Fig. 3.1 is a drawing of a single capillary seen under a microscope. Three red blood cells are shown passing through the capillary in single file. A student...
Question 1 Report
Fig. 3.1 is a drawing of a single capillary seen under a microscope. Three red blood cells are shown passing through the capillary in single file. A student used the drawing to measure the sizes of the structures shown.
(a) Measure the internal diameter of the lumen of the capillary in Fig. 3.1, in mm, at the point shown. [1]
(b) Measure the width of one red blood cell in Fig. 3.1, in mm. [1]
(c) The magnification of Fig. 3.1 is x1500. Calculate the actual width of one red blood cell, in micrometres. Show your working. (1 mm = 1000 micrometres) [3]
(d) Describe two ways the capillary shown in Fig. 3.1 is adapted for the exchange of substances. [3]
(e) Complete Table 3.1 to compare a capillary with an artery.
| Feature | Capillary | Artery |
|---|---|---|
| Number of cells across the wall | ________ | many |
| Wall thickness | ________ | thick |
| Pressure of blood carried | ________ | high |
| Muscle and elastic fibres present | ________ | yes |
(f) State why the red blood cells pass through this capillary in single file. [2]
(g) Name one substance that passes out of the blood in a capillary into the surrounding tissues. [1]
(h) Explain how the thin wall of the capillary helps its function. [2]
(i) Draw the outline of two of the red blood cells as shown in Fig. 3.1. [2]
(j) State the name of the type of blood vessel that a capillary joins to return blood towards the heart. [1]
Answer Details
Labelled answer diagram:
This question is about the structure and function of a capillary and how it is adapted for the exchange of substances between the blood and the tissues.
(a) Measure the internal (lumen) diameter of the capillary on Fig. 3.1 with a ruler; a typical reading is about 2-3 mm (accept 1-4 mm; mark your own reading). [1]
(b) Measure the width of one red blood cell in the same way; a typical reading is about 10 mm (accept 8-12 mm as drawn). [1]
(c) At a magnification of \( \times 1500 \) the actual width of a red blood cell is
\[ \frac{\text{measured width}}{\text{magnification}} = \frac{10\ \text{mm}}{1500} = 0.0067\ \text{mm} \]
and since 1 mm = 1000 µm,
\[ 0.0067\ \text{mm} \times 1000 = 6.7\ \mu m \]
(accept 6-8 µm): correct division [1], unit conversion [1], answer [1]. [3]
(d) Two ways the capillary is adapted for exchange (any two, plus development): its wall is only one cell thick, giving a very short diffusion distance [1]; the lumen is very narrow so cells move slowly in single file, allowing time for exchange [1]; the capillary network has a very large total surface area [1]. [3]
The single-file arrangement and thin wall are shown here:
(e) The completed comparison Table 3.1:
| Feature | Capillary | Artery |
|---|---|---|
| Number of cells across the wall | one | many |
| Wall thickness | very thin | thick |
| Pressure of blood carried | low | high |
| Muscle and elastic fibres present | no / none | yes |
[4]
(f) The cells pass in single file because the lumen is only just wider than one red blood cell [1], so the cells cannot pass side by side [1]. [2]
(g) One substance that passes out of the blood into the tissues (any one): oxygen, glucose, water, or amino acids (carbon dioxide passes the other way, into the blood). [1]
(h) The thin wall gives a short diffusion distance [1], so substances are exchanged quickly between the blood and the tissues [1]. [2]
(i) An outline of two of the red blood cells as they appear in Fig. 3.1:
Marks: two biconcave-disc shapes drawn [1]; neat single outlines of correct relative size [1]. [2]
(j) A capillary joins a vein (or venule) to return blood towards the heart. [1]
Examination tip: a capillary is defined by a wall one cell thick and a lumen barely wider than a red blood cell, which together make it perfect for exchange but useless for carrying blood at high pressure.
Download The App On Google Playstore
Everything you need to excel in your exams
