**Question:** Following change occurs in pregnancy:

A. Increase in blood volume
B. Increase in cardiac output
C. Increase in maternal blood pressure
D. Increase in respiratory rate

**Core Concept:** In pregnancy, physiological adaptations occur to accommodate the growing fetus and prepare the mother for childbirth. These adaptations primarily involve cardiovascular, respiratory, and renal systems.

**Why the Correct Answer is Right:**

During pregnancy, there is a significant increase in blood volume by approximately 40-50%. This increase is due to the following reasons:

1. **Extension of the renin-angiotensin-aldosterone system (RAAS):** The RAAS is activated, leading to increased production of renin, angiotensin II, and aldosterone. These hormones cause vasoconstriction, sodium and water retention, and vasopressin release (antidiuretic hormone), respectively. These actions together result in an increase in blood volume and blood pressure to support fetal growth and maintain uterine blood flow.

2. **Vasodilation of uteroplacental circulation:** The uterine arteries dilate to ensure adequate blood flow to the placenta, which plays a crucial role in nutrient and oxygen supply to the fetus.

3. **Increased cardiac output:** To compensate for the increased blood volume and maintain adequate perfusion to vital organs, the cardiac output increases by about 30%. This increase is due to the following:

a. **Increase in heart rate:** Heart rate increases to meet the increased demand for oxygen and nutrients.

b. **Increase in stroke volume:** Stroke volume increases to ensure adequate cardiac output.

**Why Each Wrong Option is Incorrect:**

A. Increase in blood volume is not solely due to increase in cardiac output. As explained above, the increase in blood volume is primarily due to activation of RAAS and vasodilation of uteroplacental circulation.

B. Increase in blood volume is not due to an increase in respiratory rate. Respiratory rate remains relatively stable during pregnancy.

C. Maternal blood pressure increases, but it is primarily due to the increase in blood volume and cardiac output, rather than a direct increase in blood pressure.

D. Increase in cardiac output is not solely due to increase in respiratory rate. As explained above, the increase in cardiac output is due to the combined effects of increased heart rate and stroke volume.

**Correct Answer Explanation:**

The correct answer is B. Increase in cardiac output in pregnancy is due to the combined effects of increased heart rate and stroke volume. The heart rate increases to meet the increased demand for oxygen and nutrients, while the stroke volume increases to ensure adequate cardiac output to support fetal growth and maintenance of uteroplacental circulation.

**Core Concept:** The increased cardiac output during pregnancy is essential for maintaining a balance between oxygen supply and demand, ensuring fetal growth, and supporting uterine blood flow to the placenta.

**Why Each Wrong Option is Incorrect:**

A. Increase in blood volume is not solely due to an increase in cardiac output. It