Renal Biochemistry PDF

BIOCHEMISTRY OF RENAL SYSTEM  Outlines  Introduction  Urine formation  The functions of kidney  Energy metabolism in kidney  Acid base balance  Acid base imbalance (acidosis and alkalosis)  Kidney function test  Introduction Nephron is the functional unit of kidney Each kidney c...

BIOCHEMISTRY OF RENAL SYSTEM  Outlines  Introduction  Urine formation  The functions of kidney  Energy metabolism in kidney  Acid base balance  Acid base imbalance (acidosis and alkalosis)  Kidney function test  Introduction Nephron is the functional unit of kidney Each kidney composed of one million nephrons Nephron contains – bowman’s capsule (blood capillaries/glomerulus), – proximal convoluted tubule (PCT), – loop of Henle, – distal convoluted tubule (DCT) and – collecting tubules DCT opens into the collecting duct which carries urine into renal pelvis from where it is carried by the ureter to bladder. Based on location, there are two different types of nephrons –cortical and juxtra-medullary nephrons Introduction  Urine Formation Urine formation has three steps: 1. Glomerular filtration – this is the passive process that results in formation ultrafiltrate of blood – Takes place through the semi- permeable wall of the glomerular capillaries. – all the unbound constituents of plasma with MW 25 mg/day).  Na and K excretion are controlled by adrenal cortex.  Ca and Mg are present in very low concentration and increased excretion is seen in pathological conditions involving bone metabolism. 13.Enzymes: Traces of certain enzymes are found in urine  Like pancreatic amylase , pepsin , trypsin and Lipase.  During pancreatic disease amylase excretion will be increased 14.Hormones – sex hormones (HCG- used for detection of pregnancy) 15. B complex and C vitamins(water soluble vitamins)  Functions of kidney Kidneys is a multifunctional organ that has different roles 1. EXCRETORY FUNCTION – Remove waste products from the bloodstream (urea, uric acid, creatinine, bilirubin, urobilin, a number of other organic acid) – Excretion of exogeneous water soluble toxic substances (food colouring dyes, drugs etc.) – Dietary intake contains a variable and usually excessive supply of sodium, potassium, chloride, calcium, phosphate, magnesium, sulfate, and bicarbonate— removed via kidney – Hormones’ metabolites (cortisol, testosterone) 2. REABSORPTIVE FUNCTION – Retention of substances vital to the body – The kidney reabsorb and retain several substance of biochemical importance E.g: glucose, aminoacids, proteins – The kidneys reabsorb filtered glucose via the sodium‐glucose cotransporters (SGLT) 1 and SGLT2, which are localized on the brush border membrane of the early PCT with immune detection of their expression in tabularized Bowman capsule – In patients with DM, the renal maximum glucose reabsorptive capacity, and the threshold for glucose passage into the urine, are higher & contribute to hyperglycemic state – The administration of SGLT2 inhibitors to diabetic patients enhances sodium and glucose excretion, leading to a reduction of glycosuria threshold and tubular maximal transport of glucose. 3. Filtration function – Preparation of an ultra filtrate 4. Secretory/endocrine function Secret important hormones Secretion of erythropoietin, regulates RBC production in bone marrow Secretion of renin, which is a key part of the RAAS system (for blood pressure regulation). Prostaglandins (PGE1, PGE2)&thromboxane synthesis Secretion of the active form of vitamin D 5. HOMEOSTATIC ROLE – Blood pressure regulation – Electrolyte and fluid (water) balance: Na, K, Ca and Mg – regulating calcium-phosphorus metabolism by secretion and reabsorbing – Acid- base balance Maintenance of blood pH (acidity/alkalinity) in a physiological range by eliminating acids, reabsorbing and forming bases – Regulation of the production of red blood cells 6. Metabolic functions – Carbohydrate, lipid and protein metabolism  Erythropoietin synthesis Erythropoietin is a 30.4KDa weight peptide hormone consisting 165 amino acids It is also known as haematopoietin or haemopoietin It is a glycoprotein cytokine secreted mainly by the kidney in response to cellular hypoxia It stimulates the