends on three main pressures• 1. glomerular blood pressure (GBP) – BP in the glomerular capillaries (55 mmHg)
– promotes filtration by forcing water and solutes through the filtration membrane• 2. caspsular hydrostatic pressure (CHP) – hydrostatic pressure exerted against the filtration
membrane by fluid already in the bowman’s capsule– opposes filtration from the blood– 15 mm Hg
• 3. blood collioid osmotic pressure (BCOP) – due to the presence of plasma proteins in the blood
– opposes filtration from the blood– 30 mmHg
• net filtration pressure (NFP) = GBP – CHP – BCOP = 10 mm Hg• loss of plasma proteins in the urine can cause edema (increased interstitial fluid)
– damage to the glomerular capillaries can increase their permeabilty – loss of the larger plasma proteins
– this increases the BCOP which draws larger amounts of water out of the blood and into the urine
– but the BCOP decreases because we are losing these plasma proteins in the urine– the overall drop in BCOP causes water to leave the blood and enter the tissues
systemically
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Glomerular filtration rate
• glomerular filtration rate (GFR) – amount of filtrate formed per minute (125 mL/min)– affected dramatically by NFP– adjusted by regulating: 1) blood flow into and out of the
glomerulus and 2) the glomerular capillary surface area available for reabsorption
– three mechanisms control GFR
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GFR• 1. renal autoregulation – two mechanisms – myogenic mechanism and
tubuloglomerular feedback– myogenic mechanism – increased blood volume can increased GFR
• by the stretching of the afferent arterioles triggers the contraction of the smooth muscle lining these arterioles
– tubulogomerular mechanism – feedback provided to the glomerulus from the renal tubules
• increase in the fluid through the PCT, LH and DCT – less time to reabsorb materials
• cells in these tubules induce vasoconstriction in the afferent arterioles• if GFR drops below normal – these cells stimulate the release of NO from the
juxtaglomerular cells – vasodilation which increases blood flow and GFR• 2. neural regulation – sympathetic ANS fibers release norepinephrine
which causes vasoconstriction of the smooth muscle in the afferent arteriole
• 3. hormonal regulation – release of angiotensin II reduces GFR by inducing vasoconstriction– also release of atrial natriureic peptide (ANP – from the cardiac cells)
increases GFP by increasing the surface area of the glomerulus
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PCT and Loop of Henle• proximal convoluted tubule: first area of reabsorption into blood ->
Loop of Henle -> distal convoluted tubule -> collecting duct -> union of ducts into ureter
• cells of these tubules are also single epithelial layers – vary as either cuboidal (PCT and DCT, descending) or squamous (ascending LH)
• PCT and DCT surrounded by the peritubular network of capillaries for reabsorption back into the blood, LH is covered with the vasa recta
• PCT is the site of water reabsorption (PASSIVE) - associated with the ACTIVE reabsorption of sodium and potassium ions– active Na+ and K+ uptake by the blood from the PCT is by sodium pumps
- sodium pumped from the PCT and chloride, bicarbonate and phosphate ions follow it - salt reabsorption
– the active transport of ions into the blood plasma increases osmotic pressure within the blood
– therefore water moves out of the PCT into the capillaries PASSIVELY!• PCT reabsorbs about 70% of filtered Na+, ions and water
– the apical surface of the PCT epithelium forms microvilli which increases the surface area of this region
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Loop of Henle• active transport of Na+ continues through the loop of Henle
and DCT• descending loop of Henle is quite permeable to water but
impermeable to solute movement – urine becomes hypertonic (increased ions within the urine, decreased water)
• ascending loop is the opposite – permeable to salt (salt pumped out of the urine back
• into the blood plasma)• the wall of the arterioles alongside the ascending portion of the
LH contain modified smooth muscle cells = juxtaglomerular cells– regulate blood pressure within the kidneys
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DCT and Collecting Duct
• two types of cells found in the DCT and CD – principal cells – receptors for ADH and aldosterone
– intercalated cells – play a role in the homeostasis of blood pH
• DCT and collecting duct are impermeable to water !!!!
