US-Lecture 23

jaxaglomular apparatus, podocytes, overview PCT structure, reabsorption in PCT, reabsorption in DCT, aldosterone, reabsorption in CD, vasopressin, RAA system, overview of fluid balance

juxtaglomerular apparatus

The glomerular capillaries are fenestrated, so water, ions, and small organics can go through

basal lamina

another layer of filtration on the pores (on the juxtaglomerular apparatus)

-filters out almost all proteins

podocytes

interlock, cells that come from Bowman’s capsule, little space between then called slit pore

Reabsorption Rates

glomerulus 180L/day (66%) —>

Bowman’s capsule 120 L/day—>

proximal convoluted tubule 60 L/day—>

Loop of Henle 40L/day (20%)—>

distal convoluted tubule 20L/day (10%)

collecting duct (2L/day)

PCT Reabsorption Rate

Reabsorption in PCT and Loop of Henle is 90%

Cells in the PCT

inside=lumen

The cells are cuboidal cells

spiky inner side- apical membrane

smooth outer layer- basolateral membrane

-to do reabsorption, you need to go through both membranes

Sodium (Na) Reabsorbtion in PCT

there is Na+ in the lumen

-has to go through sodium channel on apical surface 

-on the basolateral membrane there is a sodium potassium pump with active ATPase, take 3 NA+ out and 2 K+ in

-actively pumping sodium out, makes sure concentration gradient inside cell is low for sodium, you get movement always in= reabsorption

-the potassium channel moves K+ in and out evenly

Glucose Reabsorbed in PCT

there is a SGLT (Sodium-Glucose Lumenal Transport) on the apical membrane

-Sodium and glucose are more likely to go in at the same time cause there is a higher concentration on the outside of the apical membrane

-on the basolateral membrane there is a secondary active transport also a passive transporter cause no ATP is being used here

-GLUT (passive glucose carrier) that moves the glucose outward

-so glucose will want to move back in =reabsorbtion

Water Reabsorption in PCT

Water moves through osmosis

-water moves toward the higher osmolality outside the cell, so it goes from lumen through both membranes and out

Diabetes

Glucose in Pee

-too much glucose not enough SGLTs to move it so it gets past PCT and added into pee

Too much Pee-

When glucose gets into urine, it disturbs the osmolarity balance, putting some more inside the lumen, therefore some water moves in and increases amount of pee

Loop of Henle

There is descending limb and there is the ascending limb'

-descending- permeable to water but not solutes

-ascending loop- active transport of solutes (Na+ K+ Cl-)

-not water-permeable

-next to eachother, amplify the effect on one another

Sodium (NA+) Reabsorption in DCT

Sodium and Potassium Pump

-contains principal cells

-More Na going out (3 Na)

-reabsorption of water

Potassium (K+) Reabsorption in DCT

Potassium is moving outside the cell with a potassium pump on the apical membrane, and there is a sodium-potassium pump on the basolateral membrane —> potassium is moving into the lumen =secreting potassium

Water Reabsorption in DCT

overall movement = 3 NA+ out and 2K+ in

=osmolarity is higher outside the cell (moves through apical membrane—> basolateral membrane

Aldesterone

-more pumps on basolateral membrane (sodium and potassium pumps)

-more channels on apical membrane

reabsorb more Na+ and secrete K+

-reabsorb more volume= raise blood pressure and water 

-steroid: sternodosterone (blocks aldosterone and testosterone)

-comes from adrenal cortex

Collecting Duct

-if the collecting duct is permeable to water, reabsorb water and produce a small amount of concentrated urine

-if not permeable—> large amounts of dilute urine 

CD cells are largely impermeable to water -filled with tight junctions and cholesterol

Collecting Duct osmolarity

300 at the cortex then as you move down past the medulla there is as high as 1400

-it goes from very sensitive to less sensitive by vasopressin

Aquaphorins

water channel proteins in cells

-they are on the membrane of the basolateral side of the collecting ducts

-They are vesicles that, when cells are exposed to vasopressin—> vesicles in cell move up to the apical surface, so aquaporins are now part membrane —> now water has path through

Vaopressin

makes cells permeable to water, increases water reabsorption, turns down urine production

-released from post pituitary—> more concentrated blood—> more vasopressin needed

RRA System

Renin Angiotensin Aldosterone System

-happens in days or weeks

Low Blood Pressure

=production of renin

When renin comes in it activates angiotensinogen and converts it to angiotensin 1 (does nothing)

w/ Angiotensin Converting Enzyme

-can be made into angiotensin 2 (powerful vasoconstriction)

Fluid Balance (Obligatory)

sources of fluid loss: sweating, digestion, exhalation

Consistent

Reabsorption in PCT and Loop of Henle is consistent =90%

Variable

in DCT and CD

The amount of absorption is dependent on

Aldosterone- increases water absorption and sodium absorption

Vasopressin-increase water reabsorption

1) kidney involvement in blood pressure

low flow past macula densa—>causes secretion of prostaglandins, which then secrete renin

-caused by low blood pressure

2) kidney involvement in blood pressurelo

low stretch in afferent arterioles—>causes production of renin

-caused by low blood pressure

3) kidney involvement in blood pressure

activation B1- receptors on juxtaglomerular cells—> secrete renin 

(sympathetic cells- come from epinephrine or norepinephrine in blood)= renin production

Angiotensin 2

constricts all major blood vessels and arterioles tighten up=

increase in arterial resistance

increase in blood pressure

-also stimulates secretion of aldosterone (regulates water retention)

-stimulates vasopressin secretion (low secretion)

renin

raises blood pressure

liver

angiotensinogen

-is a pro hormone prevents from being active