Capstone (329-343) Urinalysis and body fluids
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What is the functional unit of the kidney?
The nephron. The nephron consists of the glomerulus, Bowman's capsule, the proximal convoluted tubule, the Loop of Henle, and the distal convoluted tubule.
What are the three renal functions associated with urine formation?
Filtration, reabsorption, and secretion. Filtration takes place in the glomerulit; reabsorption and secretion in the renal tubules. Plasma filtrate is concentrated and acidified in the process.
During urine formation, which substances are not allowed to filter though the glomerulus in significant amounts?
Protein and cells.
Where does water reabsorption take place in the nephron?
In the proximal and distal tubules
Where does reabsorption of glucose take place in the nephron?
In the proximal tubules.
Which hormone controls reabsorption of water in the distal tubules?
Anti-diuretic hormone (ADH). It is produced in the hypothalamus and stored and secreted by the anterior pituitary.
Which hormone controls reabsorption of sodium?
Aldosterone. It is produced in the adrenal cortex. Reabsorption of sodium is linked to excretion of K+ and H+
What are the main constituents of urine?
Water, urea, sodium, and chloride. Urea, the breakdown product of protein, accounts for about half of the dissolved solute in the urine.
What tests could be done on an unknown fluid to determine if it is urine?
Urea, creatinine, sodium, and chloride. Urine has a high concentration of these substances than other body fluids.
What is osmolality?
The measurement of the number of solute particles per solvent, irrespective of molecular weight.
How is osmolality measured in most clinical laboratories?
By freezing point depression. Each mOsm of solute lowers the freezing point of the urine by 0.00186oC.
What quality control is required for osmometers?
Osmometers should be checked each day of use with controls of known osmolality.
Why is osmolality a better measurement of kidney function than specific gravity?
It is a better reflection of the concentrating ability of the kidneys since it is not disproportionately affected by the presence of high-molecular-weight substances like glucose or protein.
What is normal urine osmolality?
It can range from 50-1400 mOsm/Kg, depending on factors such as diet and exercise. In a healthy person with a regular diet and fluid intake, urine osmolality is usually 500-850 mOsm/Kg.
What is the normal ratio of urine to plasma osmolality?
Under random conditions, at least 1:1 and after controlled fluid intake, 3:1
What is widely accepted as the best overall measure of kidney function?
Glomerular filtration rate (GFR). A decrease in GFR precedes kidney failure.
Name a test that is commonly performed to estimate the glomerular filtration rate.
Creatinine clearance. Creatinine is a good substance to use for a clearance because it is not significantly reabsorbed by the renal tubules and since it is related to muscle mass, its concentration is constant. The creatinine clearance measures the rate at which the kidneys are able to remove creatinine from the blood. Decreased levels are an indication of impaired renal function. Although the creatinine clearance becomes abnormal before the serum BUN or creatinine, it does not detect early renal disease and provides only a crude index of GFR
What is the formula for calculating creatinine clearance?
- UV/P x 1.73/A
- U=urine creatinine in mg/dL
- V=urine volume in mL/minute (24 hour volume/1440)
- P=plasma creatinine in mg/dL
- A=patient's body surface area (determined by height and weight and obtained from a nomogram)
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