A-Level Biology 9700 / Topic 14 / A2

Filter everything, keep what you need.

Every few minutes your whole blood volume passes through the kidneys. About a fifth of the plasma is squeezed out into the nephrons, and then almost all of it is taken back. What is left behind, mainly urea and excess water and ions, becomes urine.

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The key idea

Excess amino acids cannot be stored, so in the liver they are deaminated: the amine group is removed as ammonia and converted into urea, which the kidneys excrete. Each kidney has a fibrous capsule, an outer cortex, an inner medulla and a renal pelvis that drains into the ureter; it is supplied by branches of the renal artery and drained by the renal vein. Its functional units are about a million nephrons: glomerulus and Bowman's capsule, proximal convoluted tubule, loop of Henle, distal convoluted tubule and collecting duct. In ultrafiltration, high hydrostatic pressure in the glomerulus (the afferent arteriole is wider than the efferent) forces water and small solutes through the endothelium, the basement membrane and the gaps between podocytes; cells and large proteins stay in the blood. In selective reabsorption in the PCT, Na⁺/K⁺ pumps and cotransporters, with microvilli and many mitochondria, return all the glucose and amino acids, most ions and much water to the blood.

Fig. 1: a nephron and its blood supply; the Bowman's capsule and convoluted tubules lie in the cortex, the loop of Henle and collecting duct in the medulla

Section 01

Urea, and the kidney in section.

Rotate the cut kidney and click each part: fibrous capsule, cortex, medulla, renal pelvis, ureter, and the branches of the renal artery and renal vein. Read the job of each in the panel.

Deamination: in liver cells, the amine group (-NH₂) of an excess amino acid is removed, forming ammonia and a keto acid. The keto acid can be respired or converted to glucose or fat; the ammonia, which is very toxic and soluble, is combined with carbon dioxide to make the less toxic urea.
Transport: urea passes from the liver into the blood plasma and is carried to the kidneys, which remove it from the blood.

Part	Description
fibrous capsule	tough outer layer that protects the kidney
cortex	outer region: glomeruli, Bowman's capsules, proximal and distal convoluted tubules
medulla	inner region of pyramids: loops of Henle and collecting ducts
renal pelvis	cavity that collects urine from the collecting ducts and channels it into the ureter
ureter	tube that carries urine to the bladder
renal artery and branches	bring oxygenated blood containing urea to the glomeruli
renal vein and branches	carry blood with less urea away from the kidney

Examiner trap

Urea is made in the liver, not in the kidney. The kidney only removes urea that the blood brings to it.

Section 02

One nephron, six parts.

Click each part of the nephron and its blood vessels. Notice that the afferent arteriole bringing blood to the glomerulus is wider than the efferent arteriole taking it away, and that the loop of Henle and collecting duct dip into the medulla.

Part of nephron	Where	Main role
glomerulus	cortex	knot of capillaries where ultrafiltration happens
Bowman's capsule	cortex	cup around the glomerulus that collects the glomerular filtrate
proximal convoluted tubule (PCT)	cortex	selective reabsorption of glucose, amino acids, most ions and much water
loop of Henle	medulla	builds up a high solute concentration in the medulla
distal convoluted tubule (DCT)	cortex	adjusts ion concentrations and pH
collecting duct	cortex to medulla	water reabsorption under the control of ADH; carries urine to the pelvis

Blood vessels: afferent arteriole → glomerulus → efferent arteriole → a capillary network around the tubules → venules and the renal vein.
In micrographs: glomeruli look like round knots of capillaries surrounded by a thin space (the capsule); PCT sections have cuboidal cells with a fuzzy brush border of microvilli lining the lumen; collecting ducts have paler cells with clear boundaries.

Stage 1 · Learn

Check what the sim just showed you

Four quick checks. Each correct answer earns XP and records this skill on your profile.

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In the kidney simulator, which region contains the loops of Henle and the collecting ducts?

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In the nephron simulator, why is the afferent arteriole shown wider than the efferent arteriole?

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In the ultrafiltration view, which particles stay in the blood?

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In the selective reabsorption view, what powers the uptake of glucose from the PCT lumen?

Section 03

Ultrafiltration and selective reabsorption.

Choose ultrafiltration and watch which particles pass from the glomerulus into the capsule: water, glucose, urea and ions go through, but red cells and the large purple proteins stay in the blood. Then choose selective reabsorption and watch glucose, ions and water leave the PCT while most urea stays in the filtrate.

High pressure: the efferent arteriole is narrower than the afferent, so blood pressure in the glomerular capillaries is high. This hydrostatic pressure forces fluid out, against the lower water potential of the blood.
Three layers: the capillary endothelium has pores (fenestrations); the basement membrane, a mesh of collagen and glycoproteins, is the actual filter, holding back molecules with a relative molecular mass above about 69 000; the podocytes of the capsule wall have foot-like processes with slits between them so filtrate passes easily.
Glomerular filtrate: water, glucose, amino acids, urea, ions and other small solutes; no cells and almost no protein.
PCT structure: microvilli give a large surface area facing the lumen; many mitochondria supply ATP; the basal membrane is folded and next to the capillaries; tight junctions stop fluid leaking between cells.
Mechanism: Na⁺/K⁺ pumps in the basal membrane actively pump Na⁺ into the blood, keeping Na⁺ low inside the cell. Na⁺ then diffuses in from the lumen through cotransporter proteins, carrying glucose or amino acids with it (secondary active transport). These diffuse out into the blood by facilitated diffusion, and water follows by osmosis.
Outcome: all the glucose and amino acids, about 65-80 % of the sodium ions and much of the water are reabsorbed; some urea diffuses back but most stays in the filtrate.

Examiner trap

Ultrafiltration is not selective: everything small enough passes through, including glucose and amino acids. The selecting is done afterwards, by reabsorption in the PCT.

Stage 2 · Exam

Exam-style questions

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Which describes the formation of urea?

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Which component of the glomerular filtration barrier acts as the main filter?

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A drug inhibits the Na⁺/K⁺ pumps in the basal membranes of PCT cells. What is the most likely effect?

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Which feature of PCT cells is matched with its correct function?

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Six original Cambridge-style questions

Deamination and urea, the gross structure of the kidney, the parts of a nephron, ultrafiltration in the Bowman's capsule and selective reabsorption in the proximal convoluted tubule.