FSU BSC 2086 - Lesson 20: The Urinary System Part II

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BSC2086 A&P II Final Exam Study Guide – Lesson 20 1 Lesson 20: The Urinary System Part II Renal Physiology • Goal of urine production = maintain homeostasis! • Regulates volume & composition of blood o Concentrates filtrate produced by glomerular filtration  Failure  fatal dehydration – lose too much H2O o Absorbs & retains valuable materials for use by other tissues  Ex) glucose, amino acids, etc. o Excretes metabolic waste products  Organic wastes dissolved in blood • Ex) Urea, creatinine, uric acid • Kidneys – usually produce concentrated urine o 1,200-1,400 mOsm/L (4x plasma concentration) o Ways of Expressing Osmotic Concentration:  Osmolarity: Total number of solute particles per liter • Expressed in: • osmoles per liter (Osm/L) • milliosmoles per liter (mOsm/L) • Body fluids have osmotic concentration of ~300 mOsm/L  Concentrations of large organic molecules • Grams or milligrams per unit of volume of solution o mg/dL o g/dL Aldosterone and ADH • Urine osmolarity can be increased by the secretion of aldosterone & ADH • Aldosterone – Increases number of Na+/K+ exchange pumps at DCT & collecting duct to absorb Na+ in exchange for K+ o H2O is reabsorbed by osmosis (follows Na+) • ADH – increases water channels (aquaporins) in apical cell membranes of DCT & collecting duct o Concentrates 100 mOsm/L tubular fluid arriving at DCT to 1,200 mOsm/L when it reaches the minor calyx • Without ADH… o Water is NOT reabsorbed – all fluid reaching DCT is lost in urine o Produces large amounts of dilute urine (30-400 mOsm/L)  24L/day – seen in diabetes insipidus o Posterior pituitary normally is continuously secreting low levels of ADH  DCT & collecting system are always permeable to water  Collecting system reabsorbs 16.8 L/day (9.3% of filtrate) • Produces 1,200 mL per day (0.6% of filtrate) of urine (800-1,000 mOsm/L)BSC2086 A&P II Final Exam Study Guide – Lesson 20 2 Glomerular Filtration • Filtration Pressures – glomerular filtration is governed by the balance between: o Hydrostatic pressure (fluid pressure) o Colloid osmotic pressure (of materials in solution) on either side of capillary walls • Hydrostatic Pressure: Glomerular hydrostatic pressure is blood pressure in glomerular capillaries o Tends to push water & solute molecules out of plasma and into filtrate o Is significantly higher than capillary pressures in systemic circuit  Higher pressure = due to arrangement of vessels at glomerulusBSC2086 A&P II Final Exam Study Guide – Lesson 20 3 • Blood leaving glomerular capillaries flows into an efferent arteriole with a diameter smaller than afferent arteriole • Efferent arteriole produces resistance o Requires relatively high pressures to force blood into it • Capsular Hydrostatic Pressure (CsHP): Opposes glomerular hydrostatic pressure o Pushes water & solutes out of filtrate and into plasma o Results from resistance to flow along nephron & conducting system o Averages ~15 mmHg • Net Hydrostatic Pressure (NHP): The difference between glomerular hydrostatic pressure and capsular hydrostatic pressure • Colloid Osmotic Pressure: Osmotic pressure resulting from the presence of suspended proteins o Blood Colloid Osmotic Pressure (BCOP)  Tends to draw water out of filtrate and into plasma  Opposes filtration  Averages 25 mmHg • Net Filtration Pressure (NFP): Average pressure forcing water and dissolved materials out of glomerular capillaries and into capsular space o At the glomerulus, it is the difference between:  Hydrostatic pressure & blood colloid osmotic pressure across glomerular capillaries  NFP = (GHP – CsHP) – BCOP  Should NEVER be negative • Negative means kidneys are absorbing water from filtrate  If zero – no filtration is occurring • Glomerular Filtration Rate (GFR): Amount of filtrate kidneys produce each minute o ~10% of fluid delivered to kidneysBSC2086 A&P II Final Exam Study Guide – Lesson 20 4  Leaves bloodstream & enters capsular spaces  Averages 125 mL/min (at each kidney) o Glomeruli generate about 180 liters (48 gallons) of filtrate per day  99% is reabsorbed in renal tubules o Control of the GFR – three (3) interacting levels of control  Autoregulation – local level • Maintains GFR despite changes in local blood pressure & blood flow • Changes diameters of afferent arterioles, efferent arterioles, & glomerular capillaries • Reduced blood flow/glomerular blood pressure triggers: o Dilation of glomerular capillaries & afferent arteriole o Constriction of efferent arterioles • Rise in renal blood pressure… o Stretches walls of afferent arterioles o Causes smooth muscle cells to contract o Causes constriction of afferent arterioles (keeps GFR at normal rate) o Rise in renal blood pressure  stretches walls of afferent arterioles  contraction of smooth muscle cells  constriction of afferent arterioles  Hormonal Regulation (initiated by kidneys) • Renin-Angiotensin System o Constricts efferent arteriole, increasing GFP o Stimulates:  Aldosterone (adrenal cortex) & ADH (posterior pituitary) secretion  Na+ & H2O reabsorption o Increases thirst o Increase sympathetic motor tone & cardiac output o Vasoconstriction of arterioles & precapillary sphinctersBSC2086 A&P II Final Exam Study Guide – Lesson 20 5 • Natriuretic Peptides – ANP & BNP o Dilate afferent arteriole & constricts efferent arteriole to increase GFP & GFR o Increase urine production to lower blood volume/pressure  Autonomic Regulation – by sympathetic division of ANS • Mostly consists of sympathetic postganglionic fibers • Sympathetic activation o Constricts afferent arterioles o Decreases GFR & slows filtrate production • Changes in blood flow to kidneys due to sympathetic stimulation o May be opposed by autoregulation at local level Reabsorption and Secretion • Control of blood pH – important to homeostasis o By H+ removal & bicarbonate production at kidneys • Acidosis: Abnormally low (acidic) blood pH o Lactic Acidosis: Develops after exhaustive muscle activity o Ketoacidosis: Develops in starvation (from breakdown of lipids or amino acids) or diabetes mellitus • Alkalosis: Abnormally high (basic) blood pH o Can be caused by prolonged aldosterone stimulation  Stimulates secretion of H+ and


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FSU BSC 2086 - Lesson 20: The Urinary System Part II

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