Urine Formation

Urine formation

The combination of three processes: filtration, reabsorption (reabsorption) and tubular secretion - allows the kidney to remove waste but retain useful components of the blood. Filtration occurs at the point of contact of the glomerular capillaries with the wall of Bowman's capsule. Passing through these capillaries, the blood is "filtered", so that water, salts, sugar, urea and all other components of the blood, with the exception of blood cells and such large molecules as plasma protein molecules, pass at this point into the cavity of Bowman's capsule, forming glomerular filtrate.

The total blood flow through the kidneys is about 1200 ml per minute - this is a quarter of all the blood pumped by the heart! Plasma passing through the glomerulus gives about 20% of its volume to the glomerular filtrate; the rest passes from the glomerulus into the efferent blood vessel. This process is based on a purely physical filtration mechanism due to the fact that the small afferent artery is wider than the efferent artery. Therefore, the blood pressure in the glomerular capillaries is relatively high and part of the plasma is filtered into the capsule.

The amount of filtrate is also regulated by the narrowing or dilation of arterioles going to and from the glomeruli. It increases with the narrowing of the efferent arterioles and the dilation of the afferent arterioles. If the composition of excreted urine were similar to the composition of the glomerular filtrate, then excretion would be a very wasteful process and the body would lose a lot of water, glucose, amino acids and other useful substances.

However, in terms of the nature and amount of substances contained in urine, it differs sharply from plasma and glomerular filtrate. From each Bowman's capsule located in the renal cortex, the filtrate passes first through the proximal convoluted tubule (also located in the cortex), then through a long loop going into the medulla and back to the cortex, then through the second section located in the cortex - the distal convoluted tubule tubule - and finally pours into the collecting duct, through which it exits into the renal pelvis.

The walls of the renal tubules consist of a single layer of squamous or cuboidal epithelial cells. During the passage of the filtrate, these cells absorb (reabsorb) a significant part of the water and virtually all the glucose, all the amino acids and other substances needed by the body, and secrete them back into the bloodstream. This is possible due to the fact that the arteriole, having left the glomerulus, does not go directly to the vein, but connects with a second network of capillaries surrounding the proximal and distal convoluted tubules.

Thus, about 125 liters of filtrate are formed in the human kidney for every liter of urine produced; the remaining 124 liters of water are sucked back. Due to this, the concentration of waste products such as urea increases enormously as the filtrate passes through the tubules.

Renal tubular cells not only remove substances from the filtrate and return them to the blood, but also excrete additional amounts of unnecessary materials from the blood into the filtrate. This process is called tubular secretion.

When the fluid reaches the end of the distal convoluted tubule and some substances from it have been reabsorbed and others have been added to it, the conversion of the glomerular filtrate into urine is complete.