Homeostasis: Question 6

Worked solution

Part (a): Gross structure of the kidney

The kidney is divided into two main regions when cut in section: an outer cortex, which has a granular appearance because it contains the Bowman’s capsules and convoluted tubules of many nephrons, and an inner medulla, often organised into cone-shaped renal pyramids, which contains the loops of Henle and collecting ducts. Fluid draining from the tips of the medulla collects in the renal pelvis, a funnel-shaped cavity, which narrows into the ureter and carries urine towards the bladder. Blood reaches the kidney through the renal artery and leaves through the renal vein.

Because ultrafiltration happens first, the glomerulus and the Bowman’s capsule that surrounds it are both found in the cortex, at the very start of each nephron.

Part (b): The path of fluid through a nephron

Filtrate formed in the Bowman’s capsule flows, in order, through:

1. Proximal convoluted tubule (PCT), in the cortex.
2. Descending limb of the loop of Henle, dips down into the medulla.
3. Ascending limb of the loop of Henle, returns from the medulla back up to the cortex.
4. Distal convoluted tubule (DCT), in the cortex.
5. Collecting duct, runs from the cortex back down through the medulla to the renal pelvis, collecting fluid from several nephrons along the way.

So the loop of Henle and the collecting duct are the parts of the nephron that extend into the medulla; the Bowman’s capsule and both convoluted tubules stay within the cortex.

Part (c): How the loop of Henle creates the medullary water-potential gradient

The two limbs of the loop of Henle behave very differently:

• The ascending limb is impermeable to water, but its cells actively transport sodium and chloride ions out of the tubule fluid into the surrounding medulla tissue fluid.
• The descending limb is permeable to water (but not very permeable to ions), so as filtrate flows down it, water is drawn out by osmosis into the increasingly concentrated tissue fluid that surrounds it.

Because fluid in the two limbs flows in opposite directions (a countercurrent arrangement), and ion pumping out of the ascending limb continues along its entire length, this progressively concentrates the medulla tissue fluid. The deeper into the medulla, the more solute has accumulated, so water potential becomes increasingly negative towards the tip of the loop. This is the countercurrent multiplier.

This very low water potential in the medulla is essential for producing concentrated urine: the collecting duct passes straight down through this same region, and once ADH has made its wall permeable to water (by inserting aquaporins), water can only continue to leave the duct by osmosis if the surrounding tissue fluid has a lower water potential than the fluid inside the duct at every level. Without the gradient the loop of Henle creates, water reabsorption from the collecting duct would stop once the duct fluid reached the same water potential as the blood, and urine could never become more concentrated than blood plasma.

Final answers

• (a) Outer cortex (glomerulus, Bowman’s capsule, convoluted tubules) and inner medulla (loops of Henle, collecting ducts), draining into the renal pelvis and then the ureter.
• (b) Bowman’s capsule → PCT (cortex) → descending limb → ascending limb of loop of Henle (dipping into the medulla) → DCT (cortex) → collecting duct (running back down through the medulla).
• (c) Ion pumping out of the water-impermeable ascending limb, combined with osmotic water loss from the water-permeable descending limb in a countercurrent arrangement, progressively lowers the water potential of the medulla tissue fluid; this gradient is what allows water to keep leaving the collecting duct by osmosis, producing concentrated urine.