If we were the primary urine, which is pressed from the glomeruli, completely eliminated, we would "dry up" on the spot. Although the primary urine contains hardly any cells and proteins, but for the entire "rest" of the blood, the blood plasma. This fluid is routed through a complex system of canals, where it is filtered again and is taken back a large part in the blood. In the end, only about one percent of the primary urine is excreted.
The tubules of the Tubulusapparates are several kilometers long
Each nephron empties into the urinary pole Tubulusapparat. Tubulusapparat and are collectively known as renal nephron. The nephron is the functional unit of the kidney itself. The tubules, one should not just introduce as crude pipes. Every man for himself in the cortical labyrinth is a system of tortuous thick and thin sections and loops in their entirety both human kidney is several kilometers.
According to their functions, the individual sections of the Tubulusapparates arranged at different places. The same glomerulus in the subsequent tubule is called proximal tubule. Proximal means "near". It runs in the renal cortex, and is very tortuous. He concludes with a straight part, which is located in the renal medulla and is heavily entwined by blood vessels. At the proximal tubule is followed by the so-called loop of Henle. This part of the tubule is also located in the renal medulla. The loop of Henle is a very thin tubules, which has a descending and an ascending branch. The ascending branch reaches back almost to the glomeruli. There, the Henle's loop in the distal tubule through which again is a bit thicker. Distal means "located further away." The distal tubule in turn, has a short straight portion. Thereafter, the distal tubule also meanders, where the affected area in the Gefäßpols the afferent artery (afferent) of the glomeruli. In this area there is the juxtaglomerular apparatus.
The Tubulusapparat is a filter for the reabsorption of important substances.
The purpose of this system is to prolong the passage of the primary urine by the kidney. Thus, substances that should not be eliminated, are zurückresorbiert in the blood. The individual sections in Tubulusapparat have specialized in certain substances. Other substances that the body no longer needs, or are harmful, remain the primary urine.
The distal tubule empties into a collecting tube, with more and more tubules ending in a collecting tube. The ducts are continuing the urine in the renal pelvis. From then on, the urine is called Sekundärharn. From the renal pelvis from the Sekundärharn passes through the ureters into the bladder. The volume of Sekundärharns is only about one percent of the volume of primary urine. The reabsorption of fluid occurs mainly in the distal tubule and collecting duct. It is controlled by the hormone Adiuretin (ADH). The absence of this hormone or is not present in sufficient quantity, the result of the disease diabetes insipidus, in which individuals excrete up to 20 liters of fluid daily.
In the active substances from the blood filtration are released directly into the tubule.
Dissolved substances in the blood go after filtration in the glomeruli in the primary urine. But this is not the only way that substances from the blood can get into the tubule. Through active filtration area in the renal tubules, substances from the blood stream into the urine. Here's an example: you eat a meal whose main ingredient is a large amount of meat (such as the tasty barbecue plate at the Greek). In the processing and metabolism of meat very much uric acid is produced. Why is this so, you can read here. The uric acid can be excreted must be because the body can not utilize. By filtration in the glomeruli is not solved all the uric acid from the blood. Therefore, also from the blood uric acid also released directly into the tubule.
Active filtration accelerated excretion.
This process is called active or tubular secretion. With the help of the tubular secretion, mainly the rapid excretion is guaranteed by foreign substances. These include drugs such as penicillin and other such substances. The body's own breakdown products such as the above erwährte uric acid, urea, ammonia and other help to speed up excreted.
Carrier systems transport specific groups of substances. Their capacity is limited.
The excretion of drugs is ensured by means of active transporters or carriers. These carriers are not specific to a particular drug. A carrier takes on all the elements of a particular group of chemical substances belonging. Now, if to be excreted in the body of the degradation products of two drugs that belong to the same group, eg acids, they are the same carriers from the blood into the tubule. (Much like a refrigerator car. The transported chilled foods, regardless of whether they are dealing with ice or frozen vegetables or fresh fish.) It is possible that the total amount exceeds the degradation products, the available capacity of the particular carrier. (There must be more chilled food is transported as a refrigerated trucks are available.) Then remain degradation products, which continue to circulate in the blood in the body. For drugs, this can lead to the effect amplified and that an interaction occurs between different drugs. This must then be altered by dose adjustment. Circulates in the above erwährten a constant excess in the blood uric acid, so they can deposit in joints and lead to gout.
Vital substances are reabsorbed.
Many substances that are included in the primary urine are needed by the body. Therefore, they must be brought back into the blood. We say reabsorption. Substances that are reabsorbed include, for example, electrolytes such as sodium, potassium, calcium, chloride and bicarbonate. Also, carbohydrates and amino acids are reabsorbed. The electrolytes are the various ions are passively by a small amount of water "followed", which attaches itself to the ion. Two thirds of the water gets back into the primary urine in the blood.
Passive transport
The mindless followers of the water is characteristic of passive transport processes in the reabsorption. Another passive transport occurs at the Cl-ions. They follow passively over long distances to Na ions, because they are an electrical counterpart.
Active transport
An active transport process is different from passive transport, because the passage of a substance through a membrane only works when a mass transfer takes place. This mass transfer is also referred to as a pumping mechanism. An example is the sodium-hydrogen pump in which a hydrogen ion (hydrogen) that is released into the urine, while a sodium ion (Na) is taken into the cell.
The reabsorption of glucose and amino acids is limited.
When the reabsorption of amino acids and glucose can be reabsorbed only a certain amount. These substances are subject to a threshold. If the amount of dissolved substances in the primary urine that threshold, the excess is excreted in the urine. This happens, for example, diabetes mellitus also. The lack of insulin in diabetics too much glucose circulates in the blood. When exceeding the threshold value for glucose of about 180 dl \ ml glucose is then excreted in the urine
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