The Patient: Renal System

- Excretion - Homeostatic regulation - Endocrine function What are the 3 main functions of the kidneys? Formation of urine, excreting end prodWhat is involved in excretion in the kiducts of metabolism and excess inorganic neys? substances from the diet. - Electrolyte homeostasis (concentration of Na+, K+ and HCO3- and other ions) - Water homeostasis (regulation of extraWhat is regulated homeostatically in the cellular fluid - plasma and interstitial fluid kidneys? - through formation of urine). - Regulation of pH (for acid-base balance) What is the endocrine function of the kidneys? - Synthesis of hormones (erythropoietin, renin - RAAS system regulates blood pressure) - Target site for hormones What are the main structures of the urinary system? - Kidneys - Ureters - Urinary bladder - Urethra The kidneys receive their blood supply How do kidneys receive their blood sup- from the renal arteries (branch of the ply? aorta) which enters each kidney at the renal hilus. Bean-shaped organs, around the size of What is the shape and size of the kidneys a fist. Responsible for processing blood and filtering waste products and extra and what is its main function? water. - The renal cortex contains many capillaries. It can detect when oxygen levels decrease and the kidneys will secrete erythropoietin to trigger erythropoiesis. - The renal pyramids in the medulla are separated by renal columns. The pyramids contain minor calyces which unite 1/9 What is the structure of the kidneys - 3 main structures? to form a major calyx. The major calyces form the renal pelvis. - The renal pelvis collects urine and transports urine to ureters. What are nephrons and how many are there in each kidney? The urine-forming units of the kidney, there are 1 million in each kidney. - Cortical nephrons make up around 85% of total neurons. They are shorter in length, located in the cortex, and perform most of the reabsorptive and secretory functions of the kidneys. - Juxtamedullary nephrons make up around 15% of total neurons and are What are the two types of nephrons and longer in length. They run through the what are the differences? medulla and concentrate urine. What makes up the nephron system? 1. Glomerular capsule 2. Proximal convoluted tubule 3. Descending limb of loop of Henle 4. Loop of Henle 5. Ascending limb of loop of Henle (thick segment) 6. Distal convoluted tubule 7. Collecting duct 2/9 Located in the renal cortex and encloses the glomerulus (network of capillaries) Where is the Bowman's capsule and what does it enclose? Fenestrated, consisting of endothelium with large-diameter pore. Endothelial What is the structure of endothelial cells have podocytes and pores to allow cells lining capillaries of the glomerulus? movement of water, glucose and ions, What is the purpose of this? but retain erythrocytes and large plasma proteins. Filtration can only occur at high blood How does filtration in Bowman's capsule pressure. Afferent arterioles (high bp) occur? How does blood enter and leave deliver blood to be filtered and the efferthe capsule? ent arterioles take away the filtered blood from the glomerulus. the site where the initial portion of the distal tubule comes into contact with the afWhat is the juxtoglomerular apparatus? ferent and efferent arterioles. Composed What cells does it compose of? of macula densa and juxtaglomerular cells. What do macula densa cells do? Detect levels of chloride and sodium ions reaching the distal convoluted tubule. If the levels are high, the rate of filtration is high. What do juxtoglomerular cells do? Juxtaglomerular cells lining the afferent arterioles secrete renin when blood pressure drops. Renin activates the renin-angiotensin-aldosterone system to increase blood pressure. What is the renal corpuscle? Renal corpuscle is the blood-filtering component of the Bowman's capsule. 3/9 What should the net filtration pressure in the Glomerular/Bowman's capsule be? 10 mmHg - Glomerular blood hydrostatic pressure (55 mmHg) - high blood pressure for filtration. - Capsular hydrostatic pressure (15 What does the net filtration pressure in mmHg) - pressure in Bowman's capsule, Bowman's capsule depend on? that opposes filtration. - Blood colloid osmotic pressure (30 mmHg) - presence of large plasma proteins in blood tends to draw filtrate back into the glomerulus, opposing filtration. How do you calculate the net filtration pressure? NFP = GBHP - CHP - BCOP Associated with reabsorption and secreWhat occurs in the proximal convoluted tion. tubule? 