Functions of Distal Segments of the Renal Tubule

8/16/2019 Functions of Distal Segments of the Renal Tubule

1/36

RENAL MODULE

SOLA AOUN BAHOUS

Functions of Distal Segmentsof the Renal Tubule


2/36


3/36

Outline

Cell types of the renal tubule

Permeability of tubular segments to water and

solutes Effect of hormones on distal segments

Review of processes of peri-tubular capillary uptakeof solutes


4/36

Outline






5/36

Cell Types of the Renal Tubule

Before the distal convoluted tubule: cells in anysegment are homogeneous and distinct for that

segment


6/36

Cell Types of the Renal Tubule

Beginning in the 2nd half of the DCT, two cell typesare found:

> one type constitutes the majority of cellsin the particular segment: DCT cells, connectingtubule cells… = principal cells

> interspersed cells = intercalated cells


7/36

Outline


Permeability of tubular segments to water

and solutes Effect of hormones on distal segments



8/36

Permeability of Tubular Epithelium

Proximal tubules have leaky epithelia i.e.permeable tight junctions: they have high waterpermeability and low TTP

Distal convoluted tubules and collecting ducts havetight epithelia i.e. less permeable tight junctions:

they have low water permeability and high TTPdistal if asked is considered impermeable ,, bas l collecting is called relative permeability


9/36


Beyond Henle’s loop: fine tuning for mostsubstances. They determine the final amounts

excreted in urine by adjusting their rate ofreabsorption and, in a few cases, secretion. Majorsite for homeostatic control

what determines the final amount that will appear in urine is the function of the collecting


10/36


The segments of the renal tubule fall into 3categories with regard to permeability to water:

* proximal tubule and descending thin

limb have very high permeability to water

* ascending limbs and DCT are relativelyimpermeable to water

* collecting duct system has a regulated water permeability (ADH-dependent)


11/36


DCT and collecting duct system:

second half of the distal

we have overlap btwn segmentsbas iza Nacl only then pure distalwhen Na 2 cl then loop of henlebas iza Nacl w another one like hereso second half of distal


12/36



13/36


Type A intercalated cell – Adapted from www.ikcl.uniklinikum-jena.de/Research-page-10...14

only type of cell with na/k atpase pump on basolateral


14/36


Type-B intercalated cells are present only in the CCD

Type-B intercalated cell is a “flipped-around” type-Aintercalated cell

Involved mainly in bicarbonate secretiontype B secretes bicarbonate


15/36


In Summary:

DCT is almost impermeable to water fluidentering the collecting duct is even morehypoosmotic than that entering the DCT DCT isalso a diluting segment

Water permeability of the collecting duct is subject tophysiological control by the hormone vasopressin or ADH

ma khas the part that absorbs water ,, ma byirja3 bya3mila ..so it will hypososmotic bas never hyperosmotic compared toplasma


16/36


Under low ADH levels, solutes reabsorption butnot water continues to occur in the collecting ductand the final urine is dilute or hypoosmotic (waterdiuresis)

Under high ADH levels, water permeability of the

collecting duct is high

water will move followingosmotic gradients: the cortical collecting ductbehaves like the proximal tubule ya3ne isomosmotic ya3ne removes all effect of

loop of henle btirja3 300 osmolarity ,, itreabsorbs the volume salt and water andmakes concentrated medulla


17/36


Interstitial fluid in the cortex has an osmolarity ~ tothat of plasma when collecting duct is permeable to water, it undoes the dilution carried out by the diluting

segment

In the medullary collecting duct water reabsorption iseven higher (in the presence of ADH) because the

interstitial fluid is hyperosmotic in the medulla thefinal urine is hyperosmotic

antime we have ADH on board will absorb water in cortical collecting duct la seer 300 w bil medulla bitseer 1400 bcz medullaconcentrated w which made it hek is loop of henle w urea recycling there w Cl-


18/36


The tubular response to ADH is not all or none

ADH acts on principal cells via V2 receptors

always have ADH


19/36



20/36


21/36


thats why fluid hypotonic

bas major water permeability is regulated byADH


22/36

Outline






23/36

Effect of Hormones on Distal Segments

Aldosterone:

Hormone produced by the zona glomerulosa of the

adrenal cortex Acts mainly on the principal cells of the collecting

duct for fine-tuning of Na+ excretion: it stimulates Na+

reabsorption

Acts also on other epithelia/ intestinal epithelium,sweat glands, and salivary ducts (Na+ retention)

binds to its receptor mineralocorticoid w mish bas kidney it has receptors in GI

aldostersone will be responsible will be for the 4-5 hasab l table li able ,, so we ask ino kif fine tuning w hine 4-5 bas so bilcoming slides


