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14-September-2008 18:02:48 - physiology This illustration demonstrates the normal kidney physiology. It also includes illustrations showing where some types of diuretics act, and what they do. This illustration demonstrates the normal kidney physiology. It also includes illustrations showing where some types of diuretics act, and what they do. Renal physiology is the study of the physiology of the kidneys. Contents 1 Functions of the kidney 1.1 Secretion of hormones 1.2 Extracellular homeostasis 2 Mechanisms 2.1 Filtration 2.2 Reabsorption 2.2.1 Indirect reabsorption 2.2.2 Hormones 2.3 Secretion 3 Measurement of renal function 4 References Functions of the kidney The functions of the kidney can be divided into two groups: secretion of hormones, and extracellular homeostasis. The nephron is the functional unit of the kidney. Secretion of hormones Secretion of erythropoietin, which regulates red blood cell production in the bone marrow. Secretion of renin, which is a key part of the renin-angiotensin-aldosterone system. Secretion of the active form of vitamin D, calcitriol, and prostaglandins. Extracellular homeostasis The kidney is responsible for maintaining a balance of several substances: Substance Description Proximal tubule Loop of Henle Distal tubule Collecting duct Renal glucose reabsorption If glucose is not reabsorbed by the kidney, it appears in the urine, in a condition known as glucosuria. This is associated with diabetes mellitus.1. reabsorption almost 100% via sodium-glucose transport proteins2 apical and GLUT basolateral. - - - Renal oligopeptide reabsorption and renal protein reabsorption, amino acids Almost completely conserved.3 reabsorption - - - Renal urea handling Regulation of osmolality. Varies with ADH45 reabsorption 50% via passive transport secretion - reabsorption in medullary collecting ducts Renal sodium reabsorption Uses Na-H antiport, Na-glucose symport, sodium ion channels minor6 reabsorption 65%, isosmotic reabsorption 25%, thick ascending, Na-K-2Cl symporter reabsorption 5%, sodium-chloride symporter reabsorption 5%, principal cells, stimulated by aldosterone Renal chloride reabsorption Usually follows sodium. Active transcellular and passive paracellular7 reabsorption reabsorption thin ascending, thick ascending, Na-K-2Cl symporter reabsorption sodium-chloride symporter - water Uses aquaporin water channels. See also diuretic. absorbed osmotically along with solutes reabsorption descending - reabsorption regulated by ADH, via arginine vasopressin receptor 2 bicarbonate Helps maintain acid-base balance. 8 reabsorption 80-90% 9 reabsorption thick ascending 10 - reabsorption intercalated cells, via band 3 and pendrin protons Uses vacuolar H+ATPase - - - secretion intercalated cells potassium Varies upon dietary needs. reabsorption 65% reabsorption 20%, thick ascending, Na-K-2Cl symporter - secretion common, via Na+/K+-ATPase, increased by aldosterone, or reabsorption rare, hydrogen potassium ATPase calcium Uses calcium ATPase, sodium-calcium exchanger reabsorption reabsorption thick ascending via passive transport - - magnesium Calcium and magnesium compete, and an excess of one can lead to excretion of the other. reabsorption reabsorption thick ascending reabsorption - phosphate Excreted as titratable acid. reabsorption 85% via sodium/phosphate cotransporter11. Inhibited by parathyroid hormone. - - - carboxylate reabsorption 100%12 via carboxylate transporters. - - - The body is very sensitive to its pH level. Outside the range of pH that is compatible with life, proteins are denatured and digested, enzymes lose their ability to function, and the body is unable to sustain itself. The kidneys maintain acid-base homeostasis by regulating the pH of the blood plasma. Gains and losses of acid and base must be balanced. Acids are divided into volatile acids13 and nonvolatile acids.14 See also titratable acid. The major homeostatic control point for maintaining this stable balance is renal excretion. The kidney is directed to excrete or retain sodium via the action of aldosterone, antidiuretic hormone ADH, or vasopressin, atrial natriuretic peptide ANP, and other hormones. Abnormal ranges of the fractional excretion of sodium can imply acute tubular necrosis or glomerular dysfunction. Mechanisms The kidney's ability to perform many of its functions depends on the three fundamental functions of filtration, reabsorption, and secretion. Filtration Main article: Renal ultrafiltration The blood is filtered by nephrons, the functional units of the kidney. Each nephron begins in a renal corpuscle, which is composed of a glomerulus enclosed in a Bowman's capsule. Cells, proteins, and other large molecules are filtered out of the glomerulus by a process of ultrafiltration, leaving an ultrafiltrate that resembles plasma except that the ultrafiltrate has negligible plasma proteins to enter Bowman's space. Filtration is driven by Starling forces. The ultrafiltrate is passed through, in turn, the proximal tubule, the loop of Henle, the distal convoluted tubule, and a series of collecting ducts to form urine. Reabsorption Tubular reabsorption is the process by which solutes and water are removed from the tubular fluid and transported into the blood. It is called reabsorption and not absorption because these substances have already been absorbed once particularly in the intestines. Reabsorption is a two-step process beginning with the active or passive extraction of substances from the tubule fluid into the renal interstitium the connective tissue that surrounds the nephrons, and then the transport of these substances from the interstitium into the bloodstream. These transport processes are driven by Starling forces, diffusion, and active transport. Δ Indirect reabsorption In some cases, reabsorption is indirect. For example, bicarbonate