excretion i

8/2/2019 Excretion I

1/27


2/27

Renal

vein

Inferior

vena cava

Urinary

bladder

Urethra

Renal

artery

Kidney

Aorta

Ureter

Renal

pyramid

Renal

cortex

Renal

medulla

Renal

pelvis

Ureter


3/27

Figure 26.4a, b

Structure of the Kidney


4/27

Nephron

Afferent arteriole

Efferent arteriole


5/27

Blood Supply to Kidney


6/27

Two Capillary system

Glomerular capillary system - high

pressure, filtration system

Peritubular capillaries - low pressure,

absorptive system


7/27

The nephron consists of a renal

corpuscle and renal tubule

The renal corpuscle is composed of

Bowmans capsule and the glomerulus

The renal tubule consists of

Proximal convoluted tubule (PCT)

Loop of Henle: Thin descending limb, Thin ascending limband Thick ascending limb

Distal convoluted tubule (DCT)


8/27

Cortical nephrons

~85% of all nephrons

Located in the cortex Juxtamedullary nephrons

Closer to renal medulla

Loops of Henle extenddeep into renal pyramids

Two types of nephron


9/27

Urine Formation


10/27

Figure 14.8 (1)

Page 518

Afferent arteriole Efferent arteriole

Glomerulus

Bowmans

capsule

Lumen of

Bowmans

capsule

Outer layer of

Bowmans capsule

Inner layer

of Bowmans capsule

(podocytes)

Proximal convoluted tubule

Lumen of

glomerular

capillary

Endothelial

cell

Basement

membrane

Podocyte

foot process

(see

nextslide)


11/27

Podocyte

foot process

Filtrationslit

Basement

membrane

Capillarypore

(see next slide)3 Layers of Glomerular Capillary Membrane

Endothelial layer of capillaries

Basement membrane

Capsular layer with podocytes

Glomerular ultrafiltration Membrane


12/27

Podocyte

foot process

Filtration

slit

Basement

membrane

Capillary

pore

Endothelial

cell

Lumen of glomerular

capillary

Lumen of

Bowmans capsule


13/27

Mesangial Cells

Intra-mesangial cells liebetween capillary tuft and

provide support for

glomeruli. They secrete a

substance similar to basement

mebrane.

Extra mesnagial cells have

contractile properties in

response to neurohormonal

substance which regulateblood flow in glomerulus.

They are also phagocytic in

nature.


14/27

Glomerular filtration Movement of fluid through the glomerular capillaries is determined

by capillary pressure (60 mm Hg), colloidal osmotic pressure, andcapillary permeability.

125 ml of filtrate is formed each minute - (GFR) which can vary

from a few milliliters per minute to as high as 200 ml/minute.

Constriction of the efferent arteriole increases resistance to outflowfrom the glomeruli and increases the glomerular pressure and the

GFR. Constriction of the afferent arteriole causes a reduction in the

renal blood flow, glomerular filtration pressure, and GFR.

Both, afferent and the efferent arterioles are innervated by thesympathetic nervous system and are sensitive to vasoactive

hormones, such as angiotensin II.

Strong sympathetic stimulation, such as shock, constriction of the

afferent arteriole causes a marked decrease in renal blood flow and

thus glomerular filtration pressure & urine output can be zero.


15/27

Facilitated diffusion and Passive transport

Primary active transport

Secondary active transport

Cotransport (Symporter)

Countertransport (Antiporter)

Reabsorption in the kidneys

occurs by different mechanisms

R b i i PCT


16/27

Reabsorption in PCT

65% of reabsorption and

secretion occurs in PCT.

Glucose amino acids, lactateand water soluble vitamins,

ions such Na+, Cl-, K+,

HCO3- completely

reabsorbed.

As these solutes move into the

tubular cells, their

concentration in the tubular

lumen decreases, providing a

concentration gradient for theosmotic reabsorption of water

and urea.

PCT secretes H+ and organic

compounds such as penicillin,

aspirin, morphine.


17/27

Reabsorption of Bicarbonate & Na+ &

Secretion of H + Ions

Na+ antiporters reabsorb Na+

and secrete H+

PCT cells produce the H+ &

release bicarbonate ion to theperitubular capillaries

important buffering system

For every H+ secreted into the

tubular fluid, one filtered

bicarbonate eventually returns

to the blood


18/27

Reabsorption in the PCT Na+ symporters help reabsorb

materials from the tubular filtrateand each type of symporter has an

upper limit on how fast it can

work, called the transport

maximum (Tm).

The maximum amount of

substance that these transport

systems can reabsorb per unit

time is called the transport

maximum.

Tm related elated to the number

of carrier proteins that are

available for transport.

Reabsorption of Nutrients


19/27

Tm determines renal threshold for reabsorption of substances in

tubular fluid


20/27

Symporters in the Loop of Henle

Thin descending limb is

highly permeable to waterand moderately permeable to

urea, sodium, and other ions

Thick ascending limb is

impermeable to water & hasNa+ K- Cl- symporters that

reabsorb these ions.

About 20% to 25% of the

filtered load of sodium,potassium, and chloride is

reabsorbed in loop of Henle.


21/27

Reabsorption in the DCT & Collecting Duct

DCT is relatively impermeable to water

but removal of Na+ and Cl- (5%)

continues in the DCT by means of Na+

Cl- symporters

Ca++ actively reabsorbed under the

influence of parathyroid hormone and

vitamin D. ADH exerts its action on DCT.

Late part of DCT and collecting duct are

the sites for aldosterone action .

Two types of cells:

- principal cells reabsorb Na+ and secrete

K+ under the influence of aldosterone

- intercalated cells reabsorb HCO3- ions in

exchange for H+


22/27

Formation of Concentrated Urine

Urine can be up to 4 times greater osmolarity than plasma It is possible for principal cells & ADH to remove water from urine

to that extent, if interstitial fluid surrounding the loop of Henle has

high osmolarity

Long loop juxtamedullary nephrons and Na+/K+/Cl- makethat possible

Two factors contribute to building and maintaining the osmotic

gradient:

Difference in solute & water reabsorption in differentsections of the tubule

Countercurrent flow

Urea recycling causes a buildup of urea in the renal medulla

F i f C U i ADH


23/27

Formation of Con. Urine: ADH

Increases water permeability ofprincipal cells so regulatesfacultative water reabsorption

Stimulates the insertion of

aquaporin-2 channels into themembrane

water molecules move morerapidly

When osmolarity of plasma &interstitial fluid increases, moreADH is secreted and facultativewater reabsorption increases.


24/27

Tubular

lumen

filtrate Distal tubular cell

Peritubular

capillary

plasma

Water

channel

Increases permeability of

luminal membrane to H2O

by inserting newwater channels


25/27

Countercurrent Mechanism Descending limb is very permeable to water

higher osmolarity of interstitial fluid outside thedescending limb causes water to mover out of the tubuleby osmosis

at hairpin turn, osmolarity can reach 1200 mOsm/liter

Ascending limb is impermeable to water, but symportersremove Na+ and Cl- so osmolarity drops to 100mOsm/liter, but less urine is left

Vasa recta blood flowing in opposite directions than theloop of Henle -- provides nutrients & O2 without affectingosmolarity of interstitial fluid


26/27

Reabsorption within Loop of Henle


27/27

excretion i

Documents