synthesis of hemoglobin & RBCs (erythropoiesis) in the bone marrow  RAAS system Renin angiotensin-aldosterone (RAAS) system Stimulate production of angiotensin II & aldosterone- which are hormones regulating electrolyte balance  Angiotensinogen (AGT) – Is an α2-globulin synthesized in the liver – It is peptide of having 485 amino acids long (including a 33 amino-acid signal peptide) – It is a precursor for angiotensin – It is under a serpin family (called serpin A8 ) – the first 12 amino acids are the most important for activity Renin – Renin is a 340 amino acid residues (37KDa) proteolytic enzyme principally liberated by juxtaglomerular apparatus of kidney – It is released in response to a decreased renal perfusion pressure – It cleaves 10 amino acids from N-terminus of AGT to form Ang I Angiotensin-converting enzyme (ACE) – Ang I is converted to Ang II by removal of two C-terminal residues by the peptidyl-dipeptidase enzyme called ACE, primarily via ACE within lung (also present in endothelial cells, kidney epithelial cells & brain) Angiotensin II – an octapeptide formed from Angiotensin I by ACE – Acts on angiotensin types 1&2 receptors (AT1R & AT2R) It is a potent vasoconstrictor It promotes aldosterone release from adrenal cortex – It is more potent than Ang III in stimulating aldosterone secretion – Aldosterone is a mineral corticosteroid hormone that » regulates sodium and potassium homeostasis » stimulates the expression of Na channel » increases Na reabsorption and water retention » increases excretion of K &H+ ions It stimulate ADH secretion: increase water retention It also stimulated cortisol secretion unlike Ang III – Ang II is metabolized in d/t tissues by various enzymes to generate two heptapeptides (Ang III and angiotensin 1-7), which can then be catabolized into smaller peptide  Angiotensin III – It is a heptapeptide formed from Ang II by Amino peptidase A (APA) – Ang III in the brain has been shown to be more important than Ang II, in central regulation of hypertension & vasopressin release – Binds with AT1R and AT2R receptors like Ang II Angiotensin IV – Amino peptidase N (APN) remove one amino acid from Ang III to form a hexapeptide Ang IV – Ang IV stimulates AT4R and AT2R and inhibits AT1R  APA APN MasR-mitochondrial assembly receptor;MrgDR; Mas-related G-coupled receptor type D MLDAD—mononuclear leukocyte-derived aspartate decarboxylase.  Vitamin D synthesis Three organs are involved in synthesis of vitamin D-skin, liver and kidney The active form of Vitamin D3 (calcitriol) is produced is finally produced in kidney Calcitriol is important for calcium reabsorption and bones mineralization Vitamin D synthesis  Regulation of fluid and electrolytes balance Neurogenic stimuli – Thirst sensation  Osmolarity variation = 2% – Increase thirst  ECF hypertonic  decrease ICF – Decrease thirst  ECF hypotonic  increase ICF Hormonal factors Natriuretic peptides (promote sodium excretion& decrease blood pressure) ADH (detection of osmotic and mechanical stimuli) Aldosterone (Na+ reabsorption & excretion of K+ & H+)  Natriuretic peptides Natriuretic peptides involved in the regulation of fluid volume Promote sodium excretion) (H20 removal required) & decrease blood pressure Two types: atrial natriuretic peptide (ANP) & brain natriuretic peptide (BNP) 1. ANP is synthesized predominantly in cardiac atria as a 126–amino acid pro- peptide It is then cleaved into a smaller 98–amino acid N-terminal peptide and the biologically active 28–amino acid ANP 2. BNP It is synthesized in the cardiac ventricles as a 108–amino acid pro-peptide It is cleaved into a 76–amino acid N-terminal peptide and a biologically active 32–amino acid BNP – ANP & BNP are secreted in response to atrial stretch & to ventricular volume overload – They bind to G-protein-linked receptors Antidiuretic hormone [ADH] – The posterior pituitary hormone ADH (also known as the vasopressin) controls the reabsorption of water in the collecting ducts of kidney by regulation of the membrane water channels, aquaporin Aquaporin's are membrane channel proteins which transport water – ADH controls water reabsorption in collecting ducts – It binds to a receptor located on the membranes