Question

An animal’s normal stroke volume is 9 mL/beat and its normal heart rate is 125 beats/min. Immediately after a hemorrhage, its heart rate increases to 161 beats/min and its stroke volume does not change. What is its new cardiac output? a. 1.45 L/min d. 17.9 L/min b. 0.145 L/min e. 0.055 L/min c. 17.9 mL/min d. 17.9 L/min e. 0.055 L/min

Short answer

The new cardiac output of the animal after hemorrhage is approximately 1.45 L/min.

Step-by-step solution

Identify the Given Parameters

The normal stroke volume of the animal is 9 mL/beat. The normal heart rate is 125 beats/min. After the hemorrhage, the heart rate increases to 161 beats/min, but the stroke volume remains unchanged.

Use the Cardiac Output Formula

The formula to calculate cardiac output is: Cardiac output = stroke volume x heart rate. Since the stroke volume remains the same after hemorrhage, only the heart rate changes. Thus, for the new cardiac output, the unchanged stroke volume should be multiplied by the increased heart rate.

Substitute the Values into the Formula and Solve

Substitute the values into the cardiac output formula: Cardiac output = 9 mL/beat x 161 beats/min = 1449 mL/min. But in the options, the cardiac output is given in L/min. So, to convert mL/min to L/min, divide by 1000. Hence, the new cardiac output is 1.449 L/min, which rounds off to approximately 1.45 L/min.

Key concepts

Stroke Volume

Stroke volume is a crucial concept in understanding how the heart functions. It refers to the amount of blood that the heart pumps out with each beat. To put it simply, every time your heart contracts, it pushes a certain amount of blood into your circulation—this is the stroke volume. Typically, stroke volume is measured in milliliters per beat (mL/beat).
In many animals, this volume remains relatively constant under normal conditions. However, various factors such as heart disease or physical stress can influence stroke volume. In our exercise, the animal's stroke volume was 9 mL/beat, and importantly, it didn't change even after a hemorrhage occurred, which is a key factor for calculating cardiac output.

Heart Rate

Heart rate indicates the number of heartbeats per minute. It is a critical measure of heart health and efficiency. This rate can vary significantly among animals and is influenced by several factors, including activity level, stress, and medical conditions.
In the exercise scenario, the animal's normal heart rate was 125 beats per minute. However, after experiencing a hemorrhage, the heart rate increased to 161 beats per minute. This increase is a common physiological response to compensate for the loss of blood, aiming to maintain adequate blood circulation throughout the body.

Hemorrhage

A hemorrhage refers to the loss of a significant amount of blood from the circulatory system. This can arise from injuries, medical conditions, or surgical procedures. Hemorrhage can be life-threatening as it leads to decreased blood volume and reduced oxygen delivery to tissues.
When the body experiences a hemorrhage, it often triggers compensatory mechanisms such as an increase in heart rate. This response helps counteract the drop in blood pressure and maintains blood flow to essential organs. In our exercise, after the animal hemorrhaged, even though the stroke volume remained unchanged, the heart rate went up significantly.

Cardiac Output Formula

Cardiac output is a fundamental measurement in cardiovascular physiology. It represents the total volume of blood the heart pumps per minute and is crucial for ensuring that tissues in the body get enough oxygen and nutrients.

• The formula for cardiac output is:
  \[ \text{Cardiac Output} = \text{Stroke Volume} \times \text{Heart Rate} \]
• In this exercise, despite the animal's stroke volume staying at 9 mL/beat, the heart rate increased to 161 beats/min because of a hemorrhage.
• Substituting these values into the formula, we calculate the cardiac output as 9 mL/beat multiplied by 161 beats/min, equating to 1449 mL/min. To convert this to liters per minute, divide by 1000, resulting in approximately 1.45 L/min.

This calculation shows how cardiac output adjusts according to changes in heart rate, reflecting the heart's ability to adapt to bodily demands.