How does urine production take place in the renal tubules?

The kidneys are one of the most important organs in the body, responsible for making urine to help excrete waste products. The process of creating urine in the renal tubules takes place through many complicated stages.

The process of making urine by the kidneys is done through 3 stages:
Filtration in the glomerulus. The process of reabsorption of substances from the renal tubules into the blood. The process of excreting some waste products from the blood back into the renal tubules. The result of all 3 processes above is urine produced.

Every minute, more than 1 liter of blood passes through the kidneys, an adult can filter 60 liters of blood in 1 hour and have 7.5 liters of filtrate produced. Thus, with 5 liters of blood in the human body, after 24 hours, it can pass through the kidneys 288 times or once every 5 minutes. Blood filtration takes place in the glomerulus. The glomerulus is composed of a parallel network of capillaries, surrounded by Bowman's capsule. The fluid filtered from the plasma (blood) into the Bowman's capsule is called the glomerular filtrate. This fluid must pass through the glomerular membrane consisting of 3 layers:
The endothelial cell layer of the capillaries has thousands of small pores measuring 160A0 The basement membrane is a network of small slits with the size of about 110A0. Bowman's capsule epithelial cells have small, perpendicular dendrites and end on the basement membrane, creating clefts about 70A0 in size.

The membrane filter is highly selective for the molecules it passes through, depending on two factors: the size and the charge force of the molecules across the membrane. The composition of the glomerular filtrate is similar to that of blood plasma but does not contain platelets (blood cells), the amount of protein in the filtrate is very low, only about 1/200 of the plasma.
The mechanism of filtration across the glomerular membrane depends on the difference between the pressures inside the glomerular capillary and Bowman's capsule. These pressures include:
Renal capillary hydrostatic pressure (PH) Glomerular capillary colloidal pressure (PK) Bowman capsule hydrostatic pressure (PB) Effective filtration pressure (PL): PL = PH - (PK + PB), about 10mmHg. Filtering only occurs when PL > 0 or PH > P K + PB..

The first urine after being filtered in the glomerulus still has many nutrients, so it will be reabsorbed in the renal tubules. On average, about 170-180 liters of first urine are produced every day, but after reabsorption, only about 1-2 liters of urine are actually formed.
After entering Bowman's capsule, glomerular filtrate enters the renal tubular system including the proximal tubule, loop of Henle, distal tubule, and collecting duct. Here will occur the process of reabsorption and excretion of some substances to turn the filtrate into urine.
3.1 Proximal tubule Sodium reabsorption: About 65% of sodium is reabsorbed in the proximal tubule by easy diffusion with the help of a brush border carrier protein. Sugar reabsorption: when the glucose concentration is less than 180mg/100ml of plasma, the proximal tubule will reabsorb all the glucose in the filtrate. But when the concentration rises above 180, the proximal tubule cannot absorb all the glucose and glucose begins to appear in the urine (diabetes). Tubular water reabsorption: when sodium and sugar are reabsorbed by proximal tubule cells, water is also reabsorbed. About 65% of the water is reabsorbed here. Potassium reabsorption: Approximately 65% ​​of the K+ in the filtrate is actively reabsorbed in the proximal tubule. In addition, there are processes of reabsorption of proteins, amino acids, chlorides, urea as well as bicarbonates.
3.2 Loop of henle Here, water and other substances continue to be reabsorbed to pass through the distal tubule

3.3 Distal tubule Sodium reabsorption: the filtrate when reaching the distal tubule is about 10% Na+. Here, Na+ continues to be reabsorbed with the active support of an adrenocortical hormone, aldosterone. Water reabsorption: tubular water reabsorption: water is reabsorbed in the distal tubule about 18 liters/24 hours, remaining about 18 liters continues to enter the collecting duct. In addition, other substances such as chloride are also reabsorbed here. In particular, the excretion process will take place in the distal renal tubule, including substances such as potassium, H + acid radicals and ammonia.
3.4 Collectors The process of reabsorption and excretion of substances in the collecting duct is similar to that of the distal tubule. In which, water reabsorption is a very important function and has the effective support of anti-diuretic hormone ADH. As can be seen, water is continued to be reabsorbed here.
The amount of water that is reabsorbed back through the filtration process is quite large, about 16.5 liters. Finally, urine is concentrated to about 1.5 liters, poured into the renal pelvis, then down the ureter to the bladder before being excreted. The composition of urine is officially water, residues (uric acid, creatinine, urea...), metabolic products of some drugs, electrolyte ions (K+, H+...).