revew lecture 002

8/2/2019 Revew Lecture 002

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The Urinary SystemChapter 13


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The kidneys maintain thestability and composition ofthe ECF. They contribute to homeostasis. They control electrolyte and water balance of the ECF, plus urinary

output. If the ECF has an excess of water or electrolytes, the kidneys

eliminate the excess. If there is a deficiency of these substances, thekidneys can reduce the loss of these from the body.

Other functions of the kidneys include: maintaining the proper osmolarity of body fluids

maintaining proper plasma volume helping to maintain proper acid-base balance excreting wastes of body metabolism excreting many foreign compounds

producing erythropoietin and renin converting vitamin D to an active form


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The kidneys form the urine.The urinary system eliminates

urine from the body. Each kidney is supplied by a renal

artery and renal vein. The kidney

acts on the blood plasma flowingthrough it.

As urine is formed, it drains into therenal pelvis and is channeled into

the ureter. The urine is stored in the urinary

bladder. It is emptied periodicallythrough the urethra.

The urethra serves the urinary andreproductive tracts in the male.


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The nephron is the functionalunit of the kidney.

It is the smallest unit that can performall of the functions of the kidney.Each kidney has about one millionnephrons.

The cortex is the outer layer of thekidney. The inner layer of the kidney,the medulla, consists of renal

pyramids. The nephrons are arranged throughthe cortex and medulla of the kidney.

Each nephron consists of a vascularcomponent and an tubularcomponent.


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The vascular component isthe dominant portion of the

nephron. The glomerulus is a ball-like tuft of capillaries. Water and

solutes are filtered through the glomerulus as blood passes

through it. From the renal artery, inflowing blood eventually passes through

afferent arterioles. Each afferent arteriole delivers blood to theglomerulus.

The efferent arteriole transports blood from the glomerulus. The efferent arteriole breaks into peritubular capillaries. They

surround the tubular part of the nephron. They are involved withtubular changes between this part of the nephron and the blood.

The peritubular capillaries join into venules which transportblood into the renal vein.


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The tubular part of the nephron is a hollow tube

with different regions. Fluid passes through it. It begins with the Bowmans capsule

which fits around the glomerulus. Filteredfluid passes from the Bowmans capsule

into the proximal tubule. It lies entirely inthe cortex. The next segment is the loop of Henle.

Fluid passes through its descending limband is ascending limb next.

Tubular and vascular cells at this pointform a juxtaglomerular apparatus.

From the descending limb, the nexttubular part is the distal tubule.

The distal tubule empties into thecollecting duct.

The loop of Henle and collecting duct arefound in the medulla of the kidney.

The cortical and juxtamedullary nephrons

(with vasa recta) are distinguished bytheir location (cortex or medulla) andlength.


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The three basic processes of the nephrons areglomerular filtration, tubular reabsorption, andtubular secretion. Glomerular filtration is the first process. A

protein-free plasma is filtered from theglomerulus into the Bowmans capsule.Blood cells are not normally filtered.Normally about 20 % of the plasma isfiltered. Glomerular filtrate is produced atthe rate of 125 ml per minute (180 liters perday).

By tubular reabsorption, filtered substancesmove from the inside of the tubular part ofthe nephron into the blood of the peritubularcapillaries. The reabsorption rates of mostsubstances are very high.

Tubular secretion is a selective process bywhich substances from the peritubularcapillaries enter the lumen of the nephrontubule.

The 80% of the plasma not filtered passesinto the efferent arteriole and through theperitubular capillaries.

Urine excretion results from these three


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Substances filtered from the glomerulus

pass through three layers. They are wall of the glomerular capillaries, the basement membrane,

and the inner layer of the Bowmans capsule. The glomerular membrane is more permeable to water and solutes

compared to other body capillaries. The basement membrane is a gelatinous layer between the glomerulus

and Bowmans capsule. The Bowmans capsule contain podocytes that encircle the glomerulus

tuft. Normally blood cells and plasma proteins are not filtered.


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Glomerular filtration occurs bythe interaction of forces. The glomerular capillary pressure is the result of the blood

pressure pushing on the inside of the capillary wall (e.g., 55 mmHg). As blood pressure increases, this capillary pressure tends to

increase. As the blood flowing through the glomerulus works

against a small-diameter efferent arteriole, this also increasesthis pressure. The plasma-colloid osmotic pressure is due to the retention of

plasma proteins in the blood of the glomerulus.

