Learning Objectives
By the end of this section, you will be able to:
- List specific transport mechanisms occurring in different parts of the nephron, including active transport, osmosis, facilitated diffusion, and passive electrochemical gradients
- List the different membrane proteins of the nephron, including channels, transporters, and ATPase pumps
- Compare and contrast passive and active tubular reabsorption
- Explain why the differential permeability or impermeability of specific sections of the nephron tubules is necessary for urine formation
- Describe how and where water, organic compounds, and ions are reabsorbed in the nephron
- Explain the role of the loop of Henle, the vasa recta, and the countercurrent multiplication mechanisms in the concentration of urine
- List the locations in the nephron where tubular secretion occurs
- Describe how, where, and what substances are secreted by the nephron.
With up to 180 liters per day passing through the nephrons of the kidney, it is quite obvious that most of that fluid and its contents must be reabsorbed. Recall that substances that need to be removd from the body but were not yet filtered, can be secreted. This reabsorption occurs in the PCT, loop of Henle, DCT, and the collecting ducts while the majority of secretion occurs in the PCT and DCT (Table 25.5 and Figure 25.5.1). Various portions of the nephron differ in their capacity to reabsorb water and specific solutes. While much of the reabsorption and secretion occur passively based on concentration gradients, the amount of water that is reabsorbed or lost is tightly regulated. This control is exerted directly by ADH and aldosterone, and indirectly by renin. Most water is recovered in the PCT, loop of Henle, and DCT. About 10 percent (about 18 L) reaches the collecting ducts. The collecting ducts, under the influence of ADH, can recover almost all of the water passing through them, in cases of dehydration, or almost none of the water, in cases of over-hydration.
| Substances Secreted or Reabsorbed in the Nephron and Their Locations (Table 25.5) | ||||
|---|---|---|---|---|
| Substance | PCT | Loop of Henle | DCT | Collecting ducts |
| Glucose | Almost 100% reabsorbed; secondary active transport with Na+ | |||
| Oligopeptides, proteins, amino acids | Almost 100% reabsorbed; symport with Na+ | |||
| Vitamins | Reabsorbed | |||
| Lactate | Reabsorbed | |||
| Creatinine | Secreted | |||
| Urea | 50% reabsorbed by diffusion; also secreted | Secretion, diffusion in descending limb | Reabsorption in medullary collecting ducts; diffusion | |
| Sodium | 65% actively reabsorbed | 25 percent reabsorbed in thick ascending limb; active transport | 5 percent reabsorbed; active | 5 percent reabsorbed, stimulated by aldosterone; active |
| Chloride | Reabsorbed, symport with Na+, diffusion | Reabsorbed in thin and thick ascending limb; diffusion in ascending limb | Reabsorbed; diffusion | Reabsorbed; symport |
| Water | 67% reabsorbed osmotically with s