The circulatory system — practice questions

Capillaries have walls that are only one cell thick (endothelium). This shortens the diffusion distance for substances like oxygen, carbon dioxide, nutrients, and waste products between the blood and tissue cells, thereby increasing the rate of exchange.

They have a very narrow lumen, often just wide enough for red blood cells to pass through in single file. This forces the red blood cells close to the capillary wall, further reducing diffusion distance and allowing for more efficient loading and unloading of gases. It also increases the surface area to volume ratio of the blood within the capillary.

Capillaries form extensive, highly branched networks, providing a large total surface area for efficient exchange of materials with the surrounding tissues.

Step 1: State the formula

Cardiac Output (CO) is the product of Heart Rate (HR) and Stroke Volume (SV). CO = HR × SV

Step 2: Identify the given values

• Heart Rate (HR) = 50 beats min⁻¹
• Stroke Volume (SV) = 140 cm³ beat⁻¹

Step 3: Substitute the values into the formula

CO = 50 beats min⁻¹ × 140 cm³ beat⁻¹ CO = 7000 cm³ min⁻¹

Step 4: Convert the units to dm³ min⁻¹

The question requires the answer in cubic decimetres per minute (dm³ min⁻¹). We need to use the conversion factor: 1000 cm³ = 1 dm³.

CO = 7000 cm³ min⁻¹ / 1000 cm³ dm⁻³ CO = 7.0 dm³ min⁻¹

Final Answer

The athlete's cardiac output is 7.0 dm³ min⁻¹.