Renal Blood Flow & Glomerular Filtration

Question 1

The glomerulus is specialized for……..

Answer 1

Filtration of plasma not t o supply O2 or nutrients to the nephron

Question 2

Why is glomerulus pressure high & what is its significance?

Answer 2

The resistance afforded by narrow efferent arteriole compared to afferent arteriole
The renal artery is considered a direct branch from the aorta
Significance: as it is always greater than pressure in Bowman’s capsule so filtration occurs

Question 3

GR: Peritubular capillaries have low hydrostatic & high osmotic pressure

Answer 3

H: as the efferent arteriole offer some sort of resistance to blood before reaching it, the fluid reaching it is less than reaching glomerular capillary by an amount equal to that subjected to filtration (GFR)
O: as albumin reaching it become concentrated

Question 4

Describe function of peritubular capillaries

Answer 4

1. Specialized for O2 supply
2. Reabsorption of solutes & fluids from renal tubular lumen through tubular cells then to interstitial space then to capillaries
3. Secretion of substances that escaped filtration of glomerular capillaries

Question 5

Describe the function of vasa recta in JM nephron

Answer 5

1. It prevents the escape of solutes in inner medulla and so maintains the characteristic high osmolarity
   This osmolarity is essential to concentrate urine

Question 6

GR: Renal blood flow remains constant with moderate inc in ABP

Answer 6

Due to autoregulation

Question 7

What is the role of afferent arteriolar resistance?

Answer 7

Plays role in autoregulation & keeping the RBF and GFR relatively constant

Question 8

What is the role of efferent arteriolar resistance?

Answer 8

1. Generation of very high pressure in glomerular capillaries favoring filtration
2. Dec blood flow velocity in vasa recta of juxtamedullary nephrons which goes to inner medulla and this prevents the escape of solutes from interstitail space thus keep high osmolarity of medulla essential for urine conc
3. Dec blood flow to peritubular capillaries thus dec hydrostatic pressure relative to high osmotic pressure favoring reabsorption

Question 9

GR: Ang II protects the GFR

Answer 9

Because of its preferential effect on efferent arterioles in both high & low levels

Question 10

Describe function of protalglandins in kidney

Answer 10

Protective for RBF, modulate the VC produced by Ang II & sympathetic nervous system as they are produced in response to the same stimuli.

Question 11

Describe the effect of sympathtic stimulation on RBF & GFR

Answer 11

1. Moderate or mild symp stimulation has little effect on RBF & GFR due to autoregulatory capacity of kidney
2. Severe symp stimulation causes constriction of both eff & aff dec both RBF & GFR

Question 12

Describe the effect of ANP on GFR & RBF

Answer 12

Dilated afferent arteriole inc RBF & relaxes mesangial cells in walls of glomerular capillaries inc surface area for filtration

Question 13

What happens beyond 180 mmHg?

Answer 13

Pressure diuresis (loss of autoregulation)

Question 14

Autoregulatory mechanism of kidney depends on…..&……

Answer 14

Myogenic mechanism
Tubuloglomerular feedback mechanism

Question 15

Describe the myogenic mechanism of autoregulation

Answer 15

Smooth muscles in wall of afferent arterioles contract when blood pressure inc leading to VC causing dec in blood flow & dec in GFR back to normal & vice versa

Question 16

Explain tubuloglomerular feedback mechanism

Answer 16

In case of high arterial pressure:
The inc in GFR results in inc NaCl delivery to macula densa that enter via Na/K/2Cl cotransporter in their apical membranes, resulting in inc Na-K ATPase activity & inc ATP production thus:
1. ATP bind PX2 receptor & adenosine binds A1 receptor in plasma membrane of smooth muscle of afferent arteriole both inc intracellular Ca++, inducing VC in afferent arteriole, thereby dec GFR to normal levels (afferent arteriolar feedback)
2. Inhibition of renin release by granular cells as a result of inc IC Ca++ as a result of electrical coupling of the granular & vascular smooth muscle cells, thus less Ang II is produuced & eff dilates (efferent arteriolar feedback)
Both mechanisms lead to dec GFR & dec glomerular hydrostatic pressure
Opposite events occur in low BP

Question 17

What happens if autoregulation did NOT occur?

Answer 17

1. Inc GFR would lead to excess loss of water & solutes
2. Dec GFR leads to inability of kidneys to excrete wastes, excess electrolytes & other materials that should be excreted.

Question 18

The normal GFR is….., without reabsorption this would make urine output……
However, only…..is excreted, thus urine output is……

Answer 18

125 ml/min
180L/day
1 ml/min
1.5 L/day

Question 19

Glomerular filtrate contains …….anions than cations due to……

Answer 19

Less
Repulsion by negative charges in glomerular membrane esp proteocglycan of BM

Question 20

Write equation of fitration fraction & its normal value

Answer 20

FF=GFR/RPF*100
16-20%

Question 21

GR: The permeability of glomerular barrier is high

Answer 21

Bec glomerular capillaries have large porous surface 2 m2 & high pressure

Question 22

Mention factors affecting passage of molecules into Bowman’s capsule

Answer 22

1. Size of molecules, less than 4 mu (5500 mw) freely filtered, more than 8 mu (70000 mw) not filtered.
2. Electical charge of molecules, positive are more readily filtered as they are attracted by -ve charges in glomerular membrane esp proteoglycan & vice versa

Question 23

What happens in case of reduction of -ve charges on filtration barrier?

Answer 23

Loss of low molecular weight proteins esp albumin which are filtered & appear in urine (proteinuria/albuminuria)

Question 24

Mention methods of measurement of GFR which is more useful & why?

Answer 24

A. Clearance tests of inulin & creatinine (more useful)
B. Plasma concentrations of urea & creatinine (do not elevate in blood until there is significant dec in kidney function