of tubular cells in the renal collecting duct – The receptor is coupled to G-proteins and activates PKA – The PKA in turn phosphorylates aquaporin 2 (AQP2) stimulating its translocation to the cell membrane thus increasing water reabsorption in the collecting duct – AQP2 & AQP3 are present in collecting ducts of kidney &are regulated by ADH – Defects in vasopressin secretion &defective aquaporin's cause diabetes insipidus Water and sodium metabolism are closely interrelated  ENERGY METABOLISM OF KIDNEY During Normal/fed state 1. Glucose- is used by renal cortex(aerobic) and medulla (anaerobic) 2. Fatty acid- is used by renal cortex 3. Amino acid -is used by renal cortex During Starvation 1.GLUCOSE-produced by liver & renal cortex via gluconeogenesis 2.Fatty acid- directly used by renal cortex 3.Ketone bodies-Ketone bodies from liver are used by kidney  Peculiarities of biochemical processes in kidney. Kidneys have a very high level of metabolic processes They use about 10 % of all the O2 used in an organism  Metabolism of Carbohydrates Carbohydrates are the main fuel for the kidney. The process of glycolysis, ketolysis, aerobic oxidation and phosphorylation are very intense in kidney A lot of ATP is formed as a result of these processes Utilization of glucose in cortex & medulla is different Aerobic glycolysis is dominant in cortex (as a result CO2 is formed) Anaerobic glycolysis is the dominant type in medulla and lactate is produced from glucose The renal cortex, like the liver is unique in that it possess the enzymatic potential for both gluconeogenesis & glycolysis  Gluconeogenesis Gluconeogenesis is a biochemical pathway in which de novo formation of glucose occurs from non-carbohydrate precursors such as – pyruvate, lactate, glycerol, propionate, oxaloacetate, amino acid Only liver & kidney in human body possess sufficient gluconeogenic enzyme & glucose-6-phosphatase activity to enable them to release glucose into the circulation as a result of gluconeogenesis. Gluconeogenesis is confined to proximal convoluted and proximal straight tubules of mammalian kidney This pathway is operated partially in mitochondria and partially in cytosol The kidney became an important source of glucose in acidotic conditions, prolonged starvation, glucocorticoid treatment, &, possibly, PTH and catecholamines  Most of enzymes involved in gluconeogenesis are the same as those involved in glycolysis. It takes place by the reversal of glycolysis. The three irreversible steps in glycolysis are bypassed in gluconeogenesis with the help of 4 additional enzymes which are designated as the key enzymes of gluconeogenesis. i.e. 1.Pyruvate carboxylase 2.Phospho enol pyruvate carboxy-kinase 3.Fructose 1,6-bisphosphatase & 4.Glucose -6-phosphatase.  The kidney has a role in maintaining body glucose balance, not only as an organ for gluconeogenesis but by using glucose as a metabolic substrate Due to the differences in the distribution of various enzymes along the nephron, glucose utilization occurs predominantly in renal medulla, while glucose release is confined to renal cortex – Cells in the renal medulla have appreciable glucose- phosphorylating and glycolytic enzyme activity, and, like the brain, they are obligate users of glucose – These cells, however, lack glucose-6-phosphatase and other gluconeogenic enzymes. Thus, although they can take up, phosphorylate, glycolyse & accumulate glycogen, they cannot release free glucose into the circulation  The renal cortex possess gluconeogenic enzymes (including glucose-6-phosphatase), and thus they can make and release glucose into the circulation. – But these cells have little phosphorylating capacity and, under normal conditions, they cannot synthesize appreciable levels of glycogen – Therefore, the release of glucose by the normal kidney is mainly, if not exclusively, a result of renal cortical gluconeogenesis, whereas glucose uptake and utilization occur in other parts of the kidney.  