The Urinary System

1. The Urinary System Part 4: Regulation & Maintenance

2. The Urinary System • Goals of the Urinary System: Regulate chemical composition of the bodily fluids & eliminate waste products from the body. • Organs of the Urinary System: Two kidneys, two ureters, the urinary bladder, & the urethra.

4. The Kidneys • The Kidneys: Bean-shaped organs, part of the urinary system, that filter wastes & excess products from the bloodstream so they can be removed from the human body. • Retroperitoneal: Located posterior to the peritoneum of the abdominal cavity – technically outside the abdominal cavity. • 0.5% of the total body weight. • 20-25% of the total arterial blood is received by the kidneys.

6. Functions of the Kidneys • Filter waste products from the blood, returning useful materials & eliminating wastes and excesses of normal components. • Regulate the chemical composition of blood. • Regulate volume & composition of bodily fluids. • Regulate blood volume & pressure. • Regulate the osmolarity of bodily fluids. • Secrete renin (an enzyme that helps to control blood pressure & electrolyte balance). • Secrete erythropeitin (hormone that stimulates red blood cell production). • Help regulate acid-base balance of body fluids. • Synthesize calcitriol (hormone that helps regulate calcium balance). • Detoxify free radicals & some drugs. • Gluconeogensis: The synthesis of glucose from non-carbohydrates to release glucose in the blood & maintain blood glucose levels at normal. • Regulate blood ion levels. • Participate in Vitamin D synthesis.

7. External Kidney Anatomy • The Kidneys: Bean-shaped reddish-colored organs, lying just above the waist between the peritoneum & the posterior wall of the abdomen, behind the abdominal cavity. • The right kidney is slightly lower than the left due to the placement of the liver superior to the right kidney. • Lateral Surface is convex. • Medial Surface is concave. • Renal Hilus: The point at which the ureter, blood vessels, lymphatic vessels, & nerve endings connect.

9. Kidney Anatomy • Three layers of tissue surrounding each kidney: • Renal Capsule: The deep inner layer of smooth, transparent dense irregular connective tissue that is continuous with the other layer of the urethral covering. • Provides a barrier against trauma & helps maintain the shape of the organ. • Adipose Capsule: The fatty tissue surrounding the renal capsule – helps protect the kidney & hold it in place. • Renal Fascia: Outermost layer of thin, dense irregular connective tissue hat anchors the kidneys to the surrounding structure & the abdominal wall.

10. Internal Kidney Anatomy • Two internal regions: • Renal Cortex: The superficial, reddish, smooth textured area running from the renal capsule to the bases of the renal pyramids. • Divided into the cortical and juxtamedullary zones. • Renal Columns: The portions of the renal cortex that extends between the renal pyramids. • Renal Medulla: The deep, reddish-brown inner region. • Renal Pyramids: 8-18 fan-shaped structures consisting of the straight segmented renal tubules & the vasa recta. • The base facing the renal cortex and the apex (aka renal palilla) pointing toward the renal hilus. • Renal Lobe: The renal pyramid, its overlaying cortex, and ½ of each adjacent renal column.

12. Internal Kidney Anatomy • Parenchyma: Composed of the renal cortex & the renal pyramids. • Nephrons: 1 million microscopic structures designed to filter the blood – located within the parenchyma. • Papillary Ducts: Ducts that drain the urine formed in the nephrons into the… • Minor Calyces: 8-18 in each kidney, which receive urine from the papillary ducts & delivering it to the major calyces. • Major Calyces: 2-3 in each kidney, receive urine from the minor calyces & drain it into the large cavity of the renal pelvis. • Renal Pelvis: Drains the urine into the ureter to be taken to the urinary bladder.

13. Blood Supply to the Kidneys • Renal Arteries: The main blood vessel bringing blood into the renal area. • Segmental Arteries: The subdivisions of the renal arteries that branch into the parenchyma. • Interlobar Arteries: Pass through the renal columns between the renal pyramids. • Arcuate Arteries: The branches that arch between the renal medulla & renal cortex. • Interlobular Arteries: The divisions of the arcuate arteries.

14. Blood Supply to the Kidneys • Golmerulus: The ball-shaped capillary network serving the nephrons – these are part of both the cardiovascular & urinary systems. • This is a unique capillary bed since it is located between two arterioles instead of an arteriole & an a venule! • Afferent Arterioles: The divisions of the interlobular arteries that enter the renal cortex & supply the glomerulus with blood. • Efferent Arterioles: Carry blood back out of the glomerulus. • Peritubular Capillaries: Further divisions of the efferent arterioles that surround the tubular portions of the nephron & reclaim most of the filtrate the glomerulus produces. • Vasa Recta: The long loop-shaped capillaries that extend from some efferent arterioles that supply the tubular portions of the nephron in the renal medulla.

