January 21, 2011

Urine formation in Human Body


Urine Formation
Electrolytes are excreted in urine and their proper concentration in the blood is vital in maintaining the proper osmotic pressure (potential for osmosis to occur) and salt balance, which is discussed later in the chapter.
Steps in Urine Formation
Urine formation requires the following steps:
Glomerular filtration occurs at the glomerular capsule.
During glomerular filtration, water, nutrient mole­cules, and waste molecules move from the blood in the glomerulus to the fluid (filtrate) inside the glomerular capsule. The blood has been filtered be­cause blood cells and large molecules, like most proteins, remain within the blood, while small

Urine Formation 
Electrolytes are excreted in urine and their proper concentration in the blood is vital in maintaining the proper osmotic pressure (potential for osmosis to occur) and salt balance.
Steps in Urine Formation
Urine formation requires the following steps:

  • Glomerular filtration occurs at the glomerular capsule. During glomerular filtration, water, nutrient mole­cules, and waste molecules move from the blood in the glomerulus to the fluid (filtrate) inside the glomerular capsule. The blood has been filtered be­cause blood cells and large molecules, like most proteins, remain within the blood, while small molecules, such as glucose and urea, leave the blood to enter the tubule. 
  • Tubular reabsorption occurs primarily at the proximal convoluted tubule. During tubular reabsorption, nu­trient and salt (NaCl) molecules are actively reab­sorbed from the proximal convoluted tubule into the peritubular capillary, and water follows passively. 
  • Tubular secretion occurs primarily at the distal convo­luted tubule. During tubular secretion, large waste molecules, such as creatinine, are actively secreted into the distal convoluted tubule. This step in urine formation is minor in comparison to the first two steps. 

Concentrated Urine 
Humans excrete a urine that contains only waste molecules dissolved in a minimum amount of water. This concen­trated urine results because water is reabsorbed not only at the proximal convoluted tubule but also along the entire length of the nephron, particularly at the loop of the nephron and the collecting duct.
Urine has a light yellow or amber color due to the pig­ment urochrome, a breakdown product of hemoglobin, that is formed in the liver. At least 95% of urine volume is water; the remaining 5% is composed of organic nitroge­nous wastes and excess electrolytes. The chief nitrogenous waste in humans is urea, formed in the liver as a part of amino acid metabolism. Uric acid occurs when nucleotides are broken down in cells. If uric acid is present in excess, it will precipitate out of the plasma. Uric acid crystals sometimes collect in the joints, producing a painful ailment called gout. Creatinine is an end product of muscle metabolism. Ordinarily, glucose and albumin are absent from urine.
Wastes nutrients and water are all filtered into a nephron. but nutrients and water are reabsorbed so that humans excrete a concentrated solution of wastes.
Special Features of the Nephron 
The cells along the length of the nephron are specialized to carry on their respective functions.
The juxtaglomerular apparatus occurs at a region of con­tact between the afferent arteriole and the distal convo­luted tubule. Cells in this region are involved in regulating sodium (Na+) reabsorption from the distal convoluted tubules and collecting ducts and in maintaining blood vol­ume, as is discussed later in the chapter.
The inner layer of the glomerular capsule is made up of podocytes that have long cytoplasmic processes. The podocytes cling to the capillary walls of the glomerulus and leave pores through which filtration can take place.
The cells lining the proximal convoluted tubule have numerous microvilli, about 1 micron in length, that in­crease the surface area for reabsorption. In addition, the cells contain numerous mitochondria, which produce the energy necessary for active transport. Glucose is an exam­ple of a molecule that ordinarily is reabsorbed completely because the supply of carrier molecules for it is plentiful. However, every substance has a maximum rate of trans­port, and after all its carriers are in use, any excess in the fil­trate will appear in the urine. For example, if the blood glucose concentration is higher than normal, as in patients with diabetes mellitus, more glucose molecules than nor­mal will move into the glomerular capsule during filtra­tion. Not all of this glucose will be reabsorbed, and glucose will appear in the urine.
The loop of the nephron, which is lined with squa­mous epithelium, is made up of a descending and ascend­ing limb. The ascending limb extrudes sodium so that the tissues of the medulla become hypertonic to the fluid in the descending limb and the collecting duct. These features allow the loop of the nephron to perform its function of concentrating the urine.
The cells of the distal convoluted tubule also have nu­merous mitochondria, but they lack microvilli. This is con­sistent with their role in actively moving molecules from the blood into the tubule.
Each region of the nephron is anatomically suited to its task in urine formation.

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