The speed at which blood flows depends on how much room it has to move through. A scientist investigated this by measuring, for each main type of vessel, th...
Question 1 Report
The speed at which blood flows depends on how much room it has to move through. A scientist investigated this by measuring, for each main type of vessel, the total cross-sectional area of all the vessels of that type added together, and the average speed at which blood flowed through them. Total cross-sectional area matters because blood leaving one aorta later spreads out through millions of tiny capillaries. The measurements are set out in the table and the same figures are plotted on the graph, so the pattern linking area and speed can be seen clearly.
| Vessel type | Total cross-sectional area / cm2 | Speed of blood flow / cm s-1 |
|---|---|---|
| Aorta | 4 | 40 |
| Arteries | 20 | 10 |
| Capillaries | 4500 | 0.03 |
| Veins | 40 | 5 |
| Vena cava | 18 | 18 |
(a) State the type of vessel in which blood flows most slowly. [1]
(b) Using the table, describe the relationship between total cross-sectional area and the speed of blood flow. [2]
(c) Explain why slow blood flow in the capillaries is an advantage. [3]
(d) Calculate the difference in the speed of blood flow between the aorta and the veins. [1]
(e) Explain why blood flows at high speed in the aorta. [2]
(f) Suggest why the speed of blood flow increases again in the vena cava compared with the capillaries. [1]
Answer Details
(a) Blood flows most slowly in the capillaries \((0.03\ \text{cm s}^{-1})\). [1]
(b) As the total cross-sectional area increases, the speed of blood flow decreases [1]; it is an inverse (opposite) relationship [1]. The capillaries have the largest area (4500 cm\(^2\)) and the slowest flow, while the aorta has a small area (4 cm\(^2\)) and the fastest flow. [2]
(c) Slow flow in the capillaries gives more time for the exchange of substances [1], so oxygen and glucose can diffuse out to the tissues and carbon dioxide and other wastes can diffuse in [1] before the blood leaves the capillary [1]. [3]
(d) Difference between the aorta (40) and the veins (5): \( 40 - 5 = \mathbf{35\ \text{cm s}^{-1}} \). [1]
(e) Blood is pumped straight from the ventricle at high pressure [1]; and the aorta has a small total cross-sectional area, so the blood is forced through a narrow space and moves quickly [1]. [2]
(f) Moving on from the capillaries, the many capillaries join into fewer, larger veins, so the total cross-sectional area decreases and the blood speeds up again in the vena cava. [1]
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