Renal Hemodynamics Flashcards
how is the O2 consumption in the kidneys unique?
blood supply to the kidney is dictated by the need to filter, not its metabolic needs. almost everything that gets filtered needs to be reabsorbed, which takes energy. thus, oxygen consumption is dictated by blood flow.
describe the A-V difference and the pO2 in kidneys
because of the unique relationship between flow and consumption, the A-V difference is very small and independent of blood flow.
Tissue PO2 is also independent of flow and is determined by arteriole PO2
where are the resistance vessels in the kidneys?
in the afferent and efferent arterioles. the other vasculature has very little resistance
how is the resistance vessel set up unique in the kidneys?
usually, capillaries have a resistance vessel followed by segments of the capillary for filtration and reabsorption
the kidney has resistance vessels separately for filtration (afferent), and reabsorption (efferent), which gives it more control
describe regional blood flow in the kidney
blood flow initially enters the cortex and superficial cortex is the most well perfused. only about 10% reaches the juxtamedullary cells at the edge of the medulla, and a small portion of that perfuses the medulla via the vasa recta
how is Starlings equation altered for GFR and why?
it becomes K(glomularP-bowmansP) - glomularOsmotic
missing from the equation are bowmanOsmotic and the reflection coefficient. The reflection coefficient is nearly 1, meaning the filter is so strong that no proteins enter bowmans space. this means that there is no oncotic pressure pulling liquid into bowmans space, and thus it can be removed.
why is the filtration rate so high in the kidneys if there is more surface area in the rest of the bodies capillaries?
the “K” permeability constant is much higher in the kidneys d/t more numerous and larger fenestrations that allows free passage of fluid
what is the filtration fraction and how is it calculated?
FF = GFR/RBF = 20%
how does the oncotic pressure change throughout the glomeruli?
because fluid is leaving and protein is staying, the inward oncotic pressure increases, decreasing the amount of filtrate. as the RBF decreases, the severity of this gradient increases (and vice versa)
how do changes in afferent and efferent resistance affect glomerularP?
increased afferent R = decreased P
increased efferent R = increased P
what are the two major mechanisms that prevent proteins like albumin and immunoglobin from being filtered?
the main filtration barrier is the thick basal lamina, with the slits of podocytes acting as a secondary filter. the fenestrations of the epithelium are filled with glycocalyx. these mechanisms prevent large molecules from moving through
all layers are also negatively charged, which deters negatively charged proteins from entering as well
describe the hemodynamics of the efferent arteriole that allow it to reabsorb fluid efficiently
20% of the fluid is removed in the glomerulus, so the oncotic pressure is 20% greater. also, the resistance of the efferent arteriole decreases the driving pressure, allowing for more reabsorption
what is the general equation for renal clearance and how can it be adapted to show FRP and GFR
clearance = (Us * V)/Pas
where
Us- concentration of a substance in the urine
V- urine flow rate
Pas- arteriole concentration of a substance
to measure RBF- use paraaminohippuric acid (PAH) as the substance
to measure GFR- use inulin as the substance
normally, creatinine is used instead of inulin to calculate GFR. why is this a problem?
some creatinine is also secreted into the proximal tubule, so creatinine in the urine overestimates GFR. this is especially problematic when the filtration is damaged but secretion is not.
describe renal autoregulation
RBF will stay constant between 80 and 180 mmHg.
- the afferent arteriole has an intrinsic myogenic stretch response, meaning that in high pressures, the afferent resistance vessel will contract
- the macula densa can sense the amount of salt in the distal tubule. an increased BP results in an increased flow and an increased amount of salt. the macula densa senses this and sends a signal to the afferent arteriole to contract.