There are several important differences in the factors, which regulate gluconeogenesis in liver and renal cortex 1. Difference in the substrate utilized Liver utilizes predominately pyruvate, lactate and alanine as its substrate for gluconeogenesis Renal cortex utilizes pyruvate, lactate, citrate, α-ketoglutarate, glycine and glutamine 2. The effect of hydrogen ion activity H+ ion activity has little effect on hepatic gluconeogenesis while it has marked effects upon renal gluconeogenesis Thus, when intracellular fluid pH is reduced (metabolic acidosis, respiratory acidosis or potassium depletion), the rates of gluconeogenesis in renal cortex are markedly increased) The ability of the kidney to convert certain organic acids to glucose, a neutral substance, is an example of a non- excretory mechanism of pH regulation in kidney 3. The end product of gluconeogenesis in kidney it is Glucose – 6- P but in liver it’ s Glucose Metabolism of Lipids The mitochondrial β-oxidation of FFA is a major source for renal ATP production, particularly in PCT, which has a high energy demand & relatively little glycolytic capacity – FFA undergoes beta oxidation to form acetyl COA which is further oxidized in to CO2 and water in the citric acid cycle. The kidney extracts FAs from the circulation & that FFA oxidation could account for more than half of renal O2 consumption FFAs delivered to the kidney in excess of its energy needs can be esterified with glycerol and deposited as triglycerides in intracellular lipid droplets. These energy stores can be rapidly mobilized in periods of scarcity Liver produce ketone bodies from FFA & ketogenic amino acids (ketogenesis) Ketone bodies are oxidized by kidney through a process called ketolysis  Metabolism of proteins Proteins are metabolized in kidney at a high level  Transamination  Amino acid deamination  Degradation of glutamine by glutamine deaminase (glutaminase) –Glutaminase is very active & free ammonia is formed in large quantities in kidney  The Kidneys have different types of enzymes: LDH (1, 2, 3, 5), AST, ALT.  Creatine synthesis also takes place in kidney Creatine synthesis Creatine is the precursor of creatine–P , a high energy molecule required for muscle contraction The synthesis of creatine is taking place partially in kidney and partially in liver. The first step reaction takes place in the kidney in which arginine and glycine combines to form guanidoacetate  Kidney First Step Second Step Liver 1. Arginine: glycine amidinotransferase (AGAT) 2. Guanidinoacetate methyltransferase (GAMT) Acid base balance/hydrogen (H+) Homeostasis A normal healthy person has a slightly basic blood, with a pH range of 7.35 to 7.45 (called physiologic PH) and intracellular pH at approximately 7.1 (between 6.9 and 7.4) – to function properly, the body maintains blood pH close to 7.40. – throughout ones life this blood pH remains constant. Acid base balance refers to maintenance of stable level of pH of body fluids Blood's acid-base balance is precisely controlled, because even a minor deviation from the normal range can severely affect many organs During metabolic processes either acids or bases are formed. – the body produces approximately 13-22 moles of acid per day from normal metabolism. – an average rate of metabolic activity yields about 22,000mEq acid per day  Metabolism generates CO2 within cells. – CO2 dissolves in water, forming carbonic acid, which in turn dissociates releasing hydrogen ion.  The acids derived from sources other than CO2 are known as nonvolatile acids – The nonvolatile acid produced from body cannot be removed as expired CO2 via lungs & must be excreted in urine via kidney. – Most of the nonvolatile acid H+ ion is excreted as undissociated acid that generally buffers the urinary pH between 5.5 and 7.0 – The net production of nonvolatile acids is in the order of 50 mmol/24 h.  Lactate is produced during anaerobic glycolysis and its concentration in plasma is the hallmark of hypoxia  Ketoacids (acetoacetate & β-hydroxybutyrate)-produced in DM  Metabolism of sulfur-containing amino acids & phosphorus- containing compounds also generate inorganic acids (e.g., sulfuric acid).  