Nephrons are the smallest morphologic and functional units of the kidney. They consist of a renal corpuscle and the renal tubules attached to it. Blood is filtered in the nephrons, ultimately producing urine.
What is a nephron?
A nephron is a functional unit of the kidney. Each kidney contains about one million of these anatomical subunits. Each nephron consists of a renal corpuscle, also called a Malphigi corpuscle, and a renal tubule. This renal tubule is also called the tubule. It is directly connected to the renal corpuscle. The renal corpuscle in turn consists of a glomerulum and a Bowman’s capsule. The latter encloses the glomerulum.
Anatomy and structure
The glomerulum is an arterial vascular tangle approximately 0.2mm in size. The glomeruli are located in the renal cortex and are supplied with blood through branches of the renal artery. The small vascular loops have a fenestrated endothelium, which means that they are lined from the inside with a thin fenestrated cell layer. The glomeruli are surrounded by the so-called Bowman’s capsule. This consists of two sheets. The outer leaf encloses the entire renal corpuscle. The inner leaf envelops the fenestrated endothelium of the glomeruli from the outside. There are also windows in the leaf of Bowman’s capsule. This is important so that water and smaller blood components can escape through these windows, allowing urine to be filtered out. However, the windows are so small that no red blood cells or proteins can fit through them in healthy glomeruli. Thus, these components remain in the vessels and in the systemic circulation. At the so-called urinary pole, the outer leaflet of Bowman’s capsule merges into the tubule apparatus, or renal tubule. The tubular apparatus begins with the proximal tubule. Like the glomeruli, this is still located in the cortical region of the kidney. It is strongly tortuous, especially in its initial part. This part is followed by a straight section that descends into the renal medulla. Thereafter, the duct narrows and forms an arch. This transition section is called the loop of Henle. It is followed by a wider and ascending portion of the tubule, which again pulls up near the glomerulus. This part of the renal tubule is called the distal tubule.
Function and Tasks
The main function of the nephrons is to prepare urine. In order to perform the filtering function, the kidneys are very well supplied with blood. About 1700 liters of blood flow through the kidneys per day. After initial filtration by the glomeruli, about 170 liters of primary urine are produced. After further recovery processes, a quantity of 1.7 liters of final urine remains. This is then excreted via the draining urinary tract. Urine formation begins in the glomerulum. Here, an initial filtrate is squeezed out of the flowing blood through the endothelial windows. Water and small molecules such as electrolytes can pass through this so-called blood-urine barrier. Larger molecules such as proteins remain in the vascular system. This produces a protein-free ultrafiltrate, the primary urine. This primary urine now enters the tubule apparatus of the nephrons. In the tubule system, reabsorption takes place for the most part. Water, salts or glucose are reabsorbed from the primary urine into the vessels. Conversely, however, water, salts and especially urinary substances can still be secreted from the surrounding vessels into the renal tubules. Which substances and how much water ultimately enter the draining urinary tract is regulated by various systems in the body. The final filtered secondary urine then reaches the renal pelvises via the collecting tubes, which connect directly to the tubular apparatus. Finally, urinary excretion occurs via the draining urinary tract.
Diseases
When the nephrons of the kidney, or more specifically the glomeruli, are inflamed, the condition is called glomerulonephritis. Glomerulonephritis is an abacterial inflammation of the renal cortex. Abacterial means that the disease is not caused by bacteria. Acute glomerulonephritis is usually caused by an immunological reaction. In this case, the disease usually occurs about two weeks after an acute infection with ß-hemolytic group A streptococci. The body has formed antibodies against these bacteria during the infection. These bind to their adversaries, the antigens. This is how antigen-antibody complexes (immune complexes) are formed.These attach themselves to the wall of the glomeruli and cause inflammation there. The disease is therefore only indirectly caused by bacteria. At the beginning of the inflammation of the glomeruli, no more bacteria are found in the blood. Typical infections that can cause glomerulonephritis are tonsillitis, inflammation of the sinuses or ears. Certain skin diseases such as erysipelas can also be the cause of glomerulonephritis. The disease is manifested by symptoms such as blood in the urine, high blood pressure, pressure in the kidney area or edema of the eyelids. Acute glomerulonephritis can also develop into a chronic form. If left untreated, chronic glomerulonephritis leads to renal insufficiency and even renal failure. Nephrotic syndrome is a complex of symptoms that can occur as a complication of all diseases of the glomeruli. The loss of proteins and red blood cells occurs due to improper filtering efficiency. It is also referred to as protein-losing kidney. Nephrotic syndrome is characterized by proteinuria (proteins in the urine), edema, and hyperlipoproteinemia. In hyperlipoproteinemia, there is an increased amount of fat-protein compounds, called lipoproteins, in the blood. In addition to glomerulonephritis, diabetic glomerulosclerosis, intoxications, infections, plasmacytoma, or collagenosis may also cause nephrotic syndrome.