The concentration of water is higher in the glomerulus, as theproteins are absent there. Water tends to return to the blood inthe glomerulus by osmosis (30 mm Hg).

There is also a hydrostatic pressure tending to move fluid fromthe Bowmans capsule into the glomerulus (15 mm Hg). See Table 14-1


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The net filtration pressure is aninteraction of three pressures. From the previous examples:

The net pressure = glomerular blood pressure - (plasma-colloid osmotic

pressure + Bowmans capsule hydrostatic pressure) 55 - (30 +15) = 10

The net filtration pressure is 10 mm Hg by thisexample.

The glomerular filtration rate (GFR) is the product of: properties of the glomerular membrane times the net

filtration pressure

The properties include the glomerular surface for

penetration and the permeability of the glomerularmembrane.


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Controlled adjustments in the GFR

are mainly due to changes in theglomerular capillary blood pressure. Although changes in the plasma-colloid osmotic pressure and

Bowmans capsule hydrostatic pressure can change the GFR, theseare not controlled adjustments of the GFR. A higher arterial blood pressure supplying the glomerulus, and more

constriction of the efferent arterioles, can increase the GFR. For example, from the previous example, if the glomerular blood

pressure increases from 55 to 60: 60 - (30 + 15) = 15 (not 10 as in the previous example)

Note that the net filtration pressure (and GFR) changes.


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There are two main mechanisms thatcontrol the GFR. By autoregulation the kidneys regulates the GFR

by factors within these organs. They mainly alterthe caliber of the afferent arterioles.

If the GFR rises by increased arterial pressure,the afferent arterioles constrict. This lowers theGFR.

If the GFR decreases, the afferent arteriolesdilate.

The mechanisms for autoregulatory responsesare myogenic (responding to changes in thenephrons vascular component) and atubuloglomerular feedback mechanism. Thisfeedback occurs by sensing changes in flow inthe nephrons tubular component.

These two mechanisms work in unison toautoregulate the GFR. They preventinappropriate changes in the GFR.

Uncontrolled shifts in the GFR can lead to fluidand electrolyte imbalances. Changes outside arange in the arterial pressure cannot becompensated by autoregulation.


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Extrinsic sympathetic controlcan adjust the GFR. They can override the autoregulatory mechanisms.

If arterial blood pressure drops, most arterioles in the bodyconstrict by the baroreceptor reflex. These include the afferentarterioles supplying the glomeruli.

The afferent arterioles constrict by sympathetic innervation.

Less blood flows through the glomeruli, lowering the bloodpressure in these capillaries.

The resultant decrease in the GFR reduces urine volume. They helps to conserve plasma volume, increasing blood

pressure. If blood pressure is elevated, the direction of these responses is

reversed. The responses in the kidneys by the baroreceptor reflex is part

of responses throughout the body for the long-term adjustmentof blood pressure.


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The GFR can be influenced bychanges in the filtration coefficient.

This coefficient is not constant butis subject to physiological control.

The coefficient depends on surfacearea and the permeability of theglomerular membranes. Both canbe modified by contractile activity

within the membrane. The kidneys receive 20 to 25

percent of the cardiac output. The total blood flow through the

kidneys average 1,140 ml perminute. If the cardiac output is 5liters, this figure is 22 % of thecardiac output.

This kidneys need to receive thislarge blood flow to monitor andcontrol the ECF.


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By tubular reabsorption filteredsubstances are transferred from

the tubular lumen to theperitubular capillaries.

This process is highly selective and variable. The return of substances to the blood is needed to

maintain the composition of the ECF. Only excesses

of materials are eliminated. Reabsorption rates are high: 124 of 125 ml of filteredfluid per minute, 99% for water, 100% for glucose,and 99.5% for salt.


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Tubular reabsorption involvestransepithelial transport. The single layer of epithelial cells of the nephron tubule have a

luminal membrane and a basolateral membrane facing the

interstitial spaces between the tubule and peritubular capillaries. By transepithelial transport a reabsorbed substance must cross

the tubule wall, enter the interstitial fluid, and pass through thewall of the peritubular capillaries, entering the blood.

There is passive reabsorption and active reabsorption, requiringthe expenditure of energy.

Sodium reabsorption occurs at a high rate. 67% occurs in theproximal tubule. The reabsorption of glucose, amino acids,

water, chloride ions, and urea are linked to this. The reabsorption of sodium in the loop of Henle plays a role in

the production of varying concentrations and volumes of theurine.

