Hypertension (blood pressure exceeding 140 mmHg ) affects millions of people worldwide. Many modern societies consume more sodium in the form of salt (NaCl) than is required for normal physiology. The effect of changing the levels of dietary sodium has been studied in various organisms.

Scientists recorded the blood pressure changes in chimpanzees (Pan troglodytes) fed a diet without added sodium (control) or a sodium supplemented diet (experimental) as follows
- 5 grams of salt per day from December 1991 until May 1992
- 10 grams of salt per day until June 1992
- 15 grams of salt per day until August 1993
after which no more salt was added to the diet until the experiment ended.

In an investigation to compare the elasticity of arteries and veins, rings of the same diameter $(20 \mathrm{~mm})$ of artery and vein tissue were cut from blood vessels obtained from a mammal.

Each ring was attached to a clamp. Multiple masses of 10 g were added and removed. The vertical diameter of the artery and the vein was measured, both with the mass and once the mass had been removed.

The results are shown in the table.

In an investigation to compare the elasticity of arteries and veins, rings of the same diameter $(20 \mathrm{~mm})$ of artery and vein tissue were cut from blood vessels obtained from a mammal.

Each ring was attached to a clamp. Multiple masses of 10 g were added and removed. The vertical diameter of the artery and the vein was measured, both with the mass and once the mass had been removed.

The results are shown in the table.

What is a property of arteries?

In arteries, what are the functions of the lumen and the elastic fibres in the wall?

Which blood vessels have thick muscular walls that can resist pressure and assist in pumping blood?

The micrograph shows a transverse section of blood vessels in a human leg muscle.

Which is the main contributor to the movement of blood along vessel X ?

What feature of arteries is most important in maintaining sufficiently high blood pressure?

This micrograph shows a transverse section of an artery.