Urine Formation results from Glomerular Filtration Tubular Reabsorption Tubular Secretion

Glomerular Filtration As blood flows through the glomerulus protein free plasma filters through the glomerular capillaries into Bowman’s capsule Normally about 20% of the plasma that enters the glomerulus is filtered This process is known as glomerular filtration (the first step in urine formation) 125 ml/min or 180 lit/day of glomerular filtrate is formed

Tubular Reabsorption As the filtrate flows through the tubules substances of value to the body are returned to the peritubular capillary plasma This selective movement of substances from inside the tubule (the tubular lumen) into the blood is called tubular reabsorption

Tubular Secretion Tubular secretion is the selective transfer of substances from the peritubular capillary blood into the tubular lumen

Urine formation begins with fitration of large amounts of protein free fluid from the glomerular capillaries into Bowman's capsule Most substances in the plasma except for proteins are freely filtered As filtered fluid leaves Bowman's capsule and passes through the tubules it is modified by reabsorption of water and specific solutes back into the blood or by secretion of other substances from the peritubular capillaries into the tubules

Glomerular filtration, tubular reabsorption, and tubular secretion is regulated according to the needs of the body

Tubular reabsorption is quantitatively more important than tubular secretion Secretion plays an important role in determining the amounts of potassium and hydrogen ions and a few other substances that are excreted in the urine

The end products of metabolism such as urea, creatinine, uric acid, and urates, are poorly reabsorbed and are excreted in large amounts in the urine Certain foreign substances and drugs are also poorly reabsorbed but in addition are secreted from the blood into the tubules so their excretion rates are high

Electrolytes such as sodium ions, chloride ions, and bicarbonate ions are highly reabsorbed so only small amounts appear in the urine Nutritional substances such as amino acids and glucose are completely reabsorbed from the tubules and do not appear in the urine even though large amounts are filtered by the glomerular capillaries

Glomerular Capillary Membrane It has three major layers: (1)the endothelium of the capillary (2) a basement membrane (3) a layer of epithelial cells (podocytes) Despite three layers filters several hundred times as much water and solutes as the usual capillary membrane Even with this high rate of filtration the glomerular capillary membrane normally prevents filtration of plasma proteins

The high filtration rate across the glomerular capillary membrane is due to Fenestrae in the endothelial cell layer The basement membrane consists of a meshwork of collagen and proteoglycan fibrillae that have large spaces through which large amounts of water and small solutes can filter Layer of epithelial cells (podocytes) line the outer surface of the glomerulus that are not continuous(slit pores) but have long footlike processes that encircle the outer surface of the capillaries

Role of Negative Charges in Preventing Filtration of Plasma Proteins Fenestrations are large but the endothelial cells have negative charges that hinder the passage of plasma proteins The basement membrane effectively prevents filtration of plasma proteins because of strong negative electrical charges associated with the proteoglycans The foot processes are separated by gaps called slit pores through which the glomerular filtrate moves but epithelial cells also have negative charges

Minimal Change Nephropathy The negative charges on the basement membrane are lost As a result of loss of negative charges on the basement membranes some of the low- molecular-weight proteins especially albumin are filtered and appear in the urine ( proteinuria or albuminuria)