Suzie - Urinary Notes by ICSM SU

Suzie Rayner

Urinary System 1 – The urinary system Main functional components: • • • • • •

Kidneys Clayces, renal pelvis, ureters Urinary bladder Urethra and associated sphincters Neurological control systems for bladder muscle and sphincters Well-adapted blood vascular supply

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Draw a simple diagram of the urinary system indicating the following: kidney, renal pelvis, ureter, bladder, urethra, sphincter vesicae, sphincter urethrae.

Renal pelvis: funnel-like dilated proximal part of the ureter in the kidney [acts as funnel for urine flowing to the ureter

Suzie Rayner Kidneys: Position • Retroperitoneal in upper abdomen • Surrounded by dense fibrous capsule • Fascial pouch (renal fascia) is outside fibrous capsule – contains the peri-renal adipose tissue • Posteriorly overlapped by ribs 11 and 12, diaphragm and pleural cavity. Blood Supply • Abundant blood supply from renal arteries • Short direct branches from abdominal aorta • Blood pressure drives Ultrafiltration by glomerular capillaries Structure – • Cortex - granular looking – due to random organisation • Cortex consists of glomeruli, where Ultrafiltration occurs, surrounded by convoluted parts of the tubules • Medulla – striated – radial arrangement of tubules and micro-vessels • Medulla contains parallel bundles of straight tubules. • Both cortex and medulla contain distinct parts of the nephrons (urine producing units) • Kidney is multilobar • Renal columns, consisting of cortex, reach right through the medulla at the boundaries of the kidney lobes. • Each lobe drains through its own papilla and calyx • Minor calyces join to form a few major calyces which all open into renal pelvis

Suzie Rayner

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Outline the means of urine transport down the ureters into the bladder and explain the mechanism preventing reflux of urine from the bladder.

Urine production is 2 staged: • Ultrafiltration – driven by arterial blood pressure [therefore short, wide renal arteries] • Absorption and secretion to modify ultrafiltrate. Ureters: • Run vertically down posterior abdominal wall • Sites of renal colic caused by kidney stones passing down the ureters and sticking • Urine transported by peristalsis in their smooth muscle (rhythmic contraction of muscle) • Opens obliquely through bladder wall Reflux of urine prevented by sphincters (?) Plaques in the ureter prevent osmosis and exchange of ions from the urine back into the body – would occur without this due to large difference in ion concentrations. Urethra: • In females – short and simple, passes through the perineum into the vestibule (space between labia minora) • In Males – long, intra-pelvic part within the prostate gland and part within the penis in addition to the trans-perineal part •

Describe the anatomical and histological features allowing expansion of the bladder as it fills with urine.

Ureters and bladder: • Lined by urothelium (transitional epithelium) • 3-layered epithelium with slow cell turnover • Large luminal cells, highly specialized low-permeability luminal membrane Prevents dissipation of urine-plasma gradients

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Distinguish between the sphincter urethrae and sphincter vesicae muscles and their nerve supplies.

Sphincter vesicae: • At neck of bladder • Reflex opening • In response to bladder wall tension • Controlled by parasympathetic Sphincter urethrae: • In perineum • Tone maintained by somatic nerves in pudendal nerve (S2,3,4) • Opened by voluntary inhibition of nerves • Sustained closure keeps sphincter vesicae closed, reduces bladder tone. •

Describe the mechanisms involved in the reflex contraction of the bladder in response to distension. State the approximate volume of urine in the bladder that normally initiates a reflex contraction in the adult.

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Suzie Rayner

Urinary System 2 – Structural basis of kidney function

Kidney function – sensitive to body needs • Filtration of blood plasma • Selective reabsorption of contents to be retained • Tubular secretion of some components • Concentration of urine as necessary • Endocrine function – renin, erythropoietin, 1,25-dihydroxycholecalciferol •

Describe the structural organisation of the kidney, as seen at a macroscopic level.

[More detail in previous lecture] •

Draw a diagram showing the main constituent parts of a nephron.

Suzie Rayner

Mechanism of urine production in the nephron: Filtration: • Blood passing through glomerulus is filtered • Filtrate consists of all components with less than 50000 molecular weight Reabsorption: • Material to be retained is reabsorbed in proximal convoluted tubule • Includes ions, glucose, amino acids, small proteins and water Creation of hyper-osmotic extracellular fluid: • Main function of the loop of henle and vasa recta • Countercurrent mechanism

Adjustment of ion content of urine: • Occurs at distal convoluted tubule and collecting duct • Controls amounts of Na+, K+, H+, NH4+ excreted Concentration of urine: • Occurs at collecting duct • Movement of water down osmotic gradient into extracellular fluid • Controlled vasopressin • • •

Draw a diagram of the structures separating glomerular capillary plasma from the fluid in Bowman's capsule. List the features of the cellular structure of the tubules in different parts of the nephron which make possible the concentration of urine. Draw a diagram showing the pattern of blood vessels in the kidney, and state which features contribute to the filtration process, to the reabsorption process, and to the countercurrent mechanism.

