Physiology - Glomerular Filtration

Question 1

What is filtration?

Answer 1

Formation of an essentially protein-free filtrate of plasma at the glomerular capillaries

Question 2

What is reabsorption and how quickly does it occur?

Answer 2

Substances that the body wants are reabsorbed, those it doesn’t want stay in the tubule and are excreted

• Initial reabsorption begins immediately after secretion

Question 3

What is secretion?

Answer 3

Substances may be specifically removed from the body is this way

• Kidney is actively able to secrete

Question 4

What is the normal GFR?

Answer 4

• ~180l/day
  
  • Only ~2l/180l ends up in urine so almost all of it is reabsorped too

Kidney has ample opportunity to precisely regulate ECF volume and composition and eliminate “nasty” substances

Question 5

Why are the kidney’s very vulnerable to damage by vascular disease?

Answer 5

As they recieve such a high BF

• They have the highest BF/g of any tissue in the body
• In <5mins a volume equal to total BV passes through renal circulation

Question 6

What is the plasma filtration fraction at the glomerulus?

Answer 6

• 1200ml/min of blood passes through
  
  • Of which 55% = 660ml/min is plasma
  • GFR of plasma is 125ml/min
• Therefore 125/660 = ~19%

Question 7

Explain the forces behind filtration?

Answer 7

• Hydrostatic pressure favours filtration
• Oncotic pressure favours reabsorption back into glomerular capillaries

For filtration to work capillary hydrostatic pressure (Pgc) must > fluid oncotic pressure

Question 8

What factors influence filtration rate of a specific particle?

Answer 8

• Size
• Charge
• Shape

Small particles like Na+/K+/Water are almost completely filtered out.

Question 9

Describe the physical structure of the capillary walls that allows filtration?

Answer 9

1. Fenestrated (pore) of glomerular endothelial cells: prevents filtration of blood cells but allow plasma to pass
2. Basal lamina of glomerulus: prevents large proteins passing through
3. Slit membrane between pedicals under the basal lamina prevent medium proteins passing

Question 10

How is a high capillary Hydrostatic pressure maintained?

Answer 10

• Afferent arterioles
  
  • Wide and short to put up little resistance
• Efferent arterioles
  
  • Long and narrow to put up high resistance

Question 11

What is the Golden rule of circulation?

Answer 11

If you have a high resistance, hydrostatic pressure upstream is increased, while the pressure downstream is decreased

Question 12

Describe extrinsic control of Glomerular Capillary Pressure

Answer 12

• Sympathetic vasoconstrictive nerves
  
  • Afferent and efferent constriction, greater sensitivity of afferent arteriole
• Circulating catecholamines
• Angiotensin II

Question 13

How do circulating catecholamines affect glomerular Capillary Pressure (P_GC)

Answer 13

Cause vasoconstriction primarily in the afferent arteriole.

• Lowering renal blood flow and P_GC to provide more blood to other tissues e.g. heart and muscle

Question 14

How does angiotensin II affect P_GC?

Answer 14

• Constriction of the efferent at [low]
• Constriction of both efferent/afferent at [high]

Question 15

Renal vessels also have an intrinsic ability to control P_GC. What is the purpose of this?

Answer 15

• ​Autoregulation of the GFR
  
  • Keeps BF and GFR essentially constant

Question 16

How do renal vessels autoregulate GFR?

Answer 16

Afferent arterioles constrict and dilate in response to changes in blood volume in order to main P_GC.

Question 17

What happens if mean arterial P increases?

Answer 17

There is an automatic increase in afferent arteriolar constriction, preventing a rise in glomerular PGC

• DIlatation occurs if P falls

Question 18

When would autoregulation of the GFR be overridden?

Answer 18

When blood volume faces serious problems, such as a large haemorrhage.

• In order to liberate blood for more immediately essential tissues (Symp nerves and angiotensin II)

Question 19

What controls autoregulation of the GFR?

Answer 19

• The renal vessels themselves
  
  • Its independant of nerves and hormones, occuring even in denervated and isolated kidneys

Question 20

How are the peritubular capillaries capable of reabsorping almost 180l a day?

Answer 20

1. Extremely low hydrostatic pressure thanks to the high resistance efferent arteriole upstream
2. High oncotic pressure in peritubular capillaries thanks to losing 20% of the plasma but still containing 100% of normal blood protein
   
   • Net starling forces are much more in favour of reabsorption leading to reabsorption of
     
     • 50% of urea
     • 99% H2O
     • 100% Glc
     • 99.5% of Na.

Question 21

What is the equation for workingout the amount of solute excreted (E)?

Answer 21

E = F - R + S

• F (Amount Filtered)
• R (Amount Reabsorbed)
• S (Amount Secreted)