Response to the patient's request: "Explain where and how the primary urine is formed"

A person usually drinks about 2.5 liters of water per day. To this volume, add another 400 ml, which are formed as a result of the breakdown of fats, carbohydrates and proteins. The main organs that carry out the excretion of water from the body are the kidneys. A small part of it is excreted by the lungs, skin and feces. Many people have heard such concepts as primary and secondary urine. But very few people know what it is. Often patients turn to the doctor with a request. "Explain where and how the primary urine is formed," they ask. In this article we will discuss this issue in more detail.

general information

In renal nephrons, there are two phases of fluid formation. It is then transported by the excretory system of the kidneys. Let's try to explain the process of formation of fluid in the kidneys. So we will answer a common question that patients are asked in the following form: "Explain where and how the primary urine is formed." How do the phases go? Primary urine is formed in the bodies of nephrons. The second phase takes place in the tubules of the nephron. Primary urine is formed from capsules of glomeruli, capillary walls and inner sheet of which have a filtration property. Capillary blood in the glomeruli flows under greater pressure than in other organs. This is created due to the difference in diameter of the blood vessels that carry and take away blood, which in the first two times more. High blood pressure contributes to its filtration through the walls of the capillaries into the capsule of the glomerulus. Thus, the primary urine is formed as a result of the difference in pressure. During the day the kidneys pass blood through themselves, which contacts the inner walls of the vessels. Their area in the glomeruli reaches 1.5-2 m². Responding to patients' request: "Explain where and how primary urine is formed," some figures should be cited. So, a day produces about 180 liters of fluid, which is then transported from the body. At the same time, one liter of it passes through filtration with 10 liters of flowing blood.

Composition

Responding to the request: "Explain where and how the primary urine is formed," it is necessary to say about its composition, since the area of liquid formation has a direct influence on its structure. In the first phase of formation, virtually all components of the blood are contained. The exception is high-molecular proteins and uniform elements. Also in the primary urine there are exchange products, such as urea, uric acid and others. Further, it passes from the glomeruli into the tubules of the nephrons, where the amino acids, vitamins, glucose, water and salts are reabsorbed into the blood. This process is called reabsorption. In the event that there is a large number of separate substances in the blood, some part is not absorbed back.

Properties

The main characteristics of primary urine include:

1. Low osmotic pressure, which occurs in connection with membrane equilibrium.
2. Formation of a large volume per day. It can be tens of liters. Given that all the blood passes through the kidneys almost three hundred times, during the day the body is able to filter up to a half thousand liters of blood. Thus, about 180 liters of urine are produced in the first phase.

From the urine formed in the first phase, there is an absorption in the blood of substances useful to the body.

Glomerular filtration rate (GFR)

It is regulated by GFR with the help of nervous and humoral mechanisms. The speed of glomerular filtration affects the following components:

- tonus of the glomerular arterioles, affecting the amount of transmitted blood and the level of filtration pressure;

- the tone of the mesangial cells, which are the connective tissue between the capillaries of the nephron glomerulus and the surface of the filtration;

- the vigor of visceral epithelial cells and their functioning.

Some humoral effects, such as prostaglandids, adrenaline, noradrenaline, atriopeptides, adenosine and others, can increase or decrease the glomerular filtration rate. The most important role in its constancy is played by autoregulation of cortical blood flow.

The formation of secondary urine and its composition

During the day, 1.5 liters of this liquid is produced. For its formation, 150-180 liters of primary is required. Through outgoing pathways, it enters the bladder. From it, it is then removed from the body to the outside. In the tubules, about 99% of the water contained in the primary urine and the nutrients contained in it are absorbed. In its composition, the secondary liquid differs significantly from that formed in the 1 st phase. It does not contain sugar, most salts and amino acids. However, there is an increased level of sulfate, urea, uric acid, phosphate and other components.

Kidney function

In the case of a lack of salt in the body, it is not excreted in the urine. This means that the kidneys perform the function of normalizing the level of essential substances contained in the human body. That is, they help to remove unnecessary and delayed missing components. In the tubules of the nephron, in addition to the reabsorption of water and substances in it, there may be a flow into the urine of components that are unable to pass the renal filtration on the way from the capillaries to the capsule of the glomerulus. Such elements are medicines (mainly antibiotics), paints and others. Urine, which was formed in the kidneys, passes into the ureters from the kidney cups, passing the pelvis. Due to rhythmic contractions of the musculature of the transport channels, the liquid enters the bladder. However, there the urine is delayed for some time before filling it.