Renal function and water balance

What is the counter current mechanism?

System where blood flows counter to ultra-filtrate in the kidneys allowing for water and NaCl to be reabsorbed

Where is water mainly readsorbed?

The descending loop of Henle

Where is NaCl actively reabsrobed?

The descending loop of Henle

What is reabsorbed as result of NaCl active reabsorbion?

Water 

What are the function of the kidney?

• concentration of urine

• reabsorption of nutrients

• acid base and electrolyte balance

• excretion of waste

• blood pressure

• water reabsorption and concetration

What does anti Diuretic hormone (vasopressin) regulate?

Water reabsorption

What stimulates the release of ADH?

Osmoreceptors in the hypothalamus

What part of the brain releases ADH?

The posterior pituitary

What does ADH target?

The collecting ducts of the kidneys

What does increased ADH result it?

Increased water reabsorption 

What does decreased ADH result in?

Decreased water reabsorption

What disease state results in increased ADH?

Inappropriate ADH secretion

What disease state results in decreased water reabsorption

Diabetes insipidus

What is the definition of osmolality?

Concentration of solutes dissolved in the body’s water

What is osmolality expressed in?

milliosmoles (mOsm)/kilogram of water 

What can contribute to serum and urine osmolality?

Electrolytes, carbohydrates, waste products, vitamins, drugs, hormones and alcohol

What is osmolality a measure of?

The osmotic pressure across body membranes

What is osmolarity a measure of?

Osmoles of solute or dissolved particles per kilogram

What is osmometry?

Term for the analysis (or measurement of) osmolality by a lab instrument

What is the first formula for osmolality?

2.0[Na+] + [glucose]/20 + [BUN]/3

What is the second formula for osmolality?

1.86[Na+} + [glucose]/18 + [BUN]/2.8

What is the osmolar gap?

The difference between the calculated and measured osmolalities

What causes pathologic changes in osmolality?

• changes in water concentration

• changes in solute concentration 

What substances can contribute to an abnormal serum osmolality?

ETOH and ethylene glycol 

What is the reference range for osmolarity for a 24 hr collection?

300-900 mOsm/kg

What is the reference range for osmolarity for a random urine sample?

50-1200 mOsm/kg

What should osmolarity should seen after 12 hours of fluid restiction?

>900 mOsm/kg

What is the reference range of the urine to serum osmolarity ratio?

1.0-3.0

What is the reference range for osmolarity for serum?

275-295 mOsm/kg

What is the reference range for the osmolar gap?

5-10 mOsmole/kg

What are colligative properties?

Properties that are impacted by the number of dissolved particles

What are four colligative properties?

• freezing point

• vapor pressure

• boiling point

• osmotic pressure 

What properties increase with increased amounts of dissolved particles?

Boiling point and osmotic pressure

What properties decrease with increased amounts of dissolved particles?

• freezing point

• vapor pressure 

What is the principle of freezing point osmometry?

Osmometry is determined by super-cooling the sample below the freezing point

How is the freezing initiated in freezing point osmometry?

By a physical shock or a very cold stir wire

What type of calibration do freezing point osmometers use?

2 point calibration against salt solutions with a known freezing point

What does a freezing point osmometer use to measure the freezing point?

A calibrated themoresister

What is run with each assay with a freezing point osmometer?

A control solution with a known value of approximately 290 mOsmoles/kg 

What is the equation for determining the osmolarity of an unknown solution using freezing point?

mOsm (unknown) = temp unk (Celsius) / temp std (Celsius) X std contrition 

How many osmoles does it take to drop the temp by 1.86 degrees?

1000 mOsmole

What methods do most labs use to measure osmolarity?

Freezing point depression or vapor pressure depression

What is the vapor pressure?

The force exerted by the gaseous phase of a two phase system

How is vapor pressure determined in VP osmometry?

Thermometrically by a fine wire thermocouple suspended in small vapor space above the specimen in a sealed sample chamber

What is the first step of VP osmometry?

Sample is inserted and temperature and vapor pressure equilibrate. The thermocouple set this a the null point 

What is the second step of VP osmometry?

The thermocouple is cooled below the dew point temperature. Microscopic water droplets begin to condense on the thermocouple surface

What is the dew point?

The temperature below which the water vapor in a volume of humid air a a given constant barometric pressure will condense into liquid water at the same rate at which it evaporates

What is the third step of VP osmometry?

The heat of condensation causes the thermocouple temperature to rise to the point where condensation ceases which is the dew point temperature

What is the fourth step of VP osmometry?

The final instrument readout is proportional to the dew point temperature

What is the equation for the osmolar gap?

Measured osm - Calculated osm

What is the normal reference range of the osmolar gap?

