Tue. Sep 17th, 2019

Tubular Secretion

3 min read
Tubular secretion is the transfer of materials from peri-tubular capillaries to the renal tubular lumen which is reversal of reabsorption. This step is usually used to remove drugs, toxins and poisons, or other natural compounds in excessive amounts (such as potassium (K+), hydrogen (H+), and urea).
Urine Formation (Tubular Secretion)

Kidneys form urine, which passes to bladder for storage prior to excretion.

Steps involved in the process of urine formation:

  1. Glomerular filtration or ultra-filtration
  2. Selective Reabsorption
  3. Tubular Secretion

Tubular secretion

Tubular secretion is the transfer of materials from peri-tubular capillaries to the renal tubular lumen which is reversal of reabsorption. This secretion is caused mainly by active transport and passive diffusion. Usually only a few substances are secreted, and are typically waste products. Noxious substances are actively transferred from the peri-tubular capillaries into the interstitial fluid, then into the tubular lumen.

For many substances, reabsorption plays a much more important role than secretion in determining final urinary excretion rate. However, secretion accounts for significant amounts of potassium ions, hydrogen ions, and a few other substances that appears in urine. This step is usually used to remove drugs, toxins and poisons, or other natural compounds in excessive amounts (such as potassium (K+), hydrogen (H+), and urea).

Tubular Secretion in Urine Formation

 

Excess K+ ion is secreted into tubules and in exchange Na+ ion is reabsorbed otherwise it causes a clinical condition called Hyperkalemia. Tubular secretion of hydrogen ions (H+)is very important in maintaining normal blood pH. Substances such as, e.g. drugs including penicillin and aspirin, may not be entirely filtered out of blood because of the short time it remains in glomerulus. Such substances are cleared by secretion from peritubular capillaries into the filtrate within the convoluted tubules. The tubular filtrate is finally known as urine. Human urine is usually hypertonic.

Some substances are removed from blood through peri-tubular capillary network into distal convoluted tubule or collecting duct.These substances are Hydrogen ions, creatinine, and drugs.Tubular secretion takes place from blood in the peri-tubular capillaries to the filtrate in renal tubules and can ensure that wastes such as creatinine or excess H+ or excess K+ ions are actively secreted into filtrate to be excreted.

Filtration occurs as blood flows through glomerulus. Substances not required and foreign materials, e.g. drugs including penicillin and aspirin, may not be entirely filtered out of the blood because of the short time it remains in the glomerulus. Such substances are cleared by secretion from the peritubular capillaries into the filtrate within the convoluted tubules. Tubular secretion of hydrogen ions (H+) is important in maintaining normal blood pH.

Filtration occurs as blood flows through glomerulus. Substances not required and foreign materials, e.g. drugs including penicillin and aspirin, may not be entirely filtered out of the blood because of the short time it remains in the glomerulus. Such substances are cleared by secretion from the peritubular capillaries into the filtrate within the convoluted tubules. Tubular secretion of hydrogen ions (H+) is important in maintaining normal blood pH.

Tubular secretion and absorption

Urine is mainly composed of water that has not been reabsorbed, which is the way in which body lowers blood volume, by increasing amount of water that becomes urine instead of becoming reabsorbed. The other main component of urine is urea, a highly soluble molecule composed of ammonia and carbon dioxide, and provides a way for nitrogen (found in ammonia) to be removed from the body.

Urine also contains many salts and other waste components. Red blood cells and sugar are not normally found in urine but their presence may indicate glomerulus injury and diabetes mellitus respectively.

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