Under normal conditions, the body uses different mechanisms to control the blood's acid-base balance involved in maintenance of pH level 1. Buffer system function almost instantaneously 2. Respiratory mechanism take several minutes to hours 3. Renal mechanism (kidney) take several hours to days Thus, the body PH balance is maintained by buffer system, lungs and kidney Overview of Acid-Base Balance  The body Buffering system Until the acid produced from metabolism can be excreted as CO2 in expired air and as ions in the urine, it needs to be buffered in body fluids Buffers consist of a weak acid or base & its conjugate causing a solution to resist changes in pH when H+ or OH- ions are added. – They are responsible for the maintenance of pH of plasma, ICF, ECF and tissues of the body The major buffer systems in the body are: 1. Bicarbonate buffer- operates mainly in ECF (to lesser extent in ICF) 2. Phosphate buffer system-in all types of cells (in ICF) 3. Protein buffer system cells and plasma. – albumin and globulin(ECF) – Hemoglobin (in ICF of red blood cells)  The Role of Kidneys in acid base balance Kidney functions in regulating the pH of ECF Normal urine has a pH of 6. – the pH of the urine may vary from as low as 4.5 to as high as 9.8 depending upon the amount of acid excreted. – This pH is maintained by urinary buffer (phosphate, bicarbonate and ammonia buffer) The kidneys have several powerful mechanisms to control pH by the excretion of excess acid or base The major homeostatic control point for maintaining a stable pH balance is renal excretion – kidney excrete H+ or HCO3- & also can synthesize HCO3- – But kidneys make these adjustment more slower than the lungs do and compensation takes several days  The major renal mechanisms for regulation of pH /maintaining the acid–base balance are: 1. Excretion of H+ and Reabsorption of bicarbonate 2. Excretion of titrable acid(net acid excretion) 3. Excretion of ammonium ion(NH4+) 1. Secretion of H+ and reabsorption of bicarbonate Occurs in the proximal convoluted tubule  The tubule cells absorb CO2 from the blood  CO2 combines with water in the presence of Carbonic anhydrase to form carbonic acid CO2 + H2O  H2CO 3  The carbonic acid ionizes to H+ and HCO3- H2CO 3  H+ + HCO3-  The hydrogen ions are secreted into the tubular lumen in exchange for sodium ions (Na+ -H+ exchanger)  HCO3- returns to the blood  Bicarbonate (HCO3-) does not have a transporter, so its reabsorption involves a series of reactions in the tubule lumen and tubular epithelium. Reabsorption of bicarbonate within the kidney.  As the renal tubular cells transport H+ into urine, they return bicarbonate anions to blood The kidney filters approximately 4000 mmol of HCO3-per day from the plasma To reabsorb the filtered load of HCO3-, the renal tubules must, secrete 4000 mmol of H+ ions Around 80-90% of HCO3-is reabsorbed in the proximal tubules an additional 15% is reabsorbed by the thick ascending limb of Henle’s loop.  The kidney is also able to generate bicarbonate The metabolic activity of cells produces large amounts of CO2 This then reacts with water to produce HCO3– ions, which enter the plasma, and H+ ions to be transported into the lumen. This is useful as it also provides H+ ions to drive HCO3– reabsorption.  H+ secretion occurs by two apical membrane transporters, Na+/H+ antiporter & H+/ATPase Na+/H+ antiporter – known as a sodium–hydrogen exchange carrier molecule (NHE-3) – the predominant pathway for H+ secretion – As Na+ enters the cell from the luminal fluid down its electrochemical gradient via this carrier, it effectively removes H+ ions from the cell cytoplasm and adds them to the luminal fluid. – The H+ ions are generated within the cell by the action of the enzyme carbonic anhydrase (CA), which catalyzes the reaction between CO2 and water to produce H2CO3. – This rapidly breaks down to produce the hydrogen ions that are secreted into the lumen, and a bicarbonate ion which is transported across the basolateral cell membrane into plasma – H+ secretion is dependent on the lumen-to-cell Na+ gradient. Because of this coupling, factors that regulate Na+ transport in these segments will secondarily affect H+ secretion  Carbonic anhydrase also exists on the brush border membrane on the luminal surface of these cells. – here it catalyzes the breakdown of H2CO3 formed as the secreted H+ ion reacts with filtered bicarbonate, releasing water & CO2 which passes