Urine Formation
Urinary excretion rate = Filtration rate- Reabsorption rate + Secretion rateUrine formation begins when a large amount offluid is filtered from the glomerular capillaries into Bowman’s capsule. Most substances in the plasma, except for proteins, are freely filtered, so that their concentration in the glomerular filtrate in Bowman’s capsule is almost the same as in the plasma.
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.
Filtration Movement of fluid, derived from blood flowing through the glomerulus, across filtration membrane Filtrate: water, small molecules, ions that can pass through membrane Pressure difference forces filtrate across filtration membrane Glomerular filtration rate (GFR): amount of filtrate produced each minute. 180 L/day Average urine production/day: 1-2 L. Most of filtrate must be reabsorbed
Each of the processes filtration,reabsorption, and secretion—is regulated according to the needs of the body. Changes in glomerular filtration and tubular reabsorption usually act in a coordinated manner to produce the necessary changes in renal excretion.
Why Are Large Amounts of Solutes Filtered and Then Reabsorbed by the Kidneys?
1- one advantage of a high GFR is that it allows the kidneys to rapidly remove waste products from the body that depend primarily on glomerular filtration for their excretion. Most waste products are poorly reabsorbed by the tubules and, therefore, depend on a high GFR for effective removal from the body.
2- second advantage of a high GFR is that it allows all the body fluids to be filtered and processed by the kidney many times each day. Because the entire plasma volume is only about 3 liters, whereas the GFR is about 180 L/day, the entire plasma can be filtered and processed about 60 times each day. This high GFR allows the kidneys to precisely and rapidly control the volume and composition of the body fluids.
Steps of urine formation
First Step in Urine Formation---Glomerular FiltrationUrine formation begins with filtration of large amounts of fluid through the glomerular capillaries into Bowman’s capsule.The glomerular filtrate isprotein-free and devoid of cellular elements, including red blood cells.most salts and organic molecules, are similar to the concentrations in the plasma.* Exceptions to this (low-molecular-weight substances) such as calcium and fatty acids, are not freely filtered because they are partially bound to the plasma proteins.
Filtration• Fluid has to flow from capillaries (StarlingForces) into Bowman’s capsule• Filtration is size and charge selective
Proteinuria (or albuminuria) is the hallmark of glomerular injury! The nephron membrane affected by disease, age or by removal of the kidney so less surface area and less filtration. At age of 80 years 40% of these membrane not function & filtration ↓↓by 40% .
Glomerular Capillary Membrane
the endothelium of the capillary, Glomerular basement membrane, and a layer of epithelial cells (podocytes) surrounding the outer surface of the capillary basement membrane..(molecules with a weight over 68,000 are not filtered permeability 100 times more than of the capillaries in skeletal muscle(
The high filtration rate across the glomerular capillary membrane is due its special characteristics. The capillary endothelium is perforated by small holes (fenestrae) (similar to those in the liver). Although the fenestrations are relatively large, with fixed 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 final part is a layer of epithelial cells have long foot like processes (podocytes) that encircle the outer surface of the capillaries and separated by gaps called slit pores through which the glomerular filtrate moves.
The glomerular capillary membrane : *is thicker *more porous *filters fluid at a high rate *selective in determining which molecules will filter, based on their size and electrical charge. *negatively charged large molecules are filtered less easily than positively charged molecules of equal molecular size.
Glomerular Ultrafiltrate Is fluid that enters glomerular capsule, whose filtration was driven by blood pressure . Is protein-free filtrate as blood passes through the glomerular capillaries
In certain kidney diseases, the negative charges on the basement membrane are lost even before there are noticeable changes in kidney histology in (minimal change nephropathy). loss of negative charges on the basement membranes, some of the lower-molecular-weight proteins, especially albumin, are filtered and appearin the urine, a condition known as proteinuria or albuminuria.
The GFR is determined by: (1) the sum of the hydrostatic and colloid osmotic forces across the glomerular membrane, which gives the net filtration pressure, and (2) the glomerular capillary filtration coefficient GFR = Kf x Net filtration pressure
Starling Forces Involved in Filtration:
What forces favor/oppose filtration?The net filtration pressure represents the sum of the hydrostatic and colloid osmotic forces that either favor or oppose filtration across the glomerular capillaries
Under normal conditions, the concentration of protein in the glomerular filtrate is so low that the colloid osmotic pressure of the Bowman’s capsule fluid is considered to be zero.
In the average adult human, the GFR is about 125 ml/min, or180 L/day. The fraction of the renal plasma flow that is filtered (the filtration fraction) averages about 0.2; this means that about 20 % of the plasma flowing through the kidney is filtered through the glomerular capillaries. The filtration fraction is calculated as follows: Filtration fraction = GFR/Renal plasma flow
filtration fraction : Is the fraction of the renal plasma flow that becomes glomerular filtrate. Since the normal plasma flow through both kidneys is 650 ml/min and the normal GFR is 125 ml/min, the average filtration fraction is about 1/5 or 20%.
Forces that influence filtration Force inducing filtration -Glomerular blood hydrostatic pressure Opposing forces: Plasma colloid osmotic pressure Capsular hydrostatic pressure