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Marieb’s Human Anatomy and Physiology Marieb w Hoehn Chapter 25 Urinary system Lecture 15 Lecture Overview • Introduction to the Urinary System • Location and function of the kidneys • Gross anatomy • Histology • Urine formation 2 Functions of the Kidneys • Make urine • Regulate blood volume and blood pressure • Regulate plasma concentrations of Na+, K+, Cl-, HCO3-, and other ions • Help to stabilize blood pH • Conserve valuable nutrients • Assist the liver in detoxification and deamination 3 Urinary System Figure from: Hole’s Human A&P, 12th edition, 2010 Kidneys receive about 1.2 L of blood per minute and filter nearly 180 L of fluid from the bloodstream every day! 4 Location of Kidneys Figure from: Martini, Anatomy & Physiology, Prentice Hall, 2001 Located retroperitoneally from T12 to L3 Left kidney is slightly higher than right kidney Adrenal glands sit on the medial and superior part of kidneys Nephro(s) = kidney 5 Location of Kidneys Helps maintain position of kidney Figure from: Martini, Anatomy & Physiology, Prentice Hall, 2001 6 Gross Anatomy of the Kidneys Renal capsule is tough, fibrous capsule (tunica fibrosa) Hilus is entry point for renal artery, vein, and nerve (mostly sympathetic fibers) [Pyel(o)-] Bases of renal pyramids face the cortex, apices face the renal pelvis and end at renal papillae Figure from: Martini, Anatomy & Physiology, Prentice Hall, 2001 7 Renal Blood Vessels Glomeruli (Cortical radiate arteries) Minor Calyx Figures from: Martini, Anatomy & Physiology, Prentice Hall, 2001 Nerves, mainly sympathetic postganglionic fibers of the ANS, follow arteries to nephrons 8 The Nephron (80%) (20%) Nephrons are the structural and functional units of the kidney 9 Figure from: Hole’s Human A&P, 12th edition, 2010 Blood Supply of the Nephron Medulla The capillary loop of the vasa recta is a type of capillary that is closely associated with the nephron loop of juxtamedullary nephrons 10 Figure from: Hole’s Human A&P, 12th edition, 2010 Blood Flow Through Kidney and Nephron Know this! 11 Figure from: Hole’s Human A&P, 12th edition, 2010 Renal Corpuscle (Glomerulus + Capsule) Figure from: Hole’s Human A&P, 12th edition, 2010 Filtrate in capsular space Notice that the efferent arteriole is smaller than the afferent arteriole This creates a high pressure (~55-60 mm Hg) in the glomerular capillary bed 12 Visceral Glomerular Epithelium Figure from: Hole’s Human A&P, 12th edition, 2010 Material passing out of the blood must be small enough to fit through the filtration slits (slit pores) 13 Renal Cortex and Renal Medulla Figure from: Hole’s Human A&P, 12th edition, 2010 14 Specialization of the Nephron Figure from: Hole’s Human A&P, 12th edition, 2010 1. PCT – simple cuboidal with a brush border (DCT) 2. Thin segment of the descending nephron loop - simple squamous epithelium 3. Thick ascending nephron loop cuboidal/low columnar (PCT) The order of the parts of the nephron is important to know 4. DCT - simple cuboidal with no microvilli (specialized for secretion, not absorption) 15 Juxtaglomerular Apparatus Juxtaglomerular cells (JG) - modified smooth muscle cells in the wall of the afferent arteriole that contract (and secrete renin) Cells of the macula densa (MD) are osmoreceptors responding to solute concentration of filtrate MD + JG cells = juxtaglomerular apparatus Figure from: Martini, Anatomy & Physiology, Prentice Hall, 2001 16 Glomerular Filtrate and Urine Composition (1.8 L/day) Glomerular filtrate is about the same composition as plasma: H2O, glucose, amino acids, urea, uric acid, creatine, creatinine, Na, Cl, K, HCO3-, PO43-, SO42-. But notice how different the composition of urine is. Additionally, note that protein is not normally present in urine. 17 Urine Formation About 125 ml/minute (180 L/day) of the total 1200 ml/min of blood that passes through the glomerulus becomes filtrate • Glomerular Filtration (GF) *Adds to volume of urine produced • substances move from blood to glomerular capsule • Tubular Reabsorption (TR) *Subtracts from volume of urine produced • substances move from renal tubules into blood of peritubular capillaries • glucose, water, urea, proteins, creatine • amino, lactic, citric, and uric acids • phosphate, sulfate, calcium, potassium, and sodium ions • Tubular Secretion (TS) *Adds to volume of urine produced • substances move from blood of peritubular capillaries into renal tubules • drugs and ions, urea, uric acid, H+ Urine formation = GF + TS - TR 18 Glomerular Filtration Figure from: Hole’s Human A&P, 12th edition, 2010 Glomerular filtrate is plasma that passes through 1) the fenestrae of the capillary endothelium, 2) the basement membrane around the endothelium, and 3) the filtration slits (slit pores) of the pedicels This is called the ‘filtration membrane’ Glomerular filtration is a mechanical process based primarily on molecule size 19 Glomerular Filtration Rate (GFR) NFP = HPg – (HPc + OPg) Net Filtration Pressure = force favoring filtration – forces opposing filtration (*glomerular capillary ( capsular