In the distal tubule sodium reabsorption depends on hormonalcontrol.


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Sodium ion reabsorption involvesa Na-K ion ATPase carrier in thetubular cells basolateral

membrane. This pump keeps sodium concentration low in the tubular cells

and high in the lateral spaces outside the tubule.

In the proximal tubule and loop of Henle, a constant amount offiltered sodium is reabsorbed. Other substances are linked to the movement by cotransport. In the distal tubule, the reabsorption of filtered sodium is

variable and subject to hormonal control. More or less isreabsorbed, depending on the needs of the body. Water follows reabsorbed sodium by osmosis. Thus, sodium

reabsorption has a main effect on blood volume and blood

pressure.


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Granular cells of the juxtaglomerularapparatus secrete renin. Thisactivates the angiotensin-aldosterone

system. A drop in blood pressure produces a baroreceptor response with

increased sympathetic activity. This activity secretes more

renin. Renin converts angiotensinogen (plasma protein) intoangiotensin I.

The lungs, by the enzyme ACE, convert angiotensin I into

angiotensin II. Angiotensin II produces more aldosterone. Aldosterone stimulates the reabsorption of sodium in the distal

tubule and collecting duct.

Chloride ions (negative charge) follow sodium (positive charge)passively. Water follows them into the blood (reabsorption) by


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The reabsorption of water through

the renin-angiotensin-aldosteronesystem increases blood pressure.

Angiotensin II also increases bloodpressure as a vasoconstrictor. If blood pressure decreases, this system produces responses

to increase blood pressure. If blood pressure increases, renin secreted is inhibited. This

reduces blood pressure. Abnormal increases in this system can produce hypertension.

Some drugs affect reabsorption. Diuretics produce diuresis(increased urinary output). This process opposes reabsorption.ACE inhibiting drugs can treat certain kinds of hypertension.

ANP is a hormone produced by the heart that inhibits sodium

reabsorption in the distal tubules. See Figure 14 -19


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Glucose and amino acids arereabsorbed by sodium-dependent, secondary active

transport. They are cotransported with sodium. They receive a free

ride at the expense of the energy used to reabsorb sodium

in the proximal tubule. Actively reabsorbed substances have a tubular

maximum, with the exception of sodium. Each carrier molecule for reabsorption transports a specific

molecule. There is a transport maximum when all carriers for a

reabsorbed molecule are occupied. If all sodium carriers are occupied, aldosterone promotes the

synthesis of more carriers for this ion.


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Glucose is actively reabsorbed but is

not reabsorbed by the kidneys. It is filtered freely and usually reabsorbed at

a rate of 100%. The filtered load of a substance is the

quantity of that substance filtered perminute. This load for glucose is 125 mgper minute. Its tubular maximum is 375mg.

Therefore, it is usually reabsorbedcompletely.

When a substances tubular maximum isreached, and it first starts appearing in theurine, its renal threshold is reached. Thisoccurs with glucose when its plasma levelis extremely high, as during diabetesmellitus.

The kidneys do not regulate glucose atsome constant concentration.

Electrolytes such as phosphate andcalcium ions are actively reabsorbed andregulated by the kidneys.


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Active reabsorption of sodium isresponsible for the passive reabsorptionof several substances.

Oppositely-charged chloride ions follow sodium. 80% of water reabsorption is obligatory in the

proximal tubules and loop of Henle. The remaining20% is subject to hormonal control is the distal partsof the tubule.

Reabsorbed water follows sodium and otherreabsorbed solutes by osmosis. The accumulation of

sodium in the lateral spaces produces an osmoticgradient. This builds up a hydrostatic pressure therethat pushes the water into the peritubular capillaries.

The peritubular capillaries have a high plasma-colloidpressure.

The reabsorption of water in the proximal tubuleincreases the concentration of urea in the tubule, aswater is lost from the tubule. This produces aconcentration gradient for urea from the tubule intothe interstitial fluid.

Generally, unwanted waste products are notreabsorbed.


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Tubular secretion is the transfer ofsubstances from the peritubularcapillaries into the tubular lumen.

Hydrogen and potassium ions, along withorganic anions, are tubularly secreted.

Hydrogen ions are secreted and removedfrom the blood to oppose acidosis.

Potassium is almost completelyreabsorbed in the proximal tubule.

