Together with the renal corpuscle, the renal tubule forms the nephron, making it the structurally smallest element of the kidney. Together, the individual renal tubules form the tubule system, which is responsible for the reabsorption of substances such as water and the excretion of other substances. Inflammation in the tubule tissue can result in renal failure in individual cases.
What is the renal tubule?
The tissue of the human kidneys is composed of tubular structural elements. These structural elements are also known as renal tubules, renal tubules, or renal tubules. The renal tubule is a part of the nephron. This is the smallest structural element of the kidney, which contains renal corpuscles in addition to the renal tubule. The renal corpuscles of the nephrons continuously filter primary urine from the blood. Certain substances from this are reabsorbed into the tubules. The final urine is thus formed in the renal tubule. Together, the renal tubules form the tubular system of the kidney. This system absorbs various substances and especially water into the blood and releases the rest into the urine. This selection of substances is possible for the tubular system mainly due to the capillary network that surrounds it. The substances are selected on the net primarily on the basis of size. Selection also occurs with the aid of the tight junctions that connect the cells of the tubule.
Anatomy and structure
Depending on its location relative to the glomerulus, three sections of the renal tubule are distinguished. The proximal tubule is also called the proximal tubule and consists of a pars convoluta and a pars recta. The intermediate tubule is called tubulus attenuatus in technical language. It contains a descending pars descendens and an ascending pars ascendens. The distal tubule is called the distal tubule and, similar to the proximal portion, is composed of a pars recta and a pars convoluta. Thus, like the proximal, the distal tubule consists of a bulged portion, the pars convoluta, and a straight portion, the pars recta. Together with the straight portions of proximal and distal tubule, the entire intermediate tubule is functionally called loop of Henle, which forms hyperosmotic urine. The so-called connecting tubule and collecting tubules have embryologically evolved differently from the renal tubules and for this reason are not included in the nephron. Nevertheless, together with the tubule system they form a functional unit of the nephron. The tubules of the renal tubule consist of a cubic resorption epithelium. The connections of the cells are permeable tight junctions.
Function and Tasks
The function and task of each renal tubule is the reabsorption and secretion of electrolytes, carbohydrates, low molecular weight proteins, and water. Thus, the individual renal tubules are involved, for example, in the regulation of the body’s water balance. They also excrete urinary substances such as urea and creatinine from the body. The same applies to toxic substances such as drugs. The renal tubules are equally involved in the regulation of the dissolved electrolyte content in the blood. These include potassium, sodium, calcium, phosphate, magnesium and bicarbonate. The tubules take care of the reabsorption of certain substances. Reabsorption is an organic process that causes substances that have actually been excreted to be reabsorbed by living cells and tissues. In the case of the renal tubules, the substances reabsorbed are primarily water. Thus, about 99 percent of the water from the urine is reabsorbed into the blood. The capillary network that surrounds the tubule system is particularly relevant for the reabsorption of substances. The capillary network consists of a set of capillaries and forms a fine network over the tissue that intercepts or allows substances to pass through in a size-selective manner. Transcellular and paracellular reabsorption occurs primarily in the proximal tubule. Besides water, predominantly glucose, amino acids, sodium cations and carbon dioxide are reabsorbed. Paracellularly, reabsorption involves predominantly chloride anions and Ca2+ ions, which migrate detourlessly into cells via the leaky tight junctions of the system. Secretion in the proximal tubule is limited to H3O+ ions and hydrogen carbonate ions. Energy for passive mass transfer of H2O, H3O+, and hydrogen carbonate or CO2 is provided to the renal tubules by the concentration gradient maintained by high carbonic anhydrase activity.
Diseases
Proximal tubule cells in particular are relevant to various kidney diseases and dysfunctions. One example is glomerular proteinuria. Chronic graft nephropathy can also be used as an example. When proximal tubule cells are damaged or severely irritated, signaling cascades are created by second messengers. These cascades can stimulate protein production by the complement system. Chemokines or cytokines and extracellular matrix components reach the proximal renal tubule. These locally produced messengers may damage the tissue of the tubule by attracting leukocytes. Macrophages, T cells, and granulocytes may trigger inflammation in the tissues. This inflammation can impair renal function and eventually even cause renal failure. When treating inflammation that develops in this way, targeted immunosuppression in the proximal tubule cells can cause the inflammation to recede, usually preventing the consequence of insufficiency. Disorders of the renal tubules can also be genetically determined in individual cases. For example, mutations in the LRP2 gene lead to a loss of function of certain receptors. The gene codes in the DNA for the membrane protein megalin, so that the mutation results in at least a restriction of function for the receptor. The result can be proteinuria. Although Donnai-Barrow syndrome is extremely rare, it may be favored by the described mutation in individual cases.