A person usually drinks about 2.5 liters of water per day. An additional 400 ml, which are formed as a result of the breakdown of fats, carbohydrates and proteins, should be added to this volume. The main organs that remove water from the body are the kidneys. A small part of it is excreted by the lungs, skin and feces. Many have heard concepts such as primary and secondary urine. But few people know what it is. Often, patients turn to the doctor with a request. “Explain where and how primary urine is formed,” they ask. The article will cover this issue in more detail.
general information
In the renal nephrons, two phases of fluid formation are observed. It is then transported by the excretory system of the kidneys. Let's try to explain in an accessible way the process of fluid formation in the kidneys. So we will answer the common question that patients ask in the following form: "Explain where and how primary urine is formed." How do the phases go? Primary urine is formed in the bodies of nephrons. The second phase passes in the tubules of the nephron. Primary urine is formed from glomerular capsules, the walls of the capillaries and the inner leaf of which have the property of filtration. Capillary blood in the glomeruli flows under greater pressure than in other organs.
This is due to the difference in the diameter of the vessels that bring and take away blood, which is twice as large in the former. High blood pressure helps to filter it through the walls of the capillaries into the capsule of the glomerulus. So, primary urine is formed as a result of a difference in pressure. During the day, the kidneys pass blood through themselves, which is in contact with the inner walls of the vessels. Their area in glomeruli reaches 1.5-2 m². Responding to a request from patients: "Explain where and how primary urine is formed," some figures should be given. So, about 180 liters of fluid are produced per day, which is then transported from the body. At the same time, one liter of it is filtered with 10 liters of flowing blood.
Composition
Answering the request: “Explain where and how primary urine is formed”, it is necessary to say about its composition, since the area of fluid formation has a direct effect on its structure. In the 1st phase of formation, almost all blood components are contained. The exception is high molecular weight proteins and shaped elements. Primary urine also contains metabolic products such as urea, uric acid, and others. Then it passes from the glomeruli to the tubules of the nephrons, where amino acids, vitamins, glucose, water and salts are re-absorbed into the blood. This process is called reabsorption. In the event that the blood contains a large number of individual substances, some part is not absorbed back.
The properties
The main characteristics of primary urine include:
- Low osmotic pressure that occurs due to membrane equilibrium.
- The formation of a large volume per day. It can be tens of liters. Given that all blood passes through the kidneys almost three hundred times, during the day this organ is able to filter up to one and a half thousand liters of blood. Thus, about 180 liters of urine are produced in the first phase.
From urine formed in the first phase, absorption of substances useful to the body into the blood occurs.
Glomerular filtration rate (GFR)
GFR is regulated by nerve and humoral mechanisms. Glomerular filtration rate affects the following components:
- tone of glomerular arterioles, affecting the amount of passing blood and the level of filtration pressure;
- the tone of the mesangial cells, which are connective tissue located between the capillaries of the glomerulus of the nephron, and the filtration surface;
- the energy of visceral epithelial cells and their functioning.
Some humoral effects, which include prostaglandids, adrenaline, norepinephrine, 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.
Secondary urine formation and composition
During the day, 1.5 liters of this fluid are produced. For its formation requires 150-180 liters of primary. Through the excretory tract, it enters the bladder. From it, further it is excreted from the body. In the tubules, about 99% of the water contained in the primary urine and the beneficial substances contained in it are absorbed. In its composition, the secondary fluid differs significantly from that formed in the 1st phase. It does not contain sugar, most salts and amino acids. However, there is an increased level of concentration of sulfates, urea, uric acid, phosphates and other components.
Kidney function
In case of a shortage 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 necessary substances contained in the human body. That is, they contribute to the removal of excess and delay of the missing components. In the tubules of the nephron, in addition to reabsorption of water and the substances contained in it, components can enter the urine that are not able to undergo renal filtration on the way from the capillaries to the glomerular capsule. Such elements are drugs (mainly antibiotics), paints and others. Urine that forms in the kidneys passes into the ureters from the renal cups, passing the pelvis. Due to the rhythmic contractions of the muscles of the transport channels, the fluid enters the bubble. However, there the urine lingers for a while until it is full.