Renal Filtration Flashcards
What are the basic renal processes?
Filtration
Reabsorption
Secretion
What is filtration?
The formation at the glomerular capillaries of an essentially protein-free filtrate of plasma -> enters the proximal tubule
(around 20% of plasma flowing through)
What is the glomerular filtration rate (GFR)?
How much fluid is filtered per unit time
Very high = 180l/day
What does the high glomerular filtration rate allow the kidneys to do?
Ample opportunity to precisely regulate ECF volume and composition and eliminate “nasty” substances
What is reabsorption?
Substances that the body want are reabsorbed, those it doesnt want stay in the tubule and are excreted
What is reabsorbed from the proximal tubule?
NaCl
Water
Amino acids
Sugars
What is reabsorbed from the distal tubule?
NaCl
Water
What is secretion?
Substances may be specifically removed from the body this way
What is secreted in the proximal tubule?
Organic ions
Drugs
What is secreted in the distal tubule?
Potassium and Protons
How much blood flow do the kidneys actually receive?
Around 1200mls/min, i.e. 20-25% of total cardiac output
Kidneys weight
Why are the dissents vulnerable to damage by vascular disease?
Recieve such a high blood flow
Not all the blood is filtered.
What happens to the stuff that isn’t?
None of the red cells and only a fraction of the plasma is filtered through into Bowman’s capsule
The remainder passes via the efferent arterioles into the peritubular capillaries and then to the renal vein
What is the renal plasma flow?
Plasma constitutes around 55% of total blood volume
55% of 1200mls/min = 660mls/min
660mls/min is the renal plasma flow
What is the filtration fraction?
i.e. how much renal plasma becomes glomerular filtrate?
GFR is normally 125mls/min
Therefore the filtration fraction = 125/660x100 = 19%
What does glomerular filtration depend on?
Occurs in exactly the same way as fluid filteres out of any capillary in the body.
It is dependent on the balance between the hydrostatic forces favouring filtration and the oncotic pressure forces favouring reabsorption (Starling’s forces)
Primary factor is hydrostatic pressure
What determines the filterability of solutes across the glomerular filtration barrier?
Molecular size
Electrical charge
Shape
Why is 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 resisyance to flow.
Blood arriving at the glomerulous still has a gigh hydrostatic pressure
In addition the unique arrangement of the efferent anteriole which is long and narrow offers a high post-capillary resistance
TLDR; Blood can easily get into glomerulous but hard to get out
What is the “golden rule of circulation”?
If you have a high resistance, hydrostatic pressure upstream is increased, while the pressure downstream is decreased
What forces act at the glomerular filtration barrier?
Hydrostatic pressure (BP) -Works towards filtration
Colloid osmotic pressure (oncotic pressure)
- Proteins in plasma but not in bowman’s capsule
- Acts against filtration
Fluid pressure created by fluid in bowman’s capsule
-Works against filtration
how is net filtration maintained?
The hydrostatic pressure favouring filtration always exceeds the oncotic pressure (and fluid pressure)
ONLY filtration occurs at the glomerular capillaries
What does hydrostatic pressure depend on?
Afferent and efferent arteriolar diameter and therefore the resistance between them
How is hydrostatic pressure extrinsically controlled?
Sympathetic VC nerves
-Afferent and efferent, constriction, greater sensitivity of afferent arteriole
Circulating catecholamines
-Constriction of primary afferent
Angiotensin II
- Constriction of efferent at low levels
- Constriction of both afferent and efferent at high levels
How is hydrostatic pressure intinsically controlled?
Intrinsic ability to adjust its resistanvce in response to changes in arterial BP
What does the body try to do in terms of arterial BP in the kidneys?
Keep blood flow and GFR essentially constant = autoregulaton
In man its effective over a range of MBP from 60-130mmHg
Below 60mmHg, filtration falls and ceases altogether when MBP = 50mmHg
-Acute renal failure
What does the autoregulation of BF and GFR depend on?
Independent of nerves or hormones, occurs in denervated and in isolated perfused kidneys
How would the kidneys react to an incraese in mean arterial pressure?
There is an automatic incraese in afferent arteriolar constriction, preventing a rise in glomerular Pgc.
Dilation occurs if P falls
What do the kidneys do in situations where blood volume/BP face serous compromise?
e.g. haemorrhage
What if this situation continues for a prolongued period?
Activation of sympathetic VC nerves and AII, can override auto regulation liberating blood for more immediately important organs
As much as 800mls of blood per min can thus be provided to perfuse these other organs at the expense of the kidney
this is important in haemorrhage, but, prolongued reduction in renal BF can lead to irreparable damage which may then lead to death because of disruption of the kidneys role in homeostasis
What happens to the 20% of plasma fluid volume that is filtered by the kidney?
Cannot excrete all this fluid so >19% is reabsorbed into the peritubular capillaries, returning to the systemic circulation
What capillaries are responsible for reabsorption?
How does it carry out this function?
Peritubular capillaries
The unique efferent arteriole has important effects on Starling’s forces in the peritubular capillaries
Because it offers resistance along its entire length, there is a large P drop so that hydrostatic pressure is very low.
Oncotic pressure higher in comparison so only reabsorption occurs
Why is oncoic pressure higher in peritubular capillaries than hydrostatic pressure?
Hydrostatic pressure has dropped due to resistance in efferent arterioles
Oncotic pressure is higher due to higher protein content in peritubular capillaries blood
-20% less fluid but same amount of protein
How much H2O, glucose, Na and urea is reabsorbed within the tubule, mainly at the proximal tubule?
99% H2O
100% Glucose
99.5% Na+
50% Urea
