Case 3- renal blood flow Flashcards
Arteriography/angiography measure renal blood flow
You inject a dye/contrast material into the renal arteries via a catheter. An x-ray is taken. Can identify blockages in the renal arteries (hypertension, tumour, damage). Can be used when inserting a balloon or stent. Best resolution for small vessels. You have to be careful about the dye you use as it could cause contrast induced nephropathy.
Using a Doppler flowmeter/sonography to measure renal blood flow
Its non-invasive (ultrasound) so the patient is comfortable and there is a reduced complication rate. It is useful to guide catheter insertion. However, it is difficult to locate vessels due to overlaying organs.
Using a flow probe to measure renal blood flow
You insert a flow probe directly into a renal artery, invasive procedure so not preferred
PAH clearance to measure renal blood floe
Uses P amino Hippurate which can estimate the rate of renal blood flow. Non-invasive.
Amount of blood flow in the kidneys
Accounts for 22% of cardiac output, most stays within the cortex as its where the majority of nephrons are
Function of renal blood flow
Indirectly determine Glomerular filtration rate. Modifies rate of solute and water reabsorption by the proximal convoluted tubule. Participates in concentration and dilution of urine. Delivers substrates for excretion in the urine. Delivers oxygen, nutrients and hormones to cells along the nephron. Removes carbon dioxide, reabsorbed fluid and solutes.
Renal arterial branching
Renal arteries branch into segmental arteries. Interlobar arteries extend between pyramids. Arcuate arteries branch over pyramids in the medulla. Cortical radial arteries branch out into the cortex. They return in parallel venous system.
How blood is transported around the nephron
Afferent arterioles feed into the glomeruli capillaries, efferent arterioles leave the Glomeruli and wrap around the nephron. It goes to the peritubular capillaries or vasa recta then renal veins and inferior vena cava.
Vasa recta
Associated with the loop of Henle and descends into the medulla, has venous and arteriole loops. Peritubular capillaries surround the proximal convoluted tubule
Glomerular capillaries compared to systematic capillaries
Glomerular capillaries (55mmHg) have a much higher pressure then systematic capillaries but the pressure is relatively constant
How is blood transported in the blood vessels of the kidney?
For blood to be transported in a blood vessel there must be a pressure gradient. There is a sharp decrease in pressure across afferent and efferent arterioles. This is the effect of constricting afferent and efferent arterioles. The arterioles are the main site of resistance hence the pressure drop. Peritubular capillaries have a low pressure because they are the site of gas exchange.
The two capillary networks in the kidney
- High resistance arteriole (afferent) followed by a high pressure glomerular capillary network, followed by a second high-resistance arteriole (efferent)
- The low pressure capillary network of the peritubular capillaries and vasa recta that surround renal tubules. Gas exchange and fluid uptake occurs here
Vasoconstriction of the afferent arteriole
Reduces blood flow to the glomerulus due to increased resistance. This reduces Glomerular blood pressure and the forces pushing fluid across the Glomerular filtration membrane so GFR decreases. This reduces the pressure in the efferent arteriole so more flow will exit through the efferent arteriole
Vasodilation of the afferent arteriole
This will increase blood flow in the Glomerulus due to the reduced resistance. This increases Glomerular blood pressure and the forces pushing fluid across the Glomerular filtration membrane so GFR increases. Less flow will exit the Glomerulus through the efferent arteriole.
Vasoconstriction of the efferent arteriole
Increases blood flow/pressure in the glomerulus due to increased resistance leading to decreased renal blood flow out the Glomerulus. This is because pressure is higher in the efferent arterioles, which increases pressure in the Glomerulus and afferent arteriole. A decrease in renal blood flow out increases glomerular blood pressure and thus the driving force pushing fluid across the glomerular filtration membrane, so GFR increases
Vasodilation of the efferent arteriole
You have more flow through the efferent arteriole due to decreased resistance. This reduces glomerular blood pressure and thus the driving force pushing blood across the membrane so GFR decreases. As there is less resistance in the efferent arteriole due to the increased diameter
Neural control of renal blood floe
Sympathetic innervation of renal blood flow, vasoconstriction activated by renin release
Hormones and autacoids control of renal blood flow
Adrenaline from the adrenal medulla can cause vasoconstriction. Renin activates the angiotensin-aldosterone system which can affect renal vasculature. Autocoids are locally released factors and include endothelin, nitric acid, bradykinin and prostaglandins.