Suzie Rayner Renal corpuscle: Components: • Bowman’s capsule contains basement membrane, parietal epithelium and visceral epithelium (surrounds the glomerulus and high pressure of blood forces ions to filter into glomerulus) • Glomerulus (capillaries) • Podocytes (visceral epithelial cells) • Mesangial cells Blood supply: • Enters at vascular pole of corpuscle into afferent arteriole • Filters through capillary network at high pressure • Exits at efferent arteriole Filtration barrier: • Fenestrae in capillary endothelium • Specialised basal lamina • Filtration slits between foot processes ofpodocytes • Slits allow passage of ions and molecules < 50000 molecular weight Drainage of filtrate: • To proximal convoluted tubule, at urinary pole

Proximal convoluted tubule: Function: • Reabsorption of 70% of glomerular filtrate • Na+ movement by Na+ pump • Na+ movement causes water and anions (-ve) to follow • Glucose is taken up by Na+/glucose co transporter (movement of Na+ into cell

Suzie Rayner also moves glucose in) • Amino acids by Na+/amino acid co-transporter • Protein uptake by endocytosis Structure: • Cuboidal epithelium • Tight junctions • Membrane area increased to maximise rate of reabsorption • Brush border at apical surface • Interdigitations of basolateral membrane • Contains aquaporin proteins to mediate water diffusion • Prominent mitocondria (high energy requirement) Loop of Henle – the countercurrent mechanism Descending thin tubule: • Passive osmotic equilibrium • Aquaporins present • Simple squamous epithelium Ascending thick limb: • Na+ and Cl- actively pumped out of tubular fluid • Membranes lack aquaporins • Therefore, low permeability to water • Therefore, hypoosmotic tubular fluid, hyperosmotic extracellular fluid • [Creates a countercurrent mechanism - high extraceullar ion conc. - that allows water to passively move out of apparatus later on if water needs to be reabsorbed] • Cuboidal epithelium, few microvilli • High energy requirement – prominent mitochondria Vasa recta: • Blood vessels also arranged in loop • Blood in rapid equilibrium with extracellular fluid • Loop structure stabilises hyper-osmotic Distal convoluted tubule/cortical collecting duct • Adjustment of Na+/K+/H+/NH4+ • Controlled by aldosterone • Cuboidal epithelium, few microvilli • Complex lateral membrane interdigitations with Na+ pumps • Numerous large mitochondria • Specialisation of macula densa, part of juxtaglomerular apparatus Juxtaglomerular apparatus • Endocrine specialisation • Secretes renin to control blood pressure via angiotensin • Senses stretch in arteriole wall and Cl- in tubule • Cellular components are: o Macula densa of distal convoluted tubule o Juxtaglomerular cells of afferent arteriole

Suzie Rayner Medullary collecting duct • Completes ion adjustment and controls urine osmolarity • Passes through medulla – hyperosmotic extracellular fluid • Water moves down osmotic gradient to concentrate urine • Rate of water movement is due to aquaporin-2 in apical membrane o Content varied by exo/endocytosis mechanism o Under control of vasopressin (neurohypophysis) • Basolateral membrane has aquaporin-3, not under control • Duct has simple cuboidal epithelium, single cilium per cell • Cell boundaries don’t interdigitate • Smooth muscle wall for peristalsis ( 2 layers) • Cells contain organelles associated with secretory activity • Little active pumping (therefore few mitochondria) • Drains into minor calyx at papilla of medullary pyramid • Minor and major calyces and pelvis have urinary epithelium Ureters • Drain urine from the kidneys • Peristalsis movement towards the bladder • Urinary epithelium resists damage by urine Bladder • Urine storage organ (capacity of approx. 500ml) • 2 ureters enter posterior wall, urethra leaves inferiorly • Urinary epithelium resists damage and allows expansion • Smooth muscle wall (detrussor muscle) • Autonomic innervation • Sphincter vesicae at urethral exit Urinary epithelium – a.k.a. urothelium, transitional epithelium • Specialised form of epithelium – only found in urinary tract • Found in part of kidney, ureters, bladder, part of urethra • All cells contact basal lamina (but looks stratified) • Epithelium is resistant to urine and able to stretch • Cells appear squamous or cuboidal according to degree of stretch • Luminal cells are specialised for extremely low permeability

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Urinary System 3 – Renal blood flow and glomerular filtration Main functions of kidney: • Excretion of metabolic products (e.g. urea, uric acid, creatinine) • Excretion of foreign substances (drugs) • Homeostasis of body fluid, electrolytes, acid-base balance • Regulates blood pressure • Secretes hormones (renin, erythropoietin)

Filtration occurs where fluid is ‘forced’ through the semi-permeable walls of glomerular capillaries into Bowman’s capsule.

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Indicate what proportion of the cardiac output normally perfuses the kidney.

Renal blood flow: • Delivers oxygen, nutrients and substances for excretion Kidneys normally receive 20% of cardiac output (approximately 1litre/min) •

Define the term “freely filtered”.

Filtration: formation of an ultrafiltrate of plasma in the glomerulus. An abrupt fall in glomerular filtration is renal failure. Abnormalities in renal circulation and urine production lead to reduced glomerular filtration (and therefore to renal failure) Passive process of filtration (same as ‘freely filtered’?): • Fluid is ‘driven’ through semipermeable walls (fenestrated) of the glomerular capillaries

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Fluid is driven into Bowmans capsule space Driving force is the hydrostatic pressure of the heart

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State that the permeability barrier in the glomerulus discriminates mainly on the basis of size (although electrical charge also influences the filtration of charged proteins).

