Cardiac output increases by –
**Core Concept**
Cardiac output (CO) is the volume of blood the heart pumps per minute, calculated as the product of heart rate (HR) and stroke volume (SV). The regulation of cardiac output is a critical aspect of cardiovascular physiology, with various factors influencing its dynamics.
**Why the Correct Answer is Right**
Cardiac output increases in response to increased contractility of the heart muscle, which is typically achieved through the activation of the sympathetic nervous system. This leads to an increase in the release of catecholamines such as adrenaline and noradrenaline, which stimulate the beta-1 adrenergic receptors on the cardiac myocytes. Stimulation of these receptors increases the intracellular concentration of cyclic AMP (cAMP), leading to the activation of protein kinase A (PKA) and subsequent phosphorylation of key proteins involved in the contraction of cardiac muscle. This results in an increase in the force of contraction and a subsequent increase in cardiac output.
**Why Each Wrong Option is Incorrect**
**Option A:**
This option is incorrect because cardiac output is not directly influenced by the increase in peripheral vascular resistance alone. While increased peripheral resistance can lead to an increase in blood pressure, it does not directly affect the cardiac output unless it leads to a compensatory increase in heart rate or stroke volume.
**Option B:**
This option is incorrect because cardiac output is not directly influenced by the increase in venous return alone. While increased venous return can lead to an increase in stroke volume, it does not directly affect the cardiac output unless it leads to a compensatory increase in heart rate.
**Option C:**
This option is incorrect because cardiac output is not directly influenced by the increase in respiratory rate alone. While increased respiratory rate can lead to an increase in oxygenation of the blood, it does not directly affect the cardiac output unless it leads to a compensatory increase in heart rate or stroke volume.
**Clinical Pearl / High-Yield Fact**
The Frank-Starling mechanism, which describes the relationship between stroke volume and the end-diastolic volume of the heart, is a critical concept in cardiovascular physiology. Understanding this relationship is essential for appreciating the regulation of cardiac output in response to changes in preload.
**Correct Answer: A. Increased contractility of the heart muscle.**