freely across cell membrane, allowing the cycle to repeat The net outcome of this process is that the filtered sodium bicarbonate passing through the PCT is effectively reabsorbed although the bicarbonate added to the plasma in a given turn of the cycle is not the same one appearing in the lumen with sodium  This process accounts for reabsorption of some 85% of filtered bicarbonate, and operates at a high capacity but generates a low gradient of hydrogen ion concentration across the epithelium, with the luminal pH falling only slightly from 7.4 at the glomerulus to around 7.0 at the end of the proximal tubule. This is both because of the presence of carbonic anhydrase in the luminal compartment and because the epithelium is ‘leaky’ to hydrogen ions.  2. Excretion of titrable acid (net acid excretion) The fixed acids generated during metabolism are also excreted by the kidney It is important to understand that the process described above has not done anything to remove net acid from the body, since the fate of the secreted H+ in this segment is effectively to conserve most of the filtered bicarbonate  Under circumstances requiring removal of net acid from the body, the tubules must still carry out two more steps. – Secrete further acid into the tubular lumen beyond that needed to reabsorb all filtered bicarbonate. – Provide a buffer in the tubular fluid to assist in the removal of this acid (this is necessary since the maximum acidification which can be achieved in the lumen – around pH 4.5 – would not allow for excretion of the metabolic acid load needing elimination). These two requirements are fulfilled in more distal nephron segments Acid is secreted into the lumen of late distal tubule and collecting ducts by an H+/ATPase located in the apical cell membrane  This pump has been found in the intercalated cells within the cortical collecting duct and in the apical membrane of the outer medullary collecting duct cells. – The H+ undergoing secretion in this way is generated within the tubular cells by a reaction facilitated by carbonic anhydrase, as for PCT. Again, the bicarbonate generated within the cell by this process passes across the basolateral membrane (actually via a chloride–bicarbonate exchange carrier) into the plasma. However, here the bicarbonate does not replace a filtered bicarbonate molecule, but represents a ‘new’ bicarbonate, effectively counteracting the consumption of buffer which would have occurred had the excreted acid been retained in the body.  Two types of buffer are involved in excretion of this net acid. The glomerular filtrate contains a limited amount of non-bicarbonate buffer which is capable of taking up some of the H+ The main molecule involved is mono-hydrogen phosphate (HPO4 -2), which is titrated in the distal lumen to dihydrogen phosphate (H2PO4-), which is excreted in the urine with sodium. This reaction has limited capacity (removing up to 30mmol of H+/day) and tends to proceed as the urine pH falls along the distal nephron segments, typically from 7 down to 6 and below, the  3. Ammonium Excretion With increased acid load, there is increased hydrogen ion secretion, causing the urine pH to fall below 5.5. At this point, virtually all the urinary phosphate exist as H2PO4- and further buffering cannot occur unless there is an increase in urinary phosphate excretion. Phosphate excretion is mainly dependent on dietary phosphate intake and PTH levels and is not regulated in response to the need to maintain acid base balance. Without further urinary buffering, adequate acid excretion cannot take place The major adaptation to an increased acid load is increased ammonium production and excretion. This is another way by which the kidney excretes acid (and conserves Na) is via generation of ammonium from glutamine in PCT  The ability to excrete H+ ions as ammonium adds an important degree of flexibility to renal acid base regulation, because the rate of NH4+ production and excretion can be regulated in response to the acid base requirements of the body. Also of importance is the role of ammonium production in the further generation of bicarbonate ions The process of ammonium excretion takes place in 3 steps: