The kidney and the nephron
The kidney
- The kidney cleans the blood and controls its water content.
- It does this in about a million tiny tubes called nephrons.
- Urine is made in two key steps.
Practice
The nephron is:
Each kidney has about a million nephrons; each makes urine by filtration and reabsorption.
Kidney and nephron parts
- The kidney has an outer cortex, an inner medulla, and a central renal pelvis (collects urine); the ureter carries urine to the bladder.
- Along a nephron: the glomerulus (a knot of capillaries) in the Bowman's capsule, then the proximal convoluted tubule, loop of Henle, distal convoluted tubule, and collecting duct.
Step 1: Ultrafiltration
- Happens in the Bowman's capsule.
- Blood in the glomerulus is under high pressure, pushing water and small molecules (glucose, ions, urea) out to form the filtrate.
- Blood cells and large proteins are too big to pass, so they stay in the blood.
Practice
During ultrafiltration in the Bowman's capsule:
High pressure forces small molecules into the capsule; blood cells and large proteins are too big to pass.
Step 2: Selective reabsorption
- Happens in the proximal convoluted tubule.
- Useful substances are taken back into the blood — all the glucose, and much of the water and ions.
- The tubule cells have microvilli (large surface area) and many mitochondria (to power active transport).
Practice
In selective reabsorption at the proximal convoluted tubule:
The proximal tubule reabsorbs all the glucose plus much water and ions back into the blood.
Practice
The proximal tubule cells are well suited to reabsorption because they have:
Microvilli increase the surface area and the many mitochondria provide ATP for active transport.
You've got it
Key idea
- the nephron is the kidney's working unit (~1 million per kidney)
- ultrafiltration (Bowman's capsule): high pressure pushes out small molecules; cells & proteins stay
- selective reabsorption (proximal tubule): all glucose + water + ions taken back
- tubule cells: microvilli (surface area) + mitochondria (active transport)