Urinary System

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Danny M.Pincivero

Chapter 26 – Urinary System What are the functions of the urinary system? 1. Excretion - Filtration, re-absorption, secretion (formation of urine) 2. Regulation of blood volume and pressure 3. Regulation of blood solutes 4. Regulation of extracellular fluid pH - Excretion of protons 5. Regulation of RBC synthesis - Synthesis of erythropoietin 6. Regulation of vitamin D synthesis - Regulates blood calcium --------------------------------------------------------------------------------------------------- Where are the kidneys located and what are they composed of?  Location o Retroperitoneal on posterior abdominal wall on either side of vertebral column o Lumbar vertebrae and rib cage provide protection  ExternalAnatomy o Renal Capsule: fibrous connective tissue; surrounds each kidney o Adipose Tissue: engulfs renal capsule and acts as cushioning o Renal Fascia: thin layer loose connective tissue  Anchors kidneys and surrounding adipose to abdominal wall o Hilum: renal artery and nerves enter and renal vein and ureter exit kidneys  Open into renal sinus (cavity filled with fat and loose connective tissue)  InternalAnatomy o Cortex: outer area o Renal Columns: part of cortical tissue that extends into medulla o Medulla: inner area; surrounds renal sinus o Renal Pyramids: cone-shaped  Base is boundary between cortex and medulla  Renal Papilla: apex of pyramid points toward sinus o Minor Calyces: papillae extend into funnel of minor calyx o Major Calyces: converge to firm plexis o Renal Pelvis: enlarged chamber formed by major calyces o Ureter: exits at the hilum  connects to urinary bladder What is the nephron? - Functional unit of the kidney o Bowman’s capsule, proximal tubule, loop of Henle, distal tubule - Urine moves to collecting ducts, papillary ducts, minor calyses, major calyses, and renal pelvis  Juxtamedullary Nephrons o Loops of Henle extend deep into the medulla o 15% of all nephrons  Cortical Nephrons o Loops of Henle do not extend deep into the medulla --------------------------------------------------------------------------------------------------- What is the renal corpuscle composed of? • Bowman’s capsule: outer parietal and visceral layers o Site of renin production • Glomerulus: network of capillaries o Blood enters through afferent arteriole, exits through efferent arteriole --------------------------------------------------------------------------------------------------- What is the filtration membrane composed of? • Fenestrae: window-like openings in the endothelial cells of the glomerular capillaries • Filtration Slits: gaps between the cell processes of the podocytes • Filtration Membrane: capillary endothelium, basement membrane, and podocytes o First stage of urine formation --------------------------------------------------------------------------------------------------- How is filtration measured? - Movement of fluid, derived from blood flowing through the glomerulus, across filtration membrane • Filtrate: water, small molecules, ions that can pass through membrane o Pressure difference forces filtrate across filtration membrane • Renal Fraction: part of total cardiac output that passes through the kidneys o Average 21% • Renal Blood Flow Rate: 1176 mL/ min • Renal Plasma Flow Rate: renal blood flow rate x plasma fraction of blood o 650 mL/min • Filtration Fraction: part of plasma that is filtered into lumen of Bowman’s capsule o Average 19% • Glomerular Filtration Rate (GFR): amount of filtrate produced each minute o 180L/day - Average urine production/day = 1-2L o Most of filtrate must be reabsorbed Identify the arteries and veins of the kidneys. (Rather Say I Am In A Great Exam but Prefer a Vacation) 1. Renal arteries branch from abdominal aorta 2. Segmental arteries branch from renal 3. Interlobar arteries ascend within renal columns toward cortex 4. Arcuate arteries branch and arch over the base of the pyramids 5. Interlobular arteries project into cortex and give rise to afferent arterioles Part of the circulation involved w/ urine formation 6. Afferent arterioles supply blood to glomerulus 7. Glomerulus 8. EfferentArterioles exit the renal corpuscle 9. Peritubular Capillaries form a plexus around the proximal and distal tubules 10. Vasa Recta: specialized parts of peritubular capillaries that course into medulla along with loops of Henle, then back toward cortex What forces drive the filtration membrane? • Glomerular Capillary Pressure (GCP): blood pressure inside capillary o “pushes” small molecules into Bowman’s capsule • Capsule Hydrostatic Pressure (CHP): pressure of filtrate already in the lumen o Works against filtration • Blood Colloid Osmotic Pressure (BCOP): osmotic pressure caused by blood proteins o Favours fluid movement into capillary from the lumen (works against filtration) Filtration pressure = GCP (50mmHg) - CHP (10mmHg) – BCOP (30mmHg) = 10mmHg How is the glomerular filtrate rate regulated?  Autoregulation o Changes in degree of constriction in afferent arterioles o As systemic BP , afferent arterioles constrict and prevent increase in renal blood flow  Sympathetic stimulation (norepinephrine) o Constricts small arteries and afferent arterioles o  renal blood flow and filtrate formation  During intense exercise – intense sympathetic stimulation = rate of filtrate formation  ---------------------------------------------------------------------------------
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