Fenestrated capillaries are highly permeable to: • Fluid • Small solutes Impermeable to: • Cells • Proteins • Drugs •

Compare the composition of the glomerular filtrate and the plasma.

Primary urine: a clear fluid completely free from blood and protein, produced containing electrolytes and small solutes.

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Define glomerular filtration rate (GFR) and filtration fraction and give typical values for each in a normal healthy young adult.

Glomeruli of each nephron filter only plasma, not blood cells. Plasma makes up 55% of blood, thus renal plasma flow = 0.55l/min Glomerular filtration rate: the amount of plasma filtered from the glomeruli into the Bowmans capsule per minute. • •

The sum of filtration rate of all functioning nephrons, therefore loss of nephrons will reduce surface area, therefore GFR will reduce. Each nephron unit can filter 20% (therefore filtration factor = 0.2) of blood plasma in each cycle

Suzie Rayner

In typical adult GFR = 20% of 550ml/min = 110ml/min GFR is the primary indicator of renal disease → filtration fails, build up of substances in blood therefore reduction in GFR. If increased plasma conc. of urea and creatinine, markers of renal disease. • Write an equation for the net filtration pressure across the glomerular membrane in terms of the hydrostatic and osmotic pressures involved. GFR = Kf x (Pgc – Pt – πgc) Puf = Pgc – Pt – πgc Puf = net ultrafiltration pressure Pgc = pressure in glomerular capillaries Pt = hydrostatic pressure in tubules Πgc = oncotic pressure generated by plasma proteins Kf = ultrafiltration coefficient (membrane permeability and surface area)

Ultimately there is net Ultrafiltration pressure of 10-20mmHg. GFR is not a fixed value, it is subject to physiological regulation. This is achieved by neural or hormonal input to the afferent/efferent arteriole, resulting in changes in GFR. Kidney diseases may reduce number of functioning glomeruli = reduced surface area = reduced Kf. Drugs/hormones can cause dilation of glomerular arterioles, increasing Kf •

Explain how net filtration pressure will be affected by (a) a large fall in arterial blood pressure (b) a fall in plasma protein concentration and (c) ureteral obstruction

Autoregulation of GFR Myogenic mechanism: • Vascular smooth muscle constricts when stretched • This keeps the GFR constant when blood pressure rises

Suzie Rayner

Autoregulation ensures fluid and solute excretion remain constant (otherwise varying pressure will vary urine production and cause loss of important ions)

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Tubularglomerular feedback: NaCl concentration in fluid is sensed by macula densa in juxtaglomerular apparatus Macula densa signals afferent arteriole and changes its resistance and so GFR

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Describe and explain the effect of changes in renal blood flow on GFR.

Renal plasma flow: • Measured by PAH (para aminohippurate) clearance = 625ml/min • PAH is filtered and actively secreted in one pass of the kidney, therefore amount PAH excreted = amount filtered and secreted. Clearance of PAH = renal plasma flow (filtered and secreted) Amount excreted = amount filtered – amount reabsorbed + amount secreted •

Define renal clearance and explain its use in assessing renal function.

Renal clearance: • As substances in blood pass through the glomeruli they are filtered to different degrees • The extent to which the substances are removed from the blood is called clearance • Clearance is the number of litres of plasma that are completely cleared of the substance per unit of time.

Suzie Rayner

Using clearance we can estimate the GFR: • If molecule is freely filtered and neither reabsorbed or secreted in the nephrons, then amount filtered = amount excreted. • Therefore the GFR can be measured by measuring the clearance. However, hard to find such a molecule: INULIN: • Plant polysaccharide • Freely filtered, not reabsorbed or secreted • Not toxic • Measurable in urine and plasma Has to be transfused as not found in mammals → therefore use an endogenous molecule with similar clearance. Practical measurement of GFR is done using creatinine clearance: • Waste product from creatine in muscle metabolism • Amount of creatinine released is fairly constant • If renal function is stable, creatinine in urine is stable Therefore low GFR value may indicate renal failure.

Summary of clearance, reabsorption and secretion: If substance is reabsorbed, the clearance will be less than 120ml/min If substance is not reabsorbed or secreted, clearance will be 12oml/min = glomerular filtration rate. If substance is secreted, clearance will be >120ml/min (e.g. PAH = renal blood flow)

Suzie Rayner

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Suzie Rayner

Urinary System 4 – Tubular function • •

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In the context of renal function, define the terms reabsorption and secretion. Explain the meaning of transcellular and paracellular transport. Draw a diagram of the wall of the early proximal tubule showing the following: tubular fluid, luminal membrane, basolateral membrane, peritubular capillary, tight junction, Na+/K+ “pump” and one example of each of the following: an ion-selective channel, co-transport of two solutes, counter-transport of two solutes. Explain how active sodium transport acts as a driving force for the reabsorption of water and many other ions and molecules. Describe the main routes for Na+ entry into tubular cells in the thick ascending limb of the loop of Henle, in the distal convoluted tubule and in the principal cells in the cortical collecting tubule. Contrast the osmolarity of the tubular fluid (a) in Bowman’s space (b) at the end of the proximal tubule and (c) emerging from the loop of Henle.