60-70% of glomerular filtrate is reabsorbed here by epithelial cells with villi. - Over 99% of organic nutrients (glucose, amino acids) by facilitated transport and cotransport. - Ions. Na+ is actively reabsorbed, stimuWhat substances are reabsorbed in the lated by angiotensin II and HCO3. Other proximal convoluted tubule? ions such as chloride and solutes such as urea are reabsorbed passively. - Water by osmosis in response to increased solute concentration in peritubular space. - Diffusion through sodium leak channels What are the different mechanisms in and into cells of the PCT. which sodium ions can be absorbed from - Sodium-linked co-transport of gluthe lumen into the proximal convoluted cose/amino acids. tubule? - Countertransport with protons. How do sodium ions get reabsorbed Sodium ions may then be extruded from from cells of the PCT into the peritubular cells by Na+/K+ pump. K+ diffuses back fluid and then peritubular capillaries? out of the cell through leak channels. 4/9 What makes up the loop of Henle? Descending limb of loop of Henle (thin segment), loop of Henle, and ascending limb of loop of Henle (thick segment). Cells of the descending limb and initial What are the cells of the descending limb cells of the ascending limb are flat epithelike and what does this mean for reab- lial with no microvilli and few mitochonsorption of ions? dria. Thus, they are not specialised for transport and reabsorption of ions. Cells further up the ascending limb are What are the cells of the ascending limb cuboidal with lots of mitochondria so are like and what does this mean for reabspecialised for transport and secretion of sorption of ions? ions. There are lots of pumps found here. How can the kidney produce urine of varying concentrations? The loops of Henle act a countercurrent multiplier. Countercurrent means the exchange occurs between fluid in opposite direcWhat does countercurrent mean in terms tions, fluid in descending limb moves toof the loop of Henle? wards the renal pelvis and ascending limb moves towards the cortex. The thin descending limb is permeable to water but impermeable to solutes such as Na+ and Cl- (so the fluid becomes more concentrated at the hairpin). The thick ascending limb is impermeable to water but actively extrudes Na+ and ClHow does the countercurrent multiplier (permable) via active transport process. system in the loop of Henle work? Extrusion of sodium and chloride ions from the ascending limb elevates osmotic concentration of the peritubular fluid, facilitating osmotic flow of water from the descending limb into the peritubular fluid. Sodium, potassium, and chloride ions are transported out of the tubular fluid. This involves active transport transporters. - Potassium and chloride ions are 5/9 pumped into the peritubular fluid. What is reabsorbed from the ascending - Sodium ions are pumped out in exlimb of the loop of Henle? change for potassium ions (Na+/K+ exchange pump). Potassium ions diffuse back into the lumen of the tubule through potassium ion Why aren't potassium ions reabsorbed leak channels. Therefore, only sodium from the ascending limb? and chloride ions are lost into peritubular fluid. Consists of cuboidal epithelial cells, fewWhat cells make up the distal convoluted er microvilli and abundant mitochondria tubule? (to provide energy for transporters). It is involved in reabsorption of water, sodium and calcium ions from the tubuWhat happens in the distal convoluted lar fluid (macula densa cells sense that tubule? these ions need to be reabsorbed when GFR is too high). Sodium ions and chloride ions are actively transported across the distal convolutHow are sodium and chloride ions reabed tubules. Sodium ions are also reabsorbed? sorbed in exchange for potassium ions by pump. The ion pump is controlled by aldosterone. How is aldosterone involved in the ion Aldosterone stimulates synthesis and inpump? What happens when there is corporation of pump into the membrane. too much/prolonged aldosterone stimuIt also favours reabsorption of filtered lation? sodium. Prolonged aldosterone stimulation may result in hypokalaemia. What controls the regulation of calcium ion reabsorption in the distal convoluted tubule? Parathyroid hormone What is the structure of the collecting duct? Consists of connecting tubules, which connect each nephron to a collecting duct. Several collecting ducts empty into a large papillary duct. Simple cuboidal 6/9 cells line connecting tubules and columnar