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Collin County Community College BIOL. 2402 Anatomy & Physiology WEEK 12 Urinary System 1 INTRODUCTION Main functions of the kidneys are • regulate blood volume , water content • regulate blood composition e..g. Na, Cl, K, pH • remove waste products and toxins Kidneys are the guardians of the internal environment They receive 25 % of cardiac output. Thus blood is filtered every 4 minutes ! 190 liters of fluid (plasma) is filtered daily to produce 1 liter of urine 2 1 Urinary System Organization • The kidneys – Produce urine • The ureters • The urinary bladder – Stores urine • The urethra 3 Urinary System Organization • Left kidney extends slightly more superiorly than right • Both kidneys and adrenal glands are retroperitoneal • Hilus – Entry for renal artery and renal nerves – Exit for renal veins and ureter 4 2 Urinary System Organization 5 Kidney Anatomy Pyramid • Outer capsule • Under capsule is an • outer cortex area • inner medulla • Medulla contains pyramids. • They contain the functional units of the kidneys = nephrons 6 3 Kidney Anatomy 7 Kidney Anatomy • The medulla consists of 6-18 renal pyramids • The cortex is composed of roughly 1.25 million nephrons • Nephrons perform 3 basic functions that result in urine formation • filtration of blood • re-absorption of essential elements back into the blood • secretion of additional elements from blood into the forming urine 8 4 Nephron Anatomy Bowman’s Capsule • always located in cortex Proximal Convoluted Tubule • always located in cortex Loop of Henle (dips into medulla) • Descending limb • Ascending limb Distal Convoluted Tubule • located in cortex again Collecting Tubules and duct • dips back into medulla 9 Nephron Anatomy/Function • • • • Production of filtrate Reabsorption of organic nutrients Reabsorption of water and ions Secretion of waste products into tubular fluid This function of the nephron is correlated with the lining of the tubules; they are lined with simple epithelials. 10 5 Nephron Anatomy/Function 11 Nephron Anatomy/Function What the nephrons excrete is equals to what is filtered out of the blood plus what is secreted from the blood minus what is reabsorbed back into the blood. 12 6 Nephron Anatomy/Function 13 Nephron Anatomy/Function There are 2 kinds of nephrons • cortical nephrons : the loop of Henle barely reaches into the medulla • juxta medullary nephrons : the loop of Henle dips deep into the medulla 15 % of nephrons are juxta medullary nephrons ; these are very important in water reabsorption 14 7 Renal Blood Supply Since the nephrons are the filters of the blood, blood needs to be carried to the nephrons. • A Renal artery feeds the kidney. It then shows repeated branches – Segmental artery – Interlobar artery – Arcuate artery – Interlobular artery – Afferent arterioles • Renal venules follow similar opposing pattern ending with renal vein 15 Renal Blood Supply 16 8 Renal Blood Supply 17 Renal Blood Supply Bowman’s capsule of each nephron is the filtering part and located in the cortex area. Blood enters the Bowman’s capsule via the afferent arteriole. 18 9 Nephron Blood Supply Afferent arteriole • enters Bowman’s capsule Glomerulus • capillary network within Bowman’s capsule Efferent arteriole • leaves Bowman’s capsule Peritubular capillaries • surround the Proximal and Distal tubules 19 Nephron Blood Supply Vasa Recta • peritubular capillaries that surround and follow the loop of Henle in juxta medullary nephrons Thus we have 2 capillary beds associated with the nephrons • First one helps to produce the filtrate inside the lumen of the nephron (glomerulus) • Second one functions in re-absorption and secretion aspect of urine filtration (peritubular capillaries) 20 10 Nephron Blood Supply 21 Diagram of nephron with associated electron microscope pictures. Note that all along the nephron, the cell layer of the tubules is only one cell layer thick ( indicates importance of diffusion, secretion, absorption). 22 11 Vacular Resistance in Kidneys Controls of Resistance are located at entrance and exit of glomerulus; this allows for a steady pressure within the glomerulus ( which is important for homeostasis of filtering process) 23 RENAL PHYSIOLOGY • Main function of the kidneys is to regulate blood volume and composition • It does so by involving a process of excretion of waste products such as – Urea – Creatinine – Uric acid 24 12 RENAL PHYSIOLOGY Three basic mechanisms are involved : • Filtration – Blood pressure – Water and solutes across glomerular capillaries • Reabsorption – The removal of water and solutes from the filtrate • Secretion – Transport of solutes from the peritubular fluid into the tubular fluid 25 RENAL PHYSIOLOGY • Filtration occurs in the Bowmans Capsule at the Glomerulus • Filtration is very non-specific. Filtration is modified by carrier mediated transport, resulting in re-absorbption and/or additonal secretion into the filtrate. – Facilitated diffusion – Active transport – Cotransport – Countertransport • Carrier proteins have a transport maximum (Tm) – Determines renal threshold • Diffusion and osmosis aid in kidney function. 