Tubular reabsorption
The urine that is formed represent the sum of three basic renal processes Urinary excretion = Glomerular filtration - Tubular reabsorption+ Tubular secretionTubular Reabsorption
Reabsorption is selective process The rate at which each of these substances is filtered is calculated as: Filtration= Glomerular filtration rate X Plasma concentration (the substance is freely filtered and not bound to plasma proteins) e.g: if plasma glucose concentration is 1 g/L, the amount of glucose filtered each day is about 180 L/day x 1 g/L, or 180 g/day. Q. ( Filtration of glucose 180 g/day, excretion of glucose normally zero ,so The reabsorption is ------------?*glomerular filtration (relatively nonselective) and tubular reabsorption( highly selective) they are quantitatively very large. *many of the ions in the plasma, such as sodium, chloride and bicarbonate, are highly reabsorbed. *rates of reabsorption and urinary excretion are variable, depending on the needs of the body. *waste products, such as urea and creatinine are poorly reabsorbed from the tubules and they are excreted.
Tubular Reabsorption: Overview Tubular reabsorption: occurs as filtrate flows through the lumens of proximal tubule, loop of Henle, distal tubule, and collecting ducts Ways of transport Diffusion Facilitated diffusion Active transport Cotransport Osmosis Substances transported to interstitial fluid and reabsorbed into peritubular capillaries: inorganic salts, organic molecules, 99% of filtrate volume. These substances return to general circulation through venous system
Reabsorption of Salt & H20 Return of filtered molecules is called reabsorption Water is never transported Other molecules are transported & water follows by osmosis
Routes of Water and Solute Reabsorption
A good example of a primary active transport system is the reabsorption of sodium ions across the proximal tubular membrane
Filtrate in PCT is isosmotic to blood (300 mOsm/L) Thus reabsorption of H20 by osmosis cannot occur without active transport (AT) Is achieved by AT of Na+ out of filtrate Loss of + charges causes Cl- to passively follow Na+ Water follows salt by osmosis
Nonreabsorbed Substances Substances are not reabsorbed if they: -Lack carriers -Are not lipid soluble -Are too large to pass through membrane pores Urea, creatinine, and uric acid are the most important non reabsorbed substances
Transport Maximum
Transport maximum For most substances that are actively reabsorbed or secreted, there is a limit to the rate at which the solute can be transported. when the tubular load exceeds the capacity of the carrier proteins involved in the transport process this substance will appears in urine.Glucose Reabsorption: Glucose is *Freely filtered at glomerulus *Normally 100% actively reabsorbed in proximal tubule *Normally, no glucose appears in urine
Plasma [glucose] = 100 mg/dLFiltered load glucose = 125 mg/minTransport maximum for glucose reabsorption = 375 mg/minTheoretical renal threshold = 300 mg/dL(GFR x Renal Threshold = Transport Maximum)Actual renal threshold = 160–180 mg/dLFiltered load = 225 mg/min
In the adult human, the transport maximum for glucose averages about 375 mg/min, whereas the filtered load of glucose is only about 125 mg/min (GFR x plasma glucose = 125 ml/min x 1 mg/ml). With large increases in GFR and/or plasma glucose concentration that increase the filtered load of glucose above 375 mg/min, the excess glucose filtered is not reabsorbed and passes into the urine.
when the plasma glucose concentration is 100 mg/100 mL and the filtered load is at its normal level, 125 mg/min, there is no loss of glucose in the urine when the plasma concentration of glucose rises above about 200 mg/100 ml, increasing the filtered load to about 250 mg/min, a small amount of glucose begins to appear in the urine.
This is termed the threshold for glucose. Note the appearance of glucose in the urine (at the threshold) occurs before the transport maximum is reached. WHY?
The difference between threshold and transport maximum is that not all nephrons have the same transport maximum for glucose, and some of the nephrons excrete glucose before others have reached their transport maximum. The overall transport maximum for the kidneys, which is normally about 375 mg/min, is reached when all nephrons have reached their maximal capacity to reabsorb glucose.
*The plasma glucose of a healthy person almost never becomes high enough to cause excretion of glucose in the urine, even after eating a meal. *In uncontrolled diabetes mellitus, plasma glucose may rise to high levels, causing the filtered load of glucose to exceed the transport maximum and resulting in urinary glucose excretion. Note : actively transported solutes often exhibit a transport maximum.
Substances that are passively reabsorbed do not demonstrate a transport maximum)WHY?) Because their rate of transport is determined by the electrochemical gradient for diffusion of the substance across the membrane, the permeability of the membrane for the substance, the time that the fluid containing the substance remains within the tubule.
When solutes are transported out of the tubule by primary or secondary active transport, their concentrations decrease inside the tubule & increasing in the renal interstitium. This creates a concentration difference that causes osmosis of water in the same direction that the solutes are transported, from the tubular lumen to the renal interstitium.
As water moves across the tight junctions by osmosis, it can also carry with it some of the solutes, a process referred to as solvent drag.
--In the proximal tubule, the water permeability is always high.--In the ascending loop of Henle, water permeability is always low, so that almost no water is reabsorbed.--Water permeability in the distal tubules, collecting tubules, and collecting ducts—can be high or low, depending on the presence or absence of ADH.
Reabsorption of chloride, urea by passive diffusion
Active reabsorption of sodium is closely coupled to the passive reabsorption of chloride. Chloride ions can also be reabsorbed by secondary active transport Urea is also passively reabsorbed from the tubule,but to a much lesser extent than chloride ions.(urea does not permeate the tubule as readily as water. In some parts of the nephron, especially the inner medullary collecting duct, passive urea reabsorption is facilitated by specific urea transporters.--about one half of the urea that is filtered by the glomerular capillaries is reabsorbed from the tubules. --The remainder of the urea passes into the urine, allowing the kidneys to excrete large amounts of this waste product of metabolism.
Creatinine, is large molecule than urea and is essentially impermeant to the tubular membrane mean all the creatinine filtered by the glomerulus is excreted in the urine.