Understanding Urine

Author: Zou Lujia, deputy chief physician of Huashan Hospital affiliated to Fudan University

Reviewer: Jiang Haowen, Chief Physician, Huashan Hospital Affiliated to Fudan University

With the popularization of health care knowledge, the concept of "drinking 8 glasses of water every day" has gradually become popular. Drinking enough water is the "good start" of water metabolism in the human body; and clear urine flowing out of the body smoothly is the "good end" of water metabolism. However, how is urine formed? Why is urine yellow, and the color is sometimes light and sometimes dark? With these questions, let us explore the mystery of urine together.

How is urine formed?

Adults consume 1 to 2 liters of water from their diet every day. Most of this water, together with the digestive juices secreted by the gastrointestinal tract, is absorbed by the intestines into the blood, and only a small part is excreted with feces. After the nutrients and water enter the blood, they begin a long journey in the blood circulation system. They first converge to the liver, then return to the heart, and are pumped into the lungs by the heart for gas exchange. The oxygen-rich blood returns to the heart again, is pumped out and transported to the whole body, providing oxygen and necessary nutrients to organs and tissues throughout the body. At the same time, part of the blood also carries metabolic waste produced by the body and is transported to the kidneys, an important excretory organ of the human body.

Figure 1 Copyright image, no permission to reprint

The main function of the kidneys is to remove metabolic waste from the body by producing and excreting urine. The kidneys are like a "filter" that filters blood. Under the action of this filter, the two kidneys filter blood and produce urine day and night. After leaving the kidneys, the urine starts its journey to the human body's "sewer": along the ureter to the "reservoir" that temporarily stores urine - the bladder. When the bladder is full, the urethral sphincter "valve" relaxes, triggering "flood discharge" and completing the action of urination.

From the digestive system to the circulatory system, and from the circulatory system to the kidneys to form urine, then into the urinary tract, and finally excreted from the body, this is the human body's water metabolism process.

Why is urine yellow?

I believe everyone has observed a phenomenon: the color of urine changes depending on the amount of water you drink, ranging from colorless and transparent to orange-yellow, but the overall tone is yellow. This is mainly due to the urobilin in urine.

What is urobilin? Its formation process is quite tortuous: after the red blood cells in the blood circulation complete their mission of about 120 days, they will be degraded and metabolized by some cells with phagocytic function. The hemoglobin in the red blood cells is then released and becomes heme. Then, after a long metabolic transformation, it turns into bilirubin, cholinesterase, urobilinogen, and finally forms urobilin in the urine, making the urine appear light yellow. This light yellow color can be said to have undergone "seventy-two changes".

Figure 2 Copyright image, no permission to reprint

In a healthy state, the metabolic rate of red blood cells is relatively stable, and the amount of urobilin produced is relatively constant. Therefore, the amount of urine basically determines the depth of urine color. The amount of urine is precisely regulated by the human body. When the amount of blood in the body decreases, such as drinking less water, sweating a lot, bleeding, or extensive dilation of blood vessels, resulting in a relative lack of total blood volume, the body will reduce the amount of blood filtration in the kidneys through neuroendocrine regulation, thereby reducing urine volume and darkening the color of urine. Imagine that after a sweaty exercise in the hot summer, there must be little urine, and the color of the urine must be relatively yellow. On the contrary, if you drink a lot of water, the kidney filter will increase its horsepower and increase filtration, thereby producing more urine, which dilutes the yellow color of the urine.

Can the color and characteristics of urine reflect urinary system diseases?

The color, smell, turbidity and other properties of urine are important references for doctors to make diagnoses. In daily life, careful readers must have discovered such interesting phenomena: after eating red pitaya, the urine may show a red color similar to that of red pitaya, and after eating asparagus, the urine may emit a special odor. These are typical cases of food affecting the properties of urine. The former is because red pitaya contains betaine, which cannot be metabolized by the human body and will be excreted from the body with urine, thus causing the urine to turn red. But in fact, the pigment components of most foods will be metabolized by the human body, or their main body and metabolites cannot enter the urine, so it is impossible to make the urine turn the color of food. The latter is because some sulfur-containing substances in asparagus are decomposed into methyl mercaptan and enter the urine. This sulfide is volatile and has a special odor, so it will cause the urine to have a unique odor. However, these changes are mostly normal phenomena and have no direct connection with diseases.

Turbidity is also an important indicator for urine observation. Normal urine is relatively clear, but when white blood cells and some abnormal metabolites appear in the urine, such as chyle, fat particles or various crystalline components, it may become turbid, indicating the presence of infection or metabolic disease.

The redness of urine may be the change we are most worried about. In addition to the influence of food, medicine, etc. on the color of urine, hematuria may be the main reason for red urine. Under normal circumstances, the "filter" of the kidney will not miss a red blood cell. But when the "filter" is damaged or the "sewer" downstream of the kidney - the urinary system suffers from a disease, red blood cells may be mixed into the urine. However, even if the urine is red, it does not mean that a large amount of blood is mixed in. When 1000 ml of urine is mixed with 1 to 2 ml of blood, it can appear red visible to the naked eye; and when 4 ml of blood is mixed in, it is already bright red. Therefore, if you find that your urine has turned red, in most cases you don't have to worry about "death from blood loss" due to hematuria. You should seek medical attention in time to clarify the cause and treat it.

What disease conditions may cause hematuria? If the kidney "filter" is damaged, it may be a disease of the kidney itself, such as nephritis, kidney disease, etc., and it is generally recommended to go to the nephrology department for treatment. If the hematuria is caused by "sewer" abnormalities, it may be attributed to urinary system stones, infections, tumors, inflammation and other diseases, and it is generally recommended to go to the urology department for treatment. In addition, blood diseases, vascular diseases or poisoning may cause hematuria, and timely medical treatment and early diagnosis and treatment are required.

In short, urine is one of the most common body fluids in our daily lives and an important part of physiological metabolism. Knowing urine and understanding its source and destination can help us capture signals from the urinary system and even the whole body earlier in our daily lives and better identify clues of diseases.