The heart — practice questions

The left ventricle possesses a significantly thicker, more muscular wall compared to the right ventricle. This structural adaptation is critical for its function.

• Left Ventricle Function: It must generate very high pressure (around 120 mmHg during systole) to pump oxygenated blood through the systemic circulation to the entire body, overcoming the high resistance of a vast network of arteries and arterioles.

• Right Ventricle Function: It has a thinner, less muscular wall because its function is to pump deoxygenated blood only a short distance to the pulmonary circulation (the lungs) at a much lower pressure (around 25 mmHg during systole). This lower pressure is vital for two reasons:

  1. The pulmonary circuit has much lower resistance than the systemic circuit.
  2. A high pressure would damage the delicate capillary network in the lungs and could force fluid out of the capillaries into the alveoli, impairing gas exchange.

To calculate the cardiac output, we use the formula:

Cardiac Output (CO) = Heart Rate (HR) × Stroke Volume (SV)

Step 1: Identify the given values and ensure consistent units.

• Heart Rate (HR) = 70 beats/min
• Stroke Volume (SV) = 75 cm³/beat

The question asks for the answer in dm³ per minute. We need to convert the stroke volume from cm³ to dm³. Since $1 \text{ dm} = 10 \text{ cm}$, then $1 \text{ dm}^3 = (10 \text{ cm})^3 = 1000 \text{ cm}^3$.

$\text{SV in dm}^3 = \frac{75 \text{ cm}^3}{1000 \text{ cm}^3/\text{dm}^3} = 0.075 \text{ dm}^3/\text{beat}$

Step 2: Apply the formula. Now substitute the values into the cardiac output formula:

$\text{CO} = 70 \text{ beats/min} \times 0.075 \text{ dm}^3/\text{beat}$

Step 3: Calculate the final answer.

$\text{CO} = 5.25 \text{ dm}^3/\text{min}$

Final Answer: The cardiac output is 5.25 dm³ per minute.