epithelium line collecting and papillary ducts. The collecting duct is impermeable to water in the absence of antidiuretic hormone. Antidiuretic hormone (produced by the hypothalamus and secreted by What does antidiuretic hormone do and posterior pituitary) incorporates pores what produces it? into the collecting duct so that water can be retained (makes urine more concentrated). It lowers both intracellular and extracellular osmolality. It reabsorbs Na+ and HCO3-. The corWhat else, in addition to water, is reabtical region contains aldosterone-sensisorbed in the collecting duct? tive Na+/K+ pump. Involved in secretion of H+. Hydrogen ions are secreted into tubular fluid from What is secreted in the collecting duct? peritubular fluid when pH of the peritubular fluid drops. Hydrogen carbonate ions are reabsorbed when this happens. Plasma proteins enter glomerular filtrate, What happens when glomerular capillar- reducing blood colloid osmotic pressure. ies and filtration membranes in the Bow- This means more fluid moves into tissues man's capsule are damaged? and everything is reabsorbed, causing edema. What is the glomerular filtration rate (GFR)? What is the normal rate? Volume of fluid filtered from the glomeruli into Bowman's space per unit time, it is approximately 125ml/min in the average sized 70kg healthy male. It is therefore a reflection of the normal physiology as well as disease state of the kidneys. What does it mean if the GFR is too high? If the GFR is too high, needed substances cannot be reabsorbed quickly enough and are lost in urine. What does it mean if the GFR is too low? 7/9 If the GFR is too low, everything is reabsorbed, including wastes that are normally disposed of. The work carried out by the kidney. What is renal function and what can it be Renal function is used to determine the used to determine? total percentage of kidney function. - Oliguria - decreased amount of urine - Anuria - absent production of urine What is oliguria and anuria? - Important to diagnose renal diseases. Why do we assess renal function and - Most useful information obtained from what is the most useful way to do this? estimating GFR, which is used clinically to assess degree of renal impairment. How do you determine the GFR? It is determined using a substance which is filtered through the kidney, the clearance of a known substance is determined. The measurement is based on determining the volume of plasma from which a substance is removed by glomerular filtration during its passage through the kidney. What substances are used to estimate GFR? - Creatinine - Inulin Clearance = (U x V)/S How do you calculate clearance? What is creatinine? How is it filtered? - U is the urinary concentration of X - V is the rate of urine formation (ml/min) - S is the plasma concentration of X. Breakdown product of creatine phosphate in muscles and levels are maintained relatively constant because of continuous muscle catabolism and excretion by kidneys. It is filtered by the glomerulus and is not reabsorbed in the nephron. The rate of excretion is related to the GFR only, thus when GFR is reduced, 8/9 creatinine levels increase in plasma as the creatinine cannot be efficiently exWhat happens to creatinine when GFR is creted by the kidneys. Creatinine level in low? How can this be used to determine the serum is therefore determined by the kidney function? rate it is being removed, which is roughly a measure of kidney function. timed urine collection (often 24 hours) How do you determine plasma creatinine and a blood sample taken to measure concentration? plasma creatinine during that time. Creatinine clearance (ml/min) = (U (mg/dl) x V (ml/min) )/ S (mg/dl) - V is the volume of urine over a given time How do you calculate creatinine clear- U is the creatinine concentration in ance? urine - S is the creatinine concentration in serum. Values need to be adjusted by individual body surface area. Inulin is a fructose polysaccharide that is freely filtrated by the glomerulus and is neither reabsorbed nor secreted by What is inulin? Why is it used to estimate the renal tubules. It is metabolically inert GFR? and cleared only by the kidney, giving an accurate estimation of GFR (gold standard). In practice, a loading dose of inulin is given, followed by sustained infusion (usually for 3 hours), to keep plasma levels How do you measure inulin clearance? constant. Levels in urine and plasma determined together with urine flow rate, and GFR can be calculated using the equation. 9/9