The swelling in feet and ankles of an aged person due to sitting continuously for long hours during travel, is reduced by soaking the feet in warm salt water. This is because of:
(A) Reverse Osmosis
(B) Osmosis
(C) Edema
(D) Diffusion

Question

Accepted Answer

> [!TLDR]
> The salt solution is hypertonic, so water leaves the swollen tissue through the semipermeable cell membrane — plain osmosis, with no external pressure applied.

### 1. The physical situation
Sitting still for hours makes fluid accumulate in the tissue spaces of the feet and ankles (this *condition* is called **edema** — it is the disease, not the mechanism). The tissue fluid is relatively **dilute** in solutes.

Warm salt water outside is **concentrated** in solutes, i.e. **hypertonic**.

### 2. Which transport process applies
The cell membrane is a **semipermeable membrane** — it lets the solvent (water) pass but not the salt ions freely. Whenever solvent flows through a semipermeable membrane from the region of **lower solute concentration** to the region of **higher solute concentration**, the process is by definition **osmosis**:

$$	ext{water: dilute (inside tissue)} \;\longrightarrow\; 	ext{concentrated (salt bath)}$$

The driving force is the difference in osmotic pressure, $\pi = i\,CRT$; the hypertonic bath has the higher $\pi$, so water is drawn out and the swelling subsides.

### 3. Why the other options are wrong …

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