What is considered polyuria, oliguria, and anuria? What are the reasons for abnormal urine volume?

Author: Zuo Li, Chief Physician, Peking University People's Hospital

Reviewer: Mao Yonghui, Chief Physician, Beijing Hospital

In a healthy young person, the kidneys can filter about 100 ml of primary urine per minute, and then about 99% of the primary urine is reabsorbed in the renal tubules, and only 1% is finally excreted as urine. This means that about 1 ml of urine is produced every minute, and about 1440 ml of urine is produced in a day.

The daily urine production of the human body is usually maintained within a relatively stable range, which is normal due to the influence of water intake and sweating. However, if the urine volume increases or decreases significantly, it may indicate abnormal conditions in the kidneys or other related systems.

1. What is considered polyuria, oliguria and anuria?

Polyuria can be divided into two categories: physiological and pathological. Physiological polyuria is caused by increased water intake. In order to maintain the osmotic pressure balance in the body, the body will excrete excess water through a self-regulating mechanism and excrete it in the form of urine, thus manifesting as polyuria.

Under normal physiological conditions, the body will make fine adjustments based on water intake, and regulate the renal tubular reabsorption of water through hormones acting on the kidneys. However, when kidney function is impaired, especially when the renal tubular reabsorption function is impaired, it is unable to effectively reabsorb the primary urine in normal proportions, leading to polyuria. For example, if the renal tubular reabsorption rate drops to 98%, the daily urine volume can increase to 2880 ml; if it further drops to 97%, the daily urine volume can be as high as 4500 ml, far exceeding the normal range.

It is worth noting that even if a healthy individual does not consume any water in a day, his daily urine volume can usually be maintained at more than 400 ml. This is because the waste metabolized by the kidneys in the body every day requires at least 400 ml of urine to be fully dissolved and excreted.

If the daily urine volume is continuously less than 400 ml, it is called oliguria; if it further decreases to less than 100 ml, it is called anuria. Long-term maintenance of oliguria often indicates that there may be abnormalities in the kidneys.

Figure 1 Original copyright image, no permission to reprint

2. What might happen if there is excessive urine volume?

Increased urine output can usually be attributed to three main conditions:

The first case is psychogenic polydipsia, that is, the individual's daily water intake is significantly higher than the normal level, which leads to a corresponding increase in urine volume.

The second condition involves kidney dysfunction, which is manifested by the kidneys' inability to respond normally to hormones that regulate urine output, leading to an abnormal increase in urine output. This phenomenon is called nephrogenic diabetes insipidus.

The third situation is that the kidney function is normal, but the central nervous system is abnormal. Specifically, insufficient secretion of antidiuretic hormone causes the kidneys to fail to receive effective instructions from the hypothalamus, and then fail to perform the normal water reabsorption process, resulting in polyuria. This disease is called central diabetes insipidus. Its main clinical manifestations include increased drinking, significantly increased urine volume, and severe thirst, and the daily urine volume often exceeds 4 liters.

Figure 2 Original copyright image, no permission to reprint

In order to distinguish whether polyuria is caused by problems in the kidney itself or by abnormalities in the central nervous system, exogenous injection of antidiuretic hormone may be necessary for diagnosis. If the urine volume decreases significantly after injection, it may indicate central diabetes insipidus; conversely, if the urine volume does not decrease, it is more likely to point to lesions in the kidney itself. In this case, further diagnosis usually requires renal puncture biopsy, combined with pathology to clarify the specific type of disease.

3. Which kidney diseases can cause oliguria or anuria?

In the case of oliguria or anuria, the following two situations have the possibility of complete recovery.

1. Prerenal renal insufficiency: Due to insufficient blood volume, the renal perfusion pressure decreases, causing renal dysfunction. In the early stages, the renal structure has not yet undergone substantial changes, and renal function can be restored by timely replenishment of water and blood volume.

2. Obstructive nephropathy: Sudden kidney obstruction may be caused by tumors, stones or external compression leading to bilateral ureteral blockage. If the obstruction can be relieved in time, kidney function is expected to be fully restored.

Renal lesions involve multiple sites, including the glomeruli, renal tubules, renal interstitium and renal blood vessels, such as nephrotic syndrome, nephritis, acute tubular necrosis, tubular damage and acute interstitial nephritis. These lesions can all lead to acute renal damage and decreased urine volume.

If you find that your urine volume is reduced and you suspect that it may be caused by kidney disease, you should first do a urine routine test, which can reveal most signs of kidney disease. To further confirm the diagnosis, you can also test your blood creatinine level, and combined with urine routine and kidney imaging tests, you can detect most kidney diseases.