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Flashcards in Renal Filtration Deck (32):

What are the basic renal processes?



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?

Amino acids


What is reabsorbed from the distal tubule?



What is secretion?

Substances may be specifically removed from the body this way


What is secreted in the proximal tubule?

Organic ions


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


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