The kidney functions through a two-stage process: filtration and selective reabsorption. Blood enters the kidney under high pressure, forcing small molecules (water, glucose, ions, and urea) through capillary pores into the kidney tubules, while large molecules like proteins and blood cells remain in the vessels.
During selective reabsorption, the kidney identifies and recovers useful substances that the body needs. 100% of the filtered glucose is reabsorbed back into the blood via active transport, along with varying amounts of water and ions depending on the body's current requirements.
Substances that are not reabsorbed, including all of the urea and excess water/ions, continue through the tubules to form urine. This urine is then transported to the bladder for storage and eventual excretion.
The concentration of urine is controlled by Antidiuretic Hormone (ADH), which is released by the pituitary gland. ADH changes the permeability of the kidney tubules, determining how much water is reabsorbed into the bloodstream.
When the blood becomes too concentrated (low water content), the brain detects this and triggers the pituitary gland to release more ADH. This makes the kidney tubules more permeable, allowing more water to be reabsorbed, resulting in a small volume of concentrated urine.
Conversely, if the blood is too dilute (high water content), the pituitary releases less ADH. The tubules become less permeable, less water is reabsorbed, and the body produces a large volume of dilute urine to expel the excess water.
| Feature | Natural Kidney | Dialysis Machine |
|---|---|---|
| Mechanism | Active transport & filtration | Passive diffusion across a membrane |
| Control | Hormonal (ADH) feedback | Fixed concentration in dialysis fluid |
| Glucose | 100% selectively reabsorbed | Maintained by equal concentration in fluid |
| Frequency | Continuous 24/7 regulation | Intermittent (sessions several times a week) |
| Waste Removal | Highly efficient urea removal | Removes urea via concentration gradient |
Terminology Precision: Always distinguish between 'filtration' (the non-selective movement of small molecules) and 'selective reabsorption' (the active recovery of specific molecules like glucose).
ADH Logic: Remember that 'Diuretic' means increasing urine production. Therefore, 'Antidiuretic' Hormone reduces urine production by keeping water in the body. If you see 'high ADH', think 'low urine volume'.
Data Interpretation: In exams, you may be asked to compare concentrations of substances in the blood plasma vs. the filtrate vs. the urine. Note that glucose should be present in plasma and filtrate, but zero in the urine of a healthy individual.
Common Mistake: Do not say that urea 'diffuses' out of the blood in the kidney. It is forced out by high-pressure mass flow during filtration.