The red or purple lines are medically known as splinter haemorrhages.
They’re caused by tiny blood vessels becoming damaged, and bursting.
Small splodges of blood become visible underneath the nail. As the nail grows outwards, these splodges turn into lines of blood, which resemble splinters.
But, splinter haemorrhages don’t necessarily mean you have heart disease. They could also be cause by diabetes, high cholesterol, or a fungal infection.