I. Ammonium Formation (ammoniagenesis)-PCT II. Ammonium Reabsorption (Medullary Recycling)(thick ascending loop) III. Ammonium Trapping (collecting tubule)  Ammoniagenesis Generation of ammonium (ammoniagenesis) from glutamine occurs in PCT The tubular cells have two enzyme to generate NH3 in kidneys 1. Glutaminase: glutamine produced from most organs (from amino acid metabolism) is received from peritubular capillaries and degraded into glutamate & NH3 2. Glutamate dehydrogenase: glutamate produced metabolized into alpha-keto glutarate and NH4+ – Ammonium ions are major contributors to buffer urinary pH, but not blood pH. – The ammonium is secreted into the tubular lumen by substituting for H+ (in exchange for filtered sodium) on the Na+/H+ exchanger. it combines with filtered chloride to form ammonium chloride (which is then excreted). In the tubular fluid, NH4+ circulates partly in equilibrium with NH3 – The alpha-ketoglutarate is metabolized further into two HCO3- ions, which then leave the cell and enter systemic circulation by crossing the basolateral membrane. – Synthesis & excretion of NH3 is one of the major mechanisms of eliminating acidity via kidney – Cells in the kidney generate NH4+ and excrete it into urine in proportion to acidity (proton concentration) of blood Mechanism of excretion of H+ as NH4+ Ammonium Reabsorption AmmoniumTrapping  Acidosis and Alkalosis  Under pathological conditions excessive amounts of acids or bases may accumulate in body fluids and tissues leading to disturbances in acid base balance.  The two disorders of acid-base balance: – A cidosis: due to accumulation of acids & blood pH is below 7.4 – A lkalosis: due to accumulation of alkali & blood pH is above 7.4  Depending on their primary cause, acidosis and alkalosis are categorized as metabolic or respiratory 1. Metabolic acidosis & alkalosis – caused by an imbalance in production of acids or bases & their excretion 2. Respiratory acidosis & alkalosis – caused primarily by changes in CO2 exhalation due to lung or respiratory disorders  1. Metabolic acidosis Metabolic acidosis is a condition that occurs when the body produces too much acid or when the kidneys are not removing enough acid from the body. Metabolic acidosis leads to acidemia, due to increased production of hydrogen by the body or the inability of the body to form bicarbonate (HCO3-) in the kidney. Examples of conditions that can lead to production of excess acid include diabetic ketoacidosis, lactic acidosis, sepsis, and renal failure. Its causes are diverse, and its consequences can be serious, including coma and death. Together with respiratory acidosis, it is one of the two general causes of acidemia 2. METABOLIC ALKALOSIS Metabolic alkalosis may occur because of a loss of H+ or due to retention of excess HCO3 which may result from : – Loss of stomach acid through excessive vomiting. – Ingestion of alkalinizing drugs such as sodium bicarbonate – Changes in renal HCO3 balance in response to aldosterone or diuretic treatment Excess HCO3 is managed – mainly by an increase in renal HCO3 excretion. – to some extent by respiratory compensation (hypoventilation) but If the pH remains above 7.55, as in severe alkalosis, arteriolar constriction may lead to reduced cerebral blood flow, tetany, seizure or, potentially, death.  3. Respiratory Acidosis Respiratory acidosis is a condition in which a build-up of CO2 in blood produces a shift in body's pH balance & causes the body's system to become too acidic This condition is brought about by a problem either involving the lungs and respiratory system or signals from the brain that control breathing. – ventilatory failure; respiratory failure– respiratory acidosis There is primary increase in PCO2 with compensatory increase in HCO3− pH usually low but may be near normal.  