• the DCT and CD become permeable upon action of hormones
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• Tubular reabsorption– tubule cells reabsorb about 99% of the filtered water and many of the
solutes – principal materials reabsorbed – glucose, aminao acids, urea, Na+, K+,
Ca+, Cl-, HCO3- and HPO4-– return to the blood through reabsorption into the peritubular capillary
network and vasa recta– reabsorption = return to the blood– absorption = entrance of new materials into the blood (e.g. via digestive
absorption)– reabsorption routes – one of two routes before re-entering the blood
Renal Physiology
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Reabsorption Routes
• Paracellular reabsorption • between adjacent tubule cells into the
blood
– 50% of reabsorbed materialmoves between cells bydiffusion in some parts oftubule
• Transcellular reabsorption– material moves through
both the apical and basalmembranes of the tubulecell by active transport
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• Tubular secretion– tubular cells also secrete other materials –
wastes, drugs, excess ions into the urine– this also removes these materials from the
blood
Renal Physiology
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Reabsorption in the PCT• 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
Reabsorption of Nutrients
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Reabsorption of Bicarbonate, Na+ & 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
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Passive Reabsorption in the 2nd Half of PCT
• Electrochemical gradients produced by symporters & antiporters causes passive reabsorption of other solutes
• Cl-, K+, Ca+2, Mg+2 and urea passively diffuse into the peritubular capillaries
• Promotes osmosis in PCT (especially permeable due to aquaporin-1 channels
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Secretion of NH3 & NH4+ in PCT
• Ammonia (NH3) is a poisonous waste product of protein deamination in the liver– most is converted to urea which is less toxic
• Both ammonia & urea are filtered at the glomerus & secreted in the PCT– PCT cells deaminate glutamine in a process that
generates both NH3 and new bicarbonate ion.• Bicarbonate diffuses into the bloodstream
– during acidosis more bicarbonate is generated
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Reabsorption in the Loop of Henle
• Tubular fluid– PCT has reabsorbed 65% of the filtered water
so chemical composition of tubular fluid in the loop of Henle is quite different from plasma
– since many nutrients were reabsorbed as well, osmolarity of tubular fluid is close to that of blood
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Countercurrent Mechanism: Reabsorption at Loop of Henle
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Symporters in the Loop of Henle• Thick limb of loop of
Henle has Na+ K- Cl- symporters that reabsorb these ions
• 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
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Reabsorption in the DCT
• Removal of Na+ and Cl- continues in the DCT by means of Na+ Cl- symporters
• Na+ and Cl- then reabsorbed into peritubular capillaries
• DCT is major site where parathyroid hormone stimulates reabsorption of Ca+2– DCT is not very permeable to water so it is not
reabsorbed with little accompanying water
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Reabsorption & Secretion in the Collecting Duct
• By end of DCT, 95% of solutes & water have been reabsorbed and returned to the bloodstream
• Cells in the collecting duct make the final adjustments– principal cells reabsorb Na+ and secrete K+– intercalated cells reabsorb K+ & bicarbonate
ions and secrete H+
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Actions of the Principal Cells
• Na+ enters principal cellsthrough leakage channels
• Na+ pumps keep theconcentration of Na+ inthe cytosol low
• Cells secrete variableamounts of K+, to adjustfor dietary changes in K+intake– down concentration gradient due to Na+/K+ pump
• Aldosterone increases this Na+ reabsorption (and passive water reabsorption) & K+ secretion by principal cells by stimulating the synthesis of new pumps and channels.
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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 (secreted by cells of PCT), both of which combine irreversibly with H+ and are excreted
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Production of Dilute or Concentrated Urine
• Homeostasis of body fluids despite variable fluid intake
• Kidneys regulate water loss in urine• ADH controls whether dilute or concentrated urine
is formed– if lacking, urine contains high ratio of water to solutes
– dilute urine – reabsorption of ions is unchanged (normal) but ADH decreases reabsorption of water
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Summary
• H2O Reabsorption – PCT---65%– loop---15%– DCT----10-15%– collecting duct---
5-10% with ADH
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Renin-Angiotensin-Aldosterone• when blood volume and BP drop – the walls of the
afferent arterioles are stretched less – juxtaglomerular cells secrete renin into the blood (also stimulated by sympathetic stimulation)
• in the blood renin cleaves angiotensinogen (made by hepatocytes) to form angiotensin I
• the enzyme ACE (in the lung) – cleaves this even more to form angiotensin II– 1. decreases GFR by causing vasoconstriction of
afferent arterioles– 2. enhances reabsorption of Na+, Cl+ and water in the
PCT by stimulating the Na/H antiporter– 3. stimulates the release of aldosterone by the adrenal
cortex – stimulates the principal cells of the DCT collecting ducts to reabsorb more Na and Cl and secrete more K into the blood
• osmotic consequence of this causes an increased reabsorption of water
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ADH and ANP• ADH – released by the posterior pituitary
– regulated water reabsorption by increasing the permeability of the principal cells in the DCT to water
– in the absence of ADH the principal cells of the DCT and CT have low permeability to water
– within the principal cells are vesicles containing a protein called aquaporin-2
• ADH stimulates the insertion of aquaporin-2 into the apical membrane• water permeability increases• when the OP of the blood plasma increases (decreased water concentration )
via increased filtration – osmoreceptors in the hypothalamus detect this drop and stimulate the release of ADH
• increased permability to water reintroduces water back into the blood and lower the OP of the blood plasma
• ANP – inhibits the reabsorption of Na and water in the PCT and the collecting duct– also suppresses the secretion of aldosterone and ADH
• increases the excretion of Na in the urine (natriuresis) and increase urine output (diuresis) which decreases blood volume and BP and inhibits its further release