26 13 RENAL PHYSIOLOGY • Most regions of the nephron perform a combination of functions • General functions can be identified – Filtration in the renal corpuscle – Nutrient reabsorption along the PCT – Active secretion at PCT and DCT – Loops of Henle regulate final volume and solute concentration 27 RENAL PHYSIOLOGY • Most regions of the nephron perform a combination of functions • General functions can be identified – Filtration in the renal corpuscle – Nutrient re-absorption along the PCT – Active secretion at PCT and DCT – Loops of Henle regulate final volume and solute concentration 28 14 RENAL PHYSIOLOGY Glomerular Filtration • Filtration process that occurs in Bowman’s Capsule • Blood is filtered and the filtrate ends up in the tubule system of the nephron What creates the filter system ? Combination of the membrane systems of the capillaries and Bowman’s capsule cells 29 Microscopic Anatomy of Bowman’s Capsule Special cells, called Podocytes, cover the capillaries 30 15 Microscopic Anatomy of Bowman’s Capsule Pedicels of podocytes (feet extensions) create filtration slits Podocyte in Bowman’s capsule 31 Microscopic Anatomy of Bowman’s Capsule Scanning electron microscope picture of the fingerlike filtration slits of the podocytes ! Compare this with the diagram on left side and previous slide ! This system provides a filtering mechanism roughly similar to a coffee filter, but much more refined ! It houses 3 filtering mechanisms ! 32 16 Microscopic Anatomy of Bowman’s Capsule Filtration System in Bowman’s capsule Capillary endothelial cells have many pores • let everything through except blood cells and large proteins Basement membrane or Basal Lamina • Is negatively charged and repels most smaller proteins Foot process of the Podocytes • Form additional filtration slits that only let small molecules through 33 Filtration System in Bowman’s capsule What are the forces involved the filter system ? • Similar forces that are involved in capillary fluid exchange in the tissues ! • Hydrostatic pressure from the blood ( = blood pressure ) • Hydrostatic pressure in capsule from the filtrate • Osmotic (oncotic) pressure from the blood 34 17 Filtration System in Bowman’s capsule • Filtration occurs as fluids move across the glomerulus • The positive filtration pressure is the glomerular hydrostatic pressure due to blood pressure in the glomerular capillaries (GHP) – Capsular hydrostatic pressure opposes (CsHP) – Blood colloid osmotic pressure opposes (BCOP) • Net hydrostatic pressure (NHP) = GHP – CsHP • Filtration (FP) = GHP - CsHP - BCOP = NHP – BCOP 35 Filtration System in Bowman’s capsule Net filtration pressure is thus a modest 10 mm Hg Net filtration pressure is determined by the 3 forces 36 18 Bowman’s capsule Net Filtration pressure (NFP) • Pressure force that drives fluid out of the blood (out of the glomerulus) and into Bowman’s capsule • Due to the characteristics of the ‘filter’, the filtrate that passes into the tubule system of the nephron equals blood minus formed elements and minus proteins • Since proteins do not leave the blood stream, but water does, the efferent arteriole will have a higher concentration of proteins and blood cells ( will be more viscous 37 Bowman’s capsule Glomerular Filtration Rate (GFR) Total amount of filtrate formed per minute by the kidneys. Depends on : • NFP • what happens when NFP = 0 ? • what happens to NFP when BP increases ? • what happens when afferent blood osmolarity increases ? ( use next slide ) • Total filtration area ( what happens with a unilateral nephrectomy ? ) • Filtration membrane permeability ( what happens when some Glomeruli get clogged up ? ) 38 19 Bowman’s capsule Blood proteins changes affect this force Blood pressure changes affect this force 39 Regulation of GFR Regulation of the GFR is an important homeostatic process. • If GFR is too high, we would produce a high rate of filtrate and re-absorption of essential elements would not be efficient. • If GFR is too low, we would not be able to secrete important waste products fast enough Regulation of the GFR occurs via 3 mechanisms • Renal Auto-regulation • Neural regulation • Renin-Angiotensin Feedback 40 20 Regulation of GFR 1. Renal Auto-regulation a. Juxta Glomerular Apparatus (JGA) feedback • Distal Convoluted tubule makes contact with afferent and efferent arteriole • This region is called the JGA • Contains 2 groups of cells that are important in kidney function • Juxta glomerular cells : are part of the afferent arteriole wall and act as mechano-receptors and endocrine cells • Macula Densa cells : are part of the DCT and they act as chemoreceptors/endocrine cells. 41 Regulation of GFR 42 21 43 Regulation of GFR 1. Renal Auto-regulation b. Myogenic Effect • When blood vessels and smooth muscles are stretched they tend to contract • Increased blood pressure will cause vasoconstriction in the afferent arteriole and counteract a possible increase in NFP. • Overall effect of autoregulation is that GFR does not change much when a person experiences acute blood pressure changes. 44 22 Regulation of GFR 2. Neural Regulation • Mostly a sympathetic effect • Produces powerful vasoconstriction of afferent arteriole • Decreases GFR and slows production of filtrate • Important for example during blood-loss ; prevents body from excreting more urine ( fluid) • Changes the regional pattern of blood flow • Alters GFR • Also Stimulates release of renin by JGA 45 Regulation of GFR 46 23 Regulation of GFR 3. Renin-Angiotensin Feedback The following will result in a Renin release by the Juxtaglomerular cells in JGA apparatus • Drop in Blood pressure (reduced stretch in afferent arteriole) • Reduced release of the vasoconstrictor from the Macula Densa cells ( thus, reduced Na+ flow in the DCT) • Direct stimulation of JG cells by sympathetic stimuli • All these stimuli release Renin, resulting in Angiotensin II production • Efferent arterioles have more Ang II receptors than Afferent arterioles ; thus this will increase the Pressure in the glomerulus (why ? ) • Ang. II also results in release of Aldosterone and ADH ( what do they do ? ) 47 Regulation of GFR 48 24 Regulation of GFR 49 Regulation of GFR 50 25