Term
| Decribe the gross and microscopic structure of the kidney: |
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Definition
| The kidney is divided into an outer cortex and inner medulla. The medulla is composed of renal pyramids, seperated by renal columns. the renal pyramids empty urine into the calyces that drain into the renal pelvis. From there, urine flows into the ureter and is transported to the bladder to be stored. Each kidney contains more than a million microscopic functional units called nephrons. Nephrons consist of tubular and vascular components. |
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Term
| Trace the flow of blood and filtrate through the kidney: |
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Definition
| Filtration occurs in the glomerulus, which receives blood from the afferent arteriole. Glomerular blood is drained by the efferent arteriole which delivers blood to peritubular capillaries that surround the nephron tubules. The Bowman's capsule produces filtrate, which travels down the proximal convoluted tubule, descending loop of Henle, the ascending loop of Henle and the distal convoluted tubule. The filtrate from the distal convoluted tubule is drained into collecting ducts, which plunge through the medulla to empty into the calyces. |
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Term
| Describe glomerular filtration and the structures involved: |
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Definition
| A filtrate derived from plasma in the glomerulus must pass through the a basement membrane of the glomerular capillaries and through slits in the processes of the podocytes- the cells that compose the inner layer of the Bowman's capsule. The glomerular ultrfiltrate, formed under the force of blood pressure, has a low protein concentration. The glomerular filtration rate (GFR) is the volume of filtrate produced by both kidneys each minute. It ranges from 115-125 ml/min. |
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Term
| Explain the significance of the glomerular filtration rate and how its regulated: |
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Definition
| The GFR can be regulated by constriction and dilation of the afferent arterioles. Sympathetic innervation causes constriction of the afferent arterioles. Intrinsic mechanisms help to autoregulate the rate of renal blood flow and the GFR. Thses mechanisms are needed to ensure that the GFR will be high enough to allow the kidneys to eliminate wastes and regulate blood pressure, but not so high as to cause excessive water loss. |
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Term
| Describe the salt and water reabsorbtion properties of each nephron segment: |
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Definition
| Approximately 65% of the filtered Na+ and H2O is reabsorbed across the proximal convoluted tubules. An additional 20% is reabsorbed in the descending loop of Henle. So 85% of the H2O and Na+ are reabsorbed early in the tubule. Na+ is actively transported, Cl- follows passively by electrical attraction, and H2O follows Na+ out of the proximal tubule. Na+ transport in the proximal tubules is not under hormonal regulation. |
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Term
| Explain the countercurrent multiplier system: |
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Definition
| The reabsorbtion of most of the remaining H2O occurs as a result of the countercurrent multiplier system; Na+ is actively extruded from the ascending limb followed passively by Cl-. because the ascending limb is impermeable to H2O, the remaining filtrate becomes hypotonic. Because of this Na+ transport and because of countercurrent exchange in the vasa recta, the interstitial fluid of the medulla becomes hypertonic. The hypertonicity of the medulla is multiplied by a positive feedback mechanism involving the descending limb, which is passively permeable to H2O and perhaps to Na+. |
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Term
| Explain how ADH acts to promote H2O reabsorbtion: |
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Definition
| ADH is secreted by the posterior pituitary in response to dehydration. It stimulates insertion of aquaporins (water channels) into the plasma membrane of the collecting duct. When ADH is high, H2O is drawn out of the collecting duct by high osmolality of the interstitial fluid. It is then reabsorbed by the vasa recta. |
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Term
| Explain how renal plasma clearance is affected by reabsorbtion and secretion and how it is used to measure GFR and total renal blood flow: |
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Definition
| The active transport of substances from the peritubular capillaries into the tubular fluid = secretion. Secretion is the opposite in direction to that which occurs in reabsorbtion. Reabsorbtion decreases renal clearance; secretion increases renal clearance. Renal clearance refers to the ability of the kidneys to remove molecules from the blood plasma by excreting them in the urine. ex. excretion rate = (filtration rate + secterion rate) - reabsorbtion rate. This fact is used to measure the volume of blood plasma filtered per minute by the kidneys, or the GFR (glomerular filtration rate). Renal blood flow can be measured due to the fact that not all blood delivered to the glomerulus is filtered into Bowman's capsule. 20% is filtered, and the remainder passes into the efferent arteriole and back into the circulation. Since almost all of the PAH in blood going through the kidneys is cleared by filtration and secretion, the PAH clearance is a measurement of total renal blood flow. |
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Term
| Define transport maximum and explain its significance: |
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Definition
Transport maximum- carrier-mediated transport diplays the property of saturation. This means that when the transported molecule (ex. glucose) is present in sufficiently high concentrations, all the carriers become occupied and the tranposrt rate reaches a maximal value. Since the average Tm for glucose is 375 mg/min and normal glucose filtration is 175 mg/min, normall the carriers are not saturated and 100% of the glucose van be reabsorbed. If it wasn't, it would indicate a problem.
