The nephron

Fenestrated capillaries (fenestra = window) have pores through their endothelial cells on an intact basement membrane. They are found in the kidneys, the intestinal lining and endocrine glands.

• Continuous capillary:
• Correct: Fenestrated capillary:
• Sinusoid:

Arcuate arteries (arcu- = bow) arch over the bases of the pyramids, where cortex meets medulla, and give off the cortical radiate arteries.

• Correct: Arcuate artery:
• Segmental artery:
• Interlobar artery:

Renin is an enzyme released by the kidney. It cuts angiotensinogen, made by the liver, into angiotensin I. ACE then converts angiotensin I into angiotensin II, which releases aldosterone.

• It converts angiotensin I into angiotensin II:
• It makes the adrenal cortex release aldosterone:
• Correct: It cuts angiotensinogen into angiotensin I:

The glomerular capsule (Bowman's capsule) is the hollow cup around the glomerulus, the swollen start of the tubule. Filtered fluid collects in the space inside it.

• Glomerulus: The glomerulus is the tuft of capillaries inside the cup, not the cup itself.
• Correct: Glomerular capsule: Correct. The cup wrapped around the capillary tuft is the glomerular capsule.
• Proximal convoluted tubule: The proximal convoluted tubule is the coiled tube that leaves the capsule at the tubular pole.
• Afferent arteriole: The afferent arteriole is the blood vessel that carries blood into the capillary tuft.

The macula densa (dense spot) is a patch of tall, crowded cells in the wall of the tubule where it touches its own arterioles. They sense how much sodium chloride is in the fluid flowing past.

• Juxtaglomerular cells: Juxtaglomerular cells sit in the wall of the afferent arteriole, not the tubule, and release renin.
• Podocytes: Podocytes wrap the glomerular capillaries inside the capsule.
• Correct: Macula densa: Correct. These tubule cells are the macula densa.
• Mesangial cells: Mesangial cells lie between the arterioles and inside the glomerulus, not in the tubule wall.

Fluid filtered from the glomerulus collects in the capsular space of the glomerular capsule, flows into the proximal convoluted tubule, down the descending limb and up the ascending limb of the nephron loop, through the distal convoluted tubule, and into a collecting duct.

• Correct order: 1. Glomerular capsule 2. Proximal convoluted tubule 3. Descending limb of the nephron loop 4. Ascending limb of the nephron loop 5. Distal convoluted tubule 6. Collecting duct

Blood leaves the glomerulus through the efferent arteriole, which then feeds the second capillary bed, the peritubular capillaries.

• Venule: A venule would drain a capillary bed toward the veins. Here blood still has a second capillary bed to cross, so the vessel between them is an arteriole.
• Correct: Efferent arteriole: Correct. The efferent arteriole links the two capillary beds.
• Vasa recta: The vasa recta are a second capillary bed like the peritubular capillaries, fed by the efferent arteriole; they do not come between the glomerulus and the peritubular capillaries.
• Cortical radiate vein: Cortical radiate veins are much larger vessels that collect blood after both capillary beds.

This is the proximal convoluted tubule. Microvilli multiply the surface area for transport, and mitochondria supply the ATP that its pumps use. It takes back most of the filtered water and solutes.

• It stores urine between emptyings: Urine is stored in the bladder, lined by transitional epithelium. No tubule segment stores urine.
• It lets water through passively, without spending any ATP at all: Many mitochondria mean heavy ATP use. A segment built only to let water through would not need them.
• It releases renin into the blood: Renin comes from juxtaglomerular cells in the afferent arteriole wall, not from tubule cells.
• Correct: It moves large amounts of material back into the blood: Correct. A large surface and plenty of ATP point to heavy transport, which is the proximal tubule's job.

Most capillary beds drain into venules, which offer little resistance. The glomerulus drains into the efferent arteriole, a resistance vessel, so pressure backs up in the glomerular capillaries.

• The glomerulus is close to the heart: The kidneys are no closer to the heart than many other organs. The layout of their vessels, not distance, sets the pressure.
• The glomerular capillaries have thicker walls than other capillaries: Glomerular capillaries are thin, leaky fenestrated capillaries. Wall thickness does not raise the pressure inside.
• Correct: An arteriole on the way out holds pressure back in the glomerulus: Correct. Resistance downstream, in the efferent arteriole, keeps glomerular pressure high.
• The podocytes pump blood through the capillaries: Podocytes wrap the capillaries; they do not pump. The heart supplies the pressure.

The two capillary beds of a nephron are in series: afferent arteriole, glomerulus, efferent arteriole, peritubular capillaries. Blocking the first vessel starves everything downstream in that nephron but leaves its neighbors untouched.

• Blood flow into its glomerulus: down. The afferent arteriole is the only way blood enters the glomerulus, so blocking it stops the flow.
• Fluid entering its capsular space: down. With no blood and no pressure in the glomerulus, nothing is pushed into the capsule.
• Blood flow in the peritubular capillaries fed by its efferent arteriole: down. The efferent arteriole receives its blood from the glomerulus, so the second capillary bed downstream loses its supply too.
• Blood flow into the glomerulus of a neighboring nephron: no change. Each nephron has its own afferent arteriole from the cortical radiate artery, so a clot in one does not block the others.