16. Blood Supply to the Kidneys • Peritubular Venules: The first veins that the peritubular capillaries reunite into. • Interlobular Veins: The merged peritubular venules, also receiving blood from the vasa recta. • Arcuate Veins: The next largest set of veins the interlobular veins drain into. • Interlobar Veins: Drain the blood from between the renal pyramids. • Renal Vein: The large, single vein that all blood exits the kidneys through. • Exits from the renal hilus & varies venous blood back to the inferior vena cava.

18. Blood Flow to the Kidneys • Control of blood flow to the kidneys occurs via the renal nerves, formed in the celiac ganglion & pass through the renal plexus along with the renal arteries. • The sympathetic division of the Autonomic Nervous System regulates the flow of blood through the kidneys via vasodilation or vasoconstriction.

19. Blood Flow to the Kidneys • Path of Blood Flow through the kidneys: • Renal Arteries • Segmental Arteries • Arcuate Arteries • Interlobular Arteries • Afferent Arterioles • Glomerular Capillaries • Efferent Arterioles • Peritubular Capillaries • Interlobular Veins • Arcuate Veins • Interlobar Veins • Renal Vein

21. The Nephron • Nephron: The functional unit of the kidney consisting of a microscopic tube open at one end and closed at the other. • Over 1 million nephrons in each kidney. • Nephrons are the unit that form the urine. • Two Parts to a Nephron: • Renal Corpuscle: The portion where the blood plasma is filtered. • Glomerulus: The capillary network where blood enters to be filtered. • Glomerular (Bowman’s) Capsule: The double-walled epithelial cup that surrounds the glomerular capillaries. • Renal Tubule: The tube into which the filtered fluid is passed.

23. The Nephron • Glomerular (Bowman’s) Capsule: The double-walled epithelial cup that surrounds the glomerular capillaries. • This is where blood plasma is actually filtered before moving into the renal tubule. • Capsular Space aka Lumen of the Bowman’s Capsule: The place where plasma leaves the glomerular capillaries – plasma is now known as filtrate. • Filtrate: Similar to blood, but lacking blood cells & large proteins. • Two layers of the Bowman’s Capsule: • External Parietal Layer: The simple squamous epithelium layer – not involved in filtrate formation. • Visceral Layer: Made up of podocytes (highly branching epithelial cells) and is involve in filtrate formation.

25. The Nephron • Renal Tubule: The tube into which the filtered fluid is passed. 3 sections… • Proximal Convoluted Tubule (PCT): The part of the tubule that is attached to the flomerulus. Fluid flows from the PCT into the… • Loop of Henle aka the Nephron Loop: The portion of the loop that connects the proximal & distal convoluted tubules. • Descending Limb of the Loop of Henle: The portion that dips into the renal medulla. • Ascending Limb of the Loop of Henle: The portion that makes a U-turn & returns to the renal cortex. • Distal Convoluted Tubule (DCT): The part of the tubule that is farthest away from the glomerulus. Two cell types exit here: • Principal Cells: Have receptors for antidiuretic hormone & aldosterone to regulate fluid & electrolyte balance. • Intercalated Cells: Help in the homeostasis regulation of blood pH. • Collecting Duct: The vessel that collects the filtrate from the distal convoluted tubule. • Papillary Ducts: The convergence of the collecting ducts (several hundred in the body). They drain into the minor calyces.

27. The Nephron • Glomerular Tubule: The tube formed by the wall of the nephron pushing into the glomerolar capsule at the closed end of the nephron. • Renal Tubule: The final part of the ascending limb of each nephron, which makes contact with the afferent arteriole. • Juxtaglomerular Apparatus (JGA): Helps regulate the blood pressure in the kidneys. Made up of.. • Macula Densa: The crowded area of columnar tubules found in the renal tubule. • Juxtaglomerular (JG) Cells: The afferent arteriole containing modified, smooth muscle fibers that runs along side the macula densa.

28. Types of Nephrons • Cortical Nephrons: Nephrons whose renal corpuscles are located in the outer portion of the renal cortex. • Have short Loops of Henle (not entering too deeply into the renal medulla) with blood supplied by peritubular capillaries. • Vital in assuring blood has the correct chemical & ionic makeup. • The most common type - 85% of nephrons. • Juxtamedullary Nephrons: Nephrons whose renal corpuscles located deep in the cortex, close to the medulla. • Have long Loops of Henle extending deep into the medulla with blood supplied by the peritubular capillaries & the vasa recta. • Ascending limbs divide into two portions.. The thin ascending limb & thick ascending limb. • Allow the kidneys to produce very dilute or very concentrated urine. • Make up the remaining 15-20% of nephrons.

