**Question:** Changes in blood passing through the systemic capillaries are all except

A. Increase in blood volume
B. Increase in cardiac output
C. Decrease in blood volume
D. Increase in blood pH

**Core Concept:** Capillaries are the smallest and shortest blood vessels in the human body, connecting the arterioles and venules. They play a crucial role in the exchange of gases, nutrients, and waste products between the blood and body tissues. The systemic capillaries are part of the systemic circulation and are involved in the distribution and absorption of oxygen, glucose, and other essential nutrients into the tissue spaces, while removing waste products like carbon dioxide and urea from the tissue spaces back into the bloodstream.

**Why the Correct Answer is Right:** The correct answer is D (Increase in blood pH), as the pH of blood remains relatively stable, typically between 7.35 and 7.45, to maintain a suitable environment for cellular functions. Any significant change in pH would be compensated by the body through the buffering mechanisms, including the respiratory and renal systems, rather than occurring within the capillaries.

**Why Each Wrong Option is Incorrect:**

A. Increase in blood volume: While blood volume may change, such as in dehydration or hemorrhage, the capillaries are not the primary site for volume regulation. The kidneys and plasma volume regulation mechanisms primarily control blood volume changes.

B. Increase in cardiac output: Cardiac output is the product of heart rate and stroke volume, determining the amount of blood ejected by the heart per minute. Although changes in cardiac output can affect the amount of blood passing through capillaries, it is not specific to capillaries and is not directly related to the changes occurring in capillaries.

C. Decrease in blood volume: As mentioned earlier, blood volume changes are regulated primarily by the kidneys and plasma volume regulation mechanisms, not the capillaries themselves.

**Clinical Pearl:** The correct answer (D) emphasizes the importance of pH regulation in the body rather than the capillaries, which are primarily involved in gas and nutrient exchange, waste removal, and temperature control. Understanding the body's buffering mechanisms is crucial for maintaining homeostasis and preventing the capillaries from being overwhelmed with pH regulation.