Glomerular Filtration Flashcards
What are the basic renal processes?
Filtration
Reabsorption
Secretion
What is the total GFR per day?
180L/day
What is the purpose of renal reabsorption?
Bringing substances the body wants back into the body
What volume of blood do the kidneys receive?
1200mls/min
20-25% of total cardiac output
Total blood volume passes through the renal circulation in:
<5 mins
What proportion of blood volume is plasma?
55%
What is a normal GFR?
125mls/min
What percentage of the renal plasma becomes glomerular filtrate?
19%
Glomerular filtration is dependent on what?
Balance between:
Hydrostatic forces favouring filtration
Oncotic pressure forces favouring reabsorption
Which factors determine the filterability of a given solute?
Molecular size
Electrical charge
Shape
What proportion of Hemoglobin is filtered out?
3%
What proportion of serum albumin is filtered out?
<1%
Pores of glomerular endothelial cells prevent what?
Filtration of blood cells
What structure prevents filtration of larger proteins?
Basal lamina of the glomerulus
What structure prevents filtration of medium sized proteins?
Slit membrane between pedicels
Why is glomerular capillary pressure higher than most capillaries in the body?
The Afferent arterioles are short and wide - offering little resistance
Why is post-capillary resistance high?
Efferent arterioles are long and narrow
High resistance increases hydrostatic pressure where?
Upstream
High resistance decreases pressure where?
Downstream
Hydrostatic P always > oncotic pressure where?
Glomerular capillaries
Why is oncotic pressure increased downstream?
Removal of fluid causes a relative increase in concentration of substrates
What is the gross structure of the efferent arteriole?
Long and narrow
What is the gross structure of afferent arterioles?
Short and wide
What is the gromerular net filtration pressure?
10mmHg
What is the colloid osmotic pressure gradient?
Osmotic pressure gradient into blood due to presence of proteins in the plasma but not the bowmans capsule
15mmHg
What factors affect GFR?
Afferent/efferent arteriolar diameter
What are the extrinsic controls of GFR?
Sympathetic VC nerves
Circulating catecholamines
Angiotensin II
What is the effect of sympathetic VC nerves on GFR?
Afferent and efferent constriction
afferent more sensitive
What is the effect of circulating catecholamines on GFR?
Constriction of the afferent arterioles
What is the effect of angiotensin II on GFR?
Low [Angiotensin II]
Constriction of Efferent
High [Angiotensin II]
Constriction of afferent and efferent
What is the effect of efferent arteriolar constriction on GFR?
Increased GFR due to increased pressure
What is the effect of afferent arteriolar constriction on GFR?
Reduced GFR due to reduced pressure
What is the effect of afferent arteriolar dilation on GFR?
Increased GFR due to increased pressure
GFR autoregulation is effective over what range of BPs?
60-130mmHg
Filtration falls below what MBP?
60mmHg
Filtration stops below what MBP?
50mmHg
If mean arterial pressure increases, what is the effect on the afferent arteriole? Why?
Increased afferent arteriole constriction to prevent a rise in glomerular filtration
What is the role of sympathetic VC nerves in massive haemorrhage?
Override autoregulation to liberate blood for more immediately important organs
How much blood can be ‘freed up’ to perfuse other organs at expense of the kidneys?
800mls
What percentage of fluid is reabsorbed from the nephron?
> 19% (of 20% volume filtered)
Which capillaries are responsible for reabsorption?
Peritubular capillaries
Why are the peritubular capillaries in favour of reabsorption?
Higher concentrations of plasma proteins in peritubular capillaries hugely increases osmotic pressure
