The Urinary System Anatomy of kidney continued Urine formation glomerular filtration 23 1 Anatomy of Kidney Renal cortex outer zone Renal medulla inner zone and faces renal sinus Medial cavity occupied by blood and lymphatic vessels nerves adipose tissue and urine collecting structures 23 2 The Nephron The functional unit of the kidney each kidney has about 1 million nephrons Nephron composed of two principal parts renal corpuscle filters the blood plasma renal tubule long tube that converts the filtrate into urine Renal corpuscle consists of the glomerulus and a two layered glomerular Bowman capsule that encloses glomerulus parietal layer of Bowman capsule is simple squamous epithelium visceral layer of Bowman capsule with special epithelial cells podocytes Bowman capsule filtrate collecting capsular space separates the two layers of 23 3 Renal Corpuscle 23 4 Renal Tubule The simple squamous epith becomes simple cuboidal epith in the tubule The renal tube is a duct leads away from the glomerular capsule and ends at a point in the medulla Proximal convoluted tubule PCT longest most coiled simple cuboidal with microvilli brush border A large amount of absorption and some secretion occurs through the microvilli which increases the surface area 23 5 Renal Tubule Nephron loop descending and ascending limbs thick segment simple cuboidal is initial part of descending limb and most of ascending limb active transport of salts Thick cells have high metabolic rate and are loaded with mitochondria thick is impermeable to water thin segment simple squamous is most of lower limb Thin has no cuboidal because no secretion or reabsorption low metabolic rate not permeable to salt but very water permeable 23 6 Renal Tubule Distal convoluted tubule DCT cuboidal minimal microvilli some reabsorption but less than PCT and some secretion Collecting duct several DCT s join not part of nephron converge to form another duct second ducts drains into the renal pelvis Both duct types are lined with simple cuboidal epith 23 7 Renal Tubule Urine leaves the renal pelvis via the ureter that drains the urine to the urinary bladder which exists via the urethra Flow of glomerular filtrate glomerular capsule PCT nephron loop DCT collecting duct renal pelvis ureter urinary bladder urethra 23 8 Nephrons Cortical nephrons 85 Close to kidney surface in the cortex short nephron loops Some have no loops at all Juxtamedullary nephrons 15 close to the medulla very long nephron loops maintain salt gradient helps conserve water 23 9 Path of Blood Through Kidney The kidneys receive disproportionately more blood than their of total body mass Renal artery arising from the aorta afferent arteriole glomerulus efferent arteriole peritubular capillaries form a network around the renal tubule of the cortical nephron The renal tubule of cortical nephrons reabsorbs most of the water and solutes that filter out of the blood at the glomerulus and returns these to the blood via the peritubular capillaries Renal vein enter inferior vena cava 23 10 Path of Blood Through Kidney Vasa recta around nephron loop a network of vessels which supplies the renal medulla Vasa recta surrounds juxtamedullary nephrons in the deep cortex close to the medulla Capillaries from the vasa recta carry away reabsorbed water and solutes Renal vein enter inferior vena cava 23 11 Nephron Diagram 23 12 Renal Innervation Wrapped around each renal artery is a renal plexus of nerves and ganglia The renal plexus carries sympathetic innervation and parasympathetic innervation from the vagus nerves Stimulation by sympathetic fibers tends to reduce glomerular blood flow and slow the rate of urine production Symp fibers also respond to falling BP by stimulating the kidneys to secrete renin The function of the parasympathetic innervation is unknown 23 13 Urine Formation Preview The fluid in the capsular space filtrate has almost no proteins except small peptides Amount excreted in urine amount filtered amount secreted into tubule amount reabsorbed into blood 23 14 Urine Formation Some substances are filtered and secreted but not Some substances are filtered with a small fraction reabsorbed reabsorbed Some substances are filtered and 100 reabsorbed Filtration rate can vary 23 15 Filtration Membrane Turned back Blood cells Plasma proteins Large anions Protein bound minerals and hormones Most molecules 8 nm in diameter Endothelial cell of glomerular capillary Basement membrane Filtration slit Filtration pore fenestration Capsular space Bloodstream 23 16 Filtration Membrane Fenestrations Fenestrated cap are for rapid passage of small molecules but still retain most proteins and larger particles important in filtration and absorption Basement membrane Negatively charged proteoglycan gel that helps hold back particles larger than 8 nm such as the occasional small blood protein which otherwise might slip through Generally small albumins and globulins which are anionic neg charged particles are also held back in the blood because of the negative charge repulsion Filtration slits 23 17 Filtration Membrane The glomerular filtrate normally has very little protein Small molecules can pass through the filtration membrane and into the capsular space Substances that pass through filter and have about the same concentration in glomerular filtrate as in blood plasma include water electrolytes glucose amino acids nitrogenous wastes and vitamins Some substances of low molecular weight are retained in the bloodstream because they are bound to plasma proteins examples are calcium iron and thyroid hormone 23 18 Kidney disease damage Kidney infections and trauma can damage the filtration membrane and allow albumin or blood cells to filter through Kidney disease is sometimes marked by the presence of protein in urine especially albumin proteinuria or albuminuria or blood in the urine hematuria Intense exercise reduces perfusion of the kidneys and the glomerulus breaks down under extended hypoxia Glom filtration is determined by same pressures that determine filtration in other capillaries but with some sig differences in the forces involved 23 19 Filtration Pressure 23 20 Filtration Pressure BHP is highest in glomeruli cap compared to the rest of body about 60 mmHg compared to 30 at arteriole end and 10 at venule end in most capillaries in body This high BP results from the afferent arteriole is much larger than the efferent arteriole With a large inlet and small outlet BP builds up within and