Human Physiology: Osmolality and Regulation

Kidney Function

• A nephron is a working unit of the kidney
• Each kidney contains approximately one million nephrons
• Blood flows into the glomerulus, and some fluid with dissolved substances is absorbed into the tubule
• The fluid and substances needed by the body are returned to the blood in vessels alongside the tubule.
• The tubule passes waste materials on to the bladder.

Renal Function Testing and Non-Protein Nitrogen Substances

• Define terminologies applied in renal function tests
• Define nonprotein nitrogenous (NPN) compounds
• Discuss urea, creatinine, uric acid, creatinine clearance, and electrolytes
• Discuss source, metabolism, and clinical significance of these substances.
• Explain renal stones

Outline of Introduction

• Definitions of important terminologies (concepts)
• Anatomy and functions of the renal system
• Renal threshold
• Non-protein nitrogenous (NPN) compounds

Urea and BUN

• NPN compounds present in the highest concentration in blood and urine
• U = Urea
• Blood Urea Nitrogen = BUN
• Urea contains 2 nitrogen atoms: 28 g nitrogen/mole of urea
• BUN x 2.14 = urea

Introduction (Urea Synthesis)

• Liver removes toxic ammonia from circulation.
• Converts it to nontoxic, water-soluble urea
• Site of urea synthesis is the liver
• Reactions leading to urea formation from ammonia are proposed by Krebs and Henseleit
• Urea cycle is also called the Krebs-Henseleit cycle
• Urea formation in the cycle occurs in five steps:
• First reaction is not part of the urea cycle, but its product is essential for the continuation

Urea Synthesis (Continued)

• Synthesis of urea from ammonia is an energy-dependent process
• Enzymes of urea cycle are present in mitochondria and cytosol
• First two reactions of urea formation occur in mitochondria; remaining ones happen in cytosol

Urea Cycle

• Third reaction: argininosuccinate is cleaved by argininosuccinase to form arginine and fumarate
• Fifth reaction: ornithine and urea formation from arginine is the final step catalyzed by arginase.
• The formed ornithine goes into mitochondria to re-start the urea cycle
• The overall equation for urea formation is NH3 + HCO3 + aspartate + 4ATP → Urea + fumarate + 4ADP + 4Pi
• Urea has no physiological function; it is transported to kidneys for excretion in urine
• Protein catabolism makes 10-25 g of urea excreted per day, which is 80-90% of total nitrogen excreted per day.
• Blood also contains some urea

Urea Formation Regulation

• Formation of urea is regulated by the activity of carbamoyl phosphate synthetase-I.
• N-acetylglutamate regulates the activity of this enzyme, and is an allosteric activator.
• High protein intake results in increased N-acetylglutamate formation; thus, increasing urea formation
• Starvation also increases urea synthesis due to increased protein breakdown

Blood Urea N (BUN)

• BUN reference ranges
• For adults (serum/plasma): 6-20 mg/dL
• For newborns up to one week (serum/plasma): 3-25 mg/dl
• For adults over 60 (serum/plasma): 8-23 mg/dL

Urea/BUN Clinical Significance

• Plasma levels depend on diet, liver function, kidney function, and hydration status
• Increased urea (BUN): azotemia or uremia, increased protein intake, decreased kidney function, dehydration)
• Decreased urea (BUN): decreased protein intake, decreased liver function

Ammonia Toxicity and Metabolism

• Ammonia is toxic to the central nervous system, particularly glial cells
• Blood ammonia level must be within a normal range
• Symptoms of ammonia intoxication include slurred speech, blurred vision, tremors, coma, and early death in severe cases.
• Ammonia can cause brain toxicity by three ways:
• Entry into brain leads to glutamate formation, depleting alpha-ketoglutarate
• Brain utilizes ammonia for glutamine synthesis, depleting cellular ATP
• Excess glutamate overstimulates nerve cells.

Causes of Ammonia Toxicity

• Impaired hepatic function (carbon tetrachloride poisoning, heavy metal poisoning, viral infections)
• Collateral communications between portal vein and systemic blood (cirrhosis)
• Consumption of high-protein diets after gastrointestinal hemorrhage
• Ammonia metabolism
• Liver deaminates amino acids, producing urea
• Brain can convert ammonia to glutamine.