Average day consume 20-25% more salt and water than lost Therefore, this needs to be lost, as well as other waste products – in order t maintain homeostasis.

In ideal situation, all excess ions, water and waste would be pumped into bladder. However there are no pumps for water or waste products, therefore it doesn’t work. Urine is produced by passive filtration through molecular sieve BUT Cannot afford to lose all the water and small molecules that pass through filter So REABSORB.

Controlled re-absorption and secretion: Controlled by having regional specialisation of the tubule system and transport mechanisms • Allows 99% of ultra filtrate reabsorbed • How solute is balanced and plasma conc. and pH maintained. Osmolarity – ‘a measure of the osmotic pressure exerted by a solution across a semipermeable membrane’

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It is dependent on the number of particles crossing, not the nature All concentrations of the different solutes added together – each ion counts separately.

Plasma osmolarity has a small range. Urine osmolarity has a large range. [Any solute present at equal amounts on either side of semi-permeable membrane has no net movement, therefore no effect on water movement]

Reabsorption and secretion can occur paracellular (through tight junctions) or transcellular (through cell). Reabsorption: movement from lumen → capillary Secretion: movement from capillary → lumen Most important secretion is H+ and K+ (drugs can also be secreted e.g. choline, creatinine, penicillin) Types of transport in the tubules [SUMMARY OF PREVIOUS LECTURE]

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Osmosis Active transport Co-transport Counter transport Passive transport Movement down electrical gradient

Relationship between solute and rate: • Passive - Protein independent transport (lipophilic molecules)

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Passive - Protein dependent transport (hydrophilic molecules) Active – Direct ATP dependent Active – Indirect ATP dependent

Water transport – osmosis: • Paracellular – down tight junctions • Transcellular - Dependent on aquaporins Aquaporins regulate passive uptake system of water. Different types of transporters in different parts of the nephrons give the different roles. There is a limited amount of material that can be reabsorbed. HOWEVER, if limit is exceeded, excess is excreted in urine • e.g. glucose in T1DM • Vitamin B • Vitamin C

Suzie Rayner

Na+/K+ pump is most important in the kidney: • Moves H+ out of cell • Moves glucose, amino acids, bicarbonate into cell

Suzie Rayner Basolateral membrane • Na/K pump keeps intracellular Na low an K high. • Large concentration and electrical gradients favour Na movement into cell Early proximal tubule • Na+ entry down a large electrochemical gradient • Can bring ‘uphill’ entry (co-transporter) of glucose and amino acids, and exit of H+ • Carbonic anhydrase activity leads to Na+ reabsorption and increased urinary acidity Proximal convoluted tubules are affected by metabolic poisons: Passive reabsorption Active reabsorption Urea Glucose Water Amino-acids Sodium Potassium Calcium Vitamin C Uric acid Renal proximal tubular wall: Net secretion of some substances plasma → proximal tubular fluid • Important as drugs and other substances excreted in this way • Some drugs enter tubular fluid and act further down the nephrons Ascending loop of Henle: • Na/K/Cl co-transporter is blocked by diuretics Distal convoluted tubule: • Ca2+ from lumen → blood • Na+/Cl- co-transporter linked to Ca2+ reabsorption • Sodium reabsorbed depending on aldosterone (greater dependence as more distal) • Na+/K+/H+/NH4+ • Water reabsorbed under ADH(vasopressin) control Collecting duct and distal part of distal tubule: Involves: • Apical Na+ channel sensitive to aldosterone • Linked K+ channel • pH control Principle cell: • Important in Na+, K+ and H2O balance – mediated by Na/K ATP pump • Apical Na channel is aldosterone sensitive, blocked by amilioride Intercalated cell: • Important in acid-base balance

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Mediated via H-ATP pump

Cortical collecting duct principle cell has very tight epithelium, therefore little paracellular transport. Relies on vasopressin.

Proximal tubule: • Reabsorbs 60% of all solute • 100% glucose and amino acids • 90% of bicarbonate and water • 65% filtered Na Loop of Henle: • Allows urine concentration • Reabsorbs 25% of filtered Na Distal tubule: • Reabsorbs 8% of filtered Na Collecting duct: • Reabsorbs 2% Na, only if aldosterone is present ___________________________________________________________________

Suzie Rayner

Urinary system 5 – Mechanism of acid-base balance Definitions: Acid – substance that can release H+ in solution Base – substance that can accept H+ in solution Buffer – addition or removal of H+ will result in minimal change of pH pH – measure of hydrogen ion concentration, indicates acidity, pH=-log[H+] •

Give the normal arterial plasma pH and the limits compatible with life.

H+ is maintained in very narrow limits at a low conc. pH = 7.40 [Range = 7.35 – 7.45] If pH is outside range 7.2-7.6, serious pathological condition Range compatible with life = 6.80-7.80 Urine pH range = 4-8.5 •

Explain in terms of physiological buffering the importance of the bicarbonate buffer system.