How autoregulation controls renal blood flow
The blood vessels can manage and regulate their radius themselves. Maintains renal blood flow and glomerular filtration rate within narrow limits during fluctuation in mean arterial blood pressure. Two mechanisms control this the myogenic response and tubuloglomerular feedback
How the myogenic response effects renal blood flow
A type of autoregulation. When blood pressure increases this stretches the smooth muscle cells, in response there is vasoconstriction of the smooth afferent arterioles, reducing blood flow. Used for afferent arterioles as it protects glomerular capillaries from extreme changes in pressure which can damage them. This would affect filtration meaning proteins could get through
How tubuloglomerular feedback effects renal blood flow
A type of autoregulation, involves the juxtaglomerular apparatus. Glomerular filtration rate increases, increasing the flow through the nephron tubules and passed the macula densa cells. The flow carries more sodium and chloride ions. When there is an increase in flow there is an increase in sodium and chloride ion concentration, the macula densa cells sense this and release Paracrine factors. These are Adenosine, ATP and nitric oxide. These are vasoactive factors which move out of the macula densa cells and into the interstitial fluid, affecting the vascular smooth muscle cells of the afferent arteriole. Adenosine and ATP cause vasoconstriction whilst nitric oxide is a vasodilator. Resistance in the afferent arteriole will increase, decreasing the hydrostatic pressure in glomerular decreasing its filtration rate.
Paracrine signalling
When a cell influences a cell nearby to it as the signal travels through the cell wall
Glomerular ultrafiltration
The process in which plasma is filtered from the Glomerulus into the Bowman’s capsule or nephron, Glomerular filtration rate (GFR) measures this
Clearance- renal
The volume of blood per unit time needed to supply that amount of solute to the urine. Determined by glomerular filtration, tubule reabsorbtion and tubule secretion
Measuring clearance- renal
Clearance of any substance varies between 0ml.min-1 (does not appear in the urine) and 700ml.min-1 (total renal plasma flow (substance totally removed from blood the first time it goes through the kidneys. Inulin is the best way to measure GFR but is expensive so creatine is used to get an estimated GFR (eGFR).
Equation for measuring GFR
GFR= (urinary concentration of inulin x rate of urine flow) / plasma concentration of inulin
Estimating renal plasma flow (RPF)
The substance must be fully cleared by the kidneys on the first pass. Para-aminohippuric acid (PAH) is used for this but is not a 100% effective. It underestimates RPF by about 10% as about 10% of PHA passing through the kidneys remains in the plasma.
RPF equation
RPF= (urinary concentration of PAH x rate of urine flow) / plasma concentration of PAH
Calculating renal blood flow (RBF)
You need to take into account haematocrit (the proportion of blood composed of RBC)
Equation for RBF
RBF= RPF / (1-Hct)
Starling forces in the Glomerular v extrarenal capillaries
1) hydrostatic pressure is greater in a Glomerular capillary then extrarenal capillary
2) hydrostatic pressure is constant whereas it declines along the length of the extrarenal capillary
3) glomerular capillaries are less permeable to proteins so oncotic pressure is lower in the Bowman’s capsule then in the interstitium
4) Oncotic pressure increases along the length of the Glomerular capillary but is constant along an extrarenal capillary
5) hydrostatic pressure is higher in the bowmans capsule then the interstitium of other tissues
6) due to starling forces net ultrafiltration pressure is slightly greater in the Glomerular capillaries
How starling forces cause ultrafiltration
The hydrostatic pressure remains high in the Glomerular capillaries and is higher then the opposing forces in the bowmans capsule and the oncotic pressure of plasma proteins in the blood. Thus there is a net movement of fluid through the entire length favouring filtration