HCO3- does not have a transporter, so its reabsorption involves a series of reactions in the tubule lumen and tubular epithelium. It begins with the active secretion of a hydrogen ion H+ into the tubule fluid via a Na/H exchanger: In the lumen The H+ combines with HCO3- to form carbonic acid H2CO3 Luminal carbonic anhydrase enzymatically converts H2CO3 into H2O and CO2 CO2 freely diffuses into the cell In the epithelial cell Cytoplasmic carbonic anhydrase converts the CO2 and H2O which is abundant in the cell into H2CO3 H2CO3 readily dissociates into H+ and HCO3- HCO3- is facilitated out of the cell's basolateral membrane Hormones Some key regulatory hormones for reabsorption include: aldosterone, which stimulates active sodium reabsorption antidiuretic hormone, which stimulates passive water reabsorption Both hormones exert their effects principally on the collecting ducts. Secretion Main article: Clearance medicine Tubular secretion is the transfer of materials from peritubular capillaries to renal tubular lumen. Tubular secretion is caused mainly by active transport. Usually only a few substances are secreted. These substances are present in great excess, or are natural poisons. Many drugs are eliminated by tubular secretion. Further reading: Table of medication secreted in kidney Measurement of renal function Main article: Renal function A simple means of estimating renal function is to measure pH, blood urea nitrogen, creatinine, and basic electrolytes including sodium, potassium, chloride, and bicarbonate. As the kidney is the most important organ in controlling these values, any derangement in these values could suggest renal impairment. There are several more formal tests and ratios involved in estimating renal function: Measurement Calculation Details renal plasma flow RPF = \fraceffective RPFextraction ratio 15 Volume of blood plasma delivered to the kidney per unit time. PAH clearance is a renal analysis method used to provide an estimate. renal blood flow RBF = \fracRPF1 - HCT HCT is hematocrit Volume of blood delivered to the kidney per unit time. In humans, the kidneys together receive roughly 20% of cardiac output, amounting to 1 L/min in a 70-kg adult male. glomerular filtration rate GFR = KfPc - Pi - σπc - πi estimation using Starling equation Volume of fluid filtered from the renal glomerular capillaries into the Bowman's capsule per unit time. Estimated using inulin. Usually a creatinine clearance test is performed but other markers, such as the plant polysaccharide inulin or radiolabelled EDTA, may be used as well. filtration fraction FF = \fracGFRRPF 16 Measures efficiency of reabsorption. anion gap AG = Na+ - Cl- + HCO3- Cations minus anions. Excludes K+ usually, Ca2+, H2PO4-. Aids in the differential diagnosis of metabolic acidosis Clearance other than water C = \fracUVP where U = concentration, V =urine volume / time, UV = urinary excretion, and P = plasma concentration 17 Rate of removal free water clearance C = V - Cosm or V - \fracU_osmP_osmV C_H_2O 18 The volume of blood plasma that is cleared of solute-free water per unit time. Net acid excretion NEA = V U_NH_4 + U_TA - U_HCO_3 Net amount of acid excreted in the urine per unit time v d e Urinary system, physiology: renal physiology and acid base physiology Filtration Renal blood flow - Ultrafiltration - Countercurrent exchange Hormones affecting filtration Antidiuretic hormone ADH - Aldosterone - Atrial natriuretic peptide Secretion/clearance Pharmacokinetics - Clearance of medications Reabsorption Solvent drag - Na+ - Cl- - urea - glucose - oligopeptides - protein Endocrine Renin - Erythropoietin EPO - Calcitriol Active vitamin D - Prostaglandins Assessing Renal function/ Measures of dialysis Glomerular filtration rate - Creatinine clearance - Renal clearance ratio - Urea reduction ratio - Kt/V - Standardized Kt/V - Hemodialysis product - PAH clearance Effective renal plasma flow - Extraction ratio Acid base physiology Fluid balance - Darrow Yannet diagram - Body water - Interstitial fluid - Extracellular fluid - Intracellular fluid/Cytosol - Plasma - Transcellular fluid - Base excess - Davenport diagram - Anion gap - Arterial blood gas Buffering/compensation Bicarbonate buffering system - Respiratory compensation - Renal compensation References ^ Sect. 7, Ch. 6: Characteristics of Proximal Glucose Reabsorption ^ Sect. 7, Ch. 5: Cotransport Symport ^ Sect. 7, Ch. 6: Proximal Reabsorption of Amino Acids: Site of Reabsorption ^ Sect. 7, Ch. 6: Proximal Reabsorption of Urea ^ V. Excretion of Organic Molecules ^ VI. Mechanisms of Salt Water Reabsorption ^ VI. Mechanisms of Salt Water Reabsorption ^ Sect. 7, Ch. 6: Proximal Reabsorption of Bicarbonate ^ Sect. 7, Ch. 12: Proximal Tubular Reabsorption of Bicarbonate ^ Sect. 7, Ch. 12: Bicarbonate Reabsorption, Thick Limb of Henle's Loop ^ Sect. 7, Ch. 5: Cotransport Symport ^ Walter F., PhD. Boron. Medical Physiology: A Cellular And Molecular Approaoch. Elsevier/Saunders. ISBN 1-4160-2328-3. Page 799 ^ Sect. 7, Ch. 12: Physiological Definition of Acids: Volatile Acid ^ Sect. 7, Ch. 12: Nonvolatile Acids ^ Sect. 7, Ch. 4: Measurement of Renal Plasma Flow; Renal Clearance of PAH ^ Sect. 7, Ch. 4: Filtration Fraction ^ IV. Measurement of Renal Function ^ Sect. 7, Ch. 8: Free water clearance CH2O Retrieved from http://en..org/wiki/Renal_physiology Categories: Renal physiology Views Article Discussion this page History Personal tools Log in / create account Navigation Main page Contents Featured content Current events Random article Search Go Search Interaction Community portal Recent changes Contact Donate to Help Toolbox What links here Related changes Upload file Special pages Printable version Permanent link Cite this page Languages Español Português This page was last modified on 7 September 2008, at 14:47
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