Week 1 Lecture 3 Flashcards
What are the vascular aspects of regulating glomerulus filtration rate?
The primary mechanism for regulating GFR is by controlling/altering renal blood flow. this is under physiological circumstances. by doing this we affect the pressure within the glomerular capillaries.
The afferent and efferent arterioles are both resistance vessels that can be independently regulated. Like all arterioles, these arterioles contain smooth muscles in the walls so they can regulate the amount of constriction or dilation of the vessel by controlling the amount of the contractions of the smooth muscle in the vessel wall. These can happen independently of each other which allows us to regulate pressure in that capillary bed.
Vasoconstriction of the afferent arteriole will reduce blood flow into the kidney. This decreases glomerular capillary blood pressure and GFR.
Vasoconstriction of the efferent arteriole will reduce blood flow out of the kidney. This increases glomerular capillary blood pressure and GFR.
through this we can control the amount of blood flow through the glomerular capillaries, we can regulate and influence the pressure and filtration rate
Physiologically, the majority of renal blood flow regulation occurs at the afferent arteriole.
The main force that drives glomerular filtration is the hydrostatic pressure in the capillaries.
(picture) However, despite the wide variation in blood pressure that can occur throughout the day, GFR remains relatively constant.
why is autoregulation of GFR important?
– Mean arterial pressure is a critical determinant of GFR; many daily activities change mean arterial pressure (e.g. exercise), yet GFR needs to remain relatively constant.
– Autoregulation of GFR avoids imbalances in fluid, electrolyte and waste excretion due to changes in mean arterial pressure.
– Autoregulation of GFR also prevents damage to the filtration barriers by high blood pressures such a podocytes.
What are the two main mechanisms of autoregulation?
Mechanisms of GFR autoregulation include the myogenic response and tubuloglomerular feedback.
Explain the myogenic response?
myo(muscle) genic (generated)
The myogenic response is an intrinsic property of arteriolar smooth muscle.
The smooth muscle contains stretch-sensitive ion channels that can be activated by stretch (eg the stretch caused by an increase in blood pressure).
This causes depolarization of the vascular smooth muscle cells, allowing calcium influx and subsequent contraction of the vessel.
Thus, an increase in MAP (mean arterial pressure) will reduce the diameter of the afferent arteriole and therefore limit increases in glomerular capillary pressure.
Explain tubuloglomerular feedback?
Tubuloglomerular feedback autoregulates GFR in a paracrine manner.
Recall that the nephron loops back on itself to form the juxtaglomerular apparatus.
This structure allows for a local control mechanism (macula densa), where changes in the amount of Na+ and water in the tubule can alter afferent arteriole diameter, and therefore influence GFR.
(macula densa detects increase in fluid flow through the tubule, it will produce paracrine mediators that are vasoactive which can change the amount of contraction in a blood vessel. produces a vasacontrictor mediators, which constrict the afferent arteriole, you then have less blood flow into the glomerular capillaries therefore a reduction in hydrostatic pressure in glomerular capillary bed and GFR is decreased. this is how it autoregulates in response to fluid increase.
GFR can be DECREASED by:
an increase in plasma osmotic pressure
What is reabsorption?
Reabsorption is a more selective process than filtration.
The body has high reabsorptive capacity for substances it requires, and less capacity for substances it doesn’t.
The vast majority (>99%) of filtered fluid is reabsorbed; the majority of this reabsorption occurs in the proximal tubule.
The reabsorption of most substances requires specific membrane transport proteins in order transport substances across the cell membrane.
Explain the difference between passive and active tubular reabsorption?
Passive reabsorption uses electrochemical and osmotic gradients to move the substance from the lumen to the plasma; no net energy is required.
Active reabsorption requires energy to actively transport a substance against its electrochemical or osmotic gradient for at least one step.
In primary active transport, the energy comes directly from ATP. Secondary active transport uses the potential energy from the concentration gradient of another molecule.
Often, actively reabsorbed substances are those that are of particular importance to the body (e.g. Na+, glucose, amino acids, etc). Why do you think this is the case?
these are important for the body to reabsorb. active transport is an energy intensive process. the body uses ATP to reabsorb because it takes energy cost to do so.