Potassium secretion in is controlled byaldosterone in the distal tubule andcollecting duct. It is variable and subjectto regulation.

As the basolateral pump transports sodium from the lateral spaces, it pumpspotassium into the tubular cells (tubular secretion). Entering the lumen surroundedby these cells, it remains in the urinary tract for elimination.

Aldosterone stimulates the tubular cells to secrete potassium if its plasma level iselevated.


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A rise in plasma potassium stimulatesaldosterone from the adrenal cortex. A fallin plasma secretion stimulates aldosteronesecretion by the renin-angiotensin pathway.

The distal parts of the nephron can secrete either potassium orhydrogen ions in exchange for reabsorbed sodium.

Regulating potassium in the ECF is important for membraneexcitability. A rise in potassium ECF can depolarize nerve andmuscle cell membranes.

The secretion of organic anions and cations helps eliminateforeign compounds. For example, some of these anions arebound to plasma proteins and cannot be filtered. Tubularsecretion eliminates them from them from the body.

Foreign compounds eliminated by tubular secretion include foodadditives.


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Plasma clearance is the volumeof plasma cleared of a particularsubstance per minute. It is the volume of plasma from which the substance is removed,

not the amount of the substance removed. Plasma clearance can be calculated. If a substance is filtered but not reabsorbed or secreted, its

plasma clearance rate equals the GFR. Inulin is an idealsubstance, injected into the body, to compute a clearance rate.Creatine is an endogenous substance that is only filtered.

If a substance is filtered and reabsorbed, but not secreted,, itsplasma clearance rate is always less than the GFR. Forglucose, it is zero, as the reabsorption of glucose is 100 percent.

If a substance is filtered and secreted, but not reabsorbed, itsplasma clearance rate is always greater than the GFR.


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Depending on the bodys state ofhydration, the kidneys secreteurine of varying concentrations.

Too much water in the ECFestablishes a hypotonic ECF.

A water deficit establishes a hypertonic

ECF. A large, vertical osmotic gradient is

established in the interstitial fluid of themedulla (from 100 to 1200 mosm/liter

to 1200 mosm/liter). This increasefollows the hairpin loop of Henledeeper and deeper into the medulla.

This osmotic gradient exists betweenthe tubular lumen and the surroundinginterstitial fluid.


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The medullary vertical osmoticgradient is established by

countercurrent multiplication. Comparing the descending and ascending limbs of the loop of Henle: The descending ling is highly permeable to water but does not extrude

sodium for reabsorption.

The ascending limb actively transports NaCl out of the tubular lumeninto the surrounding interstitial fluid. It is impermeable to water.Therefore, water does not follow the salt by osmosis.

There is a countercurrent flow produced by the close proximity of thetwo limbs.

Refer to Fig. 14 - 28. The ascending limb produces an interstitial fluidthat becomes hypertonic to the ascending limb. It does this bypumping out sodium ions. Water does not follow. This interstitial fluidfaces against the flow of fluid (countercurrent) in the descending limb,attracting the water by osmosis for reabsorption. See Figure 14-28


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The vasa recta allows blood to themedulla and enter the renal vein at alevel that is isotonic to the incoming

arterial blood. These vessels loop back through the concentration gradient

in the interstitial fluid in reverse.

The water and solute reabsorption versus excretionare only partially coupled. In tubular segments that are permeable to water, solute

reabsorption is always accompanied by comparable water

reabsorption by osmosis. Solute excretion is always accompanied by comparable

water excretion because of osmosis. A loss or gain of pure water that is not accompanied by a

comparable solute deficit or excess in the body leads tochanges in ECF osmolarity.


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Renal failure has far-reaching

consequences in the body.

These include: infectious organisms retained in the body the accumulation of toxic agents

inappropriate immune responses obstruction of urine flow an insufficient renal blood supply


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Urine is stored in the body. Itis eliminated by micturition. The bladder stores urine.

A large volume of urine in the bladder stimulates stretchreceptors.

These receptors kick off a micturition reflex, leading to thesignaling of the smooth muscle in the bladder wall by

parasympathetic neurons. The contraction of the bladder pushes urine out of the body. Part of the micturition reflex involves the relaxation of the

external urethral sphincter muscle encircling the lumen of the

urethra. This allows the urine to pass through the urethra andleave the body (micturition or urination).

Micturition is under voluntary control, although it cannot bedelayed indefinitely.

Urinary incontinence is the inability to prevent the discharge ofurine.

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