Glomerular filtration Flashcards
Basic functions of the kidney (3)
Filtration
Re-absorption of substances the body wants
Secretion
How much blood do the kidneys receive per min on average
1200mls/min
What % of the total cardiac output do the kidneys receive? Why is this crucial in vascular disease?
20-25%
The fact that the kidneys normally receive such a high BF explains their greater vulnerability to damage by vascular disease.
What is the renal plasma flow?
Plasma constitutes 55% of total BV.
55% of 1200mls/min = 660mls/min = renal plasma flow
What is Glomerular filtration dependent on?
The balance between the hydrostatic forces favouring filtration and the oncotic pressure forces favouring reabsorption (Starling’s forces).
Glomerular Filtration occurs in exactly the same way as fluid filters out of any capillary in the body.
Which things affect the filterability of solutes across the glomerular filtration barrier?
Molecular size
Electrical charge
Shape
What prevents the filtration of larger proteins?
The Basal Lamina of the glomerulus
Why is the Glomerular capillary pressure (PGC) higher than in most of the capillaries in the body?
Because the afferent arteriole is short and wide and offers little resistance to flow. So the blood arriving at the glomerulus still has a high hydrostatic pressure (force it exerts on vessel wall)
What is the golden rule of circulation?
If you have a high resistance, hydrostatic pressure upstream is increased, while the pressure downstream is decreased.
How does only filtration occur at the glomerular capillaries?
Both the afferent and efferent arterioles contribute to the very high glomerular capillary pressure.
Therefore at the glomerular capillaries the hydrostatic P favouring filtration always exceeds the oncotic P (brings water in).
So only filtration occurs at the glomerular capillaries
What is the efferent arteriole like?
The efferent arteriole is long and narrow and offers a high post-capillary resistance.
Factor affecting the GFR
Glomerular capillary pressure
What is the Glomerular capillary pressure dependent on
The afferent and efferent arteriolar diameter and therefore the balance of resistance between them.
3 methods of extrinsic control of afferent and efferent arteriolar diameter
Sympathetic VC nerves => afferent and efferent constriction, greater sensitivity of afferent arteriole.
Circulating catecholamines e.g epinephrine => constriction primarily afferent
Angiotensin II => constriction, of efferent at [low], both afferent and efferent at [high].
If there is increased resistance in the afferent arteriole what happens?
Increased blood flow to other organs
Decreased Renal blood flow
Decreased capillary blood pressure
Decreased GFR
If there is increased resistance in the efferent arteriole what happens?
Decreased Renal blood flow
Normal flow to other organs
Increased pressure in the capillaries
Increased GFR
If there is decreased resistance in the afferent arteriole what happens?
GFR increases as the glomerular capillary pressure increases
What is MBP?
MBP = mean arterial blood pressure
You can work it out by doubling the diastolic adding it to the systolic and then dividing the whole thing by 3
At what MBP does filtration cease all together
50 mmHg
If mean arterial pressure increases how do the renal arteries respond
There is an automatic increase in afferent arteriolar constriction, preventing a rise in glomerular capillary pressure.
What is autoregulation of blood flow to the kidneys and GFR independent of?
Independent of nerves or hormones
What is the effective autoregulatory range ie what range of BPs can the GFR be maintained?
60-130mmHg
What can be overridden in situations where blood volume/BP face serious compromise, eg in haemorrhage?
Autoregulation of the kidneys therefore more blood can be given to more immediately important organs at the expense of the kidneys
What % of plasma entering the kidney returns into the systemic circulation?
> 99%
What % of volume is excreted to the external environment?
<1%
What volume is filtered through the glomerulus into the renal tubule per day?
180 L a day
only 1-2 L are excreted as urine in a day though
How is the efferent arteriole involved in reabsorption?
Because it offers resistance along its entire length, there is a large pressure drop so that the hydrostatic pressure is very low in the peritubular capillaries (extension of the efferent arteriole).
However, because 20% of the plasma has filtered into Bowman’s capsule in the glomerulus, (filtration fraction), the blood remaining in the efferent arteriole and then the peritubular capillaries has a higher concentration of plasma proteins and so IIp increases
As a consequence, the net result of the low Ppc (hydrostatic) and the high IIp (oncotic) is that the balance of Starling’s forces in the peritubular capillaries is entirely in favour of reabsorption.
