Urine is formed by the kidneys through filtration of the blood. During the process of filtration, the kidneys remove metabolic wastes and excess water from the blood to form urine. The urine formed in the kidneys then flows down two thin tubes called ureters (one from each kidney) and enters the urinary bladder. Urine formation in the kidneys and the flow of urine from the kidneys to the urinary bladder is a continuous process. As the urinary bladder fills up with urine, its walls begin to stretch.
When the bladder gets filled up to near its maximum capacity, we feel the strong urge to urinate. During the process of urination, the urine in the bladder gets excreted through the urethra. The process of urination stops when the bladder is near empty. Then the whole process of filling up the bladder with newly formed urine from the kidneys continues. Since urine is formed in the kidneys, the characteristics of the urine tend to reflect the status of the kidneys and the urinary tract.
In fact, the urinary habits and the composition of urine also reflect the status of the blood from which urine is formed in the kidneys. Since the composition of the blood depends on the state of tissue metabolism and health all over the body, the composition of the urine can be used to indirectly measure the metabolic and health status of the body. Only a small volume of urine is usually sufficient to conduct multiple diagnostic tests. Due to these factors, urine tests are an integral part of medical diagnostics.
Read more on signs of UTIs.
How to Diagnose Kidney and Bladder Disease
Diagnostic tests conducted on urine samples can shed light on the health status of the kidneys and the urinary bladder. The following are some of the signs that may indicate the presence of a kidney or bladder disease:
Abnormal urine color: Under normal conditions, urine is a clear fluid with a yellowish tinge. Depending on the concentration of solutes in the urine, the shade of yellow color may range from pale yellow to mustard-like yellow.
When the color of the urine becomes dark brown, orange, red, or black, then it indicates the presence of an underlying abnormality. Abnormal urine may also become cloudy. An orange or red color in the urine is usually a sign of bleeding. The presence of blood in the urine is technically referred to as hematuria. Blood may not always be visible in hematuria.
Read more on blood clots in urine.
However, even invisible blood can be detected by laboratory tests of urine samples. A visible red color in the urine is usually a sign of significant bleeding somewhere in the urinary system. However, a reddish color in urine may also be caused by some of the things we may have consumed (such as blackberries, beets, and even some medications).
Decreased urine output
The daily frequency of urination varies on an individual basis. For most individuals, a frequency of six times a day is normal. Others may have a slightly lower or higher frequency. We excrete around 300-500mL of urine during urination, depending on the extent to which the bladder is full. In a day, we normally excrete 1-2 liters of urine.
A urine output of less than 400mL per day is considered abnormal, and is technically referred to as oliguria. A urine output of less than 100 mL or a total cessation of urine output is technically referred to as anuria. Anuria could be caused by many different conditions, including obstruction in the urinary tract, kidney disease, heart failure and dehydration.
Abnormally high urine output
A urine output of more than 3 liters per day is also abnormal. Such a condition is technically referred to as polyuria. The maximum capacity of the urinary bladder ranges from 300-500mL. Therefore, polyuria is typically due to increased frequency of urination in a day rather than an abnormally high urine output in one episode of urination. It is important to note that not all increases in the frequency of urination are abnormal.
For example, people tend to have an increased frequency of urination in cold weather, and upon consumption of alcohol, caffeine, diuretics, and even large amounts of water. What distinguishes polyuria from a normal increase in the frequency of urination is the consistent and chronic urine output of more than 3 liters per day.
Polyuria is commonly seen in patients suffering from Cushing’s syndrome, diabetes insipidus, and diabetes mellitus. Patients who are on an intravenous saline drip also tend to have an increased frequency of urination.
Awakening at night to urinate
An increased frequency of awakening at night to urinate is a sign of some abnormality in the urinary system. Under normal circumstances, urine production decreases during sleep, and the bladder is capable of holding the urine till morning.
One typically does not wake up at night to urinate, especially if the bladder has been emptied before going to sleep. Awakening at night in order to pass urine is technically referred to as nocturia. People who have diabetes mellitus often suffer from nocturia.
Burning sensation or pain during urination
A burning sensation or pain during urination is an abnormal occurrence. These symptoms usually indicate the presence of a urinary tract infection (commonly abbreviated as UTI). Therefore, one must promptly consult a doctor when urination is accompanied by pain or burning sensation.
The burning sensation during urination is typically caused by an inflammation in the urinary tract. The passage of urine over the inflamed areas of the urinary tract intensifies the burning sensation and the pain. Both the burning sensation and the pain also tend to persist for some time after urination.
Difficulty in passing urine
Under normal circumstances, it takes only a little effort to begin urination, and almost no effort to keep the urination going. Urination stops automatically when the bladder becomes empty. Any straining to pass urine is, therefore, abnormal. Under these conditions, the urine stream may be weak even after straining.
Obstruction of the urinary tract and neuromuscular problems with the urinary bladder are some common reasons for the abnormal straining during urination. Obstruction of the urinary tract could also occur due to problems outside the urinary system (such as an enlarged prostate).