Renal Blood Flow Flashcards
Blood supply to the kidney
renal artery –> segmental artery –> interlobar artery –> arcuate arteries –>
cortical radiate/interlobular arteries –> afferent arterioles –> glomerulus 00> effernt arteriole –> peritubular capillaries –> venules 00> cortical radiate veins –> arcuate veins –> interlobar veins –> renal veins.
- *no segmental veins!
- *glomerulus is a type of capillary
how renal PLASMA flow may be measured. –> how renal BLOOD flow may be measured
Para-aminohippurate (PAH):
A substance not metabolized nor synthesized in the kidneys
An organic acid not normally present in the body
Administered through IV infusion
20% filtered in the glomerulus + 80% secreted in the tubules
C (PAH) = RPF
C (PAH) =
[U (PAH) x V]/[P (PAH)]
Why would we want to know renal plasma flow? After all, it’s whole blood that flows into the renal artery, not just plasma. The reason we want to know RPF is that’s the parameter we can measure with PAH. PAH (the marker for RPF) is dissolved only in plasma, not in RBCs. So….we measure RPF with PAH, and we calculate the RBF from the RPF, by knowing the hematocrit. If we could put a flowmeter on the renal artery, we could measure RBF directly……but we can’t, so we use the PAH method.
how can arterioles regulate GF and RBF?
arterioles are capable of vasoconstriction and through that can regulate GF and RBF
Describe the magnitude and distribution of renal blood flow within the kidney
kidney receives 20-25% of CO
kidney receives 350 ml/100g tissue/min blood flow
high renal blood flow is to allow rapid elimination of toxins and excess substances
cortex - 93%
medulla - 7%
papillla - 1%
Describe the functional significance of the variation in perfusion between the cortex and the medulla.
Approximately 90-93% of the blood flow goes to the cortex and 7-10% to the medulla.
Reason: High resistance in vasa recta
Physiological advantage: Medullary area will remain hypertonic which is important in the mechanism of producing concentrated urine.
peritubular capillaries plexus are low pressure capillaries which ____________ and ____________.
rate of filtration is ____________.
its major role is ___________
favour reabsorption
and
supply O2 and nutrients to the kidney.
Rate of filtration is slow.
Major role in excreting the waste products.
When blood goes to the peritubular capillaries, it picked up reabsorbed materials from the interstitium.
Describe autoregulation of renal blood flow
stability of blood flow/perfusion pressure helad relatively constant along glomerular capillary.
present in the kidneys, heart, and the brain.
Renal autoregulation refers to the ability of the kidney to maintain RBF and GFR within narrow limits despite variation in pressure between 80 – 170 mmHg.
A function of the AFFERENT ARTERIOLE.
Independent of the nervous system and hormonal control
Note: It is not absolutely perfect: RBF & GFR changes slightly with Arterial BP variation. They can be changed by Certain Hormones and Sympathetic nerve activity
Cortical nephron vs Medullary/Juxtamedullary nephron
Cortical:
- small size glomeruli in renal cortex
- short LoH, A-thick, D-thin
- vascular supply as peritubular capillary plexus, low resistance.
- Rate of filtration is slow.
- Major role in excreting the waste products.
Medullary/ juxtaglomerular:
- Larger size glomeruli located at the junction of cortex and medulla.
- long LoH which penetrate deep into the medulla
- Both descending & ascending LOH contains thin segments.
- vascular supply is vasa recta.
- Rate of filtration is high.
- Important in maintaining the counter current system by which kidney concentrates urine.
how renal blood flow can be altered despite autoregulation.
Innervation of Renal Vessels – Extrinsic control
- Parasympathetic – X th Nerve ( function uncertain)
- Sympathetic – T10- L2 (through splanchnic nerves)
= AE+EE and Renal tubular cells+JG cells
Stimulation via sympathetic nerve
a) Marked fall in RBF ( Mainly by α1 and less via α 2 receptors)
b) Increase Renin secretion ( Via β receptors) –> Increase ANG II
c) Increase reabsorption of Na+ and water (catecholamine direct secretory effect from adrenal medulla(via α and β receptors)
For example if you have a large drop in blood pressure, which can happen if you lose a lot of blood, your nervous system will stimulate contraction of the afferent arteriole, reducing urine production. If further measures are needed your nervous system can also activate the renin-angiotensin-aldosterone system, a hormone system that regulates blood pressure and fluid balance.
Hormonal control - atrial natriuretic peptide is a hormone that can increase the glomerular filtration rate. This hormone is produced in your heart and is secreted when your plasma volume increases, which increases urine production.
