BIOL1019 Chap.11 Kidney Structure, Filtration and the RAAS
Kidney Structure, Filtration and the RAAS
Each kidney holds about a million nephrons, and each nephron begins with a renal corpuscle where blood is filtered across fenestrated endothelium, basement membrane and podocyte slits only 6 to 9 nm wide.
This chapter explains glomerular filtration rate, which averages 125 mL/min, as the result of three forces: glomerular hydrostatic pressure of about 55 mmHg pushing fluid out against capsular hydrostatic pressure of about 15 mmHg and blood colloid osmotic pressure of about 30 mmHg, leaving a net filtration pressure of 10 mmHg.
It then compares the intrinsic controls that hold GFR steady between mean arterial pressures of 80 and 160 mmHg with the extrinsic sympathetic and hormonal controls, ending with the renin-angiotensin-aldosterone system.
The renal block runs across Weeks 8 and 9 and is examined in Test 2 and the final exam; the lectorial asks what happens to GFR when glomerular pressure falls and why the body would cut renal blood flow during severe blood loss, and both questions are worked here.
What this chapter covers
- 01
Kidney functions and internal structure
- 02
Nephron: renal corpuscle and tubule
- 03
The three-layer filtration membrane
- 04
GFR and net filtration pressure
- 05
Myogenic and tubuloglomerular autoregulation
- 06
Sympathetic control of renal blood flow
- 07
Renin, angiotensin II and aldosterone
- 08
ANP and the response to high volume
Worked example · free
Tubuloglomerular feedback when GFR rises
- +1Faster flow delivers more sodium chloride to the macula densa cells of the distal tubule.
- +1In response, the afferent arteriole constricts, reducing blood flow into the glomerulus.
- +1Glomerular pressure and therefore net filtration pressure fall, so GFR returns toward normal; the opposite happens when GFR is too low.
Key terms
- Nephron
- The structural and functional unit of the kidney, made of a renal corpuscle and a renal tubule.
- Podocyte
- A capsule cell whose foot processes leave filtration slits around the glomerular capillaries.
- Glomerular filtration rate
- The volume of filtrate the kidneys produce each minute, averaging 125 mL/min.
- Macula densa
- Tubule cells that sense sodium chloride in passing fluid and signal the afferent arteriole.
- Renin
- An enzyme released by the kidney that converts angiotensinogen to angiotensin I.
- Angiotensin II
- A hormone that constricts blood vessels and triggers aldosterone, ADH and thirst.
- Afferent arteriole
- The vessel that carries blood into the glomerulus and whose diameter intrinsic controls adjust.
Kidney Structure, Filtration and the RAAS FAQ
Why does a small fall in blood pressure affect filtration so much?
Net filtration pressure is only about 10 mmHg, the difference between glomerular hydrostatic pressure of about 55 mmHg and the opposing 15 and 30 mmHg. The opposing pressures do not fall with blood pressure, so a modest drop in glomerular pressure removes most of the net push, and GFR falls steeply unless the kidney compensates.
What is the difference between intrinsic and extrinsic control of GFR?
Intrinsic control works within the kidney to keep GFR stable: the myogenic mechanism and tubuloglomerular feedback adjust the afferent arteriole across mean arterial pressures of about 80 to 160 mmHg. Extrinsic control comes from outside, through the sympathetic nervous system and hormones such as the renin pathway, ADH and ANP, and adjusts GFR to the body's wider needs.
What does angiotensin II do?
Angiotensin II is formed when renin converts angiotensinogen to angiotensin I and ACE converts that to angiotensin II. It constricts blood vessels, including the efferent arteriole, which helps maintain glomerular pressure, and it triggers aldosterone release, ADH release and thirst, all of which help restore blood volume and pressure.
What can pass through the filtration membrane?
Filtrate must cross the fenestrated capillary endothelium, the dense basement membrane and the podocyte layer, whose filtration slits are only 6 to 9 nm wide. Water and small solutes, including glucose, amino acids, ions, vitamins and metabolic wastes, pass into the capsule, while large plasma proteins and red blood cells stay in the blood.
Exam move
Draw the nephron with its blood track and filtrate track and label where filtration, reabsorption and secretion happen. Memorise the three pressures and practise the net filtration pressure calculation with changed values until the subtraction is automatic. Then build a six-row table of GFR controls with trigger, arteriole and effect, and rehearse the renin cascade as a four-step chain.
Practise the blood-loss chain the concept checks ask for, sympathetic activity, renal blood flow, GFR and the renin pathway, until you can write it as one connected paragraph.
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