30. Urine Formation • Blood flows into the kidneys and into the glomerular capillaries. • Some of the blood in the glomerular capillaries is forced into the renal tubules, which flow alongside of the peritubular capillaries. • 99% of water forced into the renal tubules gets reabsorbed into these vessels. • Fluid is filtered from the blood into the renal tubules – any particle small enough to pass through the membranes will be filtered out. • This includes glucose, amino acids, vitamins, & minerals. • Any needed items are reabsorbed back into the blood stream while still in the renal tubules. • Waste products, including creatinine, toxins, & drugs are not reabsorbed & are excreted via urine. • Kidneys can regulate how much sodium, potassium, hydrogen, & bicarbonate remain in the plasma. • Urine formed through 3 basic processes: • Glomelular filtration • Tubular secretion • Tubular reabsorpton

32. Glomerular Filtration • Glomerular Filtration: The first step of urine formation! • Water & most dissolved solutes in the blood plasma cross the capillary walls in the glomerulus to move into the glomerular capsule & then the renal tubule. • Glomerular Filtrate: The fluid that enters the capsular space. • Filtration Fraction: The fraction of blood plasma in the afferent arterioles of the kidneys that becomes glomerular filtrate – typically 16-20% • Men produce 180 liters in filtrate daily while women produce 150 liters of filtrate daily. • 99% of filtrate returns to the blood via reabsorption. • 1-2 liters are excreted as urine.

34. Glomerular Filtration • Filtration Membrane: A layer of 3 barriers that fluid must pass through to move from the bloodstream to the capsular space. • Fenestrated Epithelium: Found in the glomerular capillaries – large pores permit entry of blood plasma into the capillaries, but prevent blood & platelets from passing. • Mesangial Cells: Cells specifically in charge of regulating glomerular filtration. • Basal Lamina: Layer of acellular material located between the endothelium & podocytes, consisting of collagen fibers & proteoglycans – prevets the filtration of larger plasma proteins. • Slit Membrane: Extends across each filtration slit to permit the passage of small diameter (less than 6-7nm) molecule • E.g., water, glucose, vitamins, amino acids, small proteins, ammonia, urea, & ions. • Pedicels: Footlike processes located on the podocytes (thousands on each) that wrap around glomerular cavities. • Filtration Slits: Spaces between the pedicles.

36. Glomerular Filtration • In order for filtration to occur, pressure is needed to force fluids & solutes through the membranes. • Renal corpuscles filter the largest volume of fluid of all capillaries due to…. • Large surface areas of the glomerular capillaries – mesangial cells can relax to maximize surface area & increase filtration or contract to decrease surface area and filtration. • Filtration membrane is thin & porous. • Glomerular capillary blood pressure is high due to afferent arteriole diameter exceeding efferent arteriole diameter.

37. Glomerular Filtration • Glomerular Pressure dependent on 3 processes: • Glomerular Blood Hydrostatic Pressure (GBHP): The blood pressure in the glomerular capillaries – promotes filtration by forcing water & solutes from the blood plasma through the filtration membrane. Normally 55 mm Hg. • Capsular Hydrostatic Pressure (CHP): The measure of the hydrostatic pressure exerted against the filtration membrane by fluid already in the capsular space & renal tubule, which opposes filtration & creates a “back pressure”. Normally 15 mm Hg. • Blood Colloid Osmotic Pressure (BCOP): Opposes filtration – also due to the presence of proteins. Normally 30 mm Hg.

38. Glomerular Filtration • Net Filtration Pressure (NFP): The total pressure that promotes filtration. Normally 10 mm Hg • Determined as… • NFP = GBHP – CHP – BCOP • 10 mm Hg = 55 – 15 – 30 mm Hg

39. Glomerular Filtration • Glomerular Filtration Rate (GFR): The amount of filtration formed in all the renal corpuscles of both kidneys each minute. • Regulated via net filtration rate, adjustment of blood flow into & out of the glomerulus, and altering of the glomerular capillary surface area. • Efferent & afferent arteriole diameters can coordinate control. • Rate increases with capillary blood flow increase. • If too high… urine output increases & dehydration & electrolyte depletion may occur due to shortened tubular reabsorption. • If too low… Tubular reabsorption is increased and wastes may be reabsorbed that should be eliminated.

