Renal blood flow and glomerular filtration

Question 1

What are the functions of the kidneys?

Answer 1

Control volume and composition of body fluids
To get rid of waste material from the body
Acid-base balance
As an endocrine organ - EPO, Renin and Vitamin D

Question 2

What is the filtration rate of the glomerulus?

Answer 2

120 ml/min

Question 3

Why is the filtration rate so high?

Answer 3

A high rate of formation of glomerular fluid is needed to wash out the waste products fast enough to keep their blood level low.

Question 4

How is glomerular fluid formed?

Answer 4

By passive ultrafiltration of plasma across the glomerular membrane, as described by Starling’s principle of capillary fluid filtration.

Question 5

What sets the glomerular filtration rate?

Answer 5

(i) autoregulation


(ii) renal sympathetic vasomotor nerve activity

Question 6

What is the concentration of small solutes in glomerular fluid?

Answer 6

For small solutes, such as NaCl, glucose and urea, 
 concentration in glomerular fluid = concentration in plasma.

Question 7

What is the concentration of proteins in glomerular fluid?

Answer 7

For plasma proteins, concentration in glomerular fluid = almost zero. Hence, urine is routinely tested on wards for protein (proteinuria). Proteinuria is a sign of renal/urinary tract disease.

Question 8

How does ultrafiltration occur?

Answer 8

A net pressure drop across the glomerular membrane drives the ultrafiltration process. An imbalance of Starling forces

Question 9

How does reabsorption work?

Answer 9

Starling force balance is reversed in peritubular capillaries. Oncotic pressure is higher than blood pressure so water goes back into the blood vessels (peritubular capillaries).

Question 10

What do podocytes do?

Answer 10

They help keep proteins in the blood vessels. They branch out pedicels/foot processes. The gaps between them are called filtration slits. They work like a sieve.

Question 11

What are fenestrae?

Answer 11

Pores in the endothelial cells in the blood capillaries.

50-100nm wide.

Question 12

Why is albumin not present in glomerular fluid?

Answer 12

Even though they around 30nm, and can easily fit between the fenestrae, the filtration slits stop them. Even though the slits are initially around 30nm as well, they subdivide into pores 4nm wide. The central spine with lateral rungs are made of proteins (nephrin and podocin).

Question 13

What is a deficiency in nephrin and podocin called?

Answer 13

Nephrotic syndrome

Question 14

What are the three sieves of the glomerular membrane?

Answer 14

3 sieves in series, of increasing fineness:
Fenestrated capillary
Basement membrane
Filtration slits of podocytes

Question 15

What is the intrinsic control of GFR?

Answer 15

GFR is in the main held constant (120ml/min)

Important for the capacity of tubules to reabsorb filtrate not be overwhelmed by excessive GFR

The mechanism holding GFR constant is an internal one called ‘autoregulation’.

Changes in urine production (diuresis, antidiuresis) are not usually due to changes in GFR, but due to changes in tubular reabsorption.

Question 16

What are the two mechanisms acting together responsible for autoregulation?

Answer 16

Bayliss myogenic response:
-Direct vasoconstriction of the afferent arteriole with an increase in perfusion pressure. Brings capillary pressure back to normal.

Tubuloglomerular feedback (TGF): 
-A flow-dependent signal detected in macula densa, that alters the tone of the afferent arteriole

Question 17

What is the failure of autoregulation in hypotension, shock (blood loss) called?

Answer 17

Oliguria.

Question 18

What is the extrinsic control (neurohumoral) of GFR?

Answer 18

Renal sympathetic nerves (vasoconstrictor, noradrenergic) can reduce the GFR, by re-setting autoregulation to a lower level.

This happens in 3 conditions-
standing upright (orthostasis)
heavy exercise
haemorrhage & other forms of clinical shock

The role is to conserve body fluid volume during physical stress.

In shock, these sympathetic actions are aided by circulating vasoconstrictor hormones such as adrenaline, angiotensin and vasopressin.

Question 19

What is nephrotic syndrome?

Answer 19

Filtration slit disorder by nephrin deficiency causing glomeruli to be too leaky to plasma protein.
Causes Proteinuria, Hypopreinaemia, Oedema.

Question 20

What is chronic glomerulonephritis?

Answer 20

When the GFR is too low due to nonfunctioning glomeruli due to infection. When GFR < 30ml/min, this is chronic renal failure.