Question

In the absence of compensations, the stroke volume will decrease when a. blood volume increases. b. venous return increases. c. contractility increases. d. arterial blood pressure increases.

Short answer

In the absence of compensations, the stroke volume will decrease when arterial blood pressure increases (option d).

Step-by-step solution

Define stroke volume

Stroke volume is the amount of blood pumped out of the left ventricle of the heart during each contraction. It is an important parameter in determining the efficiency of the heart as a pump. The major factors influencing stroke volume are preload, afterload, and contractility.

Analyze the impact of blood volume on stroke volume

When there is an increase in blood volume, it initially leads to an increase in venous return (the amount of blood returning to the heart). This increased venous return, in turn, increases the filling of the ventricles and the preload. According to the Frank-Starling law, an increase in preload will result in an increased stroke volume, not a decrease. Therefore, option a is incorrect.

Analyze the impact of venous return on stroke volume

As we mentioned in step 2, an increase in venous return increases the filling of the ventricles and preload. This increase in preload will lead to an increased stroke volume, according to the Frank-Starling law. Therefore, option b is also incorrect.

Analyze the impact of contractility on stroke volume

Contractility refers to the force generated by the heart muscle during contraction. An increase in contractility means that the heart is able to pump more blood with each contraction. This, in turn, will result in an increased stroke volume. So, option c is incorrect.

Analyze the impact of arterial blood pressure on stroke volume

Arterial blood pressure represents the afterload, which is the pressure against which the ventricles must contract to pump blood. An increase in arterial blood pressure will increase the afterload on the heart. Increased afterload makes it harder for the heart to pump the blood out of the ventricles, leading to a decrease in stroke volume. Therefore, option d is the correct answer.

State the correct answer

In the absence of compensations, the stroke volume will decrease when arterial blood pressure increases (option d).

Key concepts

Venous Return

Venous return is a critical concept in understanding how blood circulates back to the heart. It refers to the amount of blood returning to the heart from the body’s veins.

When venous return increases, it usually means that more blood is filling the heart during the resting phase. This extra filling leads to greater stretching of the heart muscle. Such stretching is essential because it enhances the force of contraction during the heart's next beat, a principle known as the Frank-Starling mechanism.

• Enhances cardiac output by increasing preload.
• Directly correlates with blood volume and vascular tone.

Understanding venous return helps predict how changes in volume or posture can affect the stroke volume and efficiency of the heart.

Contractility

Contractility is the ability of the heart muscle to contract and generate force. It plays a significant role in determining how effectively the heart can pump blood.

An increase in contractility means that the heart can eject more blood with each stroke, leading to a higher stroke volume. This is because the stronger contractions can push more blood out of the ventricles with each heartbeat.

• Affected by autonomic nervous system activity, hormones, and drugs.
• Increased contractility means a more forceful heart contraction.

Improved contractility benefits the overall circulatory efficiency and ensures adequate blood delivery to bodily tissues.

Arterial Blood Pressure

Arterial blood pressure is a crucial factor affecting stroke volume. It represents the resistance that the heart faces every time it pumps blood into the arteries.

High arterial blood pressure, or increased afterload, means that the heart has to work harder to eject blood, subsequently lowering stroke volume if the heart cannot compensate.

• Comprises two measures: systolic and diastolic pressures.
• Acts as the primary force against which the heart must contract.

Managing arterial blood pressure is vital for preventing diseases like hypertension that strain the heart and vascular system.

Frank-Starling Law

The Frank-Starling law is a fundamental principle that describes the relationship between the volume of blood filling the heart (preload) and the strength of subsequent heart contractions.

According to this law, the more the heart muscle is stretched during filling, the greater the force of contraction during the next heartbeat.

• This law explains how the heart is able to automatically adjust output to match venous return.
• It highlights the importance of maintaining optimal preload to ensure efficient cardiac function.

Understanding the Frank-Starling law helps in comprehending how certain conditions, such as heart failure, can affect the heart's ability to adjust to changes in blood volume.