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Term
| Define renal plasma threshold, and explain its significance: |
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Definition
| Renal plasma threshold- The minimum plasma concentration of a substance that results in the excretion of that substance in the urine. The renal plasma threshold for glucose is 180-200 mg/100 ml. Glucose concentration after meals usually doesn't exceed 150mg/100ml, so glucose in the urine only occurs when glucose concentration is abnormally high (hyperglycemia). |
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Term
| Explain how the renal secretion and reabsorbtion of Na+ and K+ and H+ is regulated by the renin-angiotensin-aldosterone system: |
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Definition
| Aldosterone stimulates Na+ absorbtion and K+ secretion in the late distal tubule and cortical collecting duct. Aldosterone secretion is stimulated directly by a rise in blood K+ and indirectly by a fall in blood volume. Decreased blood flow and pressure through the kidneys stimulates the secretion of the enzyme renin from the juxtoglomerular apparatus. Renin catalyzes the formation of angiotensin I, which is then converted to angiotensin II. Angiotensin II stimulates the adrenal cortex to secrete aldosterone. Aldosteone stimulates the secretion of H+, as well as K+, into the filtrate in exchange for Na+. |
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Term
| Explain how the kidneys reabsorb bicarbonate, and how the kidneys contribute to the regulation of the acid-base balance: |
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Definition
| The nephrons filter bicarbonate and reabsorb the amount required to maintain the acid-base balance. Reabsorbtion of bicarbonate, however is indirect. Filtered bicarbonate combines with H+ to form carbonic acid in the filtrate. Carbonic anhydrase in the membranes of microvilli in the tubules catalyzes the conversion of carbonic acid to CO2 and H2O. CO2 is reabsorbed and converted in either the tubule cells or the RBCs to carbonic acid, which dissociates to bicarbonate and H+. In addition to reabsorbing bicarbonate, the nephrtons filter and secrete H+, which is excreted in the urine, buffered by ammonium and phosphate buffers. |
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Term
| Explain how the different classes of diuretics act on the nephron: |
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Definition
| Diuretics are used clinically to increase the urine volume and thus to lower the blood volume and pressure. Loop diuretics and thiazides inhibit active Na+ transport in the ascending limb and early portion of the distal tubule, respectively. Osmotic diuretics are extra solutes in the filtrate that increase the osmotic pressure of the filtrate and inhibit the osmotic reabsorbtion of H2O. The K+ sparing diuretics act on the distal tubule to inhibit the reabsorbtion of Na+ and the secretion of K+. |
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Term
| Describe renal insufficiency and uremia: |
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Definition
| When nephrons are destroyed, or there is loss of a kidney or when kidney function is reduced or damaged, renal insufficiency may develop. This may cause hypertension (due to the retention of Na+ and H2O) and uremia, which is high plasma urea concentration. The inability to excrete urea is accompanied by elevated plasma H+ concentrations, and elevated K+ concentration. |
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