Function of the renal corpuscles

The functional units of the renal cortex are about one million nephrons, which in turn are composed of the renal corpuscles (Corpusculum renale) and renal tubules (Tubulus renale). The formation of the primary urine takes place in the renal corpuscles. Here the blood flows through a vascular cluster, the glomerulum, which is surrounded by the so-called Bowman capsule.

The vessels of the glomerulum have tiny pores for the filtration of toxic substances. However, these are not mere openings, but a sophisticated filter system. The components of the blood are separated according to size and charge.

Substances up to 100 nm can pass through the pores. In addition, the lining cells of the vessels carry negative charges, whereby molecules of the same polarity are rejected. As a result of these two selection mechanisms, red and white blood cells as well as blood proteins remain in the capillaries.

Other substances, such as water, electrolytes, urea, sugar and small protein molecules continue to enter the renal tubules through the pores. The renal pelvis, pelvis renalis, forms the transition from the renal calices to the ureters, the so-called ureters. It fulfils the function of a collection basin, through which the urine is directed towards the bladder.

Since the renal pelvis and renal calices form a functional unit, it is also referred to as the renal pelvic calyx system. Together with the ureters, the bladder and the urethra, it is assigned to the urinary diversion system. The renal pelvis lies in the middle of the renal medulla. The funnel-shaped extensions towards the medulla form the renal calices, while the opposite constrictions merge into the ureters.The renal pelvis collects the urine produced in the bark and marrow. Muscles contract rhythmically and allow the urine to be transported from the calices to the pelvis and further into the ureters.

Function of the renal tubules

The primary urine from the renal corpuscles flows into the tubule system consisting of the renal tubules, where most of the water is reabsorbed and various substances are released or also absorbed. This is how the actual urine is produced. The tubule system consists of four main sections.

Each of these sections fulfils different transport functions. They are divided into the proximal tubule (main section), the so-called Henle loop, the distal tubule (middle section) and the collection tube. The main section is located together with the renal corpuscles in the renal cortex, while the other sections are mainly found in the renal medulla.

The proximal tubule has a high permeability and thus enables a lively transport between the cells. Sodium ions, sugar molecules, bicarbonate and amino acids are absorbed, i.e. removed from the primary urine and returned to the bloodstream. Furthermore, an absorption or release of uric acid takes place.

In the transitional section, the so-called Henle loop, the urine is increasingly concentrated. It runs in the direction of the renal medulla and then makes a bend in the opposite direction to the renal cortex. The Henle loop is used for water reabsorption.

The distal tubule begins in the renal medulla and runs into the renal cortex before it flows into the collection tube. In the straight part, the pars recta, the urine is further concentrated. Sodium ions are actively transported via the tubule wall.

Water and chloride ions follow passively. In the tortuous pars convoluta, water reabsorption does not occur by means of a transporter, but is hormone-dependent. The steroid hormone aldosterone produced in the adrenal gland is responsible for this. ADH (antidiuretic hormone) is responsible for regulating the water balance in the last section, the collection tube. If necessary, it leads to the installation of small pores, so-called aquaporins, through which the water is reabsorbed.