Glomerular filtration

NFP = (outward) − (inward) = 35 − 25 = +10 mm Hg. Positive means fluid filters out of the capillary.

• +60 mm Hg:
• Correct: +10 mm Hg:
• −10 mm Hg:

Blood leaves the glomerulus through the efferent arteriole, which then feeds the peritubular capillaries or the vasa recta. The afferent arteriole brings blood in.

• Correct: The efferent arteriole:
• A venule:
• The afferent arteriole:

This is the myogenic response: stretch opens mechanically gated channels, calcium enters, and the smooth muscle contracts, so flow stays nearly steady.

• It relaxes, and the arteriole widens:
• Correct: It contracts, and the arteriole narrows:
• It does not respond without a nerve signal:

NFP = 55 − (25 + 30) = 55 − 55 = 0 mm Hg. With no net outward pressure, filtration stops. A blockage anywhere downstream raises capsular pressure and opposes filtration.

• It doubles, because more pressure is in the system: Capsular pressure opposes filtration, so extra pressure in the capsule lowers filtration rather than raising it.
• Correct: It stops, because net filtration pressure falls to 0: Correct. 55 − 55 = 0, so filtration stops.
• It is unchanged, because only the glomerular pressure matters: Capsular hydrostatic pressure is one of the three pressures in the equation. Raising it lowers NFP.
• It rises, because capsular pressure pushes fluid out: Capsular pressure pushes fluid back into the capillaries, not out of them.

Constricting the afferent arteriole cuts off pressure and flow before the glomerulus. Glomerular pressure, NFP and GFR all fall, along with renal blood flow and the pressure in the capillaries downstream.

• Renal blood flow: down. Narrowing a vessel in series with the whole kidney raises resistance, so less blood flows through.
• Glomerular hydrostatic pressure: down. More pressure is lost across the narrowed afferent arteriole before blood reaches the glomerulus.
• Net filtration pressure: down. NFP = GBHP − (CHP + BCOP). GBHP falls while the opposing pressures stay the same.
• GFR: down. GFR = Kf × NFP, and NFP has fallen.
• Hydrostatic pressure in the peritubular capillaries: down. Less pressure passes through the glomerulus and efferent arteriole to the capillaries downstream.

Below about 80 mm Hg the afferent arterioles have widened as far as they can, so autoregulation can no longer make up for the low pressure. The partial widening still keeps GFR above the dashed line, but GFR now falls with pressure. Strong sympathetic activation after a bleed lowers it further.

• The filtration membrane has become more permeable: A more permeable membrane would raise GFR, not lower it. Nothing in a bleed makes the filter leakier.
• Autoregulation has switched off completely below 80 mm Hg: Autoregulation is still helping at point A: the curve sits above the dashed line. It has reached its limit, not switched off.
• Correct: The afferent arterioles are already fully widened, and pressure is too low to keep GFR at normal: Correct. Autoregulation is at its limit, so GFR falls with pressure, though less than it would without it.
• Capsular hydrostatic pressure has risen above glomerular pressure: Nothing in a bleed raises capsular pressure. The problem is the low pressure reaching the glomerulus.

Fenestrations (fenestra = window) are round pores through the endothelial cells of the glomerular capillaries. They are large enough to let all dissolved substances through but stop blood cells.

• Filtration slits: Filtration slits are gaps between podocyte pedicels, on the capsule side of the basement membrane.
• Tight junctions: Tight junctions join neighboring endothelial cells at their edges; they are not pores through the cells.
• Pedicels: Pedicels are the podocytes' finger-like branches, not holes in the capillary.
• Correct: Fenestrations: Correct. The pores through the endothelium are fenestrations.

After a large bleed, the baroreceptor reflex drives strong sympathetic activity. Norepinephrine on alpha-1 receptor proteins constricts the afferent arterioles, glomerular pressure falls, and GFR and urine output drop, which saves blood volume.

• Correct: Strong sympathetic activation constricts his afferent arterioles: Correct. Strong sympathetic activation overrides autoregulation and lowers GFR.
• ANP released by stretched atria dilates his afferent arterioles: After blood loss the atria are less stretched, not more, so ANP release falls.
• His plasma proteins fall, so colloid osmotic pressure pulls fluid in: Lower plasma protein would weaken the inward pull and raise GFR, not lower it.
• Tubuloglomerular feedback dilates his afferent arterioles: Tubuloglomerular feedback dilating the afferent arterioles would help GFR, not cause it to fall.

Faster flow leaves less time to take back sodium chloride, so more reaches the macula densa. Its cells release ATP, which becomes adenosine and constricts the afferent arteriole. Less pressure reaches the glomerulus, and GFR falls back toward normal: negative feedback.

• Correct order: 1. Filtrate flows faster through the tubule 2. More sodium chloride reaches the macula densa 3. The macula densa releases ATP, broken down to adenosine 4. The afferent arteriole constricts 5. Glomerular hydrostatic pressure falls 6. GFR returns toward normal

Diabetes damages the glomerular basement membrane and podocytes, so albumin leaks into the urine (proteinuria). As plasma albumin falls, blood colloid osmotic pressure falls. In capillaries throughout the body, less protein holds fluid in the blood, filtration outpaces drainage, and edema appears.

• Protein in the urine irritates the bladder and triggers fluid retention: Protein in urine does not act on the bladder to cause fluid retention. The link is through plasma albumin.
• His glomeruli filter less fluid, so water that should leave in the urine builds up in his tissues: His problem is a leaky filter, not less filtration. The swelling comes from low plasma protein.
• Correct: Lost albumin lowers blood colloid osmotic pressure, so capillaries filter more fluid out: Correct. Low albumin weakens the inward pull in every capillary, causing edema.
• Protein in the filtrate raises capsular pressure and forces fluid into the veins: A little extra protein in the filtrate does not force fluid into the veins; the loss of protein from the blood is what matters.

About a fifth of the plasma is filtered as it passes through the glomerulus, and the filter holds back the proteins. The same amount of protein ends up in less plasma, so its concentration, and the oncotic pull it produces, rise along the capillary.

• The liver adds albumin to the blood inside the glomerulus: The liver releases albumin into the general circulation, not into the glomerulus specifically. The rise happens within one pass through the glomerulus.
• Correct: Protein-free fluid is filtered out, concentrating the proteins left behind: Correct. Removing protein-free fluid concentrates the proteins that remain.
• Podocytes secrete protein into the capillaries: Podocytes wrap the capillaries from outside and are part of the filter. They do not add protein to the blood.
• Blood cells break down and release their protein as they pass through the capillary loops: Blood cells do not break down as they pass through a healthy glomerulus.