40. Glomerular Filtration • Glomerular Filtration Rate controlled by 3 mechanisms… • Renal Auto Regulation: The ability of the kidneys to maintain a relatively stable GFR despite blood pressure changes. Occurs via.. • Myogenic Mechanism: Stretching triggers smooth muscle contractions – afferent arterioles can constrict to reduce blood flow into the glomerulus, while efferent arterioles can be dilated to allow increased outflow of blood. Can be reversed to compensate for decreased blood pressure. • Tubuloglomerular Feedback: Sodium, chlorine, & water build up in the loop of Henle due to increased blood pressure & faster blood flow through the renal tubules – Macula densa cells detect the imbalance & inhibit nitric oxide (NO) production. • Nitric Oxide (NO): Produced in the juxtaglomerular apparatus (JGA) – causes vasodilation. Decrease leads to vasoconstriction in the afferent arterioles & slows blood flow into the glomerular capillaries & decrease in GRF. Increases occur if blood pressure & GRF fall too low. • Neural Regulation: Sympathetic nervous system activation causes adrenal epinephrine to stimulate vasoconstriction in the afferent arterioles to reduce GRF & urine production. • Conserves blood volume & redirects blood to the heart, brain, & skeletal muscles. • Hormonal Regulation: Hormones are produced to change the GRF rate. • Angiotensin II: Reduces GRF through vasoconstriction of the afferent & efferent arterioles – reduces renal blood flow. • Atrial Natriuretic Peptide (ANP): Hormone secreted by cells in the atria of the heart – increases capillary surface area by relaxing the mesangial cells – GRF increases due to faster filtration.

42. Tubular Reabsorption • Tubular Reabsorption: The return of most of the filtered water & solutes back into the bloodstream from the renal tubules – 99% of the filtered water is reabsorbed. • Any substances not reabsorbed is secreted via the urine. • 3 Membranes substances must pass through to re-enter the blood stream: • Luminal Membrane of the Tubule Cell • Basolateral membrane of the Tubule Cell • Capillary Endothelium

43. Tubular Reabsorption • Two paths for fluid absorbed into the tubule lumen: • Moving between adjacent tubule cells • Passing through an individual tubule cell. • Tight junctions join neighboring cells to one another. • Atypical Membrane: Contacts the tubular fluid – lacks a sodium-potassium pump to ensure the reabsorption of Na+ only goes one way. • Most sodium ions crossing this membrane will be pumped into the interstitial fluid. • Basolateral Membrane: Contacts the interstitial fluid at the base & sides of the cells. • Cells lining the renal tubule have a low Na+ concentration in the cytosol due to sodium-potassium pumps. • Sodium-Potasium Pumps eject Na+ from the renal tubule cells.

44. Tubular Reabsorption • Paracellular Reabsorption: When fluid leaks between the cells. • Accounts for up to 50% of the reabsorption of certain ions & the water that follows via osmosis. • Transcellular Reabsorption: Where a substance passes from the fluid in the tubular lumen through the apical membrane of a tubule cell, across the cytosol, & out into the interstitial fluid through the basolateral membrane.

45. Tubular Reabsorption • Primary Active Transport: Occurs when energy is derived from hydrolysis of ATP & is used to pump a substance across a membrane. • Secondary Active Transport: Occurs when the energy stored in an ion’s electrochemical gradient drives a substance across the membrane. • Symporters: Membrane proteins that perform secondary active transport. • Antiporters: Move two or more substances in opposite directions across a membrane. • Transport Maximum (Tm): The upper limit on how fast each transporter can work – measured in mg/min.

46. Tubular Reabsorption • Osmosis: The method by which water is reabsorbed – occurs when water moves from an area of high concentration to an area of low concentration. • Obligatory Water Reabsorption: Water that is reabsorbed along with solutes – it’s “obligated” to follow solutes. • 90% of water reabsorption. • Occurs in the PCT & descending loop of Henle. • Facultative Water Reabsorption: Water reabsorption that is not dependent on solutes. • Remaining 10% of water reabsorption. • Occurs in the collecting ducts. • Regulated via antidiuretic hormone.

47. Tubular Reabsorption • PCT: The site where most reabsorption of water & solutes occurs. • Loop of Henle: Accounts for 20-35% of filtered solute reabsorption & 85% of filtered water reabsorption. • Creates the osmotic gradient between the renal cortex & renal medulla. • Aldosterone: Stimulates active NaCl reabsorption and indirectly stimulates passive water reabsorption. • Antidiuretic Hormone (ADH): Triggers water reabsorption to occur in the collecting duct & produces more concentrated urine. • If not present, more diluted urine is produced.

49. Tubular Secretion • Tubular Secretion: The transfer of materials from the blood and tubule cells into the tubular fluid – materials are transported from the peritubular capillaries into the renal tubule lumen via active transport. • Secretes hydrogen, creatine, potassium, etc. from the peritubular capillaries into the renal tubules if not filtered at the glomerulus. • Hydrogen secretion is increased to raise blood pH & decreased to lower pH – maintains homeostasis. • Eliminates other substances from the body to remove wastes and regulate blood levels of certain ions – includes substances in excessive levels & natural poisons. • Ions removed from the blood stream are deposited into the fluid within the tubules.