Control of pH is particularly important because: • Metabolic reactions are highly sensitive to pH • H+ ions change shapes of proteins – including enzymes • H+ are created and destroyed all the time • Sources of H+ ions: o Protein breakdown o CO2 o Exercise (lactic acid production) Acid-Base balance regulation: • Extra and intracellular buffers • Control of partial pressure of CO2 in blood by altering rate of alveolar ventilation • Control of plasma bicarbonate concentration by changes in renal H+ excretion [Note: Think compensatory mechanisms in acid-base] Principle buffers:

Suzie Rayner

Buffering process: • In metabolic acidosis, only 15-20% of the acid load is buffered by CO2/bicarbonate system in the extracellular fluid, most of remainder is buffered in the cells • In metabolic alkalosis, 30-35% of OH- load is buffered by cells • In respiratory acidosis/alkalosis, almost all buffering is intracellular Extracellular buffer: • CO2/HCO3- system is most important extracellular buffer • HCO3- and PCO2 are regulated independently o HCO3- regulated by changes in renal H+ o PCO2 by changes in rate of alveolar respiration Buffering at local level: • H2SO4 and HCl produced during metabolism are not circulated as free acids but are immediately buffered • These reactions minimise increase in extracellular H+ • BUT excess H+ must still be excreted by kidney to prevent progressive depletion of HCO3Sources of body H+ ions: Physiologically Produced Carbohydrates and fats H2O and CO2 Sulphur-containing amino acids e.g. Cysteine Arginine, Histidine, Lysine Further sources: Volatile acids Produced from metabolism of carbohydrates and fats Result in CO2 production

Sulphuric acid

Pathologically Hypoxia, carbohydrates and fat Diabetes, carbohydrates

Produced Lactic acid Ketoacids (βhydroxybutyric acid)

HCl

Non-volatile acids Derived from metabolism of proteins Only 50-100meq/day of acid produced this 23

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15,000mmol CO2 produced per day CO2 lost through respiration •

way H+ ions excreted by kidneys

Give the Henderson-Hasselbach equation when applied to the bicarbonate buffer system. Cite a normal value for plasma HCO3- concentration.

Overview of control of pH: Lungs: release CO2 Kidney: release H+ GI tract: release bicarbonate CO2 + H2O → H2CO3 → H+ + HCO3Regulated by carbonic anhydrase? • •

State that the kidneys help to control plasma HCO3- concentration by (a) variable reabsorption of filtered HCO3-, and (b) variable addition of new HCO3- to the blood perfusing the kidneys. Explain the mechanism and indicate the sites of HCO3- reabsorption.

Bicarbonate reabsorption: • Approximately 80% of bicarbonate is reabsorbed in proximal tubule – mostly in first 1-2mm • Remaining 20% reabsorbed in thick ascending limb of loop of Henle and outer medullary collecting tubule

H+ ion excretion is regulated by: • Extracellular pH is primary physiologic regulator 24

Suzie Rayner In pathological states these can affect acid excretion independent of systemic pH: o Blood volume o Aldosterone o Plasma potassium Effective circulating volume: • Has important effects on bicarbonate reabsorption • Most important effect is increased bicarbonate reabsorption associated with volume depletion 4 influencing factors: • Reduction in glomerular filtration rate (GFR, normally 120ml/min) • Activation of renin-angiotensin-aldosterone system • Low plasma Cl• Low plasma K+ due to urinary losses (diuretics) or GI losses (vomiting, diarrhoea) Examples of increased bicarbonate reabsorption with volume depletion: • Hypovolaemia associated with diuretics causes increased HCO3reabsorption with high levels of plasma HCO3• Eating low salt diet causes small rises in plasma HCO3• Clinically important in patients with metabolic alkalosis – patient volume depleted, not possible to excrete excess HCO3- to correct acid-base balance. [NOTE: Secretion: Blood → urine, Excretion: urine → out of body] Increased H+ secretion: Primary (directed at balancing acid-base) • Decreased plasma HCO3- conc. (reabsorbed into • Increased paCO2 (arterial partial pressure) Secondary (not directed at balancing acid-base) • Increase in filtered load of bicarbonate • Decrease in ECF volume • Increase in angiotensin II • Increase in aldosterone • Hypokalaemia Decreased H+ secretion: Primary • Increased plasma HCO3- conc. • Decreased paCO2 Secondary • Decrease in filtered load of bicarbonate • Increase in ECF • Decrease in aldosterone • Hyperkalaema •

State the limits of urine acidity and alkalinity. Thus explain why it is impossible for the kidneys to add significant amounts of new bicarbonate to

Suzie Rayner the blood simply by excreting free H+ ions. Renal H+ excretion: • Must excrete 50-100mmol of noncarbonic acid generated each day Mechanisms involved: o Proximal tubule o Thick ascending limb of loop of Henle o Collecting ducts • Mechanisms reabsorb bicarbonate filtered into urine • Secreted H+ ions are excreted with filtered buffers (phosphates, creatinine) • Secreted H+ ions are excreted with manufactured buffer (ammonia – manufactured from glutamine in proximal tubule) Problems in excreting daily acid load: • Cannot be excreted as free H+ • All filtered bicarbonate needs to be reabsorbed, as losing bicarbonate is effectively = adding H+ ions Renal H+ pump action: In proximal tube: • H+ secreted into lumen by Na+/H+ exchanger • HCO3- ions are returned to systemic circulation by Na+- HCO3- cotransporter In collecting tube: • Luminal pump mediated by active H+ - ATPase pump • And Cl- HCO3- exchanger in basolateral membrane Excretion of H+ with filtered buffer: • Lowest urine pH that can be achieved is 4 • Still represents very low free H+ • Combined H+ with filtered urinary buffer such as phosphate or ammonia

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Describe in outline the mechanisms involved in the excretion of acid phosphate and of ammonium salts. Indicate how these events contribute new bicarbonate to the blood.

