S3 GFR and Filtration

Question 1

how does blood flow from the aorta to the glomerulus and back to the aorta ?

Answer 1

aorta - renla artery - segmental artery - lobular artery - arcuate artery - interlobular artery - afferent arteriole - glomerulus - efferent arteriole - peritubular capillaries (cortical) or Vasa recta ( Juxtamedullary) - interlobular vein - arcuate vein - lobular vein - segmental vein - renal vein - IVC

Question 2

what are the differences in the cortical and juxtamedullary nephrons

Answer 2

Ratio - c - 90% J - 10%
Location - C - cortex outer J - Cortex inner
Glomerulus cortex - C - Small J - Large
Loop of Henle - C - short, next to outer cortex J - longer, goes into the inner part of the cortex, dips into medulla
Diameter of arterioles - C - AA>EA J- AA=EA
EA - C - forms peritubular capillary J - forms vasa recta
Sympathetic innervation - C - rich J - poor

Question 3

describe renal blood flow

Answer 3

renal blood flow is 1.1 L/min
all blood flows through the glomeruli in the cortex
haemotocrit is the volume percentage of red blood cells in blood and is normally ~0.45 so renal plasma flow is RPF 0.55 X 1.1L/min = 605ml/min of plasma

Question 4

what happens when blood enters the glomerulus

Answer 4

blood enters through Afferent A( pores wide enough for plasma, salts and small molecules, but not RBCs and large proteins) 20% of blood from renal artery if filtered at any one time and 80% of blood arriving exits via efferent arteriole (unfiltered)

Question 5

what is the glomerular filtrate or ultrafiltrate

Answer 5

water and solutes that have been forced out of the glomerular capillaries as they are too big pass into bowmans space

Question 6

what are the three layers to the filtration barrier in the glomerulus

Answer 6

1. Capillary endothelium
   - water, salts, glucose
   - filtrate moves between cells
2. Basement membrane
   - permeable to small proteins
   - glycoproteins (-ve charge) repel protein movement
3. Podocyte layer
   - contain filtration slits

Question 7

what substances can and cant get through the GFB

Answer 7

can - inulin (largest), Na +, K+, Cl-, H20, urea, glucose, PEG
can’t - haemoglobin, albumin

Question 8

what is the effect of charge on filtration and its clinical significance

Answer 8

neutral molecule - bigger size is likely to get through
anions - negative charge also repels, more difficult to get through
cations - positive charge allows slightly bigger molecules through
in many disease processes, the - charge on the filtration barrier is lost so that proteins are more readily filtered. the condition is called proteinuria (protein in the urine)

Question 9

why is there a higher hydrostatic pressure in the glomerulus

Answer 9

the AA is wider than the EA, so more blood can get in that can escape

Question 10

how is plasma filtered and what are the forces involved ?

Answer 10

Hydrostatic capillary forces (Pgc) : regulated, is greater than the oppositional forces so net movement is from the capillary into the bowmans space then into the tubule
Hydrostatic pressure in Bowman’s Capsule (Pbc) : where the ultra-filtrate collects

Question 11

why is auto-regulation of GFR needed ?

Answer 11

keeps GFR and renal blood flow (80-180 mmHG) within normal limits. Without regulation slight changes in BP would cause significant change in GFR

Question 12

what is the myogenic mechanism ?

Answer 12

arterial smooth muscle responds to increases and decreases in vascular wall tension

Question 13

what happens if the AA constricts ?

Answer 13

limits blood entering
Pgc falls
GFR falls

Question 14

what happens if the AA dilates ?

Answer 14

more blood enters
Pgc rises
GFR rises

Question 15

What happens if the EA constricts ?

Answer 15

less blood can leave - GFR increases

Question 16

what happens if the EA dilates ?

Answer 16

more blood can leave - GFR decreases

Question 17

what is the clinical significance of the myogenic mechanism

Answer 17

with an increased BP, AA constricts so GFR remains unchanged and with a decreased blood pressure the AA dilates so GFR remains unchanged. thus maintains GFR within physiological limits (80-180mmHG)

Question 18

describe what is tubular glomerular feedback

Answer 18

increase in arterial pressure increases glomerular capillary pressure so GFR increases
Increased GFR means more (Na+) and (Cl-) reaches the distal tubule
Macula densa cells in the DCT respond to changes in NaCl in the lumen. They stimulate juxtaglomerular apparatus to release chemicals depending on the NaCL conc

Question 19

how does the body respond to increased NaCl in the lumen

Answer 19

adenosine is released to vasodilate the EA, so decreases GFR

prostaglandins are released to vasodilate the AA so increases GFR

Question 20

how does the nervous system regulate the GFR ?

Answer 20

sympathetic nerve fibres innervate AE and EA. Normally sympathetic innervation is low (no effect on GFR). Fight or flight ischaemia can stimulate renal vessels causing vasoconstriction which conserves blood volume (haemorrhage) and can cause a fall in GFR

Question 21

what is the glomerulotubular balance

Answer 21

whilst myogenic and TGF responses are the first way to stop GFR changes, the GTB is the second line of defence which blunts Na + excretion in response to any GFR changes

Question 22

how much water and sodium is absorbed in the PT,

Answer 22

PT -s- 67% water - 65%