The kidney and the nephron · ⁨肾与肾单位⁩

The body's filter

• The kidney cleans the blood and controls its water content.
• It does this in about a million tiny tubes called nephrons 肾单位.
• Urine is made in two key steps.

Kidney and nephron parts

• The kidney has an outer cortex 皮质, an inner medulla 髓质, and a central renal pelvis (collects urine); the ureter carries urine to the bladder.
• Along a nephron: the glomerulus 肾小球 (a knot of capillaries) in the Bowman's capsule 鲍曼囊, then the proximal convoluted tubule, loop of Henle, distal convoluted tubule, and collecting duct.

A section through the kidney: the outer cortex, inner medulla and central renal pelvis

A nephron: ultrafiltration 超滤 and selective reabsorption 选择性重吸收

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.

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 微绒毛 (large surface area) and many mitochondria (to power active transport).

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