Each kidney holds about a million microscopic tubes called nephrons. Blood is filtered at the start of every nephron, and then the useful substances are taken back into the blood, leaving urine. The structure of the kidney and the work of the nephron are Extended content.
Extended: a kidney cut lengthways shows an outer cortex and an inner medulla. In each nephron, blood in the glomerulus is filtered: water, glucose, urea and ions pass into the nephron, while blood cells and proteins stay in the blood. As the filtrate flows along the nephron, all of the glucose, some of the ions and most of the water are reabsorbed into the blood. What is left is urine: urea, excess water and excess ions.
Fig. 1: a nephron (simplified); filtration in the glomerulus in the cortex, then reabsorption into the blood along the tubule, leaving urine in the collecting duct
Section 01
Cortex and medulla.
Extended. Cut a kidney in half lengthways and two regions are visible to the naked eye. Click each region of the model to see what it contains.
Cortex: the outer region. It contains the glomeruli and the first and last parts of each nephron, where filtration takes place.
Medulla: the inner region, divided into cone-shaped pyramids. It contains the loops of the nephrons and the collecting ducts, where much of the water is reabsorbed.
Pelvis: the space where urine from all the collecting ducts gathers before draining into the ureter.
Blood supply: the renal artery brings blood containing urea; the renal vein takes the filtered blood away.
Nephrons: each kidney contains about a million; each starts in the cortex and loops down into the medulla.
Examiner trap
The cortex is the outer layer and the medulla is the inner layer. The pale space in the middle, joined to the ureter, is the pelvis, not the medulla.
Section 02
Filtration in the glomerulus.
Extended. The glomerulus is a knot of blood capillaries inside the cup-shaped Bowman’s capsule at the start of the nephron. Blood arrives at high pressure. Watch which particles leave the glomerulus and enter the nephron, and read the live counts in the table.
Filtered out of the blood: water, glucose, urea and ions are small enough to pass from the glomerulus into the nephron. This liquid is the filtrate.
Stay in the blood: red blood cells and proteins are too large to be filtered, so the table shows none in the filtrate.
Associated blood vessels: blood reaches the glomerulus from a branch of the renal artery, then flows on through capillaries wrapped around the nephron, which take back the reabsorbed substances, and finally into the renal vein.
Examiner trap
Filtration does not pick out the useful substances: glucose is filtered out along with urea, because both are small. The nephron sorts useful from waste later, by reabsorption.
Stage 1 · Learn
Check what the sim just showed you
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Extended: in the kidney tab, what is the outer region of the cut kidney called?
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Extended: in the nephron tab, which two items show "none (stay in blood)" in the filtered column?
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Extended: in the nephron table, which substance always shows 0 in the in urine column?
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Extended: in the nephron table, which filtered substance is never reabsorbed?
Section 03
Reabsorption and urine.
Extended. As the filtrate flows along the nephron, the substances the body needs are taken back into the blood in the capillaries around it. The syllabus asks what is reabsorbed, not how.
Substance
Filtered in the glomerulus?
Reabsorbed?
In urine?
water
yes
most of it
excess water
glucose
yes
all of it
none
ions
yes
some of them
excess ions
urea
yes
no
yes
proteins, blood cells
no, too large
-
none
Urine contains urea, excess water and excess ions. It flows into the collecting ducts, the pelvis and then the ureter.
Water: the sim reabsorbs most of the water for clarity; in a real kidney about 99% of the filtered water is reabsorbed.
Examiner trap
Healthy urine contains no glucose and no protein. Glucose is filtered, but all of it is reabsorbed; protein is never filtered at all. Keep the two reasons separate.
Stage 2 · Exam
Exam-style questions
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Extended: which substances pass from the blood in the glomerulus into the nephron?
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Extended: which statement about reabsorption in a healthy nephron is correct?
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Extended: the inner region of the kidney, containing the pyramids, is the:
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Extended: why does the urine of a healthy person contain no protein?
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