**Core Concept**
The exchange of gases between plasma and tissue fluid is a critical process in maintaining cellular respiration and oxygenation. This process occurs through the diffusion of gases across the capillary walls, facilitated by the thin layer of plasma and tissue fluid.

**Why the Correct Answer is Right**
The exchange of gases is primarily facilitated by the **diffusion** of oxygen and carbon dioxide across the **capillary walls**. This process is driven by the concentration gradient of gases between the plasma and tissue fluid. As oxygen is more concentrated in the plasma, it diffuses into the tissue fluid, while carbon dioxide, being more concentrated in the tissue fluid, diffuses out into the plasma. This process is essential for maintaining the optimal levels of oxygen in tissue fluid and removing excess carbon dioxide.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect as it refers to the function of the pulmonary alveoli, which are responsible for gas exchange between the lungs and the bloodstream, not between plasma and tissue fluid.

**Option B:** This option is incorrect as it refers to the role of the lymphatic system in removing excess fluids and proteins from the interstitial space, not in gas exchange.

**Option C:** This option is incorrect as it refers to the function of the hepatic portal system, which transports nutrients and waste products from the digestive tract to the liver, not in gas exchange.

**Clinical Pearl / High-Yield Fact**
The rate of gas exchange is influenced by several factors, including the surface area of the capillaries, the thickness of the plasma and tissue fluid layers, and the concentration gradient of gases. A reduction in the surface area of capillaries or an increase in the thickness of the plasma and tissue fluid layers can lead to impaired gas exchange.

**Correct Answer:** C. Capillaries.