Glomerular filtration Filtrate is a plasma-derived fluid that is processed by the renal tubules to form urine. Urine is the end product of excretory activity on the filtrate, and contains mostly metabolic wastes and unneeded substances.
Net filtration pressure is responsible forfiltrate formation, and is the product of theforces acting at the glomerular bed.Glomerular hydrostatic pressure is theprimary force pushing water and solutes outof the blood across the filtration membrane.
Colloid osmotic pressure and capsularhydrostatic pressure are forces that drivefluids back into glomerular capillaries.Glomerular filtration rate is the total amountof filtrate formed per minute by the kidneys.
Renal autoregulation is a process by whichthe kidney can adjust its own resistance toblood flow, by maintaining a nearlyconstant glomerular filtration rate.The myogenic mechanism reflects thegeneral tendency of vascular smooth muscleto contract when it is stretched.
The renin-angiotensin mechanism istriggered when the juxtaglomerular cellsrelease renin in response to various stimuli.Angiotensin converting enzyme (ACE)catalyzes the reaction that convertsangiotensin I to angiotensin II.
Tubular reabsorption Tubular reabsorption is a reclamation process that prevents significant loss of plasma volume during the filtration process.
Passive tubular reabsorption is themovement of substances along theirelectrochemical gradient without the use ofATP.Because water follows salt in the osmoticprocess, a sodium-linked water flow isestablished, called obligatory waterreabsorption.
Solvent drag explains the passivereabsorption of lipid-soluble substances inthe filtrate, as a result of solutes followingthe movement of solvent.Secondary active transport is due to thegradient created by the Na+/K+ pump, whichcauses the reabsorption of glucose, aminoacids, lactate vitamins and most cations.
The transport maximum reflects the numberof carriers in the renal tubules available tocarry each substance.Urea, creatinine, and uric acid are importantsubstances not reabsorbed completely bythe nephron.
Tubule Segment Substance Reabsorbed MechansimProximal convuluted Sodium ions Active transport via sodium/potassium carrier; sets uptubule electrochemical gradient of passive solute diffusion, Aldosterone promotes Na+ retention Nutrients (glucose, amino Active transport; cotransport with sodium acids, vitamins) Cations Active transport; cotransport with sodium Anions Passive transport; paracellular diffusion by electrochemical gradient Water Osmosis Urea Passive diffusion via solvent drag Small proteins Pinocytosed by tubule cells, digested to amino acidsLoop of Henle Descending loop Water Osmosis Ascending loop Sodium, Chloride, Potassium Active transport
Tubule Segment Substance Reabsorbed MechansimDistal Convoluted Sodium Active transport; requires aldosteronetubule Calcium PTH mediated active transport Chloride Diffusion along electrochemical gradient; cotransport with sodium Water Osmosis; depends on ADH to increase porosity of tubule epitheliumCollecting Duct Sodium, Hydrogen, Potassium, Aldosterone mediated active transport of sodium; Bicarbonate, Chloride cotransport of hydrogen, potassium, chloride, bicarbonate Water Osmosis; depends on ADH to increase porosity of tubule epithelium Pinocytosed by tubule cells, digested to amino acids Urea Diffusion along electrochemical gradient; most remains in medullary insterstitial space
Tubular secretion Tubular secretion is another method, like failure of tubule cells to reabsorb filtered solutes, of clearing plasma of unwanted substances.
Regulation of urineconcentration and volume Osmolality is the number of solute particles dissolved in one liter (1000 grams) of water in a solution. An osmol is equivalent of 1 mole of a non- ionizing substance in l liter of water. A milliosmol is equal to 0.001 osmol, and is used to describe the solute concentration of body fluids.
The countercurrent mechanism allows thekidneys to keep the solute load of bodyfluids constant at about 300 mosm (= toblood plasma), by directing fluids inopposite directions through adjacentchannels.
Phases of the countercurrentmultiplier1) The descending limb of the loop of Henle is relatively impermeable to solutes and freely permeable to water.2) The ascending limb of the loop of Henle is impermeable to water and actively transports sodium chloride into the surrounding interstitial fluid.
3) The collecting ducts in the deep medullary regions are permeable to urea.4) The vasa recta as a countercurrent exchanger, maintaining the osmotic gradient while delivering the nutrient blood supply to cells in the area.
The countercurrent exchanger protects themedullary gradient set up by the loop ofHenle by preventing rapid removal of saltfrom the medullary interstitial space.
Formation of concentrated urine Antidiuretic hormone inhibits urine output (diuresis), by increasing the number of water-filled channels in the principal cells of the collecting ducts.
Water reabsorption that depends on thepresence of ADH is called facultative waterreabsorption.Diuretics are chemicals that enhanceurinary output.
Renal clearance Renal clearance is the volume of plasma from which a particular substance is completely cleared by the kidneys in a given time, usually 1 minute.
Characteristics and compositionof urine Urochrome is a pigment that results from the destruction of hemoglobin, and is responsible for the color of urine. Urine, because it is water plus solutes, has a higher specific gravity, or weighs more than water. Nitrogenous wastes include urea, uric acid, and creatinine.