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Abnormalities of Micturition

Atonic Bladder Caused by Destruction of SensoryNerve Fibers. Micturition reflex contraction cannotoccur if the sensory nerve fibers from the bladder to

the spinal cord are destroyed. When this happens, aperson loses bladder control, despite intact efferentfibers from the cord to the bladder and despiteintact neurogenic connections within the brain.Instead of emptying periodically, the bladder fills to

capacity and overflows a few drops at a timethrough the urethra. This is calledoverflowincontinence. A common cause of atonic bladder iscrush injury to the sacral region of the spinal cord

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Abnormalities of Micturition

Automatic Bladder Caused by Spinal CordDamage Above the Sacral Region. If the spinalcord is damaged above the sacral region but thesacral cord segments are still intact, typicalmicturition reflexes can still occur. However, theyareno longer controlled by the brain.

If the bladder is emptied periodically bycatheterization to prevent bladder injury caused

by overstretching of the bladder, the excitabilityof the micturition reflex gradually increases untiltypical micturition reflexes return; then, periodicbladder emptying occurs

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Reabsorption and Secretion by the

Renal Tubules

Urine that is formed and all the substances inthe urine represent the sum of three basicrenal processes-glomerular filtration, tubularreabsorption, and tubular secretion-asfollows:

Urinary excretion = glomerular filtration

tubular reabsorption + tubular secretion

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Filtration, Reabsorption, and Excretion Rates of Different

Substances by the Kidneys

AmountFiltered

AmountReabsorbed

AmountExcreted

% of FilteredLoadReabsorbed

Glucose (g/day) 180 180 0 100

Bicarbonate

(mEq/day)

4,320 4,318 2 >99.9

Sodium (mEq/day) 25,560 25,410 150 99.4

Chloride(mEq/day)

19,440 19,260 180 99.1

Potassium

(mEq/day)

756 664 92 87.8

Urea (g/day) 46.8 23.4 23.4 50

Creatinine (g/day) 1.8 0 1.8 0

tubular reabsorption is highly selective

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For a substance to be reabsorbed, it must first be transported (1)

across the tubular epithelial membranes into the renal interstitial

fluid and then (2) through the peritubular capillary membrane back

into the blood

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Active Transport

Can move a solute against an electrochemical gradient

Transport that is coupled directly to an energy source,primaryactive transport.

Sodium-potassium ATPase pump that functions throughout most

parts of the renal tubule. Transport that is coupled indirectlyto an energy source, such as

that due to an ion gradient, is referred to as secondary activetransport.

Reabsorption of glucose by the renal tubule is an example ofsecondary active transport. Solutes can be reabsorbed by active

and/or passive mechanisms by the tubule Water is always reabsorbed by a passive physical mechanism

called osmosis, which means water diffusion from a region ofhigh water concentration to one of low water concentration

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Solutes Can Be Transported Through Epithelial

Cells or Between Cells

Solutes can be reabsorbed or secreted acrossthe cells by way of the transcellular pathwayor between the cells by moving across the

tight junctions and intercellular spaces byway of theparacellular pathway.

Water is also reabsorbed across theparacellular pathway, and substancesdissolved in the water, especially potassium,magnesium, and chloride ions, are carriedwith the reabsorbed fluid between the cells

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Basic mechanism for active transport of sodium through the tubular epithelialcell. The sodium-potassium pump transports sodium from the interior of the cellacross the basolateral membrane. The low intracellular sodium concentrationand the negative electrical potential cause sodium ions to diffuse from thetubular lumen into the cell through the brush border.

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1. Sodium diffuses across the luminal membraneinto the cell down an electrochemical gradient

established by the sodium-potassium ATPasepump on the basolateral side of the membrane.

2. Sodium is transported across the basolateralmembrane against an electrochemical gradientby the sodium-potassium ATPase pump.

3. Sodium, water, and other substances arereabsorbed from the interstitial fluid into the

peritubular capillaries by ultrafiltration, apassive process driven by the hydrostatic andcolloid osmotic pressure gradients.

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Two or more substances interact a carrierprotein and are transported together acrossthe membrane

Secondary active transport does not requireenergy directly from ATP. Rather, the directsource of the energy is that liberated by the

simultaneous facilitated diffusion of anothertransported substance down its ownelectrochemical gradient

Secondary active transport

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Mechanisms of secondary active transport

co-transportof glucose and aminoacids along with sodium ions throughthe tubular epithelial cells, followed byfacilitated diffusion through thebasolateral membranes.

counter-transportof hydrogen ionsfrom the interior of the cell and into thetubular lumen; movement of sodiumions into the cell, down anelectrochemical gradient establishedby the sodium-potassium pump on the

basolateral membrane, provides theenergy for transport of the hydrogenions from inside the cell into thetubular lumen.

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Transport Maximum for Substances That Are

Actively Reabsorbed

For most substances that are activelyreabsorbed or secreted, there is a limit to therate at which the solute can be transported,often referred to as the transport maximum

This limit is due to saturation of the specifictransport systems involved when the amount

of solute delivered to the tubule exceeds thecapacity of the carrier proteins that involvedin the transport process.

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In the adult human, the transport maximum

for glucose averages about 375 mg/min The filtered load of glucose is only about 125

mg/min (GFR plasma glucose = 125 ml/min

1 mg/ml). With large increases in GFR and/or plasma

glucose concentration that increase thefiltered load of glucose above 375 mg/min,

the excess glucose filtered is not reabsorbedand passes into the urine

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Substances that are passively reabsorbed donot demonstrate a transport maximum

Transport of this type is referred to asgradient-time transportbecause the rate oftransport depends on the electrochemical

gradient and the time that the substance is inthe tubule, which in turn depends on thetubular flow rate.

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Some actively transported substances (e.g. sodiumreabsorption in the proximal tubule ) also havecharacteristics of gradient-time transport

the maximum transport capacity of the basolateralsodium-potassium ATPase pump is usually fargreater than the actual rate of net sodiumreabsorption.

This means that the greater the concentration ofsodium in the proximal tubules, the greater itsreabsorption rate. Also, the slower the flow rate oftubular fluid, the greater the percentage of sodiumthat can be reabsorbed from the proximal tubules