New bicarbonate formation: • Bicarbonate reabsorption < bicarbonate lost (during the titration of the non26

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volatile acids produced by metabolism) To maintain acid-base balance, the kidneys must replace this loss

Ammonium excretion: • Ability to excrete H+ ions as ammonium adds important degree of flexibility to renal acid-base regulation • NH3 produced in tubular cells predominantly from glutamine • Some of excess NH3 diffuses into tubular lumen • Excreted H+ combines with NH3 to form ammonia Summary of ammonia and bicarbonate production, excretion and transport:

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Explain in general terms what is meant by respiratory compensation and renal compensation for acid-base disturbances. 27

Suzie Rayner • Acid-base disturbance Respiratory acidosis

Explain the terms: respiratory acidosis, respiratory alkalosis, metabolic acidosis, metabolic alkalosis.

Plasma pH Plasma CO2 Low High

Plasma Caused by bicarbonate • Reduced alveolar ventilation

Compensation Renal compensation – increased bicarbonate and ammonia secretion (several days) Cellular buffering minimises change in intervening acute phase

Respiratory alkalosis

Metabolic acidosis

High

Low

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Low

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Metabolic alkalosis

High

Increased alveolar ventilation

Addition of nonvolatile acids Loss of non-volatile alkalis Failure to reabsorb sufficient bicarbonate

• Loss of non-volatile acid (vomiting) • Raised aldosterone

pH rises back towards, but not above, normal. Renal compensation – decreasing bicarbonate reabsorption and ammonia secretion pH falls back towards normal Respiratory compensation by raised ventilation due to peripheral chemoreceptor stimulation Renal excretion of net acid increases if possible Reduced ventilation Renal excretion of excess bicarbonate, can be limited if low blood volume with Na and Cl depletion

Suzie Rayner

Analysis of acid-base disorders • Directed at identifying the underlying cause • Treatment can be initiated • Medical history and associated physical findings often provide valuable clues about nature and origin • Require arterial blood analysis ____________________________________________________________________

Suzie Rayner

Urinary system 6 – Sodium and potassium balance •

Explain why extracellular fluid volume is determined primarily by the body's sodium content. Thus explain the importance of the renal control of sodium excretion in the control of extracellular fluid volume.

The main determinant of extracellular fluid volume is the number of osmoles present. Sodium is the most abundant of these, therefore the largest determinant. Therefore to control the ECF, sodium must be regulated. Effect of high sodium diet on body weight: Increased sodium causes an increase in body weight due the increased water volume that is retained. (1g Na+, 100g H2O)

Effects of changing sodium levels: The opposite of this occurs with decreased dietary sodium.

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Compare the daily amounts of sodium filtered with the amounts normally appearing in urine. State the approximate proportions of filtered sodium normally reabsorbed in (a) the proximal tubule and (b) the loop of Henle. 65% proximal tubules 25% thin ascending limb of loop of Henle 8% distal convoluted tubule 30

Suzie Rayner 2% collecting ducts (when in presence of aldosterone)

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State that the bulk of the glomerular filtrate is reabsorbed in the proximal tubule and loop of Henle and that the fraction of the filtered load reabsorbed in these segments is not very responsive to changes in sodium, potassium or water balance. Contrast this with the function of the distal nephron.

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Name the site of secretion of aldosterone and list three factors that influence the rate of production of this hormone.

Aldosterone: • Steroid hormone • Synthesised and released from adrenal cortex • Released in response to angiotensin II [3 factors that influence rate of synthesis: angiotensin II, atrial natriuretic peptide (ANP), Brain natriuretic peptide (BNP)] As steroid hormone, hormone passes through cell membrane, binds with protein and receptor, causing protein to be released. Hormone-receptor complex enter nucleus and act as a transcription factor [affecting production of other transcription factors, regulatory proteins, transport molecules] •

Describe the effect of aldosterone on sodium reabsorption, indicating its principal site of action.

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Acts on collecting ducts Induces formation of Na-K-ATPase pumps Induces expression of apical Na channel of the collecting duct (and probably also promotes activity)

Aldosterone stimulates: • Na reabsorption • Potassium secretion • Hydrogen ion secretion Aldosterone excess leads to hypokalaemic alkalosis (too little potassium retained) Diseases associated with aldosterone: Hypoaldosteronism Decreased Na reabsorption from distal nephrons Increased urinary loss of sodium ECF volume falls

Hyperaldosteronism Increased Na reabsorption from distal nephrons Decreased urinary loss of sodium ECF volume rises

Suzie Rayner Increased Renin, angiotensin II, ADH

Decreased renin, angiotensin II, ADH Increased ANP(atrial natriuretic peptide), BNP(brain natriuretic peptide)