5-10 mOsm/kg

What does it mean if the osmolar gap is >10mOsm/kg?

Abnormal substances are present in the blood and urine that are contributing to the measured osmolality but not accounted for in the calculation

What substances can affect the osmolar gap?

• mannitol

• glycine

• methanol

• ethylene glycol 

How does the prognosis correlate to the osmolar gap?

Higher the gap, the poor the prognosis

What major ions contribute to osmolality?

Na, BUN, and glucose

What are the two basic compartments of the body?

• intracellular fluid compartment (ICF)

• extracellular fluid compartment (ECF)

What is water movement between compartments regulated by?

The osmotic pressure set up by the differences in solute concentration across cell membranes in the body

What is the total amount of fluid typically in intracellular fluid?

25 L or 40% body weight

What is the total amount of fluid in the extracellular fluid?

15 L, 20% of the body weight

What volume of fluid makes of the plasma and interstitial fluid volume?

• Interstitial fluid: 12 L, 80% of ECF

• Plasma: 3L, 20% of ECF 

What is the typical water intake made of?

2500ml / 24 hours

• 60% drinking

• 20% moist food

• 10% water metabolsim 

What is the daily fluid output?

2500 ml/ 24 hrs

• kidneys: 1500 ml/day

• skin: 400 ml/day

• Lungs: 300 ml/day

• sweat: 200 ml/day

• GI tract: 100 ml/day

In what direction does water move across a membrane?

Passively from higher solute concentration to lower

Where is the thirst center located?

In the hypothalamus

How does the hypothalamus sense hydration level?

Osmoreceptors which are stimulated by the increase in ionic concetration

What is the definition of the glomerular filtration rate?

The ml/minute of blood filtered by the kidney

What is the equation for clearance?

(mg/dl Urine marker x volume of urine (ml))/(mg/dl plasma marker x time of collection (min))

What is the factor correction for surface area?

1.73/ surface area of patient

Why is the creatinine clearance a good test of renal function?

Creatinine is a natural marker that is excreted at a constant rate over 24 hours that is not usually reabsorbed by the kidneys

When is the inulin clearance test used?

For patients with muscle damage or wasting disorders 

What are the reference ranges for creatine clearance?

• Males: 107-139 ml/min

• Females: 87-107 ml/min 

What is the normal amount of protein in the urine?

<150 mg/24 hours secreted - not reabsorbed

What is the microalbumin test?

Assay for small levels of albumin in the urine

What is the range for albumin in a 24 hour collection?

30-300 mg/ 24 hours

What is the range for albumin in a random urine sample?

30-300 mg/L

What is the normal albumin creatine ratio (ACR)?

>30 mg/g creatinine 

What is the renal threshold?

The level at which substances filtered by the urine are reabsorbed

What is true of the renal threshold?

• each substance filtered by the urine has its unique threshold

• each substance filtered has its own threshold

• thresholds can vary 

What is a pre-renal disorder?

Due to cardiac output/or physiological output prior to kidney

What is a renal disorder?

Problem the occurs in the filtering structure

What is a post renal disorder?

Urine outflow blocked

When blood can not be adequately filtered, what waste products build up in the bloodstream?

• BUN

• Creatinine 

• Uric acid

• ammonia 

What are non-protein nitrogen compounds?

Nitrogen containing compounds that are not proteins

What are non-protein nitrogen compounds used for?

Natural markers of renal disease and Azotemia

What are the non protein nitrogen containing compounds?

• BUN

• Uric acid

• Creatinine/creatine

• Ammonia 

What is increased ammonia usually a sign of?

Liver disease 

What is increased in renal disease?

Creatinine

What is increased in muscular disorders?

Creatine

What is the Jaffe reaction?

Picric acid + NaOH → alkaline-picrate creatinine complex

What color is picric acid?

Bright yellow

What color is the alkaline-picrate creatinine complex?

reddish-yellow 

What method is the Jaffe reaction?

Colorimetric kinetic assay

Why are hemolyzed samples avoid with the colorimetric kinetic assay?

Color interferes with reading

What other methods can be used for creatinine?

Enzymatic methods that usually have a slightly lower reference range

What is the reference range for creatinine in plasma?

Adults: 0.6 - 1.2 mg/dl

What is the reference range for creatinine in a 24 hour urine?

• Male: 0.8-2.0 g/day

• Female: 0.6-1.8 g/ day

What is blood urea nitrogen referred to in the urine?

Urea

Where does urea come from?

The breakdown of protein

In the ornithine cycle what is ammonia from the breakdown of protein converted to?

BUN which is readily soluble in urine

What is azotemia?

Increase levels of BUN in plasma