glomerular filtration Flashcards
- describe the arteriolar, capillary, and epithelial components of the filtration apparatus.
Blood/plasma flow into the glomerulus via the afferent arterioles which turn into the capillary loops (where blood/plasma are filtered into bowmans capsule) and then exit the glomerulus as efferent arterioles. Granular cells are smooth muscle cells of the afferent arteriole that secrete renin.
Filterability
The ability of a solute to be filtered through the capillary into bowmans capsule. This is a function of the solutes molecular size. A freely filtered substance has a filterability of 1 and a nonfiltered substance has filterability of 0.
Molecular size cut off for filtration through glomerulus
60,000 daltons- This is just slightly lower than the size of albumins
Which components contribute to the molecular seiving properties of the glomerulus
podocyte feet slit membranes and the basal lamina
- describe the ultrastructural basis for molecular sieving during glomerular filtration.
The endothelium of capillaries are fenestrated and allow everything but RBCs to pass through . Next, the basal lamina is composed of mucoproteins that forms a mesh work that filters, and also has a negative charge that allows positive macromolecules to filter through easier than negative charged. Next, epithelial cells called podocytes are round cells with feet that are connected by slit membranes, and this acts as a molecular seive.
List the Starling forces that drive/oppose glomerular filtration
Pgc: glomerular capillary hydrostatic pressure, Pt: tubule pressure and πgc: colloid osmotic pressure.
- describe the Starling forces that drive and oppose glomerular filtration.
Hydrostatic pressure within glomerular capillary (Pgc) drives fluid filtration from the capillary into Bowmans capsule. Tubule pressure (Pt) that develops from filtrate flowing in the narrow confines of the tubule develops a backpressure that opposes glomerular filtration. Also, colloid osmotic pressure (πgc) develops from the high protein concentration (albumin) in the blood, and this results in a net negative osmotic pressure that opposes filtration .
- state the Starling equation for glomerular filtration rate.
GFR: K (Pgc - Pt - πgc) Where K is a constant for the resistance to flow, Pgc is glomerular capillary hydrostatic pressure, Pt is tubule pressure and πgc is colloid osmotic pressure.
- state the typical magnitude of each of the Starling forces and the resultant net filtration pressure.
Pgc = 46mm; Pt = 10mm; πgc= 30mm; Net filtration pressure= 6mm
Pgc = 46mm; Pt = 10mm; πgc= 30mm; Net filtration pressure= 6mm
Implications of a small net filtration pressure
Since NFP is so small, the large glomerular filtration rate is achieved by K, the total hydraulic conductivity of the kidneys. K is large b/c the total glomerular filtration area is about 1m^2
Describe regulation of glomerular filtration rate
GFR is kept constant, then the rate of tubule handling (ie. Water reabsorption) is increased or decreased depending on input. Ie. If person drank a large excess of water, GFR stays constant while water reabsorption is decreased
Describe glomerular capillary hydrostatic pressure regulation
Changes to mean arterial pressure do not cause proportionate changes in glomerular capillary pressure. Instead, Pgc is very tightly regulated by the process of autoregulation.
- define the process of autoregulation of GFR and RBF, including the structures involved, the cellular mechanisms, and physiological context and limitations under which this process operates.
When mean arterial pressure changes, the afferent arteriole of the kidney contracts or dilates the maintain constant capillary blood flow. This maintains Pgc plus glomerular filtration rate and renal blood flow
If mean arterial pressure increases, what happens at the glomerular capillary
The afferent arteriole contracts to maintain GFR, renal blood flow and Pgc.
What is the range of autoregulation
At a MAP of 75-150mmHg RBF, Pgc, and GFR all remain fairly constant (there is some residual error though). Outside of this range,such as occurs in malignant hypertension, these variables do change rather dramatically
- define the process of hypovolemic regulation of GFR and RBF including the structures involved, the cellular mechanisms, and physiological context under which this process operates.
During severe hypovolemia (in disease), GFR is maintained by co-ordinate constriction of the afferent and efferent arterioles. This decreases renal blood flow, while maintaining GFR and glomerular capillary pressure so that ECF homeostasis is not disturbed too much.
Why is hypovolemic regulation of GFR not perfect
Since renal blood flow is decreased, the filtration fraction rises, and this causes the capillary osmotic pressure to rise. This reduces GFR even if Pgc is maintained constnat
compare autoregulation vs hypovolemic response in renal arterioles when MAP drops
for a drop in MAP the autoregulatory response is to dilate the afferent arteriole, whereas the baroreceptor response in hypovolemia causes a constriction of this very same vessel
The coordinate constriction of arterioles during hypovolemia is mediated by _______________
external and renal baroreceptors.
Describe the events that occur in constriction of arterioles during hypovolemia
1) Baroreceptors in main arteries sense drop in MAP and a baroreceptor reflex occurs which increases activity of renal sympathetic nerve and causes constriction of afferent and efferent arterioles. 2) External baroreceptor reflex causes hormonally mediated constriction of the arterioles, since neural stimulation of the afferent arteriole also causes the increased release of renin from the granular cells and concurrent arteriole constriction 3) renin/angiotensin axis is stimulated by detection of reduced arteriolar pressure by intrarenal baroreceptors thought to reside on the granular cells themselves
Describe importance of angiotensin II in hypovolemia
AgII causes constriction of all arterioles, thus raising central perfusion and pressure. It is produced by two of the pathways in renal hypovolemic response
What is filtration equilibrium
The point in a capillary where net filtration pressure equals 0 and no further filtration takes place. This can occur b/c as plasma travels the length of the vessel, it is constantly losing water to filtration and blood pressure due to the resistance of the capillary.
- describe the role of renal prostaglandins in the renal response to hypovolemia.
Renal prostaglandins are produced in response to angiotensin II by the renal interstitial cells in the kidney medulla between the renal pyramids.They have a local dilatory effect on renal arterioles which serves to maintain adequate renal blood flow (to prevent renal failure in hypovolemia) and since they are selective for the afferent arteriole, they tend to restore GFR towards normal.
