Urine is produced by the filtration of blood by the kidney’s nephrons. Blood reaches the kidney via the renal artery and travels to the glomerulus via small capillaries. Solute and water are filtered through the capillary and into the Bowman capsule at this stage. This fluid moves gradually through the capsule and into the renal tubule. More capillaries surround the tubule, allowing water and other solutes to move through the tubule and into and out of the capillaries as needed by the body. The collecting system inside the renal medulla gathers all urine from each nephron and transports it to the renal pelvis. Urine then flows through the ureters and into the bladder. The urine passes through the urethra and out of the body during micturition (voiding).
Urine is almost entirely water, with just a trace of solutes. To preserve normal acid–base equilibrium, fluid and electrolyte balance, and homeostasis, all metabolic end products and potentially harmful materials are excreted in the urine.
The urine, in general, represents the blood level of every analyte. If the blood level is high and the kidneys are functioning properly, the urine level for the same substance should be high as well. If the urine level is not elevated, the kidneys may be diseased, resulting in elevated blood levels. Certain blood solute products are not altered from the blood in some cases until “threshold” levels of the solute are exceeded. Glucose, for example, is not excreted by the kidney until blood levels reach 180 mg/dL.
Reasons For Obtaining Urine Specimens
The urine specimen has been referred to as a “fluid biopsy” of the urinary tract. It is normally painless to obtain, and it offers a wealth of knowledge easily and affordably. Urine tests, like all specimen tests, must be carefully conducted and properly monitored. The majority of urine samples are done for one of the following reasons:
- To determine whether a patient has renal or urinary tract disease (e.g., proteinuria may indicate glomerulonephritis).
- To keep track of renal or urinary tract disease (e.g., urine cultures may be used to monitor the effectiveness of antibiotic therapy for urinary tract infections).
- To diagnose metabolic or systemic diseases that are not specifically linked to the kidneys (for example, glucose in the urine may indicate diabetes mellitus or Cushing syndrome).
While blood tests provide useful information about the body, urinalysis may be preferred for a variety of reasons:
- A urine specimen is needed for the diagnosis of a urinary tract infection (UTI).
- A 24-hour urine sample would better represent homeostasis and disease than a blood test taken at random during the day.
- Certain products are quickly eliminated by the kidneys and may not be detected in the blood (e.g., Bence-Jones protein). A blood test can yield normal results, while urinalysis reveals the existence of these substances.
- Renal clearance may have an effect on the serum substance being examined (e.g., sodium). As a result, obtaining a urine specimen to determine sodium concentration will provide substantial additional knowledge to a serum sodium level.