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See more from our free eBook library. The Body Online. Urinary System Structures. Give It Up for the Kidneys. Urine Storage and Elimination. Common Diseases and Disorders. When you select "Subscribe" you will start receiving our email newsletter. Use the links at the bottom of any email to manage the type of emails you receive or to unsubscribe.
See our privacy policy for additional details. Learn Site. The Glomerulus Filters Water and Other Substances from the Bloodstream Each kidney contains over 1 million tiny structures called nephrons. Get our awesome anatomy emails! About News Contact. Before urine enters the renal pelvis, it is further diluted by the removal of ions, including sodium ions, in the distal convoluted tubule and collecting duct. As a result, the osmolarity of urine can be reduced to nearly one fourth of that in the glomerular filtrate or plasma, dropping to as low as 70 mOsm.
If a person does not drink enough water, sweats a lot, or has diarrhea, water conservation is required. In such cases, the kidneys will produce a small amount of concentrated urine to preserve water, while still eliminating wastes and surplus ions.
Antidiuretic hormone ADH , as its name suggests, inhibits diuresis urine production. At maximum ADH secretion, as much as 99 percent of the water in the tubular filtrate is reabsorbed, and the kidneys produce about half a liter of highly concentrated urine per day. The production of concentrated urine is key to our ability to survive for an extended period of time without water.
The osmotic gradient needed to produce concentrated urine depends on two chief factors. First, individual areas of the nephron loop differ in permeability and reabsorption characteristics. Second, fluid flows in opposite directions through adjacent tubes in different parts of the urinary system. This process is called countercurrent flow of fluid. It occurs down and up the descending and ascending limbs of the nephron loop. Blood flowing along the ascending and descending portions of the vasa recta also follows countercurrent flow.
Two countercurrent mechanisms operate in the kidneys: countercurrent multiplication and countercurrent exchange. In countercurrent exchange , countercurrent flow enables the passive exchange of water and solutes between blood in the vasa recta and the medullary interstitial fluid.
In its descending limb, urea and sodium and chloride ions diffuse from the increasingly concentrated medullary interstitial fluid into the blood, while water diffuses from the blood into the interstitial fluid. Along the ascending limb of the vasa recta, the concentration of the interstitial fluid steadily decreases.
At this point, urea and sodium and chloride ions diffuse from the blood back into the interstitial fluid, and water diffuses in the opposite direction. Because the osmolarity of blood leaving the vasa recta is only a little higher than that of blood leaving it, it can provide oxygen and nutrients to the renal medulla without washing out the osmotic gradient.
To summarize, the osmotic gradient in the renal medulla is created by countercurrent multiplication in the nephron loop, and it is maintained by countercurrent exchange in the vasa recta. The nephron loop is responsible for the countercurrent multiplication mechanism that establishes an osmotic gradient within the medullary interstitium. This gradient is necessary for the collecting duct to create an osmotic gradient. Three factors are involved in countercurrent multiplication.
First, recall that the ascending limb is responsible for the active transport of sodium, chloride and potassium out of the tubular fluid and into the interstitium. Because this limb is relatively water impermeable, the solutes move into the interstitial fluid without additional water, increasing the osmotic pressure of the interstitium.
Second, the descending limb, which is in very close proximity to the ascending limb, is solute-impermeable and water-permeable. Because the ascending and descending limb share the same interstitial fluid, water moves along the osmotic gradient from the descending limb into the interstium, concentrating the tubular filtrate in the descending limb.
Thus, the osmolarity of the medullary interstitial fluid along this limb steadily increases, and water leaves the filtrate. The osmolarity of the filtrate is highest 1, mOsm where the nephron loop bends, and the filtrate moves from the descending limb to the ascending limb. The third factor in the countercurrent multiplication is the shift of the fluid along the length of the tubule so that fluid that participated in water loss in the descending limb, will now participate in solute loss in the ascending limb.
When the filtrate in the descending limb turns the corner and enters the ascending limb, the concentrations of sodium and chloride ions are very high in the filtrate The molecules are now actively pumped from the tubule into the interstitial fluid to maintain the high osmotic pressure in the interstitium.
Losing salt but not water makes the filtrate in the ascending limb increasingly more dilute. By the time it reaches the distal tubule, it is at mOsm and therefore has a lower osmotic pressure than the blood plasma and interstitial fluids in the renal cortex. This is what allows for the generation of a dilute urine. Another contributor to the high osmolarity of the the medullary interstitium is the recycling of urea.
Urea from the interstitial fluid diffuses into the filtrate in the thin limbs of the nephron loop. Because the thick limb and collecting duct are urea-impermeable, by the time the filtrate reaches the collecting duct, water reabsorption has created highly concentrated urea that diffuses back into the medullary interstitial fluid. The resulting pool of urea is a major contributor to the high osmolarity in this region.
