k-6 tubular reabsorption and secretion gus-k6.ppt

8/12/2019 K-6 Tubular reabsorption and secretion GUS-K6.ppt

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Early Filtrate Processing


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Gambaran seluler dari tubulusrenalis

Tubulus proximal: simple cuboidal cells(brush border cells ok terdapat microvilli)

Thin loop of henle: simple squamous cell,highly permeable to water not to solute

Thick ascending loop of henle & earlydistal tubule: cuboidal cells, highlypermeable to solutes, particularly NaCl butnot to water


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Late distal tubule and cortical collectingduct: cuboidal cells has two distinctfunction:

1. principal cells; permeability to water

and solutes are regulated by hormonesand,

2. intercalated cells; secretion of hydrogen

ion for acid/base balancing

Medullary collecting duct; principal cells;hormonally regulated permeability to

water and urea


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Tubular Reabsorption

By passive diffusion

By primary active transport: Sodium

By secondary active transport: Sugars andAmino Acids

Endositosis ; small proteins and peptide

hormones


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Reabsorption Pathways

There are two reabsorption pathways:

1. the transcellular pathway (>>)

2. the paracellular pathway


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Reabsorpsi Filtrat


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Trancellular pathway : Through luminal

and basolateral membranes of the tubularcells into the interstitial space and theninto the peritubular capillaries.

Paracellular pathway : through the tightjunctions into the lateral intercellularspace.

Water and certain ions use bothpathways, especially in the proximalconvoluted tubule.


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Diffusion of Water

Water diffuses from the lumen through thetight junctions into the interstitial space:

1. Water will move from its higher

concentration in the tubule through thetight junctions to its lower concentration inthe interstitium.

2. Water will also move through theplasma membranes of the cells that arepermeable to water

Air dapat berdifusi di seluruh bagian tubulus

kecuali di ascending limb loop of Henle


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Sodium Reabsorption

Keluar dari sel ke interstiital

Lumen

Plasma

Cells

PUMP: Na/K ATPase

Sodium

Potassium

Chloride

Water


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Tubular Secretion

Protons (acid/base balance)

Potassium

Organic ions Zat-zat lain yg tidak normal ada dalam

darah spt obat-obatan dan bahan-bahan

toksik


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Transport Maximum (Tm)

For most actively reabsorbed solutes, theamount reabsorbed in the PCT is limited only bythe number of available transport carriers forthat specific substance.

This limit is called the transport maximum, or Tm.

If the volume of a specific solute in the filtrateexceeds the transport maximum, the excess

solute continues to pass unreabsorbed throughthe tubules and is excreted in the urine.


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Reabsorption: Receptors can Limit

Figure 19-15: Glucose handling by the nephron


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The final processing of filtrate in thelate distal convoluted tubule andcollecting ducts comes under directphysiological control in response tochanging physiological conditions and

hormone levels. Membrane permeabilities and cellular

activities are altered in response to the

body's need to retain or excrete specificsubstances.


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Distal Tubule & Collecting Duct

The Late Distal Tubule & CCT arecomposed of principal cells & intercalatedcells

Intercalated cells secrete hydrogen ionsinto filtrate

Principals cells perform hormonally

regulated water & sodium reabsorption &potassium secretion


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Role of Aldosteron

Principal cells are more permeable tosodium ions and water in the presence of

Aldosterone & ADH

Low level of Aldosterone result in littlebasolateral sodium/potassium ATPase ionpump activity & few luminal sodium &

potassium channel


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Aldosteron increases the number of

basolateral Na/K pump and luminal Na& K channels

Since there are no basolateral K

channel, K ion are secreted into theinstead of returning to the interstitium

Without an increase in waterpermeability, the interstitial osmolarity

increases


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Juxtaglomerular apparatus

As the thick ascending loop of henletransition into early distal tubule, the

tubule runs adjacent to the afferent andefferent arteriole.

Where these structure are contact they

form the monitoring structure called thejuxtaglomerular apparatus (JGA), which iscomposed macula densa and JG cells


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Figure 19-9: The juxtaglomerular apparatus


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TUBULOGLOMERULAR FEEDBACK &GLOMERULOTUBULAR BALANCE

Signals from the renal tubule in each nephronfeedback to affect filtration in its glomerulus. As therate of flow through the ascending limb of the loopof Henle and first part of the distal tubule increases,glomerular filtration in the same nephron decreases,and, conversely, a decrease in flow increases theGFR

This process, which is calledtubuloglomerularfeedback, tends to maintain the constancy ofthe load delivered to the distal tubule.

The sensor for this response is the macula densa.


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Presumably, adenosine is secreted fromthe basal membrane of the cells. It acts

via adenosine A1 receptors on the maculadensa cells to increase their release ofCa2+ to the vascular smooth muscle in the

afferent arterioles. This causes afferent vasoconstriction and

a resultant decrease in GFR.

Presumably, a similar mechanismgenerates a signal that decreases reninsecretion by the adjacent juxtaglomerular

cells in the afferent arteriole but this


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The change in Na+ reabsorption occurs withinseconds after a change in filtration, so it seemsunlikely that an extrarenal humoral factor isinvolved.

One factor is the oncotic pressure in theperitubular capillaries.

When the GFR is high, there is a relatively largeincrease in the oncotic pressure of the plasmaleaving the glomeruli via the efferent arterioles andhence in their capillary branches.

This increases the reabsorption of Na+from thetubule. However, other as yet unidentifiedintrarenal mechanisms are also involved.


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Sympathetic control

In extreme stress or blood loss,sympathetic stimulation overrides the

autoregulation

Increased sympathetic discharge causeintense constriction of renal blood vessel

Blood is shunted to other vital organs GFR reduction causes minimal fluid loss

from blood


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Reduction filtration can not go indefinitely,a waste product build up & metabolicimbalances increase in blood

IV fluid increases blood volume restoresblood pressure to resting levels reduced

sympathetic stimulation allows for normal

arteriole diameterGFR & filtrate flow isnormalized


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Sympathetic Regulation of GFR

Insert fig. 17.11

k-6 tubular reabsorption and secretion gus-k6.ppt

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