**Core Concept**
Changes in blood passing through systemic capillaries refer to the exchange of oxygen, carbon dioxide, nutrients, and waste products between the blood and the tissues. This process is essential for maintaining proper cellular function and overall homeostasis.

**Why the Correct Answer is Right**
The correct answer involves the exchange of oxygen and carbon dioxide between the blood and the tissues. As blood flows through systemic capillaries, oxygen from the inhaled air is released from red blood cells and diffuses into the tissues, while carbon dioxide, a waste product of cellular metabolism, is picked up by the blood and transported back to the lungs for exhalation. This exchange is facilitated by the thin walls of the capillaries, which allow for the diffusion of small molecules.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because changes in blood pH and electrolyte composition do occur in systemic capillaries, but these changes are not the primary function of the capillary exchange process.

**Option B:** This option is incorrect because the exchange of water and ions between the blood and the tissues does occur in systemic capillaries, but it is not the primary function of the capillary exchange process.

**Option C:** This option is incorrect because the exchange of nutrients and waste products between the blood and the tissues does occur in systemic capillaries, but it is not the primary function of the capillary exchange process.

**Clinical Pearl / High-Yield Fact**
The exchange of oxygen and carbon dioxide between the blood and the tissues is a critical function of systemic capillaries, and any disruption to this process can have significant consequences for cellular function and overall health.

**Correct Answer:** A. Changes in blood pH and electrolyte composition are not the primary function of systemic capillary exchange.