BSC2086 1st Edition Lecture 22 Outline of Last Lecture I Urinary System Functions II Kidneys III Renal Tubule IV Nephron V Renal Corpuscle VI Collecting System Outline of Current Lecture I II III IV V Renal Physiology Aldosterone and ADH Glomerular Filtration Reabsorption and Secretion Transport Storage and Elimination of Urine I Renal Physiology a Goal of Producing Urine i Maintain homeostasis by regulating composition and volume of blood 1 Filtrate made by glomerular filtration is concentrated a Failure to do this will cause too much water loss or fatal dehydration 2 Absorbs and retains materials that are valuable for other tissues such as tissues and amino acids ii Includes the excretion of metabolic waste products such as urea creatinine uric acid which are all organic wastes dissolved in the blood b Kidneys i Mostly produce concentrated urine ii 1200 1400 mOsm L 1 4x plasma concentration II III iii Expression of osmotic concentration 1 Osmolarity a Total number of solute particles per liter b Expressed in osmoles per liter Osm L or milliosmoles per liter mOsm L c The osmotic concentration of body fluids about 300 mOsm L 2 Large organic molecule concentration a Grams or milligrams per unit volume of solution mg dL or g dL Aldosterone and ADH a Their secretion increases urine osmolarity b Aldosterone i Increases the number of Na K exchange pump at DCT and collecting ducts ii Promotes the reabsorption of Na in exchange for K iii Water is reabsorbed by osmosis c ADH i Increases the aquaporins or water channels in apical cell membranes of DCT and collecting duct ii Concentrates 100 mOsm L tubular fluid arriving at DCT to be concentrated to 1200 mOsm L when it reaches the minor calyx d Absence of ADH i All fluid arriving at DCT is lost in urine since water is not reabsorbed ii As seen in diabetes insipidus large amounts of dilute 20 400 mOsm L urine 24 L day iii Post Pit Normally is continuously secreting low levels of ADH 1 DCT and collecting system are both always permeable to water 2 Collecting system reabsorbs 16 8 L day 9 3 of filtrate 3 produces 1200 mL per day 0 6 of filtrate of urine 800 1000 mOsm L Glomerular Filtration a Governed by the balance between i Hydrostatic pressure 1 Fluid pressure 2 Blood pressure in glomerular capillaries 3 Usually pushes water and solutes out of plasma and into filtrate 4 Significantly higher than capillary pressures in the systemic circuit a This is due to the arrangement of vessels at the glomerulus b Blood leaving glomerular capillaries will flow into an efferent arteriole c Efferent arteriole has a small diameter than an efferent arteriole therefore producing more resistance and requiring a relatively high pressure to force blood into it ii Colloid osmotic pressure on both sides of capillary walls 1 Pressure of materials in solution trying to draw fluid in b Capsular hydrostatic pressure CsHP i Fluid stuck inside the capsule that opposes glomerular hydrostatic pressure ii Pushes water and solutes out of filtrate and into plasma iii A result of the resistance to flow along nephron and conducting system iv 15 mmHg average c Net hydrostatic pressure NHP i Glomerular hydrostatic pressure capsular hydrostatic pressure d Colloid osmotic pressure i Osmotic pressure due to suspended proteins ii Blood colloid osmotic pressure BCOP 1 Draws water out of filtrate and into plasma thus opposing filtration 2 25 mmHg average e Net filtration pressure NFP i Average pressure that forces water and dissolved materials out of glomerular capillaries and into the capsular space ii At glomerulus it accounts for the difference between hydrostatic pressure and BCOP across the glomerular capillaries iii NFP GHP CsHP BCOP 1 Should never be negative 2 If zero then no filtration is occurring f Glomerular filtration rate GFR i Amount of filtrate produced by kidneys each minute ii 10 of fluid delivered to kidneys 1 Leaves the bloodstream and enters capsular spaces 2 Averages about 125 mL min at each kidney iii About 48 gallons of filtrate are made by the glomeruli per day 1 99 is reabsorbed in the renal tubules iv Control 1 Autoregulation a Local level b Maintains GFR regardless of changes in local blood pressure and blood flow c Changes the diameters of afferent arterioles efferent arterioles and glomerular capillaries d Reduced blood flow or glomerular blood pressure causes i Dilation of glomerular capillaries and afferent arteriole ii Constriction of efferent arteriole 1 This increased resistance as blood is flowing out will increase the pressure in the glomerulus e Rise in renal blood pressure i Stretches walls of afferent arterioles ii Smooth muscle cells contract and constrict afferent arterioles iii Hydrostatic pressure drops less filtration 2 Hormonal regulation a Initiated by kidneys b Renin angiotensin system i Increases GFP by constricting efferent arteriole IV V ii Stimulates aldosterone release by the adrenal cortex and ADH by the posterior pituitary and Na and H2O reabsorption iii Increases thirst iv Increases sympathetic motor tone cardiac output and vasoconstriction of arterioles and precapillary sphincters 1 Try to increase blood pressure and blood volume v Natriuretic peptides ANP and BNP 1 Released by heart when there is too much fluid in it 2 Dilates the afferent arteriole and constricts efferent arteriole 3 Increases GFP and GFR 4 Makes more urine decreases blood volume 3 Autonomic regulation a Sympathetic division of ANS b Consists mostly of sympathetic preganglionic fibers c Activation of sympathetic nervous system i Constricts afferent arterioles ii Decreases GFR which slows filtrate production d Sympathetic stimulation also changes blood flow to the kidneys i Can be opposed by autoregulation at a local level Reabsorption and Secretion a Control of blood pH which is crucial to homeostasis i Removal of H ii Bicarbonate production b Acidosis i Lactic acidosis develops after exhaustive muscle activity ii Ketoacidosis develops during starvation or diabetes mellitus c Alkalosis i Abnormally high blood pH 1 May be caused by prolonged stimulation of aldosterone 2 Stimulates secretion of H and loss of H in urine d Vasa recta i Made by peritubular capillaries of juxtamedullary nephrons ii Functions 1 Return the water and solutes reabsorbed in the medulla to general circulation without disrupting the concentration gradient a Some solutes absorbed in the descending portion are not diffused out in the ascending portion b More water moves into the
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