Student Urinary Microanatomy Renal Physiology PDF

URINARY SYSTEM: MICROANATOMY & PHYSIOLOGY DR. JACQUELINE MOBLEY AHS 2220  MICROSCOPIC ANATOMY The NEPHRON is the basic functional unit of the kidney Located in the cortex & medulla Role: Filtration, reabsorption & secretion Amount of nephrons ranges per species & size of animal...

URINARY SYSTEM: MICROANATOMY & PHYSIOLOGY DR. JACQUELINE MOBLEY AHS 2220  MICROSCOPIC ANATOMY The NEPHRON is the basic functional unit of the kidney Located in the cortex & medulla Role: Filtration, reabsorption & secretion Amount of nephrons ranges per species & size of animal Cats: 200,000/kidney Dogs: 700,000/kidney Sheep, pigs, humans: 1,000,000 /kidney Cows: 4,000,000/kidney NEPHRON Renal corpuscle Proximal convoluted tubule Loop of Henle Distal convoluted tubule This Photo by Unknown Author is licensed under CC BY-SA-NC  RENAL CORPUSCLE Located in the cortex Renal corpuscle = glomerulus + Bowman’s capsule Glomerulus: tuft of glomerular capillaries Bowman’s capsule: double-walled capsule surrounding the glomerulus Inner layer is the visceral layer Outer layer is the parietal layer Capsular space Function: filter the blood and create urine  BOWMAN’S CAPSULE Visceral layer Adheres to the surface of glomerular capillaries Podocytes make up the visceral layer Podocytes have spaces between them for filtration of substances Capsular space – the space between visceral & parietal layers Continuous with the proximal convoluted tubule This Photo by Unkn own Auth or is licensed under CC BY-SA Glomerular filtrate – the plasma filtered through the glomerulus  PROXIMAL CONVOLUTED TUBULE (PCT) Continuation of the capsular space of Bowman’s capsule Longest part of the tubular system Histology Cuboidal epithelial cells with microvilli (brush border) Microvilli = absorption In the tubules the glomerular filtrate becomes called the “tubular filtrate” This Photo by Unkn own Auth or is licensed under CC BY  THE LOOP OF HENLE Loop of Henle continues from the PCT Loop of Henle descends from cortex → medulla → cortex Histology of the loop of Henle Descending: cuboidal epithelia with microvilli At the U-turn: simple squamous epithelia, no brush border Ascending: cuboidal epithelia, no microvilli This Photo by Unkn own Auth or is licensed under CC BY-SA-NC  DISTAL CONVOLUTED TUBULE (DCT) & COLLECTING DUCTS DCT continues from the ascending loop of Henle DCTs empty into a series of tubules called collecting ducts Collecting ducts carry the tubular filtrate through the medulla to the calyces to the renal pelvis Important things happening in the collecting ducts Urine volume is determined (ADH action occurs here) K regulation Acid-base balance This Photo by Unkn own Auth or is licensed under CC BY-SA-NC  RENAL NERVE SUPPLY Sympathetic portion of autonomic nervous system Vasoconstriction of renal vessels Vasoconstriction increases renal blood pressure & increases filtration This Photo by Unkn own Auth or is licensed under CC BY  RENAL BLOOD SUPPLY 25% of cardiac output goes to the kidneys Renal artery Branches directly from the abdominal aorta Enters at the hilus → arteries →arterioles → afferent glomerular arterioles Afferent glomerular arterioles Carry blood to the glomerular capillaries Glomerular capillaries Filter plasma (“glomerular filtrate”) out of the blood into the Bowman’s capsule BLOOD SUPPLY CONTINUED Efferent glomerular arterioles Arterioles leaving the glomerulus THIS BLOOD IS STILL OXYGENATED Peritubular Capillaries Efferent glomerular arterioles branch into peritubular capillaries Oxygen transfer takes place here Substances return to blood via tubular reabsorption here Some substances are secreted into these tubules via tubular secretion Peritubular capillaries become veins that eventually become the renal vein MECHANISMS OF RENAL ACTION Filtration of blood Reabsorption Secretion Urine volume regulation Regulation of blood pressure  FILTRATION Filtration occurs in the renal corpuscle What’s unique about the glomerular capillaries? High blood pressure Only 30% less BP than the