Absolute urinary excretion can be distinguished from FE. Absolute excretion refers to the actual excreted volume of a substance in a hour collection of urine, whereas the FE of sodium FE Na is the percentage of sodium that is filtered by the kidney and excreted in the urine. FE Na is measured in terms of plasma and urine sodium , rather than by urinary sodium concentration alone, because absolute urinary sodium concentrations can vary with water reabsorption.
The following example of the care of a critically ill patient demonstrates the use of FE Na : A sick, elderly patient has had no urinary output for 24 hours. Is the patient experiencing acute renal failure or does he have a postrenal problem or a low blood volume due to dehydration a prerenal problem? The physiologic response to a decrease in renal perfusion is an increase in sodium reabsorption to control hyponatremia, which is often caused by volume depletion or a decrease in effective circulating volume e.
Excess sodium is lost due to tubular damage or through the damaged glomeruli leading to hypovolemia and the normal response of sodium wasting. The value is lower in early disease, but with kidney damage from the obstruction, the value becomes higher. Clinicians do not routinely measure C Cr because C Cr measurement requires a timed urine collection, and the test is considered inconvenient. However, plasma creatinine concentration is typically used as a surrogate marker of C Cr and, therefore, of GFR.
Note that as GFR decreases, a greater proportion of Cr is excreted by secretion rather than by filtration. Elderly patients normally have decreased C Cr. In patients receiving nephrotoxic drugs e. Compared to other organs of the body, the kidney is very heavily supplied with blood.
Each minute, about 1. Because filtration occurs in the glomeruli, which are in the renal cortex, most of the blood that enters the kidneys only reaches the cortex. Determining RBF is useful in many diseases. The RBF can be autonomously controlled by the kidneys. This is called renal autoregulation.
RBF is maintained at a mean arterial pressure of 80— mm Hg, which is influenced by an increase in renal flow resistance. GFR also remains within this range. Table : Paracrine mechanisms that control GFR. Two principles can be attributed to autoregulation: myogenic response or Bayliss effect and tubuloglomerular feedback.
Myogenic response is defined as the vasoconstriction of the interlobular arteries with increasing pressure. Thus, increase in blood pressure does not typically affect the afferent arterioles of the glomerulus. The macula densa is the part of the kidney that is responsible for tubuloglomerular feedback, and it lies between the distal tubule and the glomerulus.
The tubular system which joins the glomerulus consists of the proximal tubule, the descending limb, the loop of Henle, the ascending limb, and the distal tubule. This is followed by the collecting duct which transitions into the papillary duct and ends in the renal papilla.
Molecules important for body functioning are reabsorbed along these structures, while harmful substances are secreted for elimination as quickly as possible. Each section of the tubular system is equipped with specific transport proteins and channels that ensure the regulation of substances that are absorbed and secreted.
Generally, tubular cells possess a luminal side which is toward the lumen and a basolateral side which communicates with the bloodstream. Image : Tubular reabsorption. The proximal tubule has a strongly defined brush border that creates an extensive area that is ideal for the absorption of water and salt.
Through the transport of a positive charge into the cells, a lumen-negative transepithelial potential is created. Due to osmosis, substance transport causes water resorption through the relatively leaky epithelium. The water, in turn, drags dissolved molecules with it, a mechanism known as solvent drag.
In the 2nd stage, a lot of chloride ions Cl — are resorbed after a quarter of the length of the proximal tubule. Because of the negative charge of Cl — , a lumen-positive transepithelial potential arises, resulting in paracellular resorption of cations. Thus, sodium is repeatedly removed from the equilibrium of the cell. Electrical and chemical gradients build up, which is crucial for the regulated transport. It should be noted that aquaporins small water channels are built into both the luminal and basolateral sides of tubular cells for water transport.
Image : Reabsorption and secretion in the PCT. When studying the loop of Henle, it is important to note that while water is absorbed in the descending part of the loop, NaCl is not. In the ascending section, the exact opposite occurs. In this part, water absorption does not take place, but there is resorption of NaCl and other ions. This is a crucial mechanism of urine concentration and an important task of the loop of Henle.
These carriers are targets for loop diuretics, such as furosemide. The basolateral side contains specific Cl — transporters, which can only be found in the kidney and the inner ear, and which contain a functional subunit, called Barttin. Image : Countercurrent multiplier system. In the distal tubule, NaCl is resorbed, and this process takes place via a NaCl cotransporter that is sensitive to thiazide and aldosterone. While thiazides can inhibit NaCl cotransporter activity, NaCl cotransporter is stimulated by aldosterone.
Tubular creatinine and inulin increase in concentration but not amount along the proximal tubule due to water reabsorption. Its relative concentration increases before it plateaus. CO 2 and H 2 O are reabsorbed into the proximal renal tubular cells. The primary function of the thick ascending limb of the distal tubule is to reabsorb water without reabsorbing sodium, which increases the osmolarity in the medulla via a countercurrent mechanism. ENaC is primarily involved in the reabsorption of sodium ions in the collecting ducts of the nephrons.
Amiloride, atrial natriuretic peptide, and prostaglandins can inhibit ENaC. Inflowing sodium from the lumen causes a lumen-negative potential. In the collecting duct and papillary duct, the resorption of water is controlled by ADH. This hormone is released in settings of water deficiency and it causes the incorporation of aquaporins in the luminal membrane. The resorption of water in the collecting duct is mostly independent of sodium absorption. Several substances such as organic cations and anions, including bile salts and uric acid, are secreted in the tubules.
The secreted substances enter the interstitial space from the peritubular capillaries and are transported by organic transporters in the basolateral membrane into the cytoplasm of tubular epithelial cells. The secreted substances are transported out of cells and into the tubular lumen by luminal membrane transporters. Most diuretics have to gain access to the tubular lumen in order to exert their effects.
Diuretics are mostly protein-bound and are, thus, not freely filtered through the glomerulus. Dunea , Chicago. Clearance studies are widely used to assess glomerular filtration rate and renal blood flow and to study the excretion of various substances by the kidney. The renal clearance of a substance represents the virtual or theoretical volume of plasma which is completely cleared of this substance in a given unit of time. Urea Clearance Since retention of urea in the blood is accompanied by decreased urinary excretion, students of renal disease in the early 's had attempted to evaluate renal function by relating the concentration of urea in urine.
Dunea G, Freedman P. Renal Clearance Studies. Coronavirus Resource Center. Our website uses cookies to enhance your experience. By continuing to use our site, or clicking "Continue," you are agreeing to our Cookie Policy Continue. Key Terms nephron : The basic structural and functional unit of the kidney that filters the blood in order to regulate chemical concentrations and thereby produce urine. Renal clearance : The rate at which a substance is removed from the plasma via renal system activity over a unit of time.
Clearance In renal physiology, clearance is a measurement of the renal excretion ability, which measures the amount of plasma from which a substance is removed from the body over an interval of time.
Clearance Mechanisms Renal clearance depends mainly on GFR, tubular absorption, and tubular secretion. These variables alter clearance through the following rules: Increased GFR will increase clearance, while decreased GFR will decrease clearance.
Increased tubular secretion will increase clearance, while decreased tubular secretion will decrease clearance. This variable is sometimes altered through changes in the expression of ATPase pumps involved in active transport. Increased tubular reabsorption will decrease clearance, while increased tubular reabsorption will increase clearance. The other types of clearance are Biliary through bile. Pulmonary clearance removed during alveolar gas exchange.
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