24/36


Aldosterone is responsible of the regulation of theexcretion of 2% of the filtered Na+

Total filtered Na+/day = GFRxPNa = 180 L/d x 145mmol/L = 26,100 mmol/day

2% of 26,100 mmol/day = 522 mmol Na+/day ~ 30 gNaCl / day: considerably more than the average personeats

plasma conc of sodium

the 2% of filtered load is more than what the person can eat


25/36


Therefore, by reflex variation of the plasmaconcentration of aldosterone between minimal and

maximal, the excretion of Na+

can be adjusted to theintake so that total-body sodium and EC volumeremain constant

GFR is a constant parameter .. what will change is adjust hemeo is absorption


26/36


aldosteroen comes and binds tomineralo receptors then goes tonucleus a w bisir fi proteins makingso by binding of aldo we increasenumber of channels available for rebasortionof sodium

primary hyperaldo: hypertension , hypokalemia


27/36


Aldosterone binds to intracellular receptors: thecomplex MR-A moves to the nucleus and binds tospecific DNA sequences, stimulating the synthesis ofmRNA which mediates translation of specific proteins

The effects of these proteins are to increase the activity

and/or the number of luminal Na+

channels and basolateral Na-K-ATPase pumps


28/36


primary hyperaldo is associated with metabolic alkalosis bcz increase [] of hydrogen ion .. kamen fi hypertension whypokalemia as said before


29/36


Control of aldosterone secretion:

ACTH controls Aldo secretion to some extent but the

secretion of ACTH is not keyed to Na+

homeostasis

The most important stimulator of Aldo secretion is Angiotensin II


30/36


Atrial natriuretic peptide (ANP)

• Secreted by cells of cardiac atria in response todistension

• Acts directly on the collecting duct system to inhibit Nareabsorption (mainly through inhibition of luminal Nachannels)

• Acts directly on granular cells to decrease reninsecretion

fi autoregulation myogenic w tubular feedback w houl occur in seconds after change bas kamenkidneys mish bas maintain internal environment it will sense the systemic changes w react to them wthis will have cross talk bil juxta w renin angiotensin which will take time to act bas kamen at level of

absorption mish level of GFR

distension is an increase in volume so iza patient ija w 3tayne volume kteer so this volume mish bas sensed bil baroreceptorsw bil renal ones,, fi kamen bil atrium of heart and releases ANP ,, lama y2el l renin, w it lowers sodium reabosprion ,, so wehave ANP so they potentiate each other by affecting reabsoprtion

principal cells


31/36


Atrial natriuretic peptide (ANP)

Acts directly on the adrenal cortex: inhibits Ang II-induced aldosterone secretion

Increases GFR by acting directly on arterioles(afferent dilation and efferent constriction) this is not sth that

perpetuates

GFR is like cardiac output tightly regulated so hata law l ANP 3emlet those stuff ha yirja3 l myogenic reflex that senses l GFRbas l effect on tubules remains

ADH is regulated by osmolarity w volume bas it is more sensitive lal osmolarity bas mish more powerful,, l volume is more

powerful ,, l volume iza fi shway change ma ha ya3mol shi ADH so sensitive more lal osmolarity bas iza l volume so huge ADHcares for that 3a 7seb l osmolarity


32/36

Outline




Review of processes of peri-tubular capillaryuptake of solutes


33/36

Peritubular Capillary Uptake


34/36


Fluid uptake by peritubular capillaries:

Major mechanism = net balance of hydrostatic and

oncotic pressures

Diffusion occurs for some substances/ glucose

Same type of dynamics that occur across all capillariesin the body

some substances diffuse w dont need transporters like glucose bas yseer interstituim ma bado pressure di8re birou7 peritubularcapillaries


35/36


Fluid uptake by peritubular capillaries:

Pnet = PInt + PC – Int – PPC

The net filtration pressure across the peritubularcapillaries always favors net movement into thecapillaries

is always toward rebasorption bcz of efferent arteriole which drops hydorstatic pressure

w kamen fi increase in oncotic in glomerulus near efferent arteriole end + efferent will drophydrostatic pressure


36/36


Date post:	06-Jul-2018
Category:	Documents
Upload:	wiam
View:	219 times
Download:	0 times

Functions of Distal Segments of the Renal Tubule

Documents