This urea is then recycled back into the thin limbs of the loop, and the cycle starts again. The presence of ADH amplifies urea recycling, which, in turn, amplifies the osmotic gradient and enables the formation of more concentrated urine. The difference in hydrostatic pressure between the glomerular capsule 10 mm Hg and the renal pelvis practically no pressure at all creates a pressure gradient that forces filtrate to flow from the glomerular capsule through the tubules and into the renal pelvis.
In contrast, no pressure gradient exists to propel the flow of urine through the ureters and into the urinary bladder. Instead, urine flow at this point is controlled by peristaltic contractions in the circular smooth muscle of the ureter walls. These peristaltic waves vary in frequency from once every few seconds to once every two to three minutes.
Their frequency is increased by parasympathetic stimulation and decreased by sympathetic stimulation. Kidney stones are one of the most painful urinary system disorders. Scientists have discovered them in 7,year-old Egyptian mummies. Kidney stones are formed when the uric acid salts, magnesium, or calcium in urine crystallize. Certain types of kidney stones run in families and appear to have a hereditary component.
Some substances in foods may also contribute to the incidence of kidney stones. Most calculi are small enough to pass through the urinary tract and be eliminated with urine. Stones larger than five millimeters in diameter, however, can prevent urine from draining.
The backed-up urine exerts increasing pressure in the kidney, causing extreme pain in the back and side near the kidney or in the lower abdomen. Dehydration can also contribute to the formation of kidney stones. Each year, about 2. Kidney stones are more common in people with a family history of calculi, or who have urine retention or frequent bacterial urinary tract infections. The most commonly used treatment for kidney stones is a procedure called extracorporeal shock wave lithotripsy.
Shock waves created outside the body pass through the skin and body tissues and break down the stones into small particles that can be eliminated in the urine. In severe cases, endoscopic or open surgery may be required to remove the stones. Much more time is spent storing urine than micturating urinating. When urine accumulates in the bladder, distension of the bladder walls activate stretch receptors.
These receptors transmit signals through visceral afferent fibers to the sacral area of the spinal cord. The reflexes also stimulate pudendal motor fibers, which cause the external urethral sphincter to contract, preventing the urine from escaping. As previously noted, micturition also called urination or voiding is the process of emptying urine from the bladder and is the result of involuntary and voluntary muscle contractions. When the amount of urine accumulated in the bladder reaches about milliliters 7 ounces though this amount varies from person to person , afferent impulses are sent to the sacral region of the spinal cord that initiate a reflexive relaxation of the internal urethral sphincter and contraction of the detrussor muscle.
The afferent signals that stimulate the urge to urinate are also sent to the brain. This allows the person to relax their external uretheral sphincter, made of skeletal muscle, so that micturition will occur. If we do not void immediately, the reflexive responses will initially weaken, but as more volume is added to the bladder, these responses will come back more strongly, creating a more urgent need to void. When we are ready to empty the bladder—a decision executed by the cerebral cortex—the micturition reflex is set in motion.
Afferent impulses activate the micturition center of the brain. This signal intregrates with parasympathetic signals of the spinal cord to allow the external sphincter to relax, thus releasing urine from the bladder.
If we need to delay micturiition, the reflex bladder contractions will taper off and stop within about one minute, and urine will continue accumulating.
The addition of another to milliliters 7 to 10 ounces of urine will prompt another micturition reflex. If voiding is still not possible, the reflexes will again subside. When more than to milliliters about 20 ounces of urine accumulates, urination will occur whether we want to or not. After micturition, about 10 milliliters 0. Despite major differences in fluid intake on a given day, the total volume of fluid within the body usually remains the same.
This is due to homeostasis. For example, if a person with healthy kidneys drinks a large volume of fluids, the kidneys will produce a large volume of urine. Conversely, if that same person does not drink enough fluids, the kidneys will produce a small amount of concentrated urine to preserve water.
The main force favoring filtration is the hydrostatic pressure of the glomerular capillary Pgc. The forces that govern filtration in the glomerular capillaries are the same as any capillary bed. What are the two main ways glomerular filtration rate can be adjusted? The two mechanisms that regulate glomerular filtration rate operate in two main ways. One, by adjusting blood flow into and out of the glomerulus and two by altering the glomerular capillary surfacearea available for filtration.
An increase in the efferent arteriolar diameter decrease in resistance causes a decrease in the glomerular capillary hydrostatic pressure and a decrease in GFR. A reduction in renal arterial pressure or renal blood flow will have the opposite effect 1. Both glomerular capillary hydrostatic pressure and renal blood flow are important determinants of the glomerular filtration rate GFR. Avoid processed foods and choose fresh fruits and vegetables instead.
Salt should be limited especially if you have high blood pressure, protein in your urine, or swelling or difficulty breathing.
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