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1

Functions of the Kidneys

1. osmoregulation = water and solute balance (ECF osmolarity -290mOsm and volume total H20
2. ion (electrolyte) balance (Na+, K+, Ca2+, etc.)
3. acid-base regulation (H+, HCO3-)
4. excretion - urea + other soluble waste products
5. endocrine function - erythropoietin

2

Regions

Cortex
medulla
renal pelvis -> ureter

3

Nephron

functional unit of the kidney

4

Renal corpuscle

glomerulus
glomerular (Bowman's) capsule

5

Renal Tubule

proximal convoluted tubule (PCT)
loop of Henle - descending limb, ascending limb
distal convoluted tubule
collecting duct

6

Blood Supply

glomerulus - afferent arteriole
glomerular capsule- efferent arteriole
renal tubule- peritubular capillaries
PCT - vasa recta

7

cortical nephrons

mostly in cortex, short loops of henle

8

juxtamedullary nephrons

long loops of Henle extend deep into the medulla

9

three primary processes of the Nephron

filtration
reabsorption
secretion

10

Filtration: Glomerulus and Glomerular Capsule

formation of protein-free filtrate
filtrate contains H20 + small to medium sized solutes (ions, glucose, urea, etc.)

11

Filtration membrane

consists of three layers
fenestrated capillaries
basement membrane
filtration slits of podocytes

12

Net filtration pressure

Blood pressure 55mm Hg (in glomerular capillaries)
-C.O.P -30 mm Hg (from plasma proteins in cap.)
-Tissue P -15mm Hg (fluid backpressure in glomerulacap)

=NFP 10 mm Hg

13

Glomerular filtration rate

GFR = 125 mL/min

14

GFR

can be estimated by measuring clearance of a substance that is filtered into the nephron but not reabsorbed or secreted. e.g. inulin, creatinine
GFR= insulin clearance = [inulin]urine X urine flow (mL/min) / [inulin]plasma

15

Reabsorption: PCT

"mass reabsorption" in the PCT, isosmotic with ECF (300 mOsm)
approx. 70% filtered fluid is reabsorbed from the PCT into the ECF (peritbular capillaries)

16

Epithelial Transport in the PCT

transport proteins (pumps, carriers, channels) in apical and basolateral membranes
microvilli on apical surface face into lumen of tubule (increase surface area)

17

Transport mechanisms
Na+

diffusion through channels in the apical membrane
primary active transport: Na+/K+ ATPase in the basolateral mambrane

18

Transport mechanisms glucose

secondary active transport with Na+ (SGLT) across apical membrane
facilitated diffusion (GLUT) across basolateral membrane

19

Transport mechanisms amino acids

cotransport with Na+ similar to glucose

20

Transport mechanisms H20

via osmosis, follows movement of solutes

21

Glucose reabsorption in the PCT

normally, all glucose filtered into nephrons is reabsorbed
at very high glucose levels, glucose transporters become saturated

22

transport maximum

maximal rate of glucose reabsorption

23

renal plasma threshold

minimum plasma [glucose] at which glucose appears in urine

24

normal plasma

normal plasma [glucose] 90 mg/dL << renal plasma threshold - 180 mg/dL

25

in diabetes mellitus

high plasma [glucose] > renal plasma threshold -> glucose in urine
-> osmotic diuresis

26

Vertical Osmotic Gradient: Loop of Henle

Counteractive multiplier mechanism
active transport of NaCl
osmotic movement of H20

27

Active transport of NaCl

NaCl pumped out of ascending limb (thick region)
decrease osmolarity of tubular fluid and increase osmolarity of surrounding ECF

28

Osmotic movement of H20

H20 moves out of descending limb due to increase ECF osmolarity
differential permeability properties along the loop of Henle:
descending limb : permeable to H20, impermeable to NaCl Ascending limb: impermeable to H20, permeable to NaCl (active transport)

29

Countercurrent flow

opposite direction of flow in descending and ascending limbs
concentrated fluid (1200 mOsm) formed at base of loop and surrounding ECF
dilute fluid (100 mOsm) at top of ascending limb enters DCT
vasa recta around loop of Henle helps maintain vertical osmotic gradient

30

Tubular secretion: PCT and DCT

active transport of ions and waste products (H+, K+, organic acids) into tubules
occurs in the PCT and DCT