4. Respiratory Alkalosis Respiratory alkalosis is a primary decrease in partial pressure of CO2(PCO2) with or without compensatory decrease in bicarbonate (HCO3−) pH may be high or near normal. Cause: occurs when you breathe too fast or too deep and carbon dioxide levels drop too low. – increase in respiratory rate or volume (hyperventilation) or both – this causes the blood pH to rise and become too alkaline Respiratory alkalosis can be acute or chronic. The Arterial Blood Gas (ABG) Abnormalities in ABG Compensation Primary Disorder Compensatory Mechanism 1. Metabolic acidosis Increased ventilation Increased renal excretion of H+ in distal tubule 2. Metabolic alkalosis Decreased ventilation 3. Respiratory acidosis Increased renal reabsorption of HCO3- in proximal tubule Increased renal excretion of H+ in distal tubule 4. Respiratory Decreased renal reabsorption of HCO3- in alkalosis proximal tubule Decreased renal excretion of H+ in distal tubule  Summary of Kidney function Filtration Preparation of an ultra filtrate Glucose, amino acids, electrolytes, proteins Re absorptive Extracellular volume, acid-base status, Homeostatic blood pressure, electrolytes Synthetic: glutathione, gluconeogenesis, ammonia Metabolic Catabolic: hormones, cytokines -Erythropoietin synthesis, Endocrine -activation of vitamin D, -renin release Renal Function Biomarkers Kidney function tests are common lab tests used to evaluate how well the kidneys are working.  Kidney function test divided into 4 groups 1. Glomerular function test – All clearance tests (inulin,creatinine, urea) 2. Tubular function test – Urine concentration or dilution test, urine acidification test 3. Blood analysis – BUN, serum creatinine, protein and electrolytes useful to asses kidney function 4. Urine analysis – routine examination-volume, PH, specific gravity, osmolality, presence of certain abnormal constituent (protein, blood, ketone bodies, glucose 1. Glomerular function test Clearance test (C) Renal clearance of the substance is defined as the volume of plasma from which the substance is completely cleared by the kidneys per minute Clearance is a pharmacokinetic measurement of the volume of the plasma from which a particular substance /metabolite is completely removed or cleared per unit time( ml /minute) 180L primary urine is formed per day, about 125 ml of primary urine per minute  Clearance (C)=UxV/P; – U-concentration of substance in urine (mg/ml) – V-volume of urine excreted in ml per minute – P-concentration of substance in plasma (mg/ml) Glucose is reabsorbed completely, C=0 Inulin is not reabsorbed absolutely, C=125ml/min If clearance is above 125ml/min, the substance is secreted actively  Glomerular Filtration Rate (GFR)  GFR is usually estimated to assess glomerular function.  Maintaining normal GFR is important for renal functioning.  Testing the GFR is the most frequently performed Renal Function Test  GFR is the best measure of glomerular function  Measurement of GFR is based on the clearance of certain substances from the blood in the urine.  The maximum rate at which the plasma cleared of any substance is equals to GFR GFR is calculated by measuring inulin clearance – A plant carbohydrate composed of fructose units – Plasma inulin is freely filtered by glomeruli but neither reabsorbed nor secreted by the tubule  GFR decrease by 30% of normal  moderate renal insufficiency, patient remains asymptomatic with evidence decline of GFR GFR decreased further  severe renal insufficiency  Patient symptomatic with uremia, acidemia, volume overload GFR decreased by 5-10% of normal  ESRD  Creatinine clearance (CC) test Creatinine is excretory product derived from creatine phosphate – 1-2% of the muscle creatine is converted to creatinine. CC is defined as the volume of plasma (ml) that would be completely cleared of creatinine per minute CC is constant that is not influenced by the body metabolism or dietary factor Creatinine is filtered by glomeruli and marginally secreted by the tubules  No re-absorption in the renal tubules.  Production depends on muscle mass, age, sex and weight  Creatinine clearance is also helpful to estimate GFR  The estimated Glomerular Filtration Rate (eGFR) will be calculated by using the creatinine equation called Cockcroft–Gault formula based on creatinine and participant’s characteristics  CC=UxV/P – U-concentration of Cr in urine – V-volume of urine excreted in ml per minute – P-concentration of Cr in plasma Normal CC is 120-145ml/min (these values are slightly lower in women) Diagnostic importance – Decrease CC value (