Reabsorption: Function, Tasks, Role & Diseases

In reabsorption, a substance that has already been excreted is reabsorbed into the body. This form of absorption primarily involves the tubular system of the kidneys. Disorders of reabsorption can manifest, for example, in cystinuria.

What is reabsorption?

In reabsorption, a substance that has already been excreted is reabsorbed into the body. This form of absorption primarily involves the tubular system of the kidneys. Reabsorption is a natural body process. It involves the absorption of substances by biological systems. In humans, absorption refers primarily to the uptake of substances from the food pulp, as it occurs in the digestive tract and especially in the intestines. As a rule, this absorption refers to cleavage products of food, such as carbohydrates, proteins, vitamins and minerals. However, water, drugs and even toxins can also be absorbed. In the human body, absorption takes place primarily via the epithelia in the small intestine. However, resorption processes can also involve the kidneys. Kidneys and the liver are considered the most important detoxification organs in humans. The kidneys filter toxins from the blood and process these substances into urine. Medicine distinguishes primary urine from secondary urine. It is in the tubular system of the kidneys that the actual urine that we excrete is formed. In this system, processes of resorption take place. This type of reabsorption is also called reabsorption or reabsorption. During reabsorption, substances are reabsorbed that have actually already been filtered out for excretion. Substances already excreted from certain organs are reabsorbed by cells during reabsorption. In the case of the kidneys, for example, the tubular system carries water and electrolytes from the urine back into the organism, giving rise to the actual urine.

Function and task

Together with the renal corpuscles, the tubules form the smallest unit of kidney tissue: the so-called nephrons. All renal tubules are interconnected to form the tubular system of the kidney. Filtration of blood takes place in the glomeruli of the kidneys and corresponds to the formation of primary urine. However, the primary urine still contains substances that the body can actually use, so the primary urine is filtered again. Therefore, people do not excrete the primary urine during micturition, but the so-called secondary urine. This secondary urine is produced by reabsorption processes in the tubular system of the kidneys. During these processes, mainly water, glucose and electrolytes are removed from the primary urine. In this way, reabsorption transports vital substances back into the blood. Glucose, for example, is actively reabsorbed into the blood. In the main body of each renal tubule, reabsorption of large amounts of sodium bicarbonate, glucose, and amino acids occurs, prompted by the symporters and antiporters. These are so-called carrier proteins, which correspond to transmembrane transport proteins and can thus transport substrates across a biomembrane. The transport processes of the proteins are substance-specific and are based on a conformational change of the molecules. Antiporters for substance transport are located in the cell membrane of the renal tubules and transport two different substances each in opposite directions. One of the substances is thus taken up into the cell, while the other substance reaches the extracellular space. The membrane-bound symporters in turn carry out transport of different substances in the same direction. These carrier proteins are found in all reabsorbing epithelia. In the main section of the renal tubules, besides the reabsorption of the mentioned substances, there is also a reabsorption or a secretion of substances such as uric acid, which is realized by anion transporters and help of the proximal tubule cells. In the other sections of the tubules, urine is concentrated by the countercurrent principle. The secondary urine is finally transferred to the bladder, where it is collected until the next micturition.

Diseases and ailments

Some diseases are associated with disorders of renal reabsorption. For example, one such disorder is cystinuria. This is an autosomal recessive hereditary and tubular-renal transport disorder that particularly affects the dibasic amino acids arginine, ornithine, lysine, and cystine. The most clinically relevant complication of the disease is the early formation of kidney stones from cystine.The prevalence of the disease is given as one affected person in 2000 to 7000 people. In the disease, the reabsorption of dibasic amino acids in the proximal tubules of the kidneys is disturbed, so that the concentration of the substances in the urine increases significantly. Because cystine is poorly soluble in water, it crystallizes in the acidic environment of the urine and manifests itself as nephrolithiasis (kidney stones). Affected individuals can thus suffer from renal cholism in early childhood. Renal tubular acidosis is also based on a reabsorption disorder. In the type II subtype, the impaired reabsorption relates, for example, to hydrogen carbonate (formerly known as bicarbonate) and is associated with a deficiency of carbon hydrases. The reabsorption defect involves the proximal tubule for bicarbonate and results in chronic metabolic acidosis. Most clinically relevant is symptomatic potassium and sodium loss. Volume depletion and the activating effect on the renin-angiotensin-aldosterone system are also crucial symptoms. Increased reabsorption of sodium occurs, so that potassium losses continue to increase. In children, this reabsorption disorder can already cause significant growth disturbances or rachitic changes. In adults, the disease leads to secondary diseases such as osteoporosis. The third subtype of renal tubular acidosis differs from type II in that it is based on decreased sodium reabsorption in the distal tubule. Renal-tubular acidosis in this disorder is due to a primary defect such as aldosterone resistance.