hydrostatic pressure hydrostatic pressure) + glomerular capillary osmotic pressure ) Figure from: Hole’s Human A&P, 12th edition, 2010 Net filtration pressure is normally positive, i.e., favors the movement of fluid out of the glomerular capillaries GFR = amount of filtrate produced each minute (~125 ml/min) 20 Afferent/Efferent Arterioles – Effect on GFR Innervated by sympathetic nerves • Afferent arteriole – Δ radius GFR – radius GFR; radius GFR • Efferent arteriole – Δ radius 1/GFR – radius GFR; radius GFR 21 Amounts of Glomerular Filtrate and Urine Figure from: Hole’s Human A&P, 12th edition, 2010 Glomerular Filtration Rate (GFR) is directly proportional to the net filtration pressure GFR 125 ml/min (180 L/day) This means that all of the plasma is filtered ~ 60x every day (How did we get this number?) Notice that urine output is only 0.6 – 2.5 L per day (an average of about 1.8 L, or about 1% of glomerular filtrate); 99% of filtrate is reabsorbed!! average amounts over a 24 hour period 22 Glomerular Filtration Rate (GFR) Net filtration pressure, although normally positive, is relatively low ( 10 mm Hg) Glomerular hydrostatic pressure is the blood pressure in the glomerular capillaries, and is usually higher than other capillary pressures Capsular hydrostatic pressure tends to push water and filtrate BACK into the capillaries Anything that alters the filtration pressures will alter GFR * Blood pressure is the most important factor altering the glomerular hydrostatic pressure (and NFP). A MAP fall of 10% will severely impair glomerular filtration; a fall of 15-20% will stop it. 23 Summary of Factors Affecting GFR Factor Effect Vasoconstriction (↑ Sympathetic stimulation) Afferent arteriole GFR Efferent arteriole ↑ GFR Vasodilation ( Sympathetic stimulation) Afferent arteriole ↑ GFR Efferent arteriole GFR Increased capillary hydrostatic pressure ↑ GFR Increased colloid osmotic pressure GFR Increased capsular hydrostatic pressure GFR Know this table – it’s important! 24 Regulation of GFR • Autoregulation – Maintains GFR despite changes in local blood pressure and blood flow (between 90 – 180 mm Hg mean systemic pressure) – Myogenic mechanism –afferent arteriolar vascular smooth muscle contracts when stretched (increased BP); relaxes when it’s not stretched (decreased BP) – Tubuloglomerular mechanism – MD cells detect flow rate and/or osmolarity of filtrate in DCT -> JG cells contract -> afferent arteriole constricts -> GFR. So… *If DCT flow increases, GFR If DCT flow decreases, GFR 25 Regulation of GFR • Neural (Autonomic) Regulation – Mostly sympathetic postganglionic fibers = vasoconstriction of afferent arterioles GFR (conserves water, redirects blood to other organs) – Stimulates juxtaglomerular apparatus to secrete renin – May override autoregulatory mechanism at afferent arteriole • Hormonal Regulation – Renin-angiotensin system – stabilizes BP and ECF volume – Atrial Natriuretic Peptide (ANP) - ↑ GFR, ↑ fluid loss (dilates afferent arteriole, constricts efferent arteriole) 26 Renin-Angiotensin System Figure from: Hole’s Human A&P, 12th edition, 2010 Renin is released by the juxtaglomerular apparatus due to: 1) Decline of BP (Renin 1/Pressure) (ACE) 2) Juxtaglomerular stimulation by sympathethic NS 3) Decline in osmotic concentration of tubular fluid at macula densa ( Renin 1/[NaCl] ) Stabilizes systemic blood pressure and extracellular fluid volume 27 Review • Kidneys – Are bean-shaped organs located retroperitoneally at level of T12 – L3 – Are enclosed by a tough, fibrous capsule – Consist of a cortex and a medulla • Cortex – glomeruli • Medulla – nephron loops, collecting ducts; renal pyramids, renal papillae – Renal sinus • Major and minor calyces • Renal blood vessels and nerves pass through the hilus and branch within the sinus 28 Review Know this! 29 Figure from: Hole’s Human A&P, 12th edition, 2010 Review • Functions of the Kidneys – Making urine – Regulating blood volume and blood pressure – Regulating plasma concentrations of Na+, K+, Cl-, HCO3-, and other ions – Helping to stabilize blood pH – Conserving valuable nutrients – Assisting the liver in detoxification and deamination 30 Review • The functional unit of the kidney is the nephron – Glomerulus (filtration) • Capillaries surrounded by podocytes • Filter blood – – – – PCT Nephron Loop DCT Peritubular capillaries • Surround proximal and distal tubules • In juxtamedullary nephrons, forms the vasa recta – Collecting ducts (technically not part of nephron) 31 Review • Glomerular filtration – Glomerular filtration rate (GFR) • • • • Amount of filtrate produced each minute Directly proportional to net filtration pressure May be determined with creatinine or inulin tests Approximately 125 ml/min (180 L/day) – Factors affecting GFR • • • • Vasoconstriction / vasodilation Capillary hydrostatic pressure Capsular hydrostatic pressure Capillary osmotic pressure 32 Review • Glomerular filtration (cont’d) – Factors controlling GFR • Autoregulation – Myogenic – Tubuloglomerular • Hormonal – Renin-Angiotensin System – ANP • Autonomic nervous system 33