Liddle’s syndrome: • Inherited disease of high BP • Due to a mutation in the aldosterone activated Na channel in the collecting duct • Defective channel is always on, therefore too much Na is reabsorbed • Results in sodium retention → hypertension How is change in BP measured? Baroreceptors: • Heart → atria • Vascular system → carotid sinus, aortic arch, pulmonary vasculature, juxtaglomerular apparatus Atrial natriuretic peptide (ANP) [also BNP from brain]: • Produced and secreted by atria of heart • Released in response to atrial stretch (high BP) Actions of ANP: • Vasodilatation of renal (and other systemic) blood vessels • Inhibition of sodium reabsorption • Inhibits renin and aldosterone release • Reduces BP Reduced blood pressure mechanism on whole nephron: Increased blood pressure (volume expansion) has opposite effect.

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Explain three ways in which an

increased concentration of Angiotensin II can influence renal function. How to measure change in Na in distal nephrons: • Macula densa monitor the Na in tubular fluid • Juxtaglomerulur cells stimulated to secrete rennin

Suzie Rayner

Effects of angiotensin II:

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Give three stimuli that lead to an increase in renin release Describe the effect of renal sympathetic nervous activity on the renal vasculature and on renin release.

Renin release is stimulated by: • Decreased renal perfusion pressure • Decreased Na in distal tubule • Decreased angiotensin II • Increased sympathetic activity (β adrenoceptors) •

Name one other hormone that can directly influence renal sodium reabsorption

ADH •

State that small variations in Na+ intake can be counterbalanced by changes in Na+ reabsorption in the collecting ducts (under the influence of aldosterone) but that more substantial variations resulting in extracellular fluid volume contraction or expansion lead to widespread co-ordinated changes in renal function Maintenance of sodium balance: • Small variations in sodum intake can be countered by changes in sodium 33

Suzie Rayner

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reabsorption in the collecting ducts (largely regulated by aldosterone) More substantial variations (and other changes in fluid balance, e.g. haemorrhage) lead to a complex set of inter-related changes in renal function

ACE inhibitors lower BP as they inhibit production of angiotensin II, effecting both vascular and kidney. •

Explain that the prime function of diuretics is to increase renal Na+ excretion (usually by reducing Na+ intake into the tubular cells). Give one example of a class of diuretic drug acting in each of the following sites: the proximal tubule, the loop of Henle, the early distal tubule and the cortical collecting ducts.

Diuretic drugs: Region acted on Proximal convoluted tubule and descending loop of Henle Proximal convoluted tubule

Drug (and example) Osmotic diuretics – glucose (as in diabetes mellitus) Carbonic anhydrase inhibitors

Thick ascending limb of loop of Henle Distal convoluted tubule Distal convoluted tubule

Loop diuretics – furosemide Thiamides K+ sparing diuretics

Action

(carbonic anhydrase leads to Na+ reabsorption through H+ secretion) Therefore: Inhibits H+ secretion, promoting Na+ and K+ excretion Blocks triple transporter (Na, 2CL, K pump) Block Na/Cl co-transporter Amiloride – block Na channels Spironolactone – aldosterone antagonist

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Compare the daily amount of potassium filtered with the amount normally appearing in the urine. Kidneys excrete 90-95% of K+ ingested from diet • • •

K+ is main intracellular ion Extracellular K+ has effects on excitable membranes: o High K+ depolarises membranes o Low K+ causes heart arrhythmias Hypokalemia ECF, one of the most common electrolyte imbalances.

Causes of hypokalaemia: • Diuretics • Surreptitious vomiting • Diarrhoea • Genetics

Suzie Rayner

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State that under normal conditions potassium ions are reabsorbed in the proximal tubule and loop of Henle but secreted into the lumen of the late distal tubule and cortical collecting duct. State that potassium excretion depends largely on the extent of this secretion.

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K+ is freely filtered in glomerulus, most reabsorbed in proximal convoluted tubule and loop of Henle Distal convoluted tubule and collecting ducts can secrete K+ when necessary (main control of excretion) Aldosterone stimulates Na+ reabsorption and K+ excretion

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Describe the cellular mechanism of potassium secretion

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In distal convoluted tubule and collecting duct, aldosterone stimulates: • Na+/K+ pump, moving K+ from blood → principle cell • Na+ channel, moving Na+ into principle cell • Inhibits K+ channel, blocking K+ from moving into lumen • K+ moved back out into blood via K+ channel in basal cell wall •

Explain how potassium secretion (and therefore excretion) is influenced by: plasma potassium concentration, aldosterone, tubular fluid flow rate, acidbase balance

Potassium secretion: Amount of filtered load reaching each point of nephron. •

K+ excretion rate is largely determined by the amount of K+ secreted in collecting ducts The secretion from the collecting ducts increases as: • Increase in K+ • Increase in aldosterone • Increase in tubular flow rate • Increase in plasma pH

Suzie Rayner

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Suzie Rayner

Urinary system 7 – Control of water excretion •

State the minimum and maximum osmolarity of the urine in humans and indicate the nephron sites responsible for the production of (a) dilute urine and (b) concentrated urine.