Creatinine

• Formation and excretion: derived spontaneously from creatine in muscle; production is constant; excreted by glomerular filtration, not reabsorbed or secreted by tubules.
• Metabolism: Involves transformations with phosphocreatine, forming creatine then creatinine.
• Reference ranges:
• Serum: 0.6-1.1 mg/dL in adult males, 0.5-0.8 mg/dL in adult females, 0-0.6 mg/dL in children
• Urine: 800-2000 mg/24 hr in adult males, 600-1800 mg/24 hr in adult females
• Amniotic fluid: 1-2 mg/dL

Creatinine Clinical Significance

• Endogenous substance
• Constant daily production (variations <10% daily); proportional to muscle mass
• Filtered by glomerulus
• Not handled by tubules
• Good test for GFR
• Not affected by diet
• Increased serum creatinine indicates renal disease (impaired renal function). 50-60% renal function loss before serum creatinine increases

BUN/Creatinine Ratio

• Calculated: serum BUN (mg/dL) / serum creatinine (mg/dL)
• Normal ratio: 10-20, with the majority around 12-16
• Increased ratio: pre-renal conditions (CHF, shock, hemorrhage, dehydration), increased protein metabolism, increased protein catabolism
• Decreased ratio: post-renal conditions (obstruction of urine flow), low protein diet, liver disease
• Renal conditions causing low CrCl or ratio: acute renal failure, chronic renal failure, glomerulonephritis, tubular necrosis

Creatinine Clearance (CrCl)

• Renal clearance expresses the volume of blood cleared of a substance per unit of time (e.g., mL of substance per minute)
• Substance used to monitor GFR must not be reabsorbed or secreted by the tubules; creatinine is commonly used
• Specimen handling: timed 24-hour urine collection, measure urine creatinine and serum creatinine
• Standard clearance formula: UV/P, where U is urine creatinine, V is urine volume, and P is serum creatinine
• Corrected clearance is calculated using UV x 1.73/P, where A is body surface area (BSA) and 1.73 is a value often used representing the average BSA.

Estimated GFR (eGFR)

• The National Kidney Foundation recommends eGFR calculation each time a serum creatinine is reported
• eGFR predicts GFR values based on patient age, sex, body size, race and serum creatinine readings.
• Timed urine collection is not needed for eGFR calculations.

Uric Acid

• Uric acid is the end product of purine catabolism.
• Purines are the important constituents of DNA and RNA, found in exogenous (ingested nucleic acids) and endogenous (tissue destruction) sources.
• Excretion: 70% of Uric acid elimination through renal excretion. The remainder goes via Gastrointestinal tract for bacterial enzyme degradation.
• Uric acid is Insoluble in plasma and mostly presents as monosodium urate.
• At high concentrations (greater than 6.8 mg/dL), uric acid precipitates in joints and tissues, causing painful inflammation.

Uric Acid Clinical Significance

• Uric acid measurements assess inherited disorders of purine metabolism, confirm gout diagnosis, monitor gout treatment, aid in diagnosing renal calculi, prevent uric acid nephropathy during chemotherapy treatment, and detect kidney dysfunction.
• Normal values are 3.4–7 mg/dL in adult males and 2.5–6 mg/dL in females.

Gout

• Gout is a disease predominantly in men, often diagnosed between 30 and 50 years old.
• Gout causes inflammation of joints, due to precipitation of sodium urates.
• Affected individuals are susceptible to the formation of Renal Calculi

Causes of Increased Uric Acid

• Increased metabolism of cell nuclei (chemotherapy for leukemia, lymphoma, multiple myeloma)
• Chronic renal disease
• Purine-rich diet
• Increased tissue catabolism (starvation)
• Inherited disorders of purine metabolism (Lesch-Nyhan syndrome)
• Toxemia of pregnancy and lactic acidosis
• Drugs (salicylates and thiazides)

Causes of Decreased Uric Acid

• Liver disease
• Defective tubular reabsorption (Fanconi syndrome)
• Chemotherapy with azathioprine or 6-mercaptopurine
• Excessive allopurinol treatment

Electrolytes

• Electrolytes are substances whose molecules dissociate into ions when in water.
• Medically important electrolytes include sodium (Na+), potassium (K+), chloride (Cl-), and bicarbonate (HCO3-) (carbon dioxide in its ionic form)
• Functions are to regulate volume, osmotic activity, myocardial rhythm, and contractility, enzyme activation, ion pump regulation, acid-base balance, blood coagulation, and neuromuscular excitability.
• They are contained in intracellular fluids (ICF), and extracellular fluids (ECF), the latter being further broken down into intravascular (plasma) and interstitial fluids (ISF)
• Regulation of electrolytes has to do with diet, kidney function (e.g., reabsorption), and hormonal systems (e.g. aldosterone)

Osmolality

• The physical property of a solution, based on solute concentration
• Water concentration is regulated via thirst and urine output
• Thirst and urine output are regulated by plasma osmolality
• Osmolality: solute concentration/kilogram of solvent
• Osmolarity: solute concentrated/liter of solution
• Determination: Freezing-point depression or vapor-pressure depression
• Serum collection preferred for measurement of osmolality