Urine Formation

The process of urine formation can be divided into three stages:

 Glomerular Filtration

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 ·Tubular Reabsorption and

 Tubular Secretion

Glomerular filtration begins when whole blood is transported into the glomerulus by the afferent arteriole. The elevated blood pressure inside the glomerulus causes water, salts, wastes, nutrients and other small molecules (called the glomerular filtrate) to pass inside the glomerular capsule. Large molecules and formed elements are unable to pass through the capillary wall and are filtered out as a result.

Filterable blood components include water, nitrogenous wastes, nutrients and salts (ions).

Non-filterable blood components include blood cells and platelets (formed elements) and plasma proteins.

The nephrons in the kidney filter about 47.5 gallons of water a day in addition to glucose, sodium, urea and other small molecules. The concentration of the molecules that are dissolved in the glomerular filtrate is about the same as that found in the plasma of the blood. Your textbook has more information on this.

Tubular reabsorption occurs when needed molecules and ions (such as nutrients and salt) are both passively and actively reabsorbed from the convoluted tubules in the nephron into the blood contained in the peritubular capillary network. Water moves passively from the tubule into the blood (passive transport). Carrier proteins are involved in the reabsorption of selected molecules (active transport). However, there is a maximum rate of transport for all molecules and if these substances are present in excessive amounts in the filtrate, they will “spill over” and be present in the urine. This is the case in diabetes mellitus in which the liver and muscles are unable to store glucose as glycogen so the levels of glucose in the blood are elevated above normal and the excess glucose spills over into the urine. Glucose is never found in the urine of healthy individuals.

The filtrate entering the proximal convoluted tubule is divided into types:

 Reabsorbed Filtrate Components – water, nutrients, and salt and

 Non reabsorbed Filtrate Components – water (a much smaller amount), the majority of the nitrogenous waste and excess salts (ions). These components continue to be passed through the nephron and then finally processed into urine.

This process is greatly enhanced by the microvilli in the epithelial cells that line the proximal convoluted tubule because they greatly increase the surface area available for reabsorption. Those substances that cannot be reabsorbed form the tubular fluid enter the loop of the nephron (loop of Henle).

Tubular Secretion is a second way that additional waste substances and excess ions are removed from the blood and become part of the tubular fluid. Certain molecules such as hydrogen, potassium and ammonium ions, creatinine, and some drugs like penicillin are transported by carrier proteins (active transport) from the blood into the kidney tubule. There are some drugs that should not be given to any one (people or animals) with kidney disease, because the kidneys just can’t process these drugs and remove them from the body. If the drug is necessary, then the dosage will often need to be adjusted in those with malfunctioning kidneys.

Ultimately, at the end of glomerular filtration, tubular reabsorption, and tubular secretion, blood leaving the kidneys has been purified and contains most of the water, nutrients and essential ions that it contained when it first entered the kidneys while the resulting urine contains the wastes and excess substances that have been filtered by the glomerulus and not been reabsorbed as well as substances that have been processed through tubular secretion. Tubular secretion occurs along the entire length of the kidney tubule.

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