Renal system: L36 - Function of glomerulus w/ calculations Flashcards
Explain features of the renal blood supply.
20-25% of cardiac output goes to the renal supply. The blood flow is at a rate of 1-1.2L/min, which is way more than heart or brain. The high flow is for filtration rather than metabolism. As blood flows through the renal system (renal arteries –> peritubular capillaries) pressure decreases. There is high pressure at the glomerular capillaries which is the driving force for filtration, high pressure allows easy control of blood flow. Renal blood flow is tightly regulated as is glomerular pressure (60mmHg) so that filtration remains constant as rate of filtration depends on blood flow.
Discuss the consequences of smooth muscles causing constriction/dilation in the arterioles in terms of glomerular pressure and flow rate.
Constrict the afferent arterioles (aa) and dilate the efferent arterioles (ea) –> decrease in pressure at glomerular capillaries, decrease in glomerular flow rate.
Dilate the afferent arterioles and constrict the efferent arterioles –> increase the pressure at glomerular capillaries, increase the glomerular flow rate.
Explain the mechanisms intrinsic to the kidneys that autoregulates blood flow.
Myogenic - vascular smooth muscle of the afferent arterioles in the kidneys constricts or dilates in response to changes in blood pressure.
Tubuloglomerular feedback - the macula densa cells (DT) of the juxtaglomerular apparatus sense the chemical composition (salt levels) of the urine and alter renal blood flow accordingly by releasing vasodilators and vasoconstrictors that act on afferent arteriole.
Explain the mechanisms extrinsic to the kidneys that autoregulates blood flow.
Sympathetic nervous system: stimulation causes vasoconstriction of blood vessels.
Angiotensin II also causes vasoconstriction, stimulated by renin release.
Explain what is hydrostatic pressure and how this affects filtration in the glomerulus and Bowman’s capsule.
The pressure exerted by a fluid. The hydrostatic pressure in the glomerular favours filtration. This is because the hydrostatic pressure ‘pushes’ blood through the filtration barrier. The hydrostatic pressure in the Bowman’s capsule on the other hand, opposes filtration by pushing fluid back into the glomerular capillary.
Explain what is osmotic pressure and how this affects filtration in the glomerulus and Bowman’s capsule.
Osmotic pressure is a pressure that depends on the relative concentrations of solutes present in solutions either side of a semipermeable membrane (either side of the filtration barrier). The osmotic pressure in the glomerular opposes filtration as water wants to rush into a saltier area so it moves back into the glomerular capillary. Osmotic pressure in the Bowman’s capsule favours filtration by pulling fluid out of the glomerular capillary, but this effect is small and much less than the glomerular osmotic pressure, therefore can mostly be ignored.
What is the effective filtration pressure and how is it calculated?
The overall gradient when the forces determining filtration are taken into account is the effective filtration pressure. EFP=(GHP+COP) - (GOP+CHP) = (60+0)-(30+20). Therefore EFP is typically about 10mmHg so fluid has a tendency to be filtered despite opposing forces. EFP can be altered by altering the forces that favour/oppose filtration. For example, dilation of the afferent arteriole will increase hydrostatic pressure in the glomerular capillary, causing an increase in EFP.
What are some other factors of glomerular filtration besides hydrostatic and osmotic pressures?
Permeability of glomerular capillary and surface area available of glomerular capillary, but these factors are usually constant under normal conditions.
What is glomerular filtration rate (GFR)? What is the normal value and how does this value vary?
The GFR is the volume of fluid that is filtered from the glomerular capillaries into the Bowman’s capsule in a period of time. This is normally 180L/day or 125ml/min. GFR shows individual variation and declines slowly from age 30.
Explain the concept of filtration fraction.
Not all blood that flows through the kidneys is filtered: 1.25L/min of blood flows through the kidneys = Renal blood flow.
625ml/min of plasma flows through the kidneys = Renal plasma flow (RPF).
125ml/min of plasma is filtered. This is GFR.
The fraction of RPF that is filtered is referred to as filtration fraction.
What is the equation for filtration fraction and what is the normal value?
FF=GFR/RPF=125/625=20%.
What is filtered load and how is it calculated?
Filtered flow is the amount of a particular substance that is filtered in one minute: Calculated by:
Filtered load = GFR x solute’s concentration in plasma.
How do we calculate GFR on a clinical level and what equation does this involve?
GFR can be indirectly measured by looking at the concentration of a particular solute in the urine compared with the plasma. The solute must neither be reabsorbed or secreted so that the concentration in the urine will represent the amount actually filtered. Creatine fulfils this criteria.
GFR x P (plasma concentration) = V (urine volume) x U (urine concentration).
GFR = (VxU)/P
Explain the concept of clearance and how it is calculated.
Clearance gives a measure of how much of a particular substance is being ‘cleared’ by the kidneys (i.e. net result of filtration, reabsorption and secretion). Clearance of creatine is unusual as it is directly proportional to its concentration in the plasma (as it is not secreted or reabsorbed) and therefore can be used to measure GFR. Clearance gives us an idea of how a particular substance is handled by the kidneys. We know that the clearance of creatine is 125ml/min. Therefore anything with a lower clearance than creatine must also be reabsorbed, and anything with a higher clearance must also be secreted. For example, the clearance of urea is 65ml/min which suggests that urea is both filtered and (partially) reabsorbed by the kidneys.
GFR = (VxU)/P