The kidney is unique : Two capillary beds (In series)
Glomerular capillaries (high pressure) Peritubular capillaries (low pressure)
Comparing the renal capillaries with the standard systemic capillaries
Starling forces – hydrostatic pressure (favors filtration) and oncotic pressure (favors absorption)
Glomerulus – high hydrostatic P –> favours filtration
Peritubular capillaries – high oncotic P (35mmHg), low hydrostatic P (20mmHg) –> favours absorption
Peritubular capillaries:
Found in both cortex and medulla (vasa recta)
Functions:
- Deliver oxygen and nutrients to the renal tubules
- They take up (reabsorb) from the interstitium fluid and solutes reabsorbed by the tubules.
THIS SITE OF THE RENAL CAPILLARY SYSTEM FAVOURS ABSORPTION!
Reasons:
Decrease hydrostatic pressure due to resistance imparted by efferent arteriole
Increase osmotic pressure because the plasma entering here has just been filtered
DEFINE renal plasma flow
Defined as the amount of plasma that perfuses the kidney per unit of time
Can be computed using:
RPF = (1-Hct) x RBF
given a Hct of 0.40
RPF = (1-0.40) x 1000 mL/min RPF = 600 mL/min
Increase in RBF –>
–> Increase in RPF –> =/=Increase in Glomerular filtration rate (GFR).
not always true as different when aa diameter or ee diameter is changed
Purpose of autoregulation by AA
Purpose:
Stabilises the filtered load of solutes that reaches the tubules
To protect glomerular capillaries from damage when perfusion pressure is increased
Explain the two mechanisms of autoregulation
- Myogenic Mechanism (Arterial Pressure Dependent)
- Tubuloglomerular Feedback (Chemical Component)
- MYOGENIC:
When arterial pressure increases –> the renal afferent arteriole is stretched –> flow increases –> stretching means increase Ca2+ influx –> So vascular smooth muscle responds by contracting (vasoconstriction) –> thus increasing resistance –> FLOW NORMALIZED. - Tubuloglomerular Feedback:
increase GFR –> Macula densa sense increase in NaCl –>
Depolarisation due to increase Cl –>
Ca enters the macula densa through non-selective ion channel –>
Macula densa release adenosine and ATP (breaks down into adenosine) –>
Adenosine triggers vasoconstriction –> resistance increases in AA –> hydrostatic pressure in glomerulus decreases. –> decrease GFR
Factors affecting sensitivity of TGF
Increase: volume decrease adenosine PGE2 Thromboxane HETE Angiotensin II
Decrease: Volume increase/expansion ANP NO cAMP PGI2 High protein diet
Kidney is ________ priority when blood volume is compromised. It will be channeled to the brain, heart and muscles.
not a
What will happen if there’s a change in effective circulating volume?
Renal Response to Haemorrhage:
MAP decrease –> renal sympathetic nerves –> n.e. –> constriction of renal arteries
OR
MAP decrease –> renin secretion –> AT2 –> constriction of renal arteries
How does RAAS work?
JGA senses decrease in perfusion –> releases Renin which actives angiotensinogen to angiotensin I –> ACE activates it to Angiotensin II –> which causes increase in SNA, tubular Na+ and Cl- reabsorption, K+ excretion, causing H2O retention, ADH secretion causing collecting duct H2O reabsorption, vasoconstriction, aldosterone secretion.
All of these lead to salt and water retention. effective circulating volume increases, increases perfusion of JGA
Differential Sensitivities and Effects of Adrenaline on Afferent and Efferent Arterioles
higher concentration at AA of A2 and adrenaline = decrease RBF, decrease GFR.
low concentration of A2 and adrenaline at EA = decrease RBF, slight increase in GFR or maintenance of GFR.
RAAS leads to
water retention. increased systemic blood pressure. increases GFR back to normal
The renin-angiotensin system or RAS regulates blood pressure and fluid balance in the body. When blood volume or sodium levels in the body are low, or blood potassium is high, cells in the kidney release the enzyme, renin. Renin converts angiotensinogen, which is produced in the liver, to the hormone angiotensin I. An enzyme known as ACE or angiotensin-converting enzyme found in the lungs metabolizes angiotensin I into angiotensin II. Angiotensin II causes blood vessels to constrict and blood pressure to increase. Angiotensin II stimulates the release of the hormone aldosterone in the adrenal glands, which causes the renal tubules to retain sodium and water and excrete potassium. Together, angiotensin II and aldosterone work to raise blood volume, blood pressure and sodium levels in the blood to restore the balance of sodium, potassium, and fluids. If the renin-angiotensin system becomes overactive, consistently high blood pressure results.
Vasoconstriction of the arterioles increases resistance and decreases blood flow. increases systemic blood pressure back to normal
through RAAS
vasoconstrictors vs vasodilators
vasoconstrictors: Noradrenaline Adrenaline AT II Endothelin
vasodilators: prostaglandin NO bradykinin dopamine