aorta How? Size difference between afferent & efferent arterioles Afferent diameter > efferent High pressure forces blood to filter into the capsular space Glomerular fenestrations (pores) of endothelium are larger than elsewhere Glomerular filtrate is similar to plasma except NO PROTEINS GLOMERULAR FILTRATION RATE GFR is the term for how fast plasma is filtered through the glomerulus GFR depends on the rate of blood flow through the kidney GFR is measured in mL/min 25% of plasma is removed from circulation every minute A 25 lb dog filters 64L (16 gallons) per 24 hours If the fluid did not reabsorb in the tubules, that would be a lot of urine Reabsorbed substances Secreted substances WHAT GETS REABSORBED? Na K Ca Mg Glucose Amino acids Cl Bicarb (HCO3) Water REABSORPTION Substances move from the renal tubules to the peritubular capillaries Osmosis Diffusion Active transport Tubular lumen → between or through tubular epithelium → interstitial fluid → endothelium → lumen of peritubular capillaries REABSORPTION Pathways for Sodium: PCT: Active transport pump in & out of epithelial cell Ascending loop of Henle: exchanged for H, ammonium or K DCT: same as ascending loop of Henle Exchange of these ions is under the influence of Aldosterone Glucose and amino acids: PCT: Cotransport with sodium (no extra energy) into epithelial cell, then diffuse passively out Sodium, glucose & amino acids diffuse into peritubular capillaries REABSORPTION Pathway for Potassium: Diffuses out of tubule, between epithelial cells, into peritubular capillaries K reabsorption occurs in the PCT, ascending loop of Henle, and the DCT Pathway for Calcium: Ca moves under the influence of vitamin D, PTH, and calcitonin Ca reabsorption occurs in the PCT, ascending loop of Henle, and DCT Pathway for Magnesium: Reabsorbed from the PCT, ascending loop of Henle, collecting duct PTH increases Mg reabsorption REABSORPTION Chloride When Na moves, it makes the interstitial space + charged and the tubular lumen negatively charged. Cl follows Na to restore electrical neutrality Water Sodium, glucose, amino acids & Cl movement encourages water to follow by osmosis Other substances (i.e. urea) Water moving out makes the filtrate more concentrated These substances are reabsorbed by passive diffusion across the concentration gradient REABSORPTION SUMMARY PCT Remaining reabsorption 65% of tubular Loop of Henle reabsorption occurs here DCT 80% of water, sodium, Collecting ducts chloride, bicarbonate 100% of glucose and amino acids SECRETION The purpose of secretion is to remove substances that were not eliminated enough by GFR alone Peritubular capillaries → interstitial fluid → tubular epithelial cells → tubular filtrate of the tubules DCT: primary site of tubular secretion H, K, ammonia Antibiotics (penicillin, some sulfas) URINE VOLUME REGULATION Antidiuretic hormone (ADH)** Posterior pituitary gland Acts on the DCT and collecting ducts to promote water reabsorption No ADH = no water reabsorption = more urine produced Aldosterone Adrenal cortex Acts on the distal tubule and collecting ducts Regulates sodium gradient in the nephron to control water reabsorption Increased aldosterone = increased sodium retention & water reabsorption REGULATION OF BLOOD PRESSURE Kidneys detect changes in blood pressure Renin-angiotensin-aldosterone system is activated with low BP Juxtaglomerular cells exist within the afferent glomerular arterioles Role: Monitor BP Macula densa within the ascending loop of Henle Role: Monitors NaCl concentration of the tubular filtrate If BP low or less Na is detected, juxtaglomerular cells release renin  RAAS Renin Enzyme that splits angiotensin I from angiotensin Angiotensin I is converted to angiotensin II by angiotensin converting enzyme (ACE) Angiotensin II Causes arterial constriction → increases BP Stimulates aldosterone release from adrenals Aldosterone stimulates Na and water This Photo by Unkn own Auth or is licensed under CC BY-SA reabsorption → increases BP THE END This Photo by Unknown Author is licensed under CC BY-ND