Anatomy and Physiology / Unit 10: The Urinary and Reproductive Systems
10.2 The Nephron and Urine Formation
Notes
Each nephron has a renal corpuscle (glomerulus, a capillary tuft, inside Bowman's capsule) and a renal tubule: proximal convoluted tubule (PCT), loop of Henle, distal convoluted tubule (DCT), which drains into a collecting duct.
- Glomerular filtration: blood pressure forces water and small solutes (glucose, amino acids, ions, urea) out of the glomerulus into Bowman's capsule. Blood cells and large proteins stay in the blood. The filtrate rate (GFR) is about 120 to 125 mL per minute (about 180 L per day).
- Tubular reabsorption: most of the filtrate (water, glucose, amino acids, ions) is returned to the blood. Normally all glucose is reabsorbed in the PCT. About 99% of the filtrate water is reabsorbed.
- Tubular secretion: additional wastes (hydrogen ions, drugs, potassium) move from blood to tubule.
- Urine = filtered + secreted - reabsorbed. Urine volume is about 1 to 2 L per day.
- Example: if proteins or blood cells appear in urine, the filtration membrane may be damaged.
Practice
Which process moves water and small solutes from the glomerulus into Bowman's capsule?
Blood pressure pushes the filtrate out through the filtration membrane.
Which substance would normally be completely reabsorbed from the filtrate in a healthy person?
Glucose is normally reabsorbed fully in the proximal tubule. Its presence in urine suggests a problem such as diabetes mellitus.
A urine test finds large amounts of protein. The most likely site of damage is the:
Large proteins should not pass into the filtrate unless the filter is damaged.
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