[Mentioned in previous lecture] Osmolarity – ‘a measure of the osmotic pressure exerted by a solution across a semipermeable membrane’ • It is dependent on the number of particles crossing, not the nature • All concentrations of the different solutes added together – each ion counts separately.

Plasma osmolarity has a small range. Urine osmolarity has a large range. Affects of excess salt, water or volume: • Excess volume – oedema and BP increase • Excess water – dilute salt, cells will swell (hypotonic) • Excess salt – cells will shrink (hypertonic) Osmolarity varies: Osmolarity Water Increased Decreased Decreased Increased Constant Decreased Constant Increased Regulation of water and salt balance are inter-related.

Salt Increased Decreased Decreased Increased

Water balance is used to regulate plasma osmolarity. The level of salt determines the extracellular fluid volume. [Revision of tissues – MCD] Extracellular fluid (lymph, plasma, interstitial, transcellular – CSF, synovial) = 15L

Suzie Rayner Intracellular fluid = 25L Excretion of water: Mechanism Skin and sweat

Controllable? Variable but uncontrollable

Volume Normally 450ml/day

Faeces

Uncontrollable

Normally 100ml/day

Respiration

Uncontrollable

Urine output

Variable and Controllable

Normally 350ml/day Normally 1500ml/day

(a) (b)

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Varies with… Fever Climate Activity Diarrhoea up to 20L/day with cholera Activity

Explain why the final concentration of the urine depends on: the osmolarity of the medullary and papillary interstitium; the permeability of the collecting ducts to water. Kidney makes different concentrations of urine by: Fraction of filtered load reaching different points in the nephron

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The fraction of the filtered load reabsorbed in the proximal tubule and loop of Henle is little changed by small variations in fluid balance • Fraction of filtered load reabsorbed in collecting ducts is very variable • This is also true for renal handling of Na+, K+, H+ Crucial factor for water is osmolarity of the plasma and the concentration of vasopressin. Interstitial osmolarity varies. But water cannot be pumped, therefore relies on gradient. 38

Suzie Rayner

Gradient is established by counter current exchange mechanism: Cortico-medullary gradient results because of: • The shape of the loop of Henle • The fact that the descending limb is water permeable, Na+ and Climpermeable and the ascending limb is water impermeable, Na+ and Clpermeable • Active transport of Na, Cl into the interstitial fluid from the ascending limb • Urea absorbed from inner medullary collecting duct into interstitial tissue • As a result solute accumulates in the medullary interstitial fluid forming a gradient.

Why don’t medullary blood flow eliminate countercurrent gradient? Vasa Recta is blood supply • Permeable to water and solutes • Water diffuses out of descending limb and solutes diffuse into descending limb • Opposite occurs in the ascending limb 39

Suzie Rayner •

Thus oxygen and nutrients are delivered without loss of Gradient.

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State that the medullary and papillary interstitium is hypertonic as a result of the accumulation of NaCl and urea.

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Describe how changes in plasma osmolarity influence the release of vasopressin (antidiuretic hormone) from the posterior pituitary, using the term 'hypothalamic osmoreceptors'. Describe the action of vasopressin on the collecting ducts, and hence explain how urine volume is regulated in accordance with the state of hydration of the body.

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Vasopressin: • Peptide hormone (Nonapeptide) • Synthesised in hypothalamus • Secreted from posterior pituitary (neurohypophysis) • Binds to specific receptors on basolateral membrane of principle cells in the collecting ducts • Leads to insertion of aquaporin 2 on luminal membrane (aquaporin 3 and 4 on capillary side), hence increasing water permeability • Also stimulates urea transport from inner medullary collecting ducts into thin ascending limb of loop of Henle and interstitial tissue. Vasopressin release is triggered by: • Osmoreceptors in the hypothalamus regulate ADH (vasopressin) release if osmolarity is > 300mOs • Also stimulated by marked fall in blood pressure or volume (via baroreceptors or stretch receptors • Ethanol inhibits ADH release, leading to dehydration as urine volume increases •

Describe how changes in plasma osmolarity and volume influence thirst.

Suzie Rayner

Water diuresis (low ADH) – solute reabsorbed, water not, lowers urine osmolarity to around 50mOsmol/l – large volume of dilute urine Maximal antidiuresis (high ADH) – osmotic equilibration in cortex and medulla leads to high urine osmolarity – small volume of concentrated urine Dehydration mechanism is opposite of water load EXCEPT that dehydration causes thirst. [Water load causes drop in salt conc. in blood – decreased plasma osmolarity]

Feedback control by ADH ensures that plasma osmolarity is kept in the normal range – and determines urine output and water balance

Suzie Rayner

Diabetes Insipidus Characterised by: • Excretion of large amounts of watery urine ( up to 30L/day) • Unremitting thirst Caused by: • Insufficient secretion of ADH • Inheritance of 2 mutant genes for ADH receptor • Inheritance of 2 mutant genes for aquaporin Why do we have concentrated urine in the morning? • Asleep therefore not drinking • Decreased water in blood, increasing osmolarity of blood and extracellular fluid • Increased osmolarity is sensed by hypothalamus (due to Na+ present), Na+ excretion into filtrate increases • ADH released • Collecting reabsorb more water from the filtrate – conc. urine, and